gcc(1)
GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for
the remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing,
compilation, assembly and linking. The "overall options"
allow you to stop this process at an intermediate stage.
For example, the -c option says not to run the linker. Then
the output consists of object files output by the assembler.
Other options are passed on to one or more stages of
processing. Some options control the preprocessor and
others the compiler itself. Yet other options control the
assembler and linker; most of these are not documented here,
since you rarely need to use any of them.
Most of the command-line options that you can use with GCC
are useful for C programs; when an option is only useful
with another language (usually C++), the explanation says so
explicitly. If the description for a particular option does
not mention a source language, you can use that option with
all supported languages.
The usual way to run GCC is to run the executable called
gcc, or machine-gcc when cross-compiling, or
machine-gcc-version to run a specific version of GCC. When
you compile C++ programs, you should invoke GCC as g++
instead.
The gcc program accepts options and file names as operands.
Many options have multi-letter names; therefore multiple
single-letter options may not be grouped: -dv is very
different from -d -v.
You can mix options and other arguments. For the most part,
the order you use doesn't matter. Order does matter when
you use several options of the same kind; for example, if
you specify -L more than once, the directories are searched
in the order specified. Also, the placement of the -l
option is significant.
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Many options have long names starting with -f or with
-W---for example, -fmove-loop-invariants, -Wformat and so
on. Most of these have both positive and negative forms;
the negative form of -ffoo is -fno-foo. This manual
documents only one of these two forms, whichever one is not
the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type.
Explanations are in the following sections.
Overall Options
-c -S -E -o file -x language -v -###
--help[=class[,...]] --target-help --version
-pass-exit-codes -pipe -specs=file -wrapper @file
-fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline
-fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm
-fno-builtin -fno-builtin-function -fgimple -fhosted
-ffreestanding -fopenacc -fopenmp -fopenmp-simd
-fms-extensions -fplan9-extensions
-fsso-struct=endianness -fallow-single-precision
-fcond-mismatch -flax-vector-conversions
-fsigned-bitfields -fsigned-char -funsigned-bitfields
-funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fcheck-new -fconstexpr-depth=n
-fconstexpr-loop-limit=n -ffriend-injection
-fno-elide-constructors -fno-enforce-eh-specs
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions
-fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt
-fno-operator-names -fno-optional-diags -fpermissive
-fno-pretty-templates -frepo -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -Wabi=n -Wabi-tag
-Wconversion-null -Wctor-dtor-privacy
-Wdelete-non-virtual-dtor -Wliteral-suffix
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-Wmultiple-inheritance -Wnamespaces -Wnarrowing
-Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor
-Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel
-Wtemplates -Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
-Wvirtual-inheritance
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime
-fnext-runtime -fno-nil-receivers -fobjc-abi-version=n
-fobjc-call-cxx-cdtors -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls
-Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fno-diagnostics-show-option
-fno-diagnostics-show-caret
-fdiagnostics-parseable-fixits
-fdiagnostics-generate-patch -fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic
-pedantic-errors -w -Wextra -Wall -Waddress
-Waggregate-return -Walloc-zero
-Walloc-size-larger-than=n -Walloca
-Walloca-larger-than=n
-Wno-aggressive-loop-optimizations -Warray-bounds
-Warray-bounds=n -Wno-attributes -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat
-Wc99-c11-compat -Wc++-compat -Wc++11-compat
-Wc++14-compat -Wcast-align -Wcast-qual
-Wchar-subscripts -Wchkp -Wclobbered -Wcomment
-Wconditionally-supported -Wconversion
-Wcoverage-mismatch -Wno-cpp -Wdangling-else
-Wdate-time -Wdelete-incomplete -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-qualifiers
-Wno-discarded-array-qualifiers -Wno-div-by-zero
-Wdouble-promotion -Wduplicated-branches
-Wduplicated-cond -Wempty-body -Wenum-compare
-Wno-endif-labels -Wexpansion-to-defined -Werror
-Werror=* -Wfatal-errors -Wfloat-equal -Wformat
-Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral
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-Wformat-overflow=n -Wformat-security
-Wformat-signedness -Wformat-truncation=n -Wformat-y2k
-Wframe-address -Wframe-larger-than=len
-Wno-free-nonheap-object -Wjump-misses-init
-Wignored-qualifiers -Wignored-attributes
-Wincompatible-pointer-types -Wimplicit
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wimplicit-function-declaration -Wimplicit-int
-Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Winvalid-memory-model -Wno-invalid-offsetof
-Winvalid-pch -Wlarger-than=len -Wlogical-op
-Wlogical-not-parentheses -Wlong-long -Wmain
-Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-braces -Wmissing-field-initializers
-Wmissing-include-dirs -Wno-multichar -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd
-Woverride-init-side-effects -Woverlength-strings
-Wpacked -Wpacked-bitfield-compat -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wplacement-new
-Wplacement-new=n -Wpointer-arith -Wpointer-compare
-Wno-pointer-to-int-cast -Wno-pragmas -Wredundant-decls
-Wrestrict -Wno-return-local-addr -Wreturn-type
-Wsequence-point -Wshadow -Wno-shadow-ivar
-Wshadow=global, -Wshadow=local,
-Wshadow=compatible-local -Wshift-overflow
-Wshift-overflow=n -Wshift-count-negative
-Wshift-count-overflow -Wshift-negative-value
-Wsign-compare -Wsign-conversion -Wfloat-conversion
-Wno-scalar-storage-order -Wsizeof-pointer-memaccess
-Wsizeof-array-argument -Wstack-protector
-Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n
-Wstringop-overflow=n
-Wsuggest-attribute=[pure|const|noreturn|format]
-Wsuggest-final-types -Wsuggest-final-methods
-Wsuggest-override -Wmissing-format-attribute
-Wsubobject-linkage -Wswitch -Wswitch-bool
-Wswitch-default -Wswitch-enum -Wswitch-unreachable
-Wsync-nand -Wsystem-headers -Wtautological-compare
-Wtrampolines -Wtrigraphs -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas
-Wunsafe-loop-optimizations -Wunsuffixed-float-constants
-Wunused -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros
-Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wunused-const-variable
-Wunused-const-variable=n -Wunused-but-set-parameter
-Wunused-but-set-variable -Wuseless-cast
-Wvariadic-macros -Wvector-operation-performance -Wvla
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-Wvla-larger-than=n -Wvolatile-register-var
-Wwrite-strings -Wzero-as-null-pointer-constant -Whsa
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes
-Wnested-externs -Wold-style-declaration
-Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gcoff -gdwarf -gdwarf-version -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstabs
-gstabs+ -gstrict-dwarf -gno-strict-dwarf
-gcolumn-info -gno-column-info -gvms -gxcoff -gxcoff+
-gz[=type] -fdebug-prefix-map=old=new
-fdebug-types-section -feliminate-dwarf2-dups
-fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly
-femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-feliminate-unused-debug-symbols
-femit-class-debug-always -fno-merge-debug-strings
-fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n]
-falign-jumps[=n] -falign-labels[=n] -falign-loops[=n]
-fassociative-math -fauto-profile
-fauto-profile[=path] -fauto-inc-dec
-fbranch-probabilities -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcombine-stack-adjustments
-fconserve-stack -fcompare-elim -fcprop-registers
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fcx-fortran-rules -fcx-limited-range -fdata-sections
-fdce -fdelayed-branch -fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively
-fdevirtualize-at-ltrans -fdse -fearly-inlining
-fipa-sra -fexpensive-optimizations -ffat-lto-objects
-ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -fforward-propagate
-ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm
-fgraphite-identity -fgcse-sm -fhoist-adjacent-loads
-fif-conversion -fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once
-finline-limit=n -finline-small-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta
-fipa-profile -fipa-pure-const -fipa-reference
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-fipa-icf -fira-algorithm=algorithm -fira-region=region
-fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam
-floop-nest-optimize -floop-parallelize-all -flra-remat
-flto -flto-compression-level -flto-partition=alg
-fmerge-all-constants -fmerge-constants -fmodulo-sched
-fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fno-branch-count-reg -fno-defer-pop
-fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock
-fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder
-fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning
-fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-values
-fprofile-reorder-functions -freciprocal-math -free
-frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling
-fselective-scheduling2 -fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap
-fshrink-wrap-separate -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-loops -fsplit-paths -fsplit-wide-types
-fssa-backprop -fssa-phiopt -fstdarg-opt
-fstore-merging -fstrict-aliasing -fstrict-overflow
-fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -fcode-hoisting -ftree-loop-if-convert
-ftree-loop-im -ftree-phiprop -ftree-loop-distribution
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-ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta
-ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra
-ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp -funconstrained-commons
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model
-fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin
--param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast
-Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fsanitize=style
-fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number
-fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcheck-pointer-bounds -fchkp-check-incomplete-type
-fchkp-first-field-has-own-bounds -fchkp-narrow-bounds
-fchkp-narrow-to-innermost-array -fchkp-optimize
-fchkp-use-fast-string-functions
-fchkp-use-nochk-string-functions
-fchkp-use-static-bounds -fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite
-fchkp-check-read -fchkp-check-read -fchkp-check-write
-fchkp-store-bounds -fchkp-instrument-calls
-fchkp-instrument-marked-only -fchkp-use-wrappers
-fchkp-flexible-struct-trailing-arrays -fstack-protector
-fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack
-fvtable-verify=[std|preinit|none] -fvtv-counts
-fvtv-debug -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC
-Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers
-fexec-charset=charset -fextended-identifiers
-finput-charset=charset -fno-canonical-system-headers
-fpch-deps -fpch-preprocess -fpreprocessed
-ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H
-imacros file -include file -M -MD -MF -MG -MM
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-MMD -MP -MQ -MT -no-integrated-cpp -P -pthread
-remap -traditional -traditional-cpp -trigraphs
-Umacro -undef -Wp,option -Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary
-nostartfiles -nodefaultlibs -nostdlib -pie -pthread
-rdynamic -s -static -static-libgcc -static-libstdc++
-static-libasan -static-libtsan -static-liblsan
-static-libubsan -static-libmpx -static-libmpxwrappers
-shared -shared-libgcc -symbolic -T script -Wl,option
-Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file
-imultilib dir -iplugindir=dir -iprefix file -iquote
dir -isysroot dir -isystem dir -iwithprefix dir
-iwithprefixbefore dir -Ldir -no-canonical-prefixes
--no-sysroot-suffix -nostdinc -nostdinc++
--sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg
-fexceptions -fnon-call-exceptions
-fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-plt -fno-jump-tables -frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-wchar
-fverbose-asm -fpack-struct[=n] -fleading-underscore
-ftls-model=model -fstack-reuse=reuse_level
-ftrampolines -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion
-dumpfullversion -fchecking -fchecking=n
-fdbg-cnt-list -fdbg-cnt=counter-value-list
-fdisable-ipa-pass_name -fdisable-rtl-pass_name
-fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-
name=range-list -fdump-noaddr -fdump-unnumbered
-fdump-unnumbered-links -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes
-fdump-rtl-pass -fdump-rtl-pass=filename
-fdump-statistics -fdump-final-insns[=file]
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-fdump-tree-all -fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
-fcompare-debug[=opts] -fcompare-debug-second
-fenable-kind-pass -fenable-kind-pass=range-list
-fira-verbose=n -flto-report -flto-report-wpa
-fmem-report-wpa -fmem-report -fpre-ipa-mem-report
-fpost-ipa-mem-report -fopt-info
-fopt-info-options[=file] -fprofile-report
-frandom-seed=string -fsched-verbose=n
-fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstats -fstack-usage
-ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps
-save-temps=cwd -save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian
-mlittle-endian -mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large -mstrict-align
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional
-mtls-size=size -mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769 -mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419 -mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt -mlow-precision-sqrt
-mno-low-precision-sqrt -mlow-precision-div
-mno-low-precision-div -march=name -mcpu=name
-mtune=name
Adapteva Epiphany Options -mhalf-reg-file
-mprefer-short-insn-regs -mbranch-cost=num -mcmove
-mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc
-mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode
-mvect-double -max-vect-align=num -msplit-vecmove-early
-m1reg-reg
ARC Options -mbarrel-shifter -mcpu=cpu -mA6 -mARC600
-mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64
-matomic -mnorm -mspfp -mspfp-compact -mspfp-fast
-msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf
-mlock -mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony
-mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata
-mvolatile-cache -mtp-regno=regno -malign-call
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-mauto-modify-reg -mbbit-peephole -mno-brcc
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec
-mearly-cbranchsi -mexpand-adddi -mindexed-loads -mlra
-mlra-priority-none -mlra-priority-compact mlra-
priority-noncompact -mno-millicode -mmixed-code
-mq-class -mRcq -mRcw -msize-level=level -mtune=cpu
-mmultcost=num -munalign-prob-threshold=probability
-mmpy-option=multo -mdiv-rem -mcode-density -mll64
-mfpu=fpu
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check
-mapcs-reentrant -mno-apcs-reentrant -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name
-mtune=name -mprint-tune-info
-mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mpoke-function-name -mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations
-mfix-cortex-m3-ldrd -munaligned-access
-mneon-for-64bits -mslow-flash-data -masm-syntax-unified
-mrestrict-it -mpure-code -mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mint8
-mn_flash=size -mno-interrupts -mrelax -mrmw
-mstrict-X -mtiny-stack -mfract-convert-truncate
-nodevicelib -Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k
-mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library
-msep-data -mno-sep-data -mlong-calls -mno-long-calls
-mfast-fp -minline-plt -mmulticore -mcorea -mcoreb
-msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu
-msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align -m32-bit
gcc-7.3.0 Last change: 2018-01-25 10
GCC(1) GNU GCC(1)
-m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32
-mbit-ops -mdata-model=model
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic
-dynamiclib -exported_symbols_list -filelist
-flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names
-iframework -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding
-nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign -sectobjectsymbols
-whyload -seg1addr -sectcreate -sectobjectsymbols
-sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table
-seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches
-whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax
-mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text
-mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double -mmedia
-mno-media -mmuladd -mno-muladd -mfdpic -minline-plt
-mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4
gcc-7.3.0 Last change: 2018-01-25 11
GCC(1) GNU GCC(1)
-macc-8 -mpack -mno-pack -mno-eflags -mcond-move
-mno-cond-move -moptimize-membar -mno-optimize-membar
-mscc -mno-scc -mcond-exec -mno-cond-exec
-mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls
-mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic
-mandroid -tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr
-mint32 -malign-300
HPPA Options -march=architecture-type -mcaller-copies
-mdisable-fpregs -mdisable-indexing
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store
-mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0
-mportable-runtime -mschedule=cpu-type -mspace-regs
-msio -mwsio -munix=unix-std -nolibdld -static
-threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as
-mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names
-msdata -mno-sdata -mconstant-gp -mauto-pic
-mfused-madd -minline-float-divide-min-latency
-minline-float-divide-max-throughput
-mno-inline-float-divide -minline-int-divide-min-latency
-minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt
-mdwarf2-asm -mearly-stop-bits -mfixed-range=register-
range -mtls-size=tls-size -mtune=cpu-type -milp32
-mlp64 -msched-br-data-spec -msched-ar-data-spec
-msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec
-msched-spec-ldc -msched-spec-control-ldc
-msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
gcc-7.3.0 Last change: 2018-01-25 12
GCC(1) GNU GCC(1)
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug
-malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type
-msdata=sdata-type -mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune
-m68000 -m68020 -m68020-40 -m68020-60 -m68030
-m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x
-m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv
-mno-div -mshort -mno-short -mhard-float -m68881
-msoft-float -mpcrel -malign-int -mstrict-align
-msep-data -mno-sep-data -mshared-library-id=n
-mid-shared-library -mno-id-shared-library -mxgot
-mno-xgot -mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n
-mbitops -mc=n -mclip -mconfig=name -mcop -mcop32
-mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile
-ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat
-ms -msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float
-msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul
-mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare
-mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1
-mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3
-mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3
-mips64r5 -mips64r6 -mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed
-minterlink-mips16 -mno-interlink-mips16 -mabi=abi
-mabicalls -mno-abicalls -mshared -mno-shared -mplt
-mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32
gcc-7.3.0 Last change: 2018-01-25 13
GCC(1) GNU GCC(1)
-mfpxx -mfp64 -mhard-float -msoft-float -mno-float
-msingle-float -mdouble-float -modd-spreg
-mno-odd-spreg -mabs=mode -mnan=encoding -mdsp
-mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva
-mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mmicromips
-mno-micromips -mmsa -mno-msa -mfpu=fpu-type
-msmartmips -mno-smartmips -mpaired-single
-mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata
-mno-local-sdata -mextern-sdata -mno-extern-sdata
-mgpopt -mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable=setting -msplit-addresses
-mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -mmemcpy
-mno-memcpy -mlong-calls -mno-long-calls -mmad
-mno-mad -mimadd -mno-imadd -mfused-madd
-mno-fused-madd -nocpp -mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely
-mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align
-msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4
-mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls
-mmcount-ra-address -mframe-header-opt
-mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon
-mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf -mbranch-predict
-mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33
-mam33 -mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw
-msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge
-msmall -mrelax -mwarn-mcu -mcode-region=
-mdata-region= -msilicon-errata= -msilicon-errata-warn=
-mhwmult= -minrt
gcc-7.3.0 Last change: 2018-01-25 14
GCC(1) GNU GCC(1)
NDS32 Options -mbig-endian -mlittle-endian
-mreduced-regs -mfull-regs -mcmov -mno-cmov -mperf-ext
-mno-perf-ext -mv3push -mno-v3push -m16bit -mno-16bit
-misr-vector-size=num -mcache-block-size=num -march=arch
-mcmodel=code-model -mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt
-mno-gpopt -mel -meb -mno-bypass-cache -mbypass-cache
-mno-cache-volatile -mcache-volatile -mno-fast-sw-div
-mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx
-mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N
-mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc
-msys-crt0=name -msys-lib=name -march=arch -mbmx
-mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0
-m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32
-mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64
-mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap -munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RISC-V Options -mbranch-cost=N-instruction -mplt
-mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv
-mno-div -march=ISA-string -mtune=processor-string
-msmall-data-limit=N-bytes -msave-restore
-mno-save-restore -mstrict-align -mno-strict-align
-mcmodel=medlow -mcmodel=medany -mexplicit-relocs
-mno-explicit-relocs
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14
-mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13
-mg14 -m64bit-doubles -m32bit-doubles
-msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-
type -mcmodel=code-model -mpowerpc64 -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf
-mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd
-mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp
-mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat
-mpe -malign-power -malign-natural -msoft-float
-mhard-float -mmultiple -mno-multiple -msingle-float
gcc-7.3.0 Last change: 2018-01-25 15
GCC(1) GNU GCC(1)
-mdouble-float -msimple-fpu -mstring -mno-string
-mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mfused-madd
-mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-maltivec -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return -mabi=abi-type
-msecure-plt -mbss-plt -mblock-move-inline-limit=num
-misel -mno-isel -misel=yes -misel=no -mspe -mno-spe
-mspe=yes -mspe=no -mpaired -mgen-cell-microcode
-mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw
-mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes
-mfloat-gprs=no -mfloat-gprs=single
-mfloat-gprs=double -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt
-mvxworks -G num -mrecip -mrecip=opt -mno-recip
-mrecip-precision -mno-recip-precision -mveclibabi=type
-mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion
-mno-mpower8-fusion -mpower8-vector -mno-power8-vector
-mcrypto -mno-crypto -mhtm -mno-htm -mdirect-move
-mno-direct-move -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic
-mcompat-align-parm -mno-compat-align-parm
-mupper-regs-df -mno-upper-regs-df -mupper-regs-sf
-mno-upper-regs-sf -mupper-regs-di -mno-upper-regs-di
-mupper-regs -mno-upper-regs -mfloat128 -mno-float128
-mfloat128-hardware -mno-float128-hardware
-mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mlra -mno-lra
RX Options -m64bit-doubles -m32bit-doubles -fpu
-nofpu -mcpu= -mbig-endian-data -mlittle-endian-data
-msmall-data -msim -mno-sim -mas100-syntax
-mno-as100-syntax -mrelax -mmax-constant-size=
-mint-register= -mpid -mallow-string-insns
-mno-allow-string-insns -mjsr
-mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-
type -mhard-float -msoft-float -mhard-dfp
-mno-hard-dfp -mlong-double-64 -mlong-double-128
gcc-7.3.0 Last change: 2018-01-25 16
GCC(1) GNU GCC(1)
-mbackchain -mno-backchain -mpacked-stack
-mno-packed-stack -msmall-exec -mno-small-exec -mmvcle
-mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace
-mno-tpf-trace -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard -mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5
-mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only
-m4a-single -m4a -m4al -mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot
-musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range
-maccumulate-outgoing-args -matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mno-zdcbranch
-mcbranch-force-delay-slot -mfused-madd -mno-fused-madd
-mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove
-mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap
-mimpure-text -mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mmemory-model=mem-model -m32 -m64
-mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return
-munaligned-doubles -mno-unaligned-doubles -muser-mode
-mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4
-mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf
-mno-fmaf -mfsmuld -mno-fsmuld -mpopc -mno-popc
-msubxc -mno-subxc -mfix-at697f -mfix-ut699
-mfix-ut700 -mfix-gr712rc -mlra -mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma
-munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem
-mstdmain -mfixed-range=register-range -mea32 -mea64
-maddress-space-conversion
-mno-address-space-conversion -mcache-size=cache-size
-matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
gcc-7.3.0 Last change: 2018-01-25 17
GCC(1) GNU GCC(1)
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian
-mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep
-mno-ep -mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs
-mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2
-mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop
-mrelax -mlong-jumps -msoft-float -mhard-float -mgcc-abi
-mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu
-mhard-float -msoft-float -mcpu=cpu-type -mtune=cpu-
type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type
-mtune-ctrl=feature-list -mdump-tune-features
-mno-default -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -m80387
-mhard-float -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf
-mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper
-mprefer-avx128 -mmmx -msse -msse2 -msse3 -mssse3
-msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512pf -mavx512er -mavx512cd -mavx512vl
-mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi
-msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c
-mfma -mprefetchwt1 -mclflushopt -mxsavec -mxsaves
-msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi
-mtbm -mfma4 -mxop -mlzcnt -mbmi2 -mfxsr -mxsave
-mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -mclzero
-mpku -mthreads -mms-bitfields -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically
-mstringop-strategy=alg -mmemcpy-strategy=strategy
-mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm
-mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64
-mpc80 -mstackrealign -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-
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GCC(1) GNU GCC(1)
model -mabi=name -maddress-mode=mode -m32 -m64 -mx32
-m16 -miamcu -mlarge-data-threshold=num -msse2avx
-mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mmitigate-rop
-mgeneral-regs-only -mindirect-branch=choice
-mfunction-return==choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin
-mdll -mnop-fun-dllimport -mthread -municode -mwin32
-mwindows -fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing,
compilation proper, assembly and linking, always in that
order. GCC is capable of preprocessing and compiling
several files either into several assembler input files, or
into one assembler input file; then each assembler input
file produces an object file, and linking combines all the
object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines
what kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with
the libobjc library to make an Objective-C program work.
file.mi
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Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with
the libobjc library to make an Objective-C++ program
work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be
preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be
turned into a precompiled header (default), or C, C++
header file to be turned into an Ada spec (via the
-fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in
.cxx, the last two letters must both be literally x.
Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be
preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header
or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be
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GCC(1) GNU GCC(1)
preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed
(with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be
preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed
(with the traditional preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.ads
Ada source code file that contains a library unit
declaration (a declaration of a package, subprogram, or
generic, or a generic instantiation), or a library unit
renaming declaration (a package, generic, or subprogram
renaming declaration). Such files are also called
specs.
file.adb
Ada source code file containing a library unit body (a
subprogram or package body). Such files are also called
bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any
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GCC(1) GNU GCC(1)
file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x
option:
-x language
Specify explicitly the language for the following input
files (rather than letting the compiler choose a default
based on the file name suffix). This option applies to
all following input files until the next -x option.
Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that
subsequent files are handled according to their file
name suffixes (as they are if -x has not been used at
all).
If you only want some of the stages of compilation, you can
use -x (or filename suffixes) to tell gcc where to start,
and one of the options -c, -S, or -E to say where gcc is to
stop. Note that some combinations (for example, -x cpp-
output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link.
The linking stage simply is not done. The ultimate
output is in the form of an object file for each source
file.
By default, the object file name for a source file is
made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or
assembly, are ignored.
-S Stop after the stage of compilation proper; do not
assemble. The output is in the form of an assembler
code file for each non-assembler input file specified.
By default, the assembler file name for a source file is
made by replacing the suffix .c, .i, etc., with .s.
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Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the
compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard
output.
Input files that don't require preprocessing are
ignored.
-o file
Place output in file file. This applies to whatever
sort of output is being produced, whether it be an
executable file, an object file, an assembler file or
preprocessed C code.
If -o is not specified, the default is to put an
executable file in a.out, the object file for
source.suffix in source.o, its assembler file in
source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on
standard output.
-v Print (on standard error output) the commands executed
to run the stages of compilation. Also print the
version number of the compiler driver program and of the
preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and
arguments are quoted unless they contain only
alphanumeric characters or "./-_". This is useful for
shell scripts to capture the driver-generated command
lines.
--help
Print (on the standard output) a description of the
command-line options understood by gcc. If the -v
option is also specified then --help is also passed on
to the various processes invoked by gcc, so that they
can display the command-line options they accept. If
the -Wextra option has also been specified (prior to the
--help option), then command-line options that have no
documentation associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-
specific command-line options for each tool. For some
targets extra target-specific information may also be
printed.
--help={class|[^]qualifier}[,...]
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GCC(1) GNU GCC(1)
Print (on the standard output) a description of the
command-line options understood by the compiler that fit
into all specified classes and qualifiers. These are
the supported classes:
optimizers
Display all of the optimization options supported by
the compiler.
warnings
Display all of the options controlling warning
messages produced by the compiler.
target
Display target-specific options. Unlike the
--target-help option however, target-specific
options of the linker and assembler are not
displayed. This is because those tools do not
currently support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where
language is the name of one of the languages
supported in this version of GCC.
common
Display the options that are common to all
languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears
after an equal sign in the same continuous piece of
text, such as: --help=target.
separate
Display options taking an argument that appears as a
separate word following the original option, such
as: -o output-file.
Thus for example to display all the undocumented
target-specific switches supported by the compiler, use:
--help=target,undocumented
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GCC(1) GNU GCC(1)
The sense of a qualifier can be inverted by prefixing it
with the ^ character, so for example to display all
binary warning options (i.e., ones that are either on or
off and that do not take an argument) that have a
description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of
inverted qualifiers.
Combining several classes is possible, although this
usually restricts the output so much that there is
nothing to display. One case where it does work,
however, is when one of the classes is target. For
example, to display all the target-specific optimization
options, use:
--help=target,optimizers
The --help= option can be repeated on the command line.
Each successive use displays its requested class of
options, skipping those that have already been
displayed.
If the -Q option appears on the command line before the
--help= option, then the descriptive text displayed by
--help= is changed. Instead of describing the displayed
options, an indication is given as to whether the option
is enabled, disabled or set to a specific value
(assuming that the compiler knows this at the point
where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous
command-line options, so for example it is possible to
find out which optimizations are enabled at -O2 by
using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary
optimizations are enabled by -O3 by using:
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GCC(1) GNU GCC(1)
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked
GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any
phase of the compiler returns a non-success return code.
If you specify -pass-exit-codes, the gcc program instead
returns with the numerically highest error produced by
any phase returning an error indication. The C, C++,
and Fortran front ends return 4 if an internal compiler
error is encountered.
-pipe
Use pipes rather than temporary files for communication
between the various stages of compilation. This fails
to work on some systems where the assembler is unable to
read from a pipe; but the GNU assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard
specs file, in order to override the defaults which the
gcc driver program uses when determining what switches
to pass to cc1, cc1plus, as, ld, etc. More than one
-specs=file can be specified on the command line, and
they are processed in order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The
name of the wrapper program and its parameters are
passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args,
thus the invocation of cc1 is gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a
shared object to be dlopen'd by the compiler. The base
name of the shared object file is used to identify the
plugin for the purposes of argument parsing (See
-fplugin-arg-name-key=value below). Each plugin should
define the callback functions specified in the Plugins
API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for
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GCC(1) GNU GCC(1)
the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate
corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above,
generate Ada specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding
Go declarations in file. This generates Go "const",
"type", "var", and "func" declarations which may be a
useful way to start writing a Go interface to code
written in some other language.
@file
Read command-line options from file. The options read
are inserted in place of the original @file option. If
file does not exist, or cannot be read, then the option
will be treated literally, and not removed.
Options in file are separated by whitespace. A
whitespace character may be included in an option by
surrounding the entire option in either single or double
quotes. Any character (including a backslash) may be
included by prefixing the character to be included with
a backslash. The file may itself contain additional
@file options; any such options will be processed
recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C,
.cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often
use .hh, .hpp, .H, or (for shared template code) .tcc; and
preprocessed C++ files use the suffix .ii. GCC recognizes
files with these names and compiles them as C++ programs
even if you call the compiler the same way as for compiling
C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++
is a program that calls GCC and automatically specifies
linking against the C++ library. It treats .c, .h and .i
files as C++ source files instead of C source files unless
-x is used. This program is also useful when precompiling a
C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with
the name c++.
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GCC(1) GNU GCC(1)
When you compile C++ programs, you may specify many of the
same command-line options that you use for compiling
programs in any language; or command-line options meaningful
for C and related languages; or options that are meaningful
only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages
derived from C, such as C++, Objective-C and Objective-C++)
that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode,
it is equivalent to -std=c++98.
This turns off certain features of GCC that are
incompatible with ISO C90 (when compiling C code), or of
standard C++ (when compiling C++ code), such as the
"asm" and "typeof" keywords, and predefined macros such
as "unix" and "vax" that identify the type of system you
are using. It also enables the undesirable and rarely
used ISO trigraph feature. For the C compiler, it
disables recognition of C++ style // comments as well as
the "inline" keyword.
The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C
program, of course, but it is useful to put them in
header files that might be included in compilations done
with -ansi. Alternate predefined macros such as
"__unix__" and "__vax__" are also available, with or
without -ansi.
The -ansi option does not cause non-ISO programs to be
rejected gratuitously. For that, -Wpedantic is required
in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi
option is used. Some header files may notice this macro
and refrain from declaring certain functions or defining
certain macros that the ISO standard doesn't call for;
this is to avoid interfering with any programs that
might use these names for other things.
Functions that are normally built in but do not have
semantics defined by ISO C (such as "alloca" and "ffs")
are not built-in functions when -ansi is used.
-std=
Determine the language standard. This option is
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GCC(1) GNU GCC(1)
currently only supported when compiling C or C++.
The compiler can accept several base standards, such as
c90 or c++98, and GNU dialects of those standards, such
as gnu90 or gnu++98. When a base standard is specified,
the compiler accepts all programs following that
standard plus those using GNU extensions that do not
contradict it. For example, -std=c90 turns off certain
features of GCC that are incompatible with ISO C90, such
as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such
as omitting the middle term of a "?:" expression. On
the other hand, when a GNU dialect of a standard is
specified, all features supported by the compiler are
enabled, even when those features change the meaning of
the base standard. As a result, some strict-conforming
programs may be rejected. The particular standard is
used by -Wpedantic to identify which features are GNU
extensions given that version of the standard. For
example -std=gnu90 -Wpedantic warns about C++ style //
comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible
values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions
that conflict with ISO C90 are disabled). Same as
-ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely
supported, modulo bugs and floating-point issues
(mainly but not entirely relating to optional C99
features from Annexes F and G). See
<http://gcc.gnu.org/c99status.html> for more
information. The names c9x and iso9899:199x are
deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard.
This standard is substantially completely supported,
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GCC(1) GNU GCC(1)
modulo bugs, floating-point issues (mainly but not
entirely relating to optional C11 features from
Annexes F and G) and the optional Annexes K
(Bounds-checking interfaces) and L (Analyzability).
The name c1x is deprecated.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99
features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is
deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. This is the default for C
code. The name gnu1x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical
corrigendum and some additional defect reports. Same
as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name
c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is
deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name
c++1y is deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for
C++ code. The name gnu++1y is deprecated.
c++1z
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GCC(1) GNU GCC(1)
The next revision of the ISO C++ standard,
tentatively planned for 2017. Support is highly
experimental, and will almost certainly change in
incompatible ways in future releases.
gnu++1z
GNU dialect of -std=c++1z. Support is highly
experimental, and will almost certainly change in
incompatible ways in future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the
traditional GNU semantics for "inline" functions when in
C99 mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use
the C99 semantics for "inline" when in C99 or gnu99 mode
(i.e., it specifies the default behavior). This option
is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which
semantics are in effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that operations and
constants with a semantic type that is an interchange or
extended format should be evaluated to the precision and
range of that type. These new values are a superset of
those permitted under C99/C11, which does not specify
the meaning of other positive values of
"FLT_EVAL_METHOD". As such, code conforming to C11 may
not have been written expecting the possibility of the
new values.
-fpermitted-flt-eval-methods specifies whether the
compiler should allow only the values of
"FLT_EVAL_METHOD" specified in C99/C11, or the extended
set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11
or similar) is -fpermitted-flt-eval-methods=c11. The
default when in a GNU dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
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Output to the given filename prototyped declarations for
all functions declared and/or defined in a translation
unit, including those in header files. This option is
silently ignored in any language other than C.
Besides declarations, the file indicates, in comments,
the origin of each declaration (source file and line),
whether the declaration was implicit, prototyped or
unprototyped (I, N for new or O for old, respectively,
in the first character after the line number and the
colon), and whether it came from a declaration or a
definition (C or F, respectively, in the following
character). In the case of function definitions, a
K&R-style list of arguments followed by their
declarations is also provided, inside comments, after
the declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this
is not very useful as it is not possible to read the
arguments. This is only supported for C as this
construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a
keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__",
"__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword,
since "asm" and "inline" are standard keywords. You may
want to use the -fno-gnu-keywords flag instead, which
has the same effect. In C99 mode (-std=c99 or
-std=gnu99), this switch only affects the "asm" and
"typeof" keywords, since "inline" is a standard keyword
in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin
with __builtin_ as prefix.
GCC normally generates special code to handle certain
built-in functions more efficiently; for instance, calls
to "alloca" may become single instructions which adjust
the stack directly, and calls to "memcpy" may become
inline copy loops. The resulting code is often both
smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on
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GCC(1) GNU GCC(1)
those calls, nor can you change the behavior of the
functions by linking with a different library. In
addition, when a function is recognized as a built-in
function, GCC may use information about that function to
warn about problems with calls to that function, or to
generate more efficient code, even if the resulting code
still contains calls to that function. For example,
warnings are given with -Wformat for bad calls to
"printf" when "printf" is built in and "strlen" is known
not to modify global memory.
With the -fno-builtin-function option only the built-in
function function is disabled. function must not begin
with __builtin_. If a function is named that is not
built-in in this version of GCC, this option is ignored.
There is no corresponding -fbuiltin-function option; if
you wish to enable built-in functions selectively when
using -fno-builtin or -ffreestanding, you may define
macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with
"__GIMPLE". This is an experimental feature that allows
unit testing of GIMPLE passes.
-fhosted
Assert that compilation targets a hosted environment.
This implies -fbuiltin. A hosted environment is one in
which the entire standard library is available, and in
which "main" has a return type of "int". Examples are
nearly everything except a kernel. This is equivalent
to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding
environment. This implies -fno-builtin. A freestanding
environment is one in which the standard library may not
exist, and program startup may not necessarily be at
"main". The most obvious example is an OS kernel. This
is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in
C/C++ and "!$acc" in Fortran. When -fopenacc is
specified, the compiler generates accelerated code
according to the OpenACC Application Programming
Interface v2.0 <http://www.openacc.org/>. This option
implies -pthread, and thus is only supported on targets
that have support for -pthread.
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-fopenacc-dim=geom
Specify default compute dimensions for parallel offload
regions that do not explicitly specify. The geom value
is a triple of ':'-separated sizes, in order 'gang',
'worker' and, 'vector'. A size can be omitted, to use a
target-specific default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in
C/C++ and "!$omp" in Fortran. When -fopenmp is
specified, the compiler generates parallel code
according to the OpenMP Application Program Interface
v4.5 <http://www.openmp.org/>. This option implies
-pthread, and thus is only supported on targets that
have support for -pthread. -fopenmp implies
-fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with
"#pragma omp" in C/C++ and "!$omp" in Fortran. Other
OpenMP directives are ignored.
-fcilkplus
Enable the usage of Cilk Plus language extension
features for C/C++. When the option -fcilkplus is
specified, enable the usage of the Cilk Plus Language
extension features for C/C++. The present
implementation follows ABI version 1.2. This is an
experimental feature that is only partially complete,
and whose interface may change in future versions of GCC
as the official specification changes. Currently, all
features but "_Cilk_for" have been implemented.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler
generates code for the Linux variant of Intel's current
Transactional Memory ABI specification document
(Revision 1.1, May 6 2009). This is an experimental
feature whose interface may change in future versions of
GCC, as the official specification changes. Please note
that not all architectures are supported for this
feature.
For more information on GCC's support for transactional
memory,
Note that the transactional memory feature is not
supported with non-call exceptions
(-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft
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header files.
In C++ code, this allows member names in structures to
be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions
are only accepted with this option.
Note that this option is off for all targets but x86
targets using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers
to structures with anonymous fields to functions that
expect pointers to elements of the type of the field,
and permits referring to anonymous fields declared using
a typedef. This is only supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in
the second and third arguments. The value of such an
expression is void. This option is not supported for
C++.
-flax-vector-conversions
Allow implicit conversions between vectors with
differing numbers of elements and/or incompatible
element types. This option should not be used for new
code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char"
should be. It is either like "unsigned char" by default
or like "signed char" by default.
Ideally, a portable program should always use "signed
char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been
written to use plain "char" and expect it to be signed,
or expect it to be unsigned, depending on the machines
they were written for. This option, and its inverse,
let you make such a program work with the opposite
default.
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The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its
behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char,
which is the negative form of -funsigned-char.
Likewise, the option -fno-signed-char is equivalent to
-funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or
unsigned, when the declaration does not use either
"signed" or "unsigned". By default, such a bit-field is
signed, because this is consistent: the basic integer
types such as "int" are signed types.
-fsso-struct=endianness
Set the default scalar storage order of structures and
unions to the specified endianness. The accepted values
are big-endian, little-endian and native for the native
endianness of the target (the default). This option is
not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate
code that is not binary compatible with code generated
without it if the specified endianness is not the native
endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are
only meaningful for C++ programs. You can also use most of
the GNU compiler options regardless of what language your
program is in. For example, you might compile a file
firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only
for C++ programs; you can use the other options with any
language supported by GCC.
Some options for compiling C programs, such as -std, are
also relevant for C++ programs.
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Here is a list of options that are only for compiling C++
programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely
to the C++ ABI specification. Therefore, the ABI
obtained using version 0 will change in different
versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first
appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first
appeared in G++ 3.4, and was the default through G++
4.9.
Version 3 corrects an error in mangling a constant
address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a
standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the
mangling of attribute const/volatile on function pointer
types, decltype of a plain decl, and use of a function
parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the
promotion behavior of C++11 scoped enums and the
mangling of template argument packs, const/static_cast,
prefix ++ and --, and a class scope function used as a
template argument.
Version 7, which first appeared in G++ 4.8, that treats
nullptr_t as a builtin type and corrects the mangling of
lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the
substitution behavior of function types with
function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the
alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds
mangling of attributes that affect type identity, such
as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the
mangling of sizeof... expressions and operator names.
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For multiple entities with the same name within a
function, that are declared in different scopes, the
mangling now changes starting with the twelfth
occurrence. It also implies -fnew-inheriting-ctors.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around
mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect
mangled name. This switch specifies which ABI version
to use for the alias.
With -fabi-version=0 (the default), this defaults to 8
(GCC 5 compatibility). If another ABI version is
explicitly selected, this defaults to 0. For
compatibility with GCC versions 3.2 through 4.9, use
-fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that
version is used for compatibility aliases. If this
option is provided along with -Wabi (without the
version), the version from this option is used for the
warning.
-fno-access-control
Turn off all access checking. This switch is mainly
useful for working around bugs in the access control
code.
-faligned-new
Enable support for C++17 "new" of types that require
more alignment than "void* ::operator new(std::size_t)"
provides. A numeric argument such as "-faligned-new=32"
can be used to specify how much alignment (in bytes) is
provided by that function, but few users will need to
override the default of "alignof(std::max_align_t)".
-fcheck-new
Check that the pointer returned by "operator new" is
non-null before attempting to modify the storage
allocated. This check is normally unnecessary because
the C++ standard specifies that "operator new" only
returns 0 if it is declared "throw()", in which case the
compiler always checks the return value even without
this option. In all other cases, when "operator new"
has a non-empty exception specification, memory
exhaustion is signalled by throwing "std::bad_alloc".
See also new (nothrow).
-fconcepts
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Enable support for the C++ Extensions for Concepts
Technical Specification, ISO 19217 (2015), which allows
code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11
constexpr functions to n. A limit is needed to detect
endless recursion during constant expression evaluation.
The minimum specified by the standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14
constexpr functions to n. A limit is needed to detect
infinite loops during constant expression evaluation.
The default is 262144 (1<<18).
-fdeduce-init-list
Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed
initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension
to the originally proposed semantics for the C++11
standard, but was not part of the final standard, so it
is disabled by default. This option is deprecated, and
may be removed in a future version of G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so
that they are visible outside the scope of the class in
which they are declared. Friend functions were
documented to work this way in the old Annotated C++
Reference Manual. However, in ISO C++ a friend function
that is not declared in an enclosing scope can only be
found using argument dependent lookup. GCC defaults to
the standard behavior.
This option is for compatibility, and may be removed in
a future release of G++.
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-fno-elide-constructors
The C++ standard allows an implementation to omit
creating a temporary that is only used to initialize
another object of the same type. Specifying this option
disables that optimization, and forces G++ to call the
copy constructor in all cases. This option also causes
G++ to call trivial member functions which otherwise
would be expanded inline.
In C++17, the compiler is required to omit these
temporaries, but this option still affects trivial
member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the
C++ standard, but may be useful for reducing code size
in production builds, much like defining "NDEBUG". This
does not give user code permission to throw exceptions
in violation of the exception specifications; the
compiler still optimizes based on the specifications, so
throwing an unexpected exception results in undefined
behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a
variable goes through a wrapper function that performs
any necessary initialization. When the use and
definition of the variable are in the same translation
unit, this overhead can be optimized away, but when the
use is in a different translation unit there is
significant overhead even if the variable doesn't
actually need dynamic initialization. If the programmer
can be sure that no use of the variable in a non-
defining TU needs to trigger dynamic initialization
(either because the variable is statically initialized,
or a use of the variable in the defining TU will be
executed before any uses in another TU), they can avoid
this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support
symbol aliases, the default is -fno-extern-tls-init.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables
declared in a for-init-statement is limited to the "for"
loop itself, as specified by the C++ standard. If
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-fno-for-scope is specified, the scope of variables
declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of
G++, and other (traditional) implementations of C++.
If neither flag is given, the default is to follow the
standard, but to allow and give a warning for old-style
code that would otherwise be invalid, or have different
behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can
use this word as an identifier. You can use the keyword
"__typeof__" instead. This option is implied by the
strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11,
etc.
-fno-implicit-templates
Never emit code for non-inline templates that are
instantiated implicitly (i.e. by use); only emit code
for explicit instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline
templates, either. The default is to handle inlines
differently so that compiles with and without
optimization need the same set of explicit
instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline
functions controlled by "#pragma implementation". This
causes linker errors if these functions are not inlined
everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC,
such as implicit int and getting a pointer to member
function via non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11
constructor inheritance. This is part of C++17 but also
considered to be a Defect Report against C++11 and
C++14. This flag is enabled by default unless
-fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template
template parameters and default arguments: this allows a
template with default template arguments as an argument
for a template template parameter with fewer template
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parameters. This flag is enabled by default for
-std=c++1z.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not
mandated by ANSI/ISO C. These include "ffs", "alloca",
"_exit", "index", "bzero", "conjf", and other related
functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were
a "noexcept" specification to reduce or eliminate the
text size overhead relative to a function with no
exception specification. If the function has local
variables of types with non-trivial destructors, the
exception specification actually makes the function
smaller because the EH cleanups for those variables can
be optimized away. The semantic effect is that an
exception thrown out of a function with such an
exception specification results in a call to "terminate"
rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand",
"bitor", "compl", "not", "or" and "xor" as synonyms as
keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler
does not need to issue. Currently, the only such
diagnostic issued by G++ is the one for a name having
multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive allows
some nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a
function template, the compiler normally prints the
signature of the template followed by the template
arguments and any typedefs or typenames in the signature
(e.g. "void f(T) [with T = int]" rather than "void
f(int)") so that it's clear which template is involved.
When an error message refers to a specialization of a
class template, the compiler omits any template
arguments that match the default template arguments for
that template. If either of these behaviors make it
harder to understand the error message rather than
easier, you can use -fno-pretty-templates to disable
them.
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-frepo
Enable automatic template instantiation at link time.
This option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with
virtual functions for use by the C++ run-time type
identification features ("dynamic_cast" and "typeid").
If you don't use those parts of the language, you can
save some space by using this flag. Note that exception
handling uses the same information, but G++ generates it
as needed. The "dynamic_cast" operator can still be used
for casts that do not require run-time type information,
i.e. casts to "void *" or to unambiguous base classes.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example,
use the size of the object to make deallocation faster.
Enabled by default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want
to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that
a value of enumerated type can only be one of the values
of the enumeration (as defined in the C++ standard;
basically, a value that can be represented in the
minimum number of bits needed to represent all the
enumerators). This assumption may not be valid if the
program uses a cast to convert an arbitrary integer
value to the enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift
expressions in left-to-right order, and evaluate
assignment in right-to-left order, as adopted for C++17.
Enabled by default with -std=c++1z.
-fstrong-eval-order=some enables just the ordering of
member access and shift expressions, and is the default
without -std=c++1z.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes
for a single warning or error to n. The default value
is 10.
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-ftemplate-depth=n
Set the maximum instantiation depth for template classes
to n. A limit on the template instantiation depth is
needed to detect endless recursions during template
class instantiation. ANSI/ISO C++ conforming programs
must not rely on a maximum depth greater than 17
(changed to 1024 in C++11). The default value is 900,
as the compiler can run out of stack space before
hitting 1024 in some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified
in the C++ ABI for thread-safe initialization of local
statics. You can use this option to reduce code size
slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage
duration with the "__cxa_atexit" function rather than
the "atexit" function. This option is required for
fully standards-compliant handling of static
destructors, but only works if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine.
This causes "std::uncaught_exception" to be incorrect,
but is necessary if the runtime routine is not
available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to
compare pointers to inline functions or methods where
the addresses of the two functions are taken in
different shared objects.
The effect of this is that GCC may, effectively, mark
inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear in the export
table of a DSO and do not require a PLT indirection when
used within the DSO. Enabling this option can have a
dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table
when the library makes heavy use of templates.
The behavior of this switch is not quite the same as
marking the methods as hidden directly, because it does
not affect static variables local to the function or
cause the compiler to deduce that the function is
defined in only one shared object.
You may mark a method as having a visibility explicitly
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to negate the effect of the switch for that method. For
example, if you do want to compare pointers to a
particular inline method, you might mark it as having
default visibility. Marking the enclosing class with
explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by
this option as their linkage might otherwise cross a
shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make
GCC's C++ linkage model compatible with that of
Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by
default.
3. The One Definition Rule is relaxed for types without
explicit visibility specifications that are defined
in more than one shared object: those declarations
are permitted if they are permitted when this option
is not used.
In new code it is better to use -fvisibility=hidden and
export those classes that are intended to be externally
visible. Unfortunately it is possible for code to rely,
perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static
data members of the same type with the same name but
defined in different shared objects are different, so
changing one does not change the other; and that
pointers to function members defined in different shared
objects may not compare equal. When this flag is given,
it is a violation of the ODR to define types with the
same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided
by the linker. By default, G++ uses weak symbols if
they are available. This option exists only for
testing, and should not be used by end-users; it results
in inferior code and has no benefits. This option may
be removed in a future release of G++.
-nostdinc++
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Do not search for header files in the standard
directories specific to C++, but do still search the
other standard directories. (This option is used when
building the C++ library.)
In addition, these optimization, warning, and code
generation options have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not
compatible with the vendor-neutral C++ ABI. Since G++
now defaults to updating the ABI with each major
release, normally -Wabi will warn only if there is a
check added later in a release series for an ABI issue
discovered since the initial release. -Wabi will warn
about more things if an older ABI version is selected
(with -fabi-version=n).
-Wabi can also be used with an explicit version number
to warn about compatibility with a particular
-fabi-version level, e.g. -Wabi=2 to warn about changes
relative to -fabi-version=2.
If an explicit version number is provided and
-fabi-compat-version is not specified, the version
number from this option is used for compatibility
aliases. If no explicit version number is provided with
this option, but -fabi-compat-version is specified, that
version number is used for ABI warnings.
Although an effort has been made to warn about all such
cases, there are probably some cases that are not warned
about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even
though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if
you are concerned about the fact that code generated by
G++ may not be binary compatible with code generated by
other compilers.
Known incompatibilities in -fabi-version=2 (which was
the default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of
reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
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* SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way
that does not allow for overloading of functions
taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled
as type qualifiers, and "decltype" of a plain
declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic
function are promoted like unscoped enumerators,
causing "va_arg" to complain. On most targets this
does not actually affect the parameter passing ABI,
as there is no way to pass an argument smaller than
"int".
Also, the ABI changed the mangling of template
argument packs, "const_cast", "static_cast", prefix
increment/decrement, and a class scope function used
as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled
incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with
function-cv-qualifiers, the un-qualified function
type was incorrectly treated as a substitution
candidate.
This was fixed in -fabi-version=8, the default for
GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of
1, leading to unaligned accesses. Note that this
did not affect the ABI of a function with a
"nullptr_t" parameter, as parameters have a minimum
alignment.
This was fixed in -fabi-version=9, the default for
GCC 5.2.
* Target-specific attributes that affect the identity
of a type, such as ia32 calling conventions on a
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function type (stdcall, regparm, etc.), did not
affect the mangled name, leading to name collisions
when function pointers were used as template
arguments.
This was fixed in -fabi-version=10, the default for
GCC 6.1.
It also warns about psABI-related changes. The known
psABI changes at this point include:
* For SysV/x86-64, unions with "long double" members
are passed in memory as specified in psABI. For
example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context
that does not have that ABI tag. See C++ Attributes for
more information about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the
constructors or destructors in that class are private,
and it has neither friends nor public static member
functions. Also warn if there are no non-private
methods, and there's at least one private member
function that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a
class that has virtual functions and non-virtual
destructor. It is unsafe to delete an instance of a
derived class through a pointer to a base class if the
base class does not have a virtual destructor. This
warning is enabled by -Wall.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a
ud-suffix which does not begin with an underscore. As a
conforming extension, GCC treats such suffixes as
separate preprocessing tokens in order to maintain
backwards compatibility with code that uses formatting
macros from "<inttypes.h>". For example:
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#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate
preprocessing token.
Additionally, warn when a user-defined literal operator
is declared with a literal suffix identifier that
doesn't begin with an underscore. Literal suffix
identifiers that don't begin with an underscore are
reserved for future standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type
mismatches in global declarations from different
compilation units. Requires -flto to be enabled.
Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are
diagnosed by default, as required by the standard. A
narrowing conversion from a constant produces an error,
and a narrowing conversion from a non-constant produces
a warning, but -Wno-narrowing suppresses the diagnostic.
Note that this does not affect the meaning of well-
formed code; narrowing conversions are still considered
ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing
conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false
because of a call to a function that does not have a
non-throwing exception specification (i.e. "throw()" or
"noexcept") but is known by the compiler to never throw
an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++1z feature making "noexcept" part of a
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function type changes the mangled name of a symbol
relative to C++14. Enabled by -Wabi and -Wc++1z-compat.
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); } // in C++14 calls f<void(*)()>, in C++1z calls f<void(*)()noexcept>
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an
accessible non-virtual destructor itself or in an
accessible polymorphic base class, in which case it is
possible but unsafe to delete an instance of a derived
class through a pointer to the class itself or base
class. This warning is automatically enabled if
-Weffc++ is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier,
except when it is part of the GNU Explicit Register
Variables extension. The use of the "register" keyword
as storage class specifier has been deprecated in C++11
and removed in C++17. Enabled by default with
-std=c++1z.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the
code does not match the order in which they must be
executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i"
and "j" to match the declaration order of the members,
emitting a warning to that effect. This warning is
enabled by -Wall.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined
literal number suffixes as GNU extensions. When this
option is turned off these suffixes are treated as C++11
user-defined literal numeric suffixes. This is on by
default for all pre-C++11 dialects and all GNU dialects:
-std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14.
This option is off by default for ISO C++11 onwards
(-std=c++11, ...).
The following -W... options are not affected by -Wall.
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-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines
from Scott Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator
for classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return
an object.
* Distinguish between prefix and postfix forms of
increment and decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is
also one of the effective C++ recommendations. However,
the check is extended to warn about the lack of virtual
destructor in accessible non-polymorphic bases classes
too.
When selecting this option, be aware that the standard
library headers do not obey all of these guidelines; use
grep -v to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel.
When compiling only with GCC this is a valid sentinel,
as "NULL" is defined to "__null". Although it is a null
pointer constant rather than a null pointer, it is
guaranteed to be of the same size as a pointer. But
this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are
declared within a template. In very old versions of GCC
that predate implementation of the ISO standard,
declarations such as friend int foo(int), where the name
of the friend is an unqualified-id, could be interpreted
as a particular specialization of a template function;
the warning exists to diagnose compatibility problems,
and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type
is used within a C++ program. The new-style casts
("dynamic_cast", "static_cast", "reinterpret_cast", and
"const_cast") are less vulnerable to unintended effects
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and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions
from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code
like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to
member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from
unsigned or enumerated type to a signed type, over a
conversion to an unsigned type of the same size.
Previous versions of G++ tried to preserve unsignedness,
but the standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered.
Some coding rules disallow templates, and this may be
used to enforce that rule. The warning is inactive
inside a system header file, such as the STL, so one can
still use the STL. One may also instantiate or
specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base
classes. Some coding rules disallow multiple
inheritance, and this may be used to enforce that rule.
The warning is inactive inside a system header file,
such as the STL, so one can still use the STL. One may
also define classes that indirectly use multiple
inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base
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class. Some coding rules disallow multiple inheritance,
and this may be used to enforce that rule. The warning
is inactive inside a system header file, such as the
STL, so one can still use the STL. One may also define
classes that indirectly use virtual inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding
rules disallow namespaces, and this may be used to
enforce that rule. The warning is inactive inside a
system header file, such as the STL, so one can still
use the STL. One may also use using directives and
qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will
immediately result in a call to "terminate".
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and
Objective-C++ languages themselves.
This section describes the command-line options that are
only meaningful for Objective-C and Objective-C++ programs.
You can also use most of the language-independent GNU
compiler options. For example, you might compile a file
some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for
Objective-C and Objective-C++ programs; you can use the
other options with any language supported by GCC.
Note that since Objective-C is an extension of the C
language, Objective-C compilations may also use options
specific to the C front-end (e.g., -Wtraditional).
Similarly, Objective-C++ compilations may use C++-specific
options (e.g., -Wabi).
Here is a list of options that are only for compiling
Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate
for each literal string specified with the syntax
"@"..."". The default class name is "NXConstantString"
if the GNU runtime is being used, and "NSConstantString"
if the NeXT runtime is being used (see below). The
-fconstant-cfstrings option, if also present, overrides
the -fconstant-string-class setting and cause "@"...""
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literals to be laid out as constant CoreFoundation
strings.
-fgnu-runtime
Generate object code compatible with the standard GNU
Objective-C runtime. This is the default for most types
of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This
is the default for NeXT-based systems, including Darwin
and Mac OS X. The macro "__NEXT_RUNTIME__" is
predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches
("[receiver message:arg]") in this translation unit
ensure that the receiver is not "nil". This allows for
more efficient entry points in the runtime to be used.
This option is only available in conjunction with the
NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected
runtime. This option is currently supported only for
the NeXT runtime. In that case, Version 0 is the
traditional (32-bit) ABI without support for properties
and other Objective-C 2.0 additions. Version 1 is the
traditional (32-bit) ABI with support for properties and
other Objective-C 2.0 additions. Version 2 is the
modern (64-bit) ABI. If nothing is specified, the
default is Version 0 on 32-bit target machines, and
Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance
variables is a C++ object with a non-trivial default
constructor. If so, synthesize a special "- (id)
.cxx_construct" instance method which runs non-trivial
default constructors on any such instance variables, in
order, and then return "self". Similarly, check if any
instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which runs all such default
destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct"
methods thusly generated only operate on instance
variables declared in the current Objective-C class, and
not those inherited from superclasses. It is the
responsibility of the Objective-C runtime to invoke all
such methods in an object's inheritance hierarchy. The
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"- (id) .cxx_construct" methods are invoked by the
runtime immediately after a new object instance is
allocated; the "- (void) .cxx_destruct" methods are
invoked immediately before the runtime deallocates an
object instance.
As of this writing, only the NeXT runtime on Mac OS X
10.4 and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin
this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception
handling in Objective-C, similar to what is offered by
C++. This option is required to use the Objective-C
keywords @try, @throw, @catch, @finally and
@synchronized. This option is available with both the
GNU runtime and the NeXT runtime (but not available in
conjunction with the NeXT runtime on Mac OS X 10.2 and
earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and
Objective-C++ programs. This option is only available
with the NeXT runtime; the GNU runtime has a different
garbage collection implementation that does not require
special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check
for a nil receiver in method invocations before doing
the actual method call. This is the default and can be
disabled using -fno-objc-nilcheck. Class methods and
super calls are never checked for nil in this way no
matter what this flag is set to. Currently this flag
does nothing when the GNU runtime, or an older version
of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the
language recognized by GCC 4.0. This only affects the
Objective-C additions to the C/C++ language; it does not
affect conformance to C/C++ standards, which is
controlled by the separate C/C++ dialect option flags.
When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is
not recognized by GCC 4.0 is rejected. This is useful
if you need to make sure that your Objective-C code can
be compiled with older versions of GCC.
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-freplace-objc-classes
Emit a special marker instructing lldd(1) not to
statically link in the resulting object file, and allow
ddyylldd(1) to load it in at run time instead. This is used
in conjunction with the Fix-and-Continue debugging mode,
where the object file in question may be recompiled and
dynamically reloaded in the course of program execution,
without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only
available in conjunction with the NeXT runtime on Mac OS
X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler
ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time)
with static class references that get initialized at
load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be
retained. This is useful in Zero-Link debugging mode,
since it allows for individual class implementations to
be modified during program execution. The GNU runtime
currently always retains calls to
"objc_get_class("...")" regardless of command-line
options.
-fno-local-ivars
By default instance variables in Objective-C can be
accessed as if they were local variables from within the
methods of the class they're declared in. This can lead
to shadowing between instance variables and other
variables declared either locally inside a class method
or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus
avoiding variable shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the
specified option so that instance variables declared
outside the scope of any access modifier directives
default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the
source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being
intercepted by the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
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If a class is declared to implement a protocol, a
warning is issued for every method in the protocol that
is not implemented by the class. The default behavior
is to issue a warning for every method not explicitly
implemented in the class, even if a method
implementation is inherited from the superclass. If you
use the -Wno-protocol option, then methods inherited
from the superclass are considered to be implemented,
and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same
selector are found during compilation. The check is
performed on the list of methods in the final stage of
compilation. Additionally, a check is performed for
each selector appearing in a "@selector(...)"
expression, and a corresponding method for that selector
has been found during compilation. Because these checks
scan the method table only at the end of compilation,
these warnings are not produced if the final stage of
compilation is not reached, for example because an error
is found during compilation, or because the
-fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or
return types are found for a given selector when
attempting to send a message using this selector to a
receiver of type "id" or "Class". When this flag is off
(which is the default behavior), the compiler omits such
warnings if any differences found are confined to types
that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an
undeclared selector is found. A selector is considered
undeclared if no method with that name has been declared
before the "@selector(...)" expression, either
explicitly in an @interface or @protocol declaration, or
implicitly in an @implementation section. This option
always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its
checks in the final stage of compilation. This also
enforces the coding style convention that methods and
selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that
is passed by value, if any.
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Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted
irrespective of the output device's aspect (e.g. its width,
...). You can use the options described below to control
the formatting algorithm for diagnostic messages, e.g. how
many characters per line, how often source location
information should be reported. Note that some language
front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines
of about n characters. If n is zero, then no line-
wrapping is done; each error message appears on a single
line. This is the default for all front ends.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit source location
information once; that is, in case the message is too
long to fit on a single physical line and has to be
wrapped, the source location won't be emitted (as
prefix) again, over and over, in subsequent continuation
lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit the same source
location information (as prefix) for physical lines that
result from the process of breaking a message which is
too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or
auto. The default depends on how the compiler has been
configured, it can be any of the above WHEN options or
also never if GCC_COLORS environment variable isn't
present in the environment, and auto otherwise. auto
means to use color only when the standard error is a
terminal. The forms -fdiagnostics-color and
-fno-diagnostics-color are aliases for
-fdiagnostics-color=always and
-fdiagnostics-color=never, respectively.
The colors are defined by the environment variable
GCC_COLORS. Its value is a colon-separated list of
capabilities and Select Graphic Rendition (SGR)
substrings. SGR commands are interpreted by the terminal
or terminal emulator. (See the section in the
documentation of your text terminal for permitted values
and their meanings as character attributes.) These
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substring values are integers in decimal representation
and can be concatenated with semicolons. Common values
to concatenate include 1 for bold, 4 for underline, 5
for blink, 7 for inverse, 39 for default foreground
color, 30 to 37 for foreground colors, 90 to 97 for
16-color mode foreground colors, 38;5;0 to 38;5;255 for
88-color and 256-color modes foreground colors, 49 for
default background color, 40 to 47 for background
colors, 100 to 107 for 16-color mode background colors,
and 48;5;0 to 48;5;255 for 88-color and 256-color modes
background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is
bold cyan, 32 is green, 34 is blue, 01 is bold, and 31
is red. Setting GCC_COLORS to the empty string disables
colors. Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be
inserted or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be
deleted.
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"diff-filename="
SGR substring for filename headers within generated
patches.
"diff-hunk="
SGR substring for the starts of hunks within
generated patches.
"diff-delete="
SGR substring for deleted lines within generated
patches.
"diff-insert="
SGR substring for inserted lines within generated
patches.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text
indicating the command-line option that directly
controls the diagnostic (if such an option is known to
the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that
behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the
original source line and a caret ^ indicating the
column. This option suppresses this information. The
source line is truncated to n characters, if the
-fmessage-length=n option is given. When the output is
done to the terminal, the width is limited to the width
given by the COLUMNS environment variable or, if not
set, to the terminal width.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format,
suitable for consumption by IDEs. For each fix-it, a
line will be printed after the relevant diagnostic,
starting with the string "fix-it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range,
expressed as a count of bytes, starting at byte 1 for
the initial column. In the above example, bytes 3
through 20 of line 45 of "test.c" are to be replaced
with the given string:
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00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as
"\\", tab as "\t", newline as "\n", double quotes as
"\"", non-printable characters as octal (e.g. vertical
tab as "\013").
An empty replacement string indicates that the given
range is to be removed. An empty range (e.g.
"45:3-45:3") indicates that the string is to be inserted
at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format,
after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same
rules as for diagnostics (see -fdiagnostics-color).
-fno-show-column
Do not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program
that does not understand the column numbers, such as
dejagnu.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions
that are not inherently erroneous but that are risky or
suggest there may have been an error.
The following language-independent options do not enable
specific warnings but control the kinds of diagnostics
produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything
beyond that.
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-fmax-errors=n
Limits the maximum number of error messages to n, at
which point GCC bails out rather than attempting to
continue processing the source code. If n is 0 (the
default), there is no limit on the number of error
messages produced. If -Wfatal-errors is also specified,
then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier
for a warning is appended; for example -Werror=switch
turns the warnings controlled by -Wswitch into errors.
This switch takes a negative form, to be used to negate
-Werror for specific warnings; for example
-Wno-error=switch makes -Wswitch warnings not be errors,
even when -Werror is in effect.
The warning message for each controllable warning
includes the option that controls the warning. That
option can then be used with -Werror= and -Wno-error= as
described above. (Printing of the option in the warning
message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies
-Wfoo. However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on
the first error occurred rather than trying to keep
going and printing further error messages.
You can request many specific warnings with options
beginning with -W, for example -Wimplicit to request
warnings on implicit declarations. Each of these specific
warning options also has a negative form beginning -Wno- to
turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the
default. For further language-specific options also refer
to C++ Dialect Options and Objective-C and Objective-C++
Dialect Options.
Some options, such as -Wall and -Wextra, turn on other
options, such as -Wunused, which may turn on further
options, such as -Wunused-value. The combined effect of
positive and negative forms is that more specific options
have priority over less specific ones, independently of
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their position in the command-line. For options of the same
specificity, the last one takes effect. Options enabled or
disabled via pragmas take effect as if they appeared at the
end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the
option is not recognized. However, if the -Wno- form is
used, the behavior is slightly different: no diagnostic is
produced for -Wno-unknown-warning unless other diagnostics
are being produced. This allows the use of new -Wno-
options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO
C++; reject all programs that use forbidden extensions,
and some other programs that do not follow ISO C and ISO
C++. For ISO C, follows the version of the ISO C
standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly
with or without this option (though a rare few require
-ansi or a -std option specifying the required version
of ISO C). However, without this option, certain GNU
extensions and traditional C and C++ features are
supported as well. With this option, they are rejected.
-Wpedantic does not cause warning messages for use of
the alternate keywords whose names begin and end with
__. Pedantic warnings are also disabled in the
expression that follows "__extension__". However, only
system header files should use these escape routes;
application programs should avoid them.
Some users try to use -Wpedantic to check programs for
strict ISO C conformance. They soon find that it does
not do quite what they want: it finds some non-ISO
practices, but not all---only those for which ISO C
requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C
might be useful in some instances, but would require
considerable additional work and would be quite
different from -Wpedantic. We don't have plans to
support such a feature in the near future.
Where the standard specified with -std represents a GNU
extended dialect of C, such as gnu90 or gnu99, there is
a corresponding base standard, the version of ISO C on
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which the GNU extended dialect is based. Warnings from
-Wpedantic are given where they are required by the base
standard. (It does not make sense for such warnings to
be given only for features not in the specified GNU C
dialect, since by definition the GNU dialects of C
include all features the compiler supports with the
given option, and there would be nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see
-Wpedantic) requires a diagnostic, in some cases where
there is undefined behavior at compile-time and in some
other cases that do not prevent compilation of programs
that are valid according to the standard. This is not
equivalent to -Werror=pedantic, since there are errors
enabled by this option and not enabled by the latter and
vice versa.
-Wall
This enables all the warnings about constructions that
some users consider questionable, and that are easy to
avoid (or modify to prevent the warning), even in
conjunction with macros. This also enables some
language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect
Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2)
-Wbool-compare -Wbool-operation -Wc++11-compat
-Wc++14-compat -Wchar-subscripts -Wcomment
-Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat -Wint-in-bool-context -Wimplicit (C and
Objective-C only) -Wimplicit-int (C and Objective-C
only) -Wimplicit-function-declaration (C and Objective-C
only) -Winit-self (only for C++)
-Wlogical-not-parentheses -Wmain (only for C/ObjC and
unless -ffreestanding) -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation (only for C/C++)
-Wmissing-braces (only for C/ObjC) -Wnarrowing (only for
C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd
-Wparentheses -Wpointer-sign -Wreorder -Wreturn-type
-Wsequence-point -Wsign-compare (only in C++)
-Wsizeof-pointer-memaccess -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -Wtautological-compare
-Wtrigraphs -Wuninitialized -Wunknown-pragmas
-Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvolatile-register-var
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Note that some warning flags are not implied by -Wall.
Some of them warn about constructions that users
generally do not consider questionable, but which
occasionally you might wish to check for; others warn
about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the
code to suppress the warning. Some of them are enabled
by -Wextra but many of them must be enabled
individually.
-Wextra
This enables some extra warning flags that are not
enabled by -Wall. (This option used to be called -W.
The older name is still supported, but the newer name is
more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init
-Wsign-compare (C only) -Wtype-limits -Wuninitialized
-Wshift-negative-value (in C++03 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the
following cases:
* A pointer is compared against integer zero with "<",
"<=", ">", or ">=".
* (C++ only) An enumerator and a non-enumerator both
appear in a conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been
declared "register".
* (C++ only) Taking the address of a variable that has
been declared "register".
* (C++ only) A base class is not initialized in the
copy constructor of a derived class.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a
common cause of error, as programmers often forget that
this type is signed on some machines. This warning is
enabled by -Wall.
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-Wchkp
Warn about an invalid memory access that is found by
Pointer Bounds Checker (-fcheck-pointer-bounds).
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the
-fprofile-use option. If a source file is changed
between compiling with -fprofile-gen and with
-fprofile-use, the files with the profile feedback can
fail to match the source file and GCC cannot use the
profile feedback information. By default, this warning
is enabled and is treated as an error.
-Wno-coverage-mismatch can be used to disable the
warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning
can result in poorly optimized code and is useful only
in the case of very minor changes such as bug fixes to
an existing code-base. Completely disabling the warning
is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning"
directives.
-Wdouble-promotion (C, C++, Objective-
C and Objective-C++ only)
Give a warning when a value of type "float" is
implicitly promoted to "double". CPUs with a 32-bit
"single-precision" floating-point unit implement "float"
in hardware, but emulate "double" in software. On such
a machine, doing computations using "double" values is
much more expensive because of the overhead required for
software emulation.
It is easy to accidentally do computations with "double"
because floating-point literals are implicitly of type
"double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with
"double" because the floating-point literal is a
"double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile",
"restrict" or "_Atomic" specifier. This warning is
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enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure
that the arguments supplied have types appropriate to
the format string specified, and that the conversions
specified in the format string make sense. This
includes standard functions, and others specified by
format attributes, in the "printf", "scanf", "strftime"
and "strfmon" (an X/Open extension, not in the C
standard) families (or other target-specific families).
Which functions are checked without format attributes
having been specified depends on the standard version
selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or
-fno-builtin.
The formats are checked against the format features
supported by GNU libc version 2.2. These include all
ISO C90 and C99 features, as well as features from the
Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not
support all these features; GCC does not support warning
about features that go beyond a particular library's
limitations. However, if -Wpedantic is used with
-Wformat, warnings are given about format features not
in the selected standard version (but not for "strfmon"
formats, since those are not in any version of the C
standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and
-Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for
several functions, -Wformat also implies -Wnonnull.
Some aspects of this level of format checking can be
disabled by the options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length.
-Wformat is enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format
strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess
arguments to a "printf" or "scanf" format function.
The C standard specifies that such arguments are
ignored.
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Where the unused arguments lie between used
arguments that are specified with $ operand number
specifications, normally warnings are still given,
since the implementation could not know what type to
pass to "va_arg" to skip the unused arguments.
However, in the case of "scanf" formats, this option
suppresses the warning if the unused arguments are
all pointers, since the Single Unix Specification
says that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions
such as "sprintf" and "vsprintf" that might overflow
the destination buffer. When the exact number of
bytes written by a format directive cannot be
determined at compile-time it is estimated based on
heuristics that depend on the level argument and on
optimization. While enabling optimization will in
most cases improve the accuracy of the warning, it
may also result in false positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat
employs a conservative approach that warns only
about calls that most likely overflow the
buffer. At this level, numeric arguments to
format directives with unknown values are
assumed to have the value of one, and strings of
unknown length to be empty. Numeric arguments
that are known to be bounded to a subrange of
their type, or string arguments whose output is
bounded either by their directive's precision or
by a finite set of string literals, are assumed
to take on the value within the range that
results in the most bytes on output. For
example, the call to "sprintf" below is
diagnosed because even with both a and b equal
to zero, the terminating NUL character ('\0')
appended by the function to the destination
buffer will be written past its end. Increasing
the size of the buffer by a single byte is
sufficient to avoid the warning, though it may
not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [12];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
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-Wformat-overflow=2
Level 2 warns also about calls that might
overflow the destination buffer given an
argument of sufficient length or magnitude. At
level 2, unknown numeric arguments are assumed
to have the minimum representable value for
signed types with a precision greater than 1,
and the maximum representable value otherwise.
Unknown string arguments whose length cannot be
assumed to be bounded either by the directive's
precision, or by a finite set of string literals
they may evaluate to, or the character array
they may point to, are assumed to be 1 character
long.
At level 2, the call in the example above is
again diagnosed, but this time because with a
equal to a 32-bit "INT_MIN" the first %i
directive will write some of its digits beyond
the end of the destination buffer. To make the
call safe regardless of the values of the two
variables, the size of the destination buffer
must be increased to at least 34 bytes. GCC
includes the minimum size of the buffer in an
informational note following the warning.
An alternative to increasing the size of the
destination buffer is to constrain the range of
formatted values. The maximum length of string
arguments can be bounded by specifying the
precision in the format directive. When numeric
arguments of format directives can be assumed to
be bounded by less than the precision of their
type, choosing an appropriate length modifier to
the format specifier will reduce the required
buffer size. For example, if a and b in the
example above can be assumed to be within the
precision of the "short int" type then using
either the %hi format directive or casting the
argument to "short" reduces the maximum required
size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-
length formats. The C standard specifies that
zero-length formats are allowed.
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-Wformat=2
Enable -Wformat plus additional format checks.
Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format
string is not a string literal and so cannot be
checked, unless the format function takes its format
arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of
format functions that represent possible security
problems. At present, this warns about calls to
"printf" and "scanf" functions where the format
string is not a string literal and there are no
format arguments, as in "printf (foo);". This may
be a security hole if the format string came from
untrusted input and contains %n. (This is currently
a subset of what -Wformat-nonliteral warns about,
but in future warnings may be added to
-Wformat-security that are not included in
-Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format
string requires an unsigned argument and the
argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions
such as "snprintf" and "vsnprintf" that might result
in output truncation. When the exact number of
bytes written by a format directive cannot be
determined at compile-time it is estimated based on
heuristics that depend on the level argument and on
optimization. While enabling optimization will in
most cases improve the accuracy of the warning, it
may also result in false positives. Except as noted
otherwise, the option uses the same logic
-Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by
-Wformat employs a conservative approach that
warns only about calls to bounded functions
whose return value is unused and that will most
likely result in output truncation.
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-Wformat-truncation=2
Level 2 warns also about calls to bounded
functions whose return value is used and that
might result in truncation given an argument of
sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime"
formats that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked
as requiring a non-null value by the "nonnull" function
attribute.
-Wnonnull is included in -Wall and -Wformat. It can be
disabled with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the
"nonnull" function attribute against null inside the
function.
-Wnonnull-compare is included in -Wall. It can be
disabled with the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger
erroneous or undefined behavior due to dereferencing a
null pointer. This option is only active when
-fdelete-null-pointer-checks is active, which is enabled
by optimizations in most targets. The precision of the
warnings depends on the optimization options used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized
with themselves. Note this option can only be used with
the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in
the following snippet only when -Winit-self has been
specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
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Warn when a declaration does not specify a type. This
warning is enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being
declared. In C99 mode (-std=c99 or -std=gnu99), this
warning is enabled by default and it is made into an
error by -pedantic-errors. This warning is also enabled
by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and
-Wimplicit-function-declaration. This warning is
enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as
-Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough
is the same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a
case cannot fall through, e.g. when there is a return
statement or a call to function declared with the
noreturn attribute. -Wimplicit-fallthrough= also takes
into account control flow statements, such as ifs, and
only warns when appropriate. E.g.
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GCC(1) GNU GCC(1)
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall
through is desirable, GCC provides an attribute,
"__attribute__ ((fallthrough))", that is to be used
along with a null statement to suppress this warning
that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the
-Wimplicit-fallthrough warning using "[[fallthrough]];"
instead of the GNU attribute. In C++11 or C++14 users
can use "[[gnu::fallthrough]];", which is a GNU
extension. Instead of the these attributes, it is also
possible to add a fallthrough comment to silence the
warning. The whole body of the C or C++ style comment
should match the given regular expressions listed below.
The option argument n specifies what kind of comments
are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough
comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
the>
*<-Wimplicit-fallthrough=3 case sensitively matches one of
following regular expressions:
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GCC(1) GNU GCC(1)
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
|-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
the>
*<-Wimplicit-fallthrough=4 case sensitively matches one of
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the
warning.
The comment needs to be followed after optional
whitespace and other comments by "case" or "default"
keywords or by a user label that precedes some "case" or
"default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by
-Wextra.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type
qualifier such as "const". For ISO C such a type
qualifier has no effect, since the value returned by a
function is not an lvalue. For C++, the warning is only
emitted for scalar types or "void". ISO C prohibits
qualified "void" return types on function definitions,
so such return types always receive a warning even
without this option.
This warning is also enabled by -Wextra.
-Wignored-attributes (C and C++ only)
Warn when an attribute is ignored. This is different
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GCC(1) GNU GCC(1)
from the -Wattributes option in that it warns whenever
the compiler decides to drop an attribute, not that the
attribute is either unknown, used in a wrong place, etc.
This warning is enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should
be a function with external linkage, returning int,
taking either zero arguments, two, or three arguments of
appropriate types. This warning is enabled by default
in C++ and is enabled by either -Wall or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect
the block structure. Specifically, a warning is issued
for "if", "else", "while", and "for" clauses with a
guarded statement that does not use braces, followed by
an unguarded statement with the same indentation.
In the following example, the call to "bar" is
misleadingly indented as if it were guarded by the "if"
conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses
the -ftabstop= option to determine if the statements
line up (defaulting to 8).
The warning is not issued for code involving multiline
preprocessor logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive,
since this typically indicates autogenerated code, and
no assumptions can be made about the layout of the file
that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully
bracketed. In the following example, the initializer
for "a" is not fully bracketed, but that for "b" is
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fully bracketed. This warning is enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
only)
-Wmissing-include-dirs (C, C++, Objective-
C and Objective-C++
Warn if a user-supplied include directory does not
exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts,
such as when there is an assignment in a context where a
truth value is expected, or when operators are nested
whose precedence people often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this
is equivalent to "(x<=y ? 1 : 0) <= z", which is a
different interpretation from that of ordinary
mathematical notation.
Also warn for dangerous uses of the GNU extension to
"?:" with omitted middle operand. When the condition in
the "?": operator is a boolean expression, the omitted
value is always 1. Often programmers expect it to be a
value computed inside the conditional expression
instead.
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics
because of violations of sequence point rules in the C
and C++ standards.
The C and C++ standards define the order in which
expressions in a C/C++ program are evaluated in terms of
sequence points, which represent a partial ordering
between the execution of parts of the program: those
executed before the sequence point, and those executed
after it. These occur after the evaluation of a full
expression (one which is not part of a larger
expression), after the evaluation of the first operand
of a "&&", "||", "? :" or "," (comma) operator, before a
function is called (but after the evaluation of its
arguments and the expression denoting the called
function), and in certain other places. Other than as
expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not
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specified. All these rules describe only a partial
order rather than a total order, since, for example, if
two functions are called within one expression with no
sequence point between them, the order in which the
functions are called is not specified. However, the
standards committee have ruled that function calls do
not overlap.
It is not specified when between sequence points
modifications to the values of objects take effect.
Programs whose behavior depends on this have undefined
behavior; the C and C++ standards specify that "Between
the previous and next sequence point an object shall
have its stored value modified at most once by the
evaluation of an expression. Furthermore, the prior
value shall be read only to determine the value to be
stored.". If a program breaks these rules, the results
on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;",
"a[n] = b[n++]" and "a[i++] = i;". Some more
complicated cases are not diagnosed by this option, and
it may give an occasional false positive result, but in
general it has been found fairly effective at detecting
this sort of problem in programs.
The C++17 standard will define the order of evaluation
of operands in more cases: in particular it requires
that the right-hand side of an assignment be evaluated
before the left-hand side, so the above examples are no
longer undefined. But this warning will still warn
about them, to help people avoid writing code that is
undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is
some debate over the precise meaning of the sequence
point rules in subtle cases. Links to discussions of
the problem, including proposed formal definitions, may
be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a
reference) to a variable that goes out of scope after
the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type
that defaults to "int". Also warn about any "return"
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statement with no return value in a function whose
return type is not "void" (falling off the end of the
function body is considered returning without a value).
For C only, warn about a "return" statement with an
expression in a function whose return type is "void",
unless the expression type is also "void". As a GNU
extension, the latter case is accepted without a warning
unless -Wpedantic is used.
For C++, a function without return type always produces
a diagnostic message, even when -Wno-return-type is
specified. The only exceptions are "main" and functions
defined in system headers.
This warning is enabled by -Wall.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled
by default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is
enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is
enabled by -Wextra in C99 and C++11 modes (and newer).
-Wshift-overflow
-Wshift-overflow=n
Warn about left shift overflows. This warning is
enabled by default in C99 and C++11 modes (and newer).
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is
enabled by default in C99 and C++11 modes (and
newer). This warning level does not warn about
left-shifting 1 into the sign bit. (However, in C,
such an overflow is still rejected in contexts where
an integer constant expression is required.)
-Wshift-overflow=2
This warning level also warns about left-shifting 1
into the sign bit, unless C++14 mode is active.
-Wswitch
Warn whenever a "switch" statement has an index of
enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. (The presence of a
"default" label prevents this warning.) "case" labels
outside the enumeration range also provoke warnings when
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this option is used (even if there is a "default"
label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a
"default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of
enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. "case" labels
outside the enumeration range also provoke warnings when
this option is used. The only difference between
-Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there
is a "default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of
boolean type and the case values are outside the range
of a boolean type. It is possible to suppress this
warning by casting the controlling expression to a type
other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++
programs.
-Wswitch-unreachable
Warn whenever a "switch" statement contains statements
between the controlling expression and the first case
label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement
between the controlling expression and the first case
label is just a declaration:
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switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++
programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and
"__sync_nand_and_fetch" built-in functions are used.
These functions changed semantics in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but
otherwise unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with
-Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but
otherwise unused (aside from its declaration). This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is
enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not
defined or a non-inline static function is unused. This
warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
only)
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++
Warn when a typedef locally defined in a function is not
used. This warning is enabled by -Wall.
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-Wunused-parameter
Warn whenever a function parameter is unused aside from
its declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with
attribute "warn_unused_result" does not use its return
value. The default is -Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside
from its declaration. This option implies
-Wunused-const-variable=1 for C, but not for C++. This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside
from its declaration. -Wunused-const-variable=1 is
enabled by -Wunused-variable for C, but not for C++. In
C this declares variable storage, but in C++ this is not
an error since const variables take the place of
"#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by
-Wunused-variable for C. It warns only about unused
static const variables defined in the main
compilation unit, but not about static const
variables declared in any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant
static variables in headers (excluding system
headers). This is the warning level of
-Wunused-const-variable and must be explicitly
requested since in C++ this isn't an error and in C
it might be harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is
explicitly not used. To suppress this warning cast the
unused expression to "void". This includes an
expression-statement or the left-hand side of a comma
expression that contains no side effects. For example,
an expression such as "x[i,j]" causes a warning, while
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"x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function
parameter, you must either specify -Wextra -Wunused
(note that -Wall implies -Wunused), or separately
specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first
being initialized or if a variable may be clobbered by a
"setjmp" call. In C++, warn if a non-static reference or
non-static "const" member appears in a class without
constructors.
If you want to warn about code that uses the
uninitialized value of the variable in its own
initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or
clobbered elements of structure, union or array
variables as well as for variables that are
uninitialized or clobbered as a whole. They do not
occur for variables or elements declared "volatile".
Because these warnings depend on optimization, the exact
variables or elements for which there are warnings
depends on the precise optimization options and version
of GCC used.
Note that there may be no warning about a variable that
is used only to compute a value that itself is never
used, because such computations may be deleted by data
flow analysis before the warnings are printed.
-Winvalid-memory-model
Warn for invocations of __atomic Builtins, __sync
Builtins, and the C11 atomic generic functions with a
memory consistency argument that is either invalid for
the operation or outside the range of values of the
"memory_order" enumeration. For example, since the
"__atomic_store" and "__atomic_store_n" built-ins are
only defined for the relaxed, release, and sequentially
consistent memory orders the following code is
diagnosed:
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void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from
the function entry to a use of the variable that is
initialized, but there exist some other paths for which
the variable is not initialized, the compiler emits a
warning if it cannot prove the uninitialized paths are
not executed at run time. These warnings are made
optional because GCC is not smart enough to see all the
reasons why the code might be correct in spite of
appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is
always initialized, but GCC doesn't know this. To
suppress the warning, you need to provide a default case
with assert(0) or similar code.
This option also warns when a non-volatile automatic
variable might be changed by a call to "longjmp". These
warnings as well are possible only in optimizing
compilation.
The compiler sees only the calls to "setjmp". It cannot
know where "longjmp" will be called; in fact, a signal
handler could call it at any point in the code. As a
result, you may get a warning even when there is in fact
no problem because "longjmp" cannot in fact be called at
the place that would cause a problem.
Some spurious warnings can be avoided if you declare all
the functions you use that never return as "noreturn".
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This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is
not understood by GCC. If this command-line option is
used, warnings are even issued for unknown pragmas in
system header files. This is not the case if the
warnings are only enabled by the -Wall command-line
option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between
pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is
active. It warns about code that might break the strict
aliasing rules that the compiler is using for
optimization. The warning does not catch all cases, but
does attempt to catch the more common pitfalls. It is
included in -Wall. It is equivalent to
-Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is
active. It warns about code that might break the strict
aliasing rules that the compiler is using for
optimization. Higher levels correspond to higher
accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels do not warn but
-fstrict-aliasing still breaks the code, as it has very
few false negatives. However, it has many false
positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced.
Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still
have many false positives (not as many as level 1
though), and few false negatives (but possibly more than
level 1). Unlike level 1, it only warns when an address
is taken. Warns about incomplete types. Runs in the
front end only.
Level 3 (default for -Wstrict-aliasing): Should have
very few false positives and few false negatives.
Slightly slower than levels 1 or 2 when optimization is
enabled. Takes care of the common pun+dereference
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pattern in the front end: "*(int*)&some_float". If
optimization is enabled, it also runs in the back end,
where it deals with multiple statement cases using
flow-sensitive points-to information. Only warns when
the converted pointer is dereferenced. Does not warn
about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is
active. It warns about cases where the compiler
optimizes based on the assumption that signed overflow
does not occur. Note that it does not warn about all
cases where the code might overflow: it only warns about
cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does
not occur is perfectly safe if the values of the
variables involved are such that overflow never does, in
fact, occur. Therefore this warning can easily give a
false positive: a warning about code that is not
actually a problem. To help focus on important issues,
several warning levels are defined. No warnings are
issued for the use of undefined signed overflow when
estimating how many iterations a loop requires, in
particular when determining whether a loop will be
executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy
to avoid. For example, with -fstrict-overflow, the
compiler simplifies "x + 1 > x" to 1. This level of
-Wstrict-overflow is enabled by -Wall; higher levels
are not, and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is
simplified to a constant. For example: "abs (x) >=
0". This can only be simplified when
-fstrict-overflow is in effect, because "abs
(INT_MIN)" overflows to "INT_MIN", which is less
than zero. -Wstrict-overflow (with no level) is the
same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is
simplified. For example: "x + 1 > 1" is simplified
to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by
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the above cases. For example: "(x * 10) / 5" is
simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison.
For example: "x + 2 > y" is simplified to "x + 1 >=
y". This is reported only at the highest warning
level because this simplification applies to many
comparisons, so this warning level gives a very
large number of false positives.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as
"memcpy" and "strcpy" that are determined to overflow
the destination buffer. The optional argument is one
greater than the type of Object Size Checking to perform
to determine the size of the destination. The argument
is meaningful only for functions that operate on
character arrays but not for raw memory functions like
"memcpy" which always make use of Object Size type-0.
The option also warns for calls that specify a size in
excess of the largest possible object or at most
"SIZE_MAX / 2" bytes. The option produces the best
results with optimization enabled but can detect a small
subset of simple buffer overflows even without
optimization in calls to the GCC built-in functions like
"__builtin_memcpy" that correspond to the standard
functions. In any case, the option warns about just a
subset of buffer overflows detected by the corresponding
overflow checking built-ins. For example, the option
will issue a warning for the "strcpy" call below because
it copies at least 5 characters (the string "blue"
including the terminating NUL) into the buffer of size
4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
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Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero
Object Size Checking to determine the sizes of
destination objects. This is the default setting of
the option. At this setting the option will not
warn for writes past the end of subobjects of larger
objects accessed by pointers unless the size of the
largest surrounding object is known. When the
destination may be one of several objects it is
assumed to be the largest one of them. On Linux
systems, when optimization is enabled at this
setting the option warns for the same code as when
the "_FORTIFY_SOURCE" macro is defined to a non-zero
value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one
Object Size Checking to determine the sizes of
destination objects. At this setting the option
will warn about overflows when writing to members of
the largest complete objects whose exact size is
known. It will, however, not warn for excessive
writes to the same members of unknown objects
referenced by pointers since they may point to
arrays containing unknown numbers of elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two
Object Size Checking to determine the sizes of
destination objects. At this setting the option
warns about overflowing the smallest object or data
member. This is the most restrictive setting of the
option that may result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three
Object Size Checking to determine the sizes of
destination objects. At this setting the option
will warn about overflowing any data members, and
when the destination is one of several objects it
uses the size of the largest of them to decide
whether to issue a warning. Similarly to
-Wstringop-overflow=3 this setting of the option may
result in warnings for benign code.
-Wsuggest-attribute=[pure|const|noreturn|format]
Warn for cases where adding an attribute may be
beneficial. The attributes currently supported are
listed below.
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-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for
attributes "pure", "const" or "noreturn". The
compiler only warns for functions visible in other
compilation units or (in the case of "pure" and
"const") if it cannot prove that the function
returns normally. A function returns normally if it
doesn't contain an infinite loop or return
abnormally by throwing, calling "abort" or trapping.
This analysis requires option -fipa-pure-const,
which is enabled by default at -O and higher.
Higher optimization levels improve the accuracy of
the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be
candidates for "format" attributes. Note these are
only possible candidates, not absolute ones. GCC
guesses that function pointers with "format"
attributes that are used in assignment,
initialization, parameter passing or return
statements should have a corresponding "format"
attribute in the resulting type. I.e. the left-hand
side of the assignment or initialization, the type
of the parameter variable, or the return type of the
containing function respectively should also have a
"format" attribute to avoid the warning.
GCC also warns about function definitions that might
be candidates for "format" attributes. Again, these
are only possible candidates. GCC guesses that
"format" attributes might be appropriate for any
function that calls a function like "vprintf" or
"vscanf", but this might not always be the case, and
some functions for which "format" attributes are
appropriate may not be detected.
-Wsuggest-final-types
Warn about types with virtual methods where code quality
would be improved if the type were declared with the
C++11 "final" specifier, or, if possible, declared in an
anonymous namespace. This allows GCC to more
aggressively devirtualize the polymorphic calls. This
warning is more effective with link time optimization,
where the information about the class hierarchy graph is
more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be
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improved if the method were declared with the C++11
"final" specifier, or, if possible, its type were
declared in an anonymous namespace or with the "final"
specifier. This warning is more effective with link-
time optimization, where the information about the class
hierarchy graph is more complete. It is recommended to
first consider suggestions of -Wsuggest-final-types and
then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not
marked with the override keyword.
-Walloc-zero
Warn about calls to allocation functions decorated with
attribute "alloc_size" that specify zero bytes,
including those to the built-in forms of the functions
"aligned_alloc", "alloca", "calloc", "malloc", and
"realloc". Because the behavior of these functions when
called with a zero size differs among implementations
(and in the case of "realloc" has been deprecated)
relying on it may result in subtle portability bugs and
should be avoided.
-Walloc-size-larger-than=n
Warn about calls to functions decorated with attribute
"alloc_size" that attempt to allocate objects larger
than the specified number of bytes, or where the result
of the size computation in an integer type with infinite
precision would exceed "SIZE_MAX / 2". The option
argument n may end in one of the standard suffixes
designating a multiple of bytes such as "kB" and "KiB"
for kilobyte and kibibyte, respectively, "MB" and "MiB"
for megabyte and mebibyte, and so on.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=n
This option warns on calls to "alloca" that are not
bounded by a controlling predicate limiting its argument
of integer type to at most n bytes, or calls to "alloca"
where the bound is unknown. Arguments of non-integer
types are considered unbounded even if they appear to be
constrained to the expected range.
For example, a bounded case of "alloca" could be:
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void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing
"-Walloca-larger-than=1000" would not issue a warning
because the call to "alloca" is known to be at most 1000
bytes. However, if "-Walloca-larger-than=500" were
passed, the compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca"
with no controlling predicate constraining its integer
argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above
would trigger a warning, but this time because of the
lack of bounds checking.
Note, that even seemingly correct code involving signed
integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a
larger than expected argument to be implicitly cast into
the "alloca" call.
This option also warns when "alloca" is used in a loop.
This warning is not enabled by -Wall, and is only active
when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=n.
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-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active
(default for -O2 and above). It warns about subscripts
to arrays that are always out of bounds. This warning is
enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is
enabled by -Wall; higher levels are not, and must be
explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds
access for arrays at the end of a struct and for
arrays accessed through pointers. This warning level
may give a larger number of false positives and is
deactivated by default.
-Wbool-compare
Warn about boolean expression compared with an integer
value different from "true"/"false". For instance, the
following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a
boolean type. For instance, bitwise negation of a
boolean is very likely a bug in the program. For C,
this warning also warns about incrementing or
decrementing a boolean, which rarely makes sense. (In
C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++1z, and
deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This
warning detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null
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statement. This warning also warn for conditional
operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain.
For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or
__builtin_return_address is called with an argument
greater than 0. Such calls may return indeterminate
values or crash the program. The warning is included in
-Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being
discarded. Typically, the compiler warns if a "const
char *" variable is passed to a function that takes a
"char *" parameter. This option can be used to suppress
such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are
pointer targets are being discarded. Typically, the
compiler warns if a "const int (*)[]" variable is passed
to a function that takes a "int (*)[]" parameter. This
option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers
that have incompatible types. This warning is for cases
not covered by -Wno-pointer-sign, which warns for
pointer argument passing or assignment with different
signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and
pointer to integer conversions. This warning is about
implicit conversions; for explicit conversions the
warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero.
Floating-point division by zero is not warned about, as
it can be a legitimate way of obtaining infinities and
NaNs.
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-Wsystem-headers
Print warning messages for constructs found in system
header files. Warnings from system headers are normally
suppressed, on the assumption that they usually do not
indicate real problems and would only make the compiler
output harder to read. Using this command-line option
tells GCC to emit warnings from system headers as if
they occurred in user code. However, note that using
-Wall in conjunction with this option does not warn
about unknown pragmas in system headers---for that,
-Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or
false. This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested
functions. A trampoline is a small piece of data or
code that is created at run time on the stack when the
address of a nested function is taken, and is used to
call the nested function indirectly. For some targets,
it is made up of data only and thus requires no special
treatment. But, for most targets, it is made up of code
and thus requires the stack to be made executable in
order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality
comparisons.
The idea behind this is that sometimes it is convenient
(for the programmer) to consider floating-point values
as approximations to infinitely precise real numbers.
If you are doing this, then you need to compute (by
analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when
producing output, but that's a different problem). In
particular, instead of testing for equality, you should
check to see whether the two values have ranges that
overlap; and this is done with the relational operators,
so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
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traditional and ISO C. Also warn about ISO C constructs
that have no traditional C equivalent, and/or
problematic constructs that should be avoided.
* Macro parameters that appear within string literals
in the macro body. In traditional C macro
replacement takes place within string literals, but
in ISO C it does not.
* In traditional C, some preprocessor directives did
not exist. Traditional preprocessors only
considered a line to be a directive if the #
appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that
traditional C understands but ignores because the #
does not appear as the first character on the line.
It also suggests you hide directives like "#pragma"
not understood by traditional C by indenting them.
Some traditional implementations do not recognize
"#elif", so this option suggests avoiding it
altogether.
* A function-like macro that appears without
arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L
floating-point constant suffixes. (Traditional C
does support the L suffix on integer constants.)
Note, these suffixes appear in macros defined in the
system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these
macros in user code might normally lead to spurious
warnings, however GCC's integrated preprocessor has
enough context to avoid warning in these cases.
* A function declared external in one block and then
used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a
"static" one. This construct is not accepted by
some traditional C compilers.
* The ISO type of an integer constant has a different
width or signedness from its traditional type. This
warning is only issued if the base of the constant
is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned
about.
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* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C
lacks a separate namespace for labels.
* Initialization of unions. If the initializer is
zero, the warning is omitted. This is done under
the assumption that the zero initializer in user
code appears conditioned on e.g. "__STDC__" to avoid
missing initializer warnings and relies on default
initialization to zero in the traditional C case.
* Conversions by prototypes between
fixed/floating-point values and vice versa. The
absence of these prototypes when compiling with
traditional C causes serious problems. This is a
subset of the possible conversion warnings; for the
full set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This
warning intentionally is not issued for prototype
declarations or variadic functions because these ISO
C features appear in your code when using
libiberty's traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also
bypassed for nested functions because that feature
is already a GCC extension and thus not relevant to
traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is
different from what would happen to the same argument in
the absence of a prototype. This includes conversions
of fixed point to floating and vice versa, and
conversions changing the width or signedness of a
fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a
block. This construct, known from C++, was introduced
with ISO C99 and is by default allowed in GCC. It is
not supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration
shadows another variable, parameter, type, class member
(in C++), or instance variable (in Objective-C) or
whenever a built-in function is shadowed. Note that in
C++, the compiler warns if a local variable shadows an
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explicit typedef, but not if it shadows a
struct/class/enum. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an
instance variable in an Objective-C method.
-Wshadow=global
The default for -Wshadow. Warns for any (global)
shadowing.
-Wshadow=local
Warn when a local variable shadows another local
variable or parameter. This warning is enabled by
-Wshadow=global.
-Wshadow=compatible-local
Warn when a local variable shadows another local
variable or parameter whose type is compatible with that
of the shadowing variable. In C++, type compatibility
here means the type of the shadowing variable can be
converted to that of the shadowed variable. The creation
of this flag (in addition to -Wshadow=local) is based on
the idea that when a local variable shadows another one
of incompatible type, it is most likely intentional, not
a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have
incompatible types, enabling only
-Wshadow=compatible-local will not emit a warning.
Because their types are incompatible, if a programmer
accidentally uses one in place of the other, type
checking will catch that and emit an error or warning.
So not warning (about shadowing) in this case will not
lead to undetected bugs. Use of this flag instead of
-Wshadow=local can possibly reduce the number of
warnings triggered by intentional shadowing.
This warning is enabled by -Wshadow=local.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is
defined.
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-Wframe-larger-than=len
Warn if the size of a function frame is larger than len
bytes. The computation done to determine the stack
frame size is approximate and not conservative. The
actual requirements may be somewhat greater than len
even if you do not get a warning. In addition, any
space allocated via "alloca", variable-length arrays, or
related constructs is not included by the compiler when
determining whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was
not allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger
than len bytes. The computation done to determine the
stack usage is conservative. Any space allocated via
"alloca", variable-length arrays, or related constructs
is included by the compiler when determining whether or
not to issue a warning.
The message is in keeping with the output of
-fstack-usage.
* If the stack usage is fully static but exceeds the
specified amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded,
it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not
bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the
compiler cannot assume anything on the bounds of the
loop indices. With -funsafe-loop-optimizations warn if
the compiler makes such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic
without GNU extensions, this option disables the
warnings about non-ISO "printf" / "scanf" format width
specifiers "I32", "I64", and "I" used on Windows
targets, which depend on the MS runtime.
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-Waligned-new
Warn about a new-expression of a type that requires
greater alignment than the "alignof(std::max_align_t)"
but uses an allocation function without an explicit
alignment parameter. This option is enabled by -Wall.
Normally this only warns about global allocation
functions, but -Waligned-new=all also warns about class
member allocation functions.
-Wplacement-new
-Wplacement-new=n
Warn about placement new expressions with undefined
behavior, such as constructing an object in a buffer
that is smaller than the type of the object. For
example, the placement new expression below is diagnosed
because it attempts to construct an array of 64 integers
in a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of
-Wplacement-new. At this level the warning is not
issued for some strictly undefined constructs that
GCC allows as extensions for compatibility with
legacy code. For example, the following "new"
expression is not diagnosed at this level even
though it has undefined behavior according to the
C++ standard because it writes past the end of the
one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the
same constructs as at level 1, a diagnostic is also
issued for placement new expressions that construct
an object in the last member of structure whose type
is an array of a single element and whose size is
less than the size of the object being constructed.
While the previous example would be diagnosed, the
following construct makes use of the flexible member
array extension to avoid the warning at level 2.
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struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wpointer-arith
Warn about anything that depends on the "size of" a
function type or of "void". GNU C assigns these types a
size of 1, for convenience in calculations with "void *"
pointers and pointers to functions. In C++, warn also
when an arithmetic operation involves "NULL". This
warning is also enabled by -Wpedantic.
-Wpointer-compare
Warn if a pointer is compared with a zero character
constant. This usually means that the pointer was meant
to be dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due
to the limited range of the data type, but do not warn
for constant expressions. For example, warn if an
unsigned variable is compared against zero with "<" or
">=". This warning is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a
/* comment, or whenever a backslash-newline appears in a
// comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change
the meaning of the program. Trigraphs within comments
are not warned about, except those that would form
escaped newlines.
This option is implied by -Wall. If -Wall is not given,
this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings,
but get the other -Wall warnings, use -trigraphs -Wall
-Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if"
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directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of
a macro (including the case where the macro is expanded
by an #if directive). Such usage is not portable. This
warning is also enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are
unused. A macro is used if it is expanded or tested for
existence at least once. The preprocessor also warns if
the macro has not been used at the time it is redefined
or undefined.
Built-in macros, macros defined on the command line, and
macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in
skipped conditional blocks, then the preprocessor
reports it as unused. To avoid the warning in such a
case, you might improve the scope of the macro's
definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy
use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are
followed by text. This sometimes happens in older
programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This
warning is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching
type. For example, warn if a call to a function
returning an integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present
in ISO C99. For instance, warn about use of variable
length arrays, "long long" type, "bool" type, compound
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literals, designated initializers, and so on. This
option is independent of the standards mode. Warnings
are disabled in the expression that follows
"__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present
in ISO C11. For instance, warn about use of anonymous
structures and unions, "_Atomic" type qualifier,
"_Thread_local" storage-class specifier, "_Alignas"
specifier, "Alignof" operator, "_Generic" keyword, and
so on. This option is independent of the standards
mode. Warnings are disabled in the expression that
follows "__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the
common subset of ISO C and ISO C++, e.g. request for
implicit conversion from "void *" to a pointer to
non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between
ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO
C++ 1998 that are keywords in ISO C++ 2011. This
warning turns on -Wnarrowing and is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between
ISO C++ 2011 and ISO C++ 2014. This warning is enabled
by -Wall.
-Wc++1z-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between
ISO C++ 2014 and the forthoming ISO C++ 2017(?). This
warning is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type
qualifier from the target type. For example, warn if a
"const char *" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type
qualifier in an unsafe way. For example, casting "char
**" to "const char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
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-Wcast-align
Warn whenever a pointer is cast such that the required
alignment of the target is increased. For example, warn
if a "char *" is cast to an "int *" on machines where
integers can only be accessed at two- or four-byte
boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a
non-"const" "char *" pointer produces a warning. These
warnings help you find at compile time code that can try
to write into a string constant, but only if you have
been very careful about using "const" in declarations
and prototypes. Otherwise, it is just a nuisance. This
is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion
from string literals to "char *". This warning is
enabled by default for C++ programs.
-Wclobbered
Warn for variables that might be changed by "longjmp" or
"vfork". This warning is also enabled by -Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs])
constructs.
-Wconversion
Warn for implicit conversions that may alter a value.
This includes conversions between real and integer, like
"abs (x)" when "x" is "double"; conversions between
signed and unsigned, like "unsigned ui = -1"; and
conversions to smaller types, like "sqrtf (M_PI)". Do
not warn for explicit casts like "abs ((int) x)" and "ui
= (unsigned) -1", or if the value is not changed by the
conversion like in "abs (2.0)". Warnings about
conversions between signed and unsigned integers can be
disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for
user-defined conversions; and conversions that never use
a type conversion operator: conversions to "void", the
same type, a base class or a reference to them. Warnings
about conversions between signed and unsigned integers
are disabled by default in C++ unless -Wsign-conversion
is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-
pointer types. -Wconversion-null is enabled by default.
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-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant.
This can be useful to facilitate the conversion to
"nullptr" in C++11.
-Wsubobject-linkage (C++ and Objective-C++ only)
Warn if a class type has a base or a field whose type
uses the anonymous namespace or depends on a type with
no linkage. If a type A depends on a type B with no or
internal linkage, defining it in multiple translation
units would be an ODR violation because the meaning of B
is different in each translation unit. If A only
appears in a single translation unit, the best way to
silence the warning is to give it internal linkage by
putting it in an anonymous namespace as well. The
compiler doesn't give this warning for types defined in
the main .C file, as those are unlikely to have multiple
definitions. -Wsubobject-linkage is enabled by default.
-Wdangling-else
Warn about constructions where there may be confusion to
which "if" statement an "else" branch belongs. Here is
an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost
possible "if" statement, which in this example is "if
(b)". This is often not what the programmer expected,
as illustrated in the above example by indentation the
programmer chose. When there is the potential for this
confusion, GCC issues a warning when this flag is
specified. To eliminate the warning, add explicit
braces around the innermost "if" statement so there is
no way the "else" can belong to the enclosing "if". The
resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
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This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or
"__TIMESTAMP__" are encountered as they might prevent
bit-wise-identical reproducible compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which
may cause undefined behavior at runtime. This warning
is enabled by default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do
while" statement. This warning is also enabled by
-Wextra.
-Wenum-compare
Warn about a comparison between values of different
enumerated types. In C++ enumerated type mismatches in
conditional expressions are also diagnosed and the
warning is enabled by default. In C this warning is
enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps
forward across the initialization of a variable, or
jumps backward to a label after the variable has been
initialized. This only warns about variables that are
initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort
of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can
be disabled with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned
values could produce an incorrect result when the signed
value is converted to unsigned. In C++, this warning is
also enabled by -Wall. In C, it is also enabled by
-Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign
of an integer value, like assigning a signed integer
expression to an unsigned integer variable. An explicit
cast silences the warning. In C, this option is enabled
also by -Wconversion.
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-Wfloat-conversion
Warn for implicit conversions that reduce the precision
of a real value. This includes conversions from real to
integer, and from higher precision real to lower
precision real values. This option is also enabled by
-Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse
scalar storage order.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation
function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized
deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with
-fsized-deallocation.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string
and memory built-in functions if the argument uses
"sizeof". This warning warns e.g. about "memset (ptr,
0, sizeof (ptr));" if "ptr" is not an array, but a
pointer, and suggests a possible fix, or about "memcpy
(&foo, ptr, sizeof (&foo));". This warning is enabled
by -Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a
parameter that is declared as an array in a function
definition. This warning is enabled by default for C
and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in
function, if the first argument references an array, and
the third argument is a number equal to the number of
elements, but not equal to the size of the array in
memory. This indicates that the user has omitted a
multiplication by the element size. This warning is
enabled by -Wall.
-Wmemset-transposed-args
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Warn for suspicious calls to the "memset" built-in
function, if the second argument is not zero and the
third argument is zero. This warns e.g.@ about "memset
(buf, sizeof buf, 0)" where most probably "memset (buf,
0, sizeof buf)" was meant instead. The diagnostics is
only emitted if the third argument is literal zero. If
it is some expression that is folded to zero, a cast of
zero to some type, etc., it is far less likely that the
user has mistakenly exchanged the arguments and no
warning is emitted. This warning is enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These
include using the address of a function in a conditional
expression, such as "void func(void); if (func)", and
comparisons against the memory address of a string
literal, such as "if (x == "abc")". Such uses typically
indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional
usually indicate that the programmer forgot the
parentheses in a function call; and comparisons against
string literals result in unspecified behavior and are
not portable in C, so they usually indicate that the
programmer intended to use "strcmp". This warning is
enabled by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in
expressions. This includes using logical operators in
contexts where a bit-wise operator is likely to be
expected. Also warns when the operands of a logical
operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side
operand of a comparison. This option does not warn if
the right operand is considered to be a boolean
expression. Its purpose is to detect suspicious code
like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the
LHS into parentheses:
if ((!a) > 1) { ... }
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This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions
are defined or called. (In languages where you can
return an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the
compiler detects undefined behavior in some statement
during one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used,
such as unrecognized attributes, function attributes
applied to variables, etc. This does not stop errors
for incorrect use of supported attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with the wrong
signature. This warning is enabled by default.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined.
This suppresses warnings for redefinition of
"__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and
"__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without
specifying the argument types. (An old-style function
definition is permitted without a warning if preceded by
a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C
Standard, in a declaration. For example, warn if
storage-class specifiers like "static" are not the first
things in a declaration. This warning is also enabled
by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A
warning is given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type
specifier in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
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-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous
prototype declaration. This warning is issued even if
the definition itself provides a prototype. Use this
option to detect global functions that do not have a
matching prototype declaration in a header file. This
option is not valid for C++ because all function
declarations provide prototypes and a non-matching
declaration declares an overload rather than conflict
with an earlier declaration. Use -Wmissing-declarations
to detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself
provides a prototype. Use this option to detect global
functions that are not declared in header files. In C,
no warnings are issued for functions with previous non-
prototype declarations; use -Wmissing-prototypes to
detect missing prototypes. In C++, no warnings are
issued for function templates, or for inline functions,
or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields
missing. For example, the following code causes such a
warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers,
so the following modification does not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C++ this option does not warn either about the empty
{ } initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other
-Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is
used. Usually they indicate a typo in the user's code,
as they have implementation-defined values, and should
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not be used in portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if
they are different sequences of characters. However,
sometimes when characters outside the basic ASCII
character set are used, you can have two different
character sequences that look the same. To avoid
confusion, the ISO 10646 standard sets out some
normalization rules which when applied ensure that two
sequences that look the same are turned into the same
sequence. GCC can warn you if you are using identifiers
that have not been normalized; this option controls that
warning.
There are four levels of warning supported by GCC. The
default is -Wnormalized=nfc, which warns about any
identifier that is not in the ISO 10646 "C" normalized
form, NFC. NFC is the recommended form for most uses.
It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in
identifiers by ISO C and ISO C++ that, when turned into
NFC, are not allowed in identifiers. That is, there's
no way to use these symbols in portable ISO C or C++ and
have all your identifiers in NFC. -Wnormalized=id
suppresses the warning for these characters. It is
hoped that future versions of the standards involved
will correct this, which is why this option is not the
default.
You can switch the warning off for all characters by
writing -Wnormalized=none or -Wno-normalized. You
should only do this if you are using some other
normalization scheme (like "D"), because otherwise you
can easily create bugs that are literally impossible to
see.
Some characters in ISO 10646 have distinct meanings but
look identical in some fonts or display methodologies,
especially once formatting has been applied. For
instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N",
displays just like a regular "n" that has been placed in
a superscript. ISO 10646 defines the NFKC normalization
scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you
use -Wnormalized=nfkc. This warning is comparable to
warning about every identifier that contains the letter
O because it might be confused with the digit 0, and so
is not the default, but may be useful as a local coding
convention if the programming environment cannot be
fixed to display these characters distinctly.
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-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and
types marked as deprecated by using the "deprecated"
attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant
expressions.
-Wno-odr
Warn about One Definition Rule violations during link-
time optimization. Requires -flto-odr-type-merging to
be enabled. Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP
or the Cilk Plus simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax
the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is
overridden when using designated initializers.
This warning is included in -Wextra. To get other
-Wextra warnings without this one, use -Wextra
-Wno-override-init.
-Woverride-init-side-effects (C and Objective-C only)
Warn if an initialized field with side effects is
overridden when using designated initializers. This
warning is enabled by default.
-Wpacked
Warn if a structure is given the packed attribute, but
the packed attribute has no effect on the layout or size
of the structure. Such structures may be mis-aligned
for little benefit. For instance, in this code, the
variable "f.x" in "struct bar" is misaligned even though
"struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
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-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed"
attribute on bit-fields of type "char". This has been
fixed in GCC 4.4 but the change can lead to differences
in the structure layout. GCC informs you when the
offset of such a field has changed in GCC 4.4. For
example there is no longer a 4-bit padding between field
"a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to
align an element of the structure or to align the whole
structure. Sometimes when this happens it is possible
to rearrange the fields of the structure to reduce the
padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same
scope, even in cases where multiple declaration is valid
and changes nothing.
-Wrestrict
Warn when an argument passed to a restrict-qualified
parameter aliases with another argument.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a
function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting
constructors when the base class inherited from has a C
variadic constructor; the warning is on by default
because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be
inlined. Even with this option, the compiler does not
warn about failures to inline functions declared in
system headers.
The compiler uses a variety of heuristics to determine
whether or not to inline a function. For example, the
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compiler takes into account the size of the function
being inlined and the amount of inlining that has
already been done in the current function. Therefore,
seemingly insignificant changes in the source program
can cause the warnings produced by -Winline to appear or
disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to
a non-POD type. According to the 2014 ISO C++ standard,
applying "offsetof" to a non-standard-layout type is
undefined. In existing C++ implementations, however,
"offsetof" typically gives meaningful results. This
flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to
ignore the warning about it.
The restrictions on "offsetof" may be relaxed in a
future version of the C++ standard.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean
values are expected, such as conditional expressions
(?:) using non-boolean integer constants in boolean
context, like "if (a <= b ? 2 : 3)". Or left shifting
of signed integers in boolean context, like "for (a = 0;
1 << a; a++);". Likewise for all kinds of
multiplications regardless of the data type. This
warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an
integer of a different size. In C++, casting to a
pointer type of smaller size is an error. Wint-to-
pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an
integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path
but cannot be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by
either -Wpedantic or -Wtraditional in ISO C90 and C++98
modes. To inhibit the warning messages, use
-Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if
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the GNU alternate syntax is used in ISO C99 mode. This
is enabled by either -Wpedantic or -Wtraditional. To
inhibit the warning messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to
handle variable arguments like "va_start". This is
default. To inhibit the warning messages, use
-Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD
capabilities of the architecture. Mainly useful for the
performance tuning. Vector operation can be implemented
"piecewise", which means that the scalar operation is
performed on every vector element; "in parallel", which
means that the vector operation is implemented using
scalars of wider type, which normally is more
performance efficient; and "as a single scalar", which
means that vector fits into a scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base
with a non-trivial C++11 move assignment operator. This
is dangerous because if the virtual base is reachable
along more than one path, it is moved multiple times,
which can mean both objects end up in the moved-from
state. If the move assignment operator is written to
avoid moving from a moved-from object, this warning can
be disabled.
-Wvla
Warn if a variable-length array is used in the code.
-Wno-vla prevents the -Wpedantic warning of the
variable-length array.
-Wvla-larger-than=n
If this option is used, the compiler will warn on uses
of variable-length arrays where the size is either
unbounded, or bounded by an argument that can be larger
than n bytes. This is similar to how
-Walloca-larger-than=n works, but with variable-length
arrays.
Note that GCC may optimize small variable-length arrays
of a known value into plain arrays, so this warning may
not get triggered for such arrays.
This warning is not enabled by -Wall, and is only active
when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=n.
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-Wvolatile-register-var
Warn if a register variable is declared volatile. The
volatile modifier does not inhibit all optimizations
that may eliminate reads and/or writes to register
variables. This warning is enabled by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This
warning does not generally indicate that there is
anything wrong with your code; it merely indicates that
GCC's optimizers are unable to handle the code
effectively. Often, the problem is that your code is
too big or too complex; GCC refuses to optimize programs
when the optimization itself is likely to take
inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with
different signedness. This option is only supported for
C and Objective-C. It is implied by -Wall and by
-Wpedantic, which can be disabled with
-Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is
active. It warns about functions that are not protected
against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the
"minimum maximum" length specified in the C standard.
Modern compilers generally allow string constants that
are much longer than the standard's minimum limit, but
very portable programs should avoid using longer
strings.
The limit applies after string constant concatenation,
and does not count the trailing NUL. In C90, the limit
was 509 characters; in C99, it was raised to 4095.
C++98 does not specify a normative minimum maximum, so
we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be
disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not
have a suffix. When used together with -Wsystem-headers
it warns about such constants in system header files.
This can be useful when preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the decimal
floating-point extension to C99.
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-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used
to initialize a structure that has been marked with the
"designated_init" attribute.
-Whsa
Issue a warning when HSAIL cannot be emitted for the
compiled function or OpenMP construct.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger,
in almost all cases you need only to add -g to your other
options.
GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally be surprising: some
variables you declared may not exist at all; flow of control
may briefly move where you did not expect it; some
statements may not be executed because they compute constant
results or their values are already at hand; some statements
may execute in different places because they have been moved
out of loops. Nevertheless it is possible to debug
optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
If you are not using some other optimization option,
consider using -Og with -g. With no -O option at all, some
compiler passes that collect information useful for
debugging do not run at all, so that -Og may result in a
better debugging experience.
-g Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF). GDB can
work with this debugging information.
On most systems that use stabs format, -g enables use of
extra debugging information that only GDB can use; this
extra information makes debugging work better in GDB but
probably makes other debuggers crash or refuse to read
the program. If you want to control for certain whether
to generate the extra information, use -gstabs+,
-gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This
means to use the most expressive format available
(DWARF, stabs, or the native format if neither of those
are supported), including GDB extensions if at all
possible.
-gdwarf
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-gdwarf-version
Produce debugging information in DWARF format (if that
is supported). The value of version may be either 2, 3,
4 or 5; the default version for most targets is 4.
DWARF Version 5 is only experimental.
Note that with DWARF Version 2, some ports require and
always use some non-conflicting DWARF 3 extensions in
the unwind tables.
Version 4 may require GDB 7.0 and
-fvar-tracking-assignments for maximum benefit.
GCC no longer supports DWARF Version 1, which is
substantially different than Version 2 and later. For
historical reasons, some other DWARF-related options
(including -feliminate-dwarf2-dups and
-fno-dwarf2-cfi-asm) retain a reference to DWARF Version
2 in their names, but apply to all currently-supported
versions of DWARF.
-gstabs
Produce debugging information in stabs format (if that
is supported), without GDB extensions. This is the
format used by DBX on most BSD systems. On MIPS, Alpha
and System V Release 4 systems this option produces
stabs debugging output that is not understood by DBX or
SDB. On System V Release 4 systems this option requires
the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that
is supported), using GNU extensions understood only by
the GNU debugger (GDB). The use of these extensions is
likely to make other debuggers crash or refuse to read
the program.
-gcoff
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most
System V systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that
is supported). This is the format used by the DBX
debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that
is supported), using GNU extensions understood only by
the GNU debugger (GDB). The use of these extensions is
likely to make other debuggers crash or refuse to read
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the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.
-gvms
Produce debugging information in Alpha/VMS debug format
(if that is supported). This is the format used by
DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to
specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0
negates -g.
Level 1 produces minimal information, enough for making
backtraces in parts of the program that you don't plan
to debug. This includes descriptions of functions and
external variables, and line number tables, but no
information about local variables.
Level 3 includes extra information, such as all the
macro definitions present in the program. Some
debuggers support macro expansion when you use -g3.
-gdwarf does not accept a concatenated debug level, to
avoid confusion with -gdwarf-level. Instead use an
additional -glevel option to change the debug level for
DWARF.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that
is supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++
class in only one object file, emit it in all object
files using the class. This option should be used only
with debuggers that are unable to handle the way GCC
normally emits debugging information for classes because
using this option increases the size of debugging
information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the
debugging information that are identical in different
object files. Merging is not supported by all
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assemblers or linkers. Merging decreases the size of
the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by
default.
-fdebug-prefix-map=old=new
When compiling files in directory old, record debugging
information describing them as in new instead.
-fvar-tracking
Run variable tracking pass. It computes where variables
are stored at each position in code. Better debugging
information is then generated (if the debugging
information format supports this information).
It is enabled by default when compiling with
optimization (-Os, -O, -O2, ...), debugging information
(-g) and the debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the
compilation and attempt to carry the annotations over
throughout the compilation all the way to the end, in an
attempt to improve debug information while optimizing.
Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in
which case annotations are created and maintained, but
discarded at the end. By default, this flag is enabled
together with -fvar-tracking, except when selective
scheduling is enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible
into a separate output file with the extension .dwo.
This option allows the build system to avoid linking
files with debug information. To be useful, this option
requires a debugger capable of reading .dwo files.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes"
sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes"
sections in a format suitable for conversion into a GDB
index. This option is only useful with a linker that
can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be
put into their own ".debug_types" section instead of
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making them part of the ".debug_info" section. It is
more efficient to put them in a separate comdat sections
since the linker can then remove duplicates. But not
all DWARF consumers support ".debug_types" sections yet
and on some objects ".debug_types" produces larger
instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to
invoke the compiler that may affect code generation to
be appended to the DW_AT_producer attribute in DWARF
debugging information. The options are concatenated
with spaces separating them from each other and from the
compiler version. It is enabled by default. See also
-frecord-gcc-switches for another way of storing
compiler options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard
version than selected with -gdwarf-version. On most
targets using non-conflicting DWARF extensions from
later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version
than selected with -gdwarf-version.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging
information, rather than just file and line. This
option is disabled by default.
-gz[=type]
Produce compressed debug sections in DWARF format, if
that is supported. If type is not given, the default
type depends on the capabilities of the assembler and
linker used. type may be one of none (don't compress
debug sections), zlib (use zlib compression in ELF gABI
format), or zlib-gnu (use zlib compression in
traditional GNU format). If the linker doesn't support
writing compressed debug sections, the option is
rejected. Otherwise, if the assembler does not support
them, -gz is silently ignored when producing object
files.
-feliminate-dwarf2-dups
Compress DWARF debugging information by eliminating
duplicated information about each symbol. This option
only makes sense when generating DWARF debugging
information.
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-femit-struct-debug-baseonly
Emit debug information for struct-like types only when
the base name of the compilation source file matches the
base name of file in which the struct is defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type
information to the debugger. See
-femit-struct-debug-reduced for a less aggressive
option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when
the base name of the compilation source file matches the
base name of file in which the type is defined, unless
the struct is a template or defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type
information to the debugger. See
-femit-struct-debug-baseonly for a more aggressive
option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler
generates debug information. The intent is to reduce
duplicate struct debug information between different
object files within the same program.
This option is a detailed version of
-femit-struct-debug-reduced and
-femit-struct-debug-baseonly, which serves for most
needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to
structs that are used directly (dir:) or used indirectly
(ind:). A struct type is used directly when it is the
type of a variable, member. Indirect uses arise through
pointers to structs. That is, when use of an incomplete
struct is valid, the use is indirect. An example is
struct one direct; struct two * indirect;.
The optional second word limits the specification to
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ordinary structs (ord:) or generic structs (gen:).
Generic structs are a bit complicated to explain. For
C++, these are non-explicit specializations of template
classes, or non-template classes within the above.
Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement
them.
The third word specifies the source files for those
structs for which the compiler should emit debug
information. The values none and any have the normal
meaning. The value base means that the base of name of
the file in which the type declaration appears must
match the base of the name of the main compilation file.
In practice, this means that when compiling foo.c, debug
information is generated for types declared in that file
and foo.h, but not other header files. The value sys
means those types satisfying base or declared in system
or compiler headers.
You may need to experiment to determine the best
settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame"
section instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids
producing debug symbol output for types that are nowhere
used in the source file being compiled. Sometimes it is
useful to have GCC emit debugging information for all
types declared in a compilation unit, regardless of
whether or not they are actually used in that
compilation unit, for example if, in the debugger, you
want to cast a value to a type that is not actually used
in your program (but is declared). More often, however,
this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to
reduce the cost of compilation and to make debugging produce
the expected results. Statements are independent: if you
stop the program with a breakpoint between statements, you
can then assign a new value to any variable or change the
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program counter to any other statement in the function and
get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to
improve the performance and/or code size at the expense of
compilation time and possibly the ability to debug the
program.
The compiler performs optimization based on the knowledge it
has of the program. Compiling multiple files at once to a
single output file mode allows the compiler to use
information gained from all of the files when compiling each
of them.
Not all optimizations are controlled directly by a flag.
Only optimizations that have a flag are listed in this
section.
Most optimizations are only enabled if an -O level is set on
the command line. Otherwise they are disabled, even if
individual optimization flags are specified.
Depending on the target and how GCC was configured, a
slightly different set of optimizations may be enabled at
each -O level than those listed here. You can invoke GCC
with -Q --help=optimizers to find out the exact set of
optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more
time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and
execution time, without performing any optimizations
that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg
-fcombine-stack-adjustments -fcompare-elim
-fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability
-fif-conversion2 -fif-conversion
-finline-functions-called-once -fipa-pure-const
-fipa-profile -fipa-reference -fmerge-constants
-fmove-loop-invariants -freorder-blocks -fshrink-wrap
-fshrink-wrap-separate -fsplit-wide-types -fssa-backprop
-fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
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-O also turns on -fomit-frame-pointer on machines where
doing so does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed
tradeoff. As compared to -O, this option increases both
compilation time and the performance of the generated
code.
-O2 turns on all optimization flags specified by -O. It
also turns on the following optimization flags:
-fthread-jumps -falign-functions -falign-jumps
-falign-loops -falign-labels -fcaller-saves
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-small-functions -findirect-inlining -fipa-cp
-fipa-bit-cp -fipa-vrp -fipa-sra -fipa-icf
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen
-fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2 -fstore-merging
-fstrict-aliasing -fstrict-overflow
-ftree-builtin-call-dce -ftree-switch-conversion
-ftree-tail-merge -fcode-hoisting -ftree-pre -ftree-vrp
-fipa-ra
Please note the warning under -fgcse about invoking -O2
on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations
specified by -O2 and also turns on the
-finline-functions, -funswitch-loops,
-fpredictive-commoning, -fgcse-after-reload,
-ftree-loop-vectorize, -ftree-loop-distribute-patterns,
-fsplit-paths -ftree-slp-vectorize, -fvect-cost-model,
-ftree-partial-pre, -fpeel-loops and -fipa-cp-clone
options.
-O0 Reduce compilation time and make debugging produce the
expected results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations
that do not typically increase code size. It also
performs further optimizations designed to reduce code
size.
-Os disables the following optimization flags:
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-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks
-freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -fprefetch-loop-arrays
-Ofast
Disregard strict standards compliance. -Ofast enables
all -O3 optimizations. It also enables optimizations
that are not valid for all standard-compliant programs.
It turns on -ffast-math and the Fortran-specific
-fno-protect-parens and -fstack-arrays.
-Og Optimize debugging experience. -Og enables
optimizations that do not interfere with debugging. It
should be the optimization level of choice for the
standard edit-compile-debug cycle, offering a reasonable
level of optimization while maintaining fast compilation
and a good debugging experience.
If you use multiple -O options, with or without level
numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent
flags. Most flags have both positive and negative forms;
the negative form of -ffoo is -fno-foo. In the table below,
only one of the forms is listed---the one you typically use.
You can figure out the other form by either removing no- or
adding it.
The following options control specific optimizations. They
are either activated by -O options or are related to ones
that are. You can use the following flags in the rare cases
when "fine-tuning" of optimizations to be performed is
desired.
-fno-defer-pop
Always pop the arguments to each function call as soon
as that function returns. For machines that must pop
arguments after a function call, the compiler normally
lets arguments accumulate on the stack for several
function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass
tries to combine two instructions and checks if the
result can be simplified. If loop unrolling is active,
two passes are performed and the second is scheduled
after loop unrolling.
This option is enabled by default at optimization levels
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-O, -O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression
contraction. -ffp-contract=fast enables floating-point
expression contraction such as forming of fused
multiply-add operations if the target has native support
for them. -ffp-contract=on enables floating-point
expression contraction if allowed by the language
standard. This is currently not implemented and treated
equal to -ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions
that don't need one. This avoids the instructions to
save, set up and restore frame pointers; it also makes
an extra register available in many functions. It also
makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no
effect, because the standard calling sequence
automatically handles the frame pointer and nothing is
saved by pretending it doesn't exist. The machine-
description macro "FRAME_POINTER_REQUIRED" controls
whether a target machine supports this flag.
The default setting (when not optimizing for size) for
32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is
-fomit-frame-pointer. You can configure GCC with the
--enable-frame-pointer configure option to change the
default.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g.
"strlen", "strchr" or "strcpy") and their
"_FORTIFY_SOURCE" counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those
marked with the "always_inline" attribute. This is the
default when not optimizing.
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Single functions can be exempted from inlining by
marking them with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body
is smaller than expected function call code (so overall
size of program gets smaller). The compiler
heuristically decides which functions are simple enough
to be worth integrating in this way. This inlining
applies to all functions, even those not declared
inline.
Enabled at level -O2.
-findirect-inlining
Inline also indirect calls that are discovered to be
known at compile time thanks to previous inlining. This
option has any effect only when inlining itself is
turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
-finline-functions
Consider all functions for inlining, even if they are
not declared inline. The compiler heuristically decides
which functions are worth integrating in this way.
If all calls to a given function are integrated, and the
function is declared "static", then the function is
normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining
into their caller even if they are not marked "inline".
If a call to a given function is integrated, then the
function is not output as assembler code in its own
right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions
whose body seems smaller than the function call overhead
early before doing -fprofile-generate instrumentation
and real inlining pass. Doing so makes profiling
significantly cheaper and usually inlining faster on
programs having large chains of nested wrapper
functions.
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Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of
aggregates, removal of unused parameters and replacement
of parameters passed by reference by parameters passed
by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be
inlined. This flag allows coarse control of this limit.
n is the size of functions that can be inlined in number
of pseudo instructions.
Inlining is actually controlled by a number of
parameters, which may be specified individually by using
--param name=value. The -finline-limit=n option sets
some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual
parameters controlling inlining and for the defaults of
these parameters.
Note: there may be no value to -finline-limit that
results in default behavior.
Note: pseudo instruction represents, in this particular
context, an abstract measurement of function's size. In
no way does it represent a count of assembly
instructions and as such its exact meaning might change
from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of
-fkeep-inline-functions, which applies only to functions
that are declared using the "dllexport" attribute or
declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline"
into the object file, even if the function has been
inlined into all of its callers. This switch does not
affect functions using the "extern inline" extension in
GNU C90. In C++, emit any and all inline functions into
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the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if
the function is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization
isn't turned on, even if the variables aren't
referenced.
GCC enables this option by default. If you want to
force the compiler to check if a variable is referenced,
regardless of whether or not optimization is turned on,
use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants
and floating-point constants) across compilation units.
This option is the default for optimized compilation if
the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical
variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables
with integral or floating-point types. Languages like C
or C++ require each variable, including multiple
instances of the same variable in recursive calls, to
have distinct locations, so using this option results in
non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the
first scheduling pass. This pass looks at innermost
loops and reorders their instructions by overlapping
different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with
register moves allowed. By setting this flag certain
anti-dependences edges are deleted, which triggers the
generation of reg-moves based on the life-range
analysis. This option is effective only with
-fmodulo-sched enabled.
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-fno-branch-count-reg
Avoid running a pass scanning for opportunities to use
"decrement and branch" instructions on a count register
instead of generating sequences of instructions that
decrement a register, compare it against zero, and then
branch based upon the result. This option is only
meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and
S/390. Note that the -fno-branch-count-reg option
doesn't remove the decrement and branch instructions
from the generated instruction stream introduced by
other optimization passes.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each
instruction that calls a constant function contain the
function's address explicitly.
This option results in less efficient code, but some
strange hacks that alter the assembler output may be
confused by the optimizations performed when this option
is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default
puts variables that are initialized to zero into BSS.
This can save space in the resulting code.
This option turns off this behavior because some
programs explicitly rely on variables going to the data
section---e.g., so that the resulting executable can
find the beginning of that section and/or make
assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump
branches to a location where another comparison subsumed
by the first is found. If so, the first branch is
redirected to either the destination of the second
branch or a point immediately following it, depending on
whether the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
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-fsplit-wide-types
When using a type that occupies multiple registers, such
as "long long" on a 32-bit system, split the registers
apart and allocate them independently. This normally
generates better code for those types, but may make
debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through
jump instructions when the target of the jump is not
reached by any other path. For example, when CSE
encounters an "if" statement with an "else" clause, CSE
follows the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps that conditionally skip over blocks. When
CSE encounters a simple "if" statement with no else
clause, -fcse-skip-blocks causes CSE to follow the jump
around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop
optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy
propagation.
Note: When compiling a program using computed gotos, a
GCC extension, you may get better run-time performance
if you disable the global common subexpression
elimination pass by adding -fno-gcse to the command
line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression
elimination attempts to move loads that are only killed
by stores into themselves. This allows a loop
containing a load/store sequence to be changed to a load
outside the loop, and a copy/store within the loop.
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Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run
after global common subexpression elimination. This
pass attempts to move stores out of loops. When used in
conjunction with -fgcse-lm, loops containing a
load/store sequence can be changed to a load before the
loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common
subexpression elimination pass eliminates redundant
loads that come after stores to the same memory location
(both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load
elimination pass is performed after reload. The purpose
of this pass is to clean up redundant spilling.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language
constraints to derive bounds for the number of
iterations of a loop. This assumes that loop code does
not invoke undefined behavior by for example causing
signed integer overflows or out-of-bound array accesses.
The bounds for the number of iterations of a loop are
used to guide loop unrolling and peeling and loop exit
test optimizations. This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared
in common blocks (e.g. Fortran) may later be overridden
with longer trailing arrays. This prevents certain
optimizations that depend on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This
transformation unifies equivalent code and saves code
size. The resulting code may or may not perform better
than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with
memory accesses. This pass is always skipped on
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architectures that do not have instructions to support
this. Enabled by default at -O and higher on
architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by
default at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by
default at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This includes use of conditional moves,
min, max, set flags and abs instructions, and some
tricks doable by standard arithmetics. The use of
conditional execution on chips where it is available is
controlled by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for
constructors and destructors: one for a base subobject,
one for a complete object, and one for a virtual
destructor that calls operator delete afterwards. For a
hierarchy with virtual bases, the base and complete
variants are clones, which means two copies of the
function. With this option, the base and complete
variants are changed to be thunks that call a common
implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null
pointers, and that no code or data element resides at
address zero. This option enables simple constant
folding optimizations at all optimization levels. In
addition, other optimization passes in GCC use this flag
to control global dataflow analyses that eliminate
useless checks for null pointers; these assume that a
memory access to address zero always results in a trap,
so that if a pointer is checked after it has already
been dereferenced, it cannot be null.
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Note however that in some environments this assumption
is not true. Use -fno-delete-null-pointer-checks to
disable this optimization for programs that depend on
that behavior.
This option is enabled by default on most targets. On
Nios II ELF, it defaults to off. On AVR and CR16, this
option is completely disabled.
Passes that use the dataflow information are enabled
independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct
calls. This is done both within a procedure and
interprocedurally as part of indirect inlining
(-findirect-inlining) and interprocedural constant
propagation (-fipa-cp). Enabled at levels -O2, -O3,
-Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to
speculative direct calls. Based on the analysis of the
type inheritance graph, determine for a given call the
set of likely targets. If the set is small, preferably
of size 1, change the call into a conditional deciding
between direct and indirect calls. The speculative
calls enable more optimizations, such as inlining. When
they seem useless after further optimization, they are
converted back into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive
devirtualization when running the link-time optimizer in
local transformation mode. This option enables more
devirtualization but significantly increases the size of
streamed data. For this reason it is disabled by
default.
-fexpensive-optimizations
Perform a number of minor optimizations that are
relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions.
This is especially helpful for the x86-64 architecture,
which implicitly zero-extends in 64-bit registers after
writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3,
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-Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by
changes within its lifetime: when the constructor
begins, the object has an indeterminate value, and any
changes during the lifetime of the object are dead when
the object is destroyed. Normally dead store
elimination will take advantage of this; if your code
relies on the value of the object storage persisting
beyond the lifetime of the object, you can use this flag
to disable this optimization. To preserve stores before
the constructor starts (e.g. because your operator new
clears the object storage) but still treat the object as
dead after the destructor you, can use -flifetime-dse=1.
The default behavior can be explicitly selected with
-flifetime-dse=2. -flifetime-dse=0 is equivalent to
-fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register
live range shrinkage. This is helpful for fast
processors with small or moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated
register allocator. The algorithm argument can be
priority, which specifies Chow's priority coloring, or
CB, which specifies Chaitin-Briggs coloring. Chaitin-
Briggs coloring is not implemented for all
architectures, but for those targets that do support it,
it is the default because it generates better code.
-fira-region=region
Use specified regions for the integrated register
allocator. The region argument should be one of the
following:
all Use all loops as register allocation regions. This
can give the best results for machines with a small
and/or irregular register set.
mixed
Use all loops except for loops with small register
pressure as the regions. This value usually gives
the best results in most cases and for most
architectures, and is enabled by default when
compiling with optimization for speed (-O, -O2,
...).
one Use all functions as a single region. This typically
results in the smallest code size, and is enabled by
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default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code
hoisting pass for decisions to hoist expressions. This
option usually results in smaller code, but it can slow
the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for
decisions to move loop invariants. This option usually
results in generation of faster and smaller code on
machines with large register files (>= 32 registers),
but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used
hard registers living through a call. Each hard
register gets a separate stack slot, and as a result
function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for
pseudo-registers. Each pseudo-register that does not
get a hard register gets a separate stack slot, and as a
result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead
of loading values of spilled pseudos, LRA tries to
rematerialize (recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available
after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to
required data being unavailable. This helps machines
that have slow floating point or memory load
instructions by allowing other instructions to be issued
until the result of the load or floating-point
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instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional
pass of instruction scheduling after register allocation
has been done. This is especially useful on machines
with a relatively small number of registers and where
memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This
is normally enabled by default when scheduling before
register allocation, i.e. with -fschedule-insns or at
-O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions.
This is normally enabled by default when scheduling
before register allocation, i.e. with -fschedule-insns
or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling
before register allocation. This only makes sense when
scheduling before register allocation is enabled, i.e.
with -fschedule-insns or at -O2 or higher. Usage of
this option can improve the generated code and decrease
its size by preventing register pressure increase above
the number of available hard registers and subsequent
spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions.
This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or
higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions.
This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or
higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely
from the queue of stalled insns into the ready list
during the second scheduling pass.
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-fno-sched-stalled-insns means that no insns are moved
prematurely, -fsched-stalled-insns=0 means there is no
limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a
dependency on a stalled insn that is a candidate for
premature removal from the queue of stalled insns. This
has an effect only during the second scheduling pass,
and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep
without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use
superblock scheduling. This allows motion across basic
block boundaries, resulting in faster schedules. This
option is experimental, as not all machine descriptions
used by GCC model the CPU closely enough to avoid
unreliable results from the algorithm.
This only makes sense when scheduling after register
allocation, i.e. with -fschedule-insns2 or at -O2 or
higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This
heuristic favors the instruction that belongs to a
schedule group. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler.
This heuristic favors instructions on the critical path.
This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the
scheduler. This heuristic favors speculative
instructions with greater dependency weakness. This is
enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
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-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This
heuristic favors the instruction belonging to a basic
block with greater size or frequency. This is enabled
by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler.
This heuristic favors the instruction that is less
dependent on the last instruction scheduled. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler.
This heuristic favors the instruction that has more
instructions depending on it. This is enabled by
default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional
scheduling. If a loop is modulo scheduled, later
scheduling passes may change its schedule. Use this
option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling
algorithm. Selective scheduling runs instead of the
first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling
algorithm. Selective scheduling runs instead of the
second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during
selective scheduling. This option has no effect unless
one of -fselective-scheduling or -fselective-scheduling2
is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also
pipeline outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
-fsemantic-interposition
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Some object formats, like ELF, allow interposing of
symbols by the dynamic linker. This means that for
symbols exported from the DSO, the compiler cannot
perform interprocedural propagation, inlining and other
optimizations in anticipation that the function or
variable in question may change. While this feature is
useful, for example, to rewrite memory allocation
functions by a debugging implementation, it is expensive
in the terms of code quality. With
-fno-semantic-interposition the compiler assumes that if
interposition happens for functions the overwriting
function will have precisely the same semantics (and
side effects). Similarly if interposition happens for
variables, the constructor of the variable will be the
same. The flag has no effect for functions explicitly
declared inline (where it is never allowed for
interposition to change semantics) and for symbols
explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the
function that need it, rather than at the top of the
function. This flag is enabled by default at -O and
higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue
separately, so that those parts are only executed when
needed. This option is on by default, but has no effect
unless -fshrink-wrap is also turned on and the target
supports this.
-fcaller-saves
Enable allocation of values to registers that are
clobbered by function calls, by emitting extra
instructions to save and restore the registers around
such calls. Such allocation is done only when it seems
to result in better code.
This option is always enabled by default on certain
machines, usually those which have no call-preserved
registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack
memory references and then tries to find ways to combine
them.
Enabled by default at -O1 and higher.
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-fipa-ra
Use caller save registers for allocation if those
registers are not used by any called function. In that
case it is not necessary to save and restore them around
calls. This is only possible if called functions are
part of same compilation unit as current function and
they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is
disabled if generated code will be instrumented for
profiling (-p, or -pg) or if callee's register usage
cannot be known exactly (this happens on targets that do
not expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts
to use less stack space, even if that makes the program
slower. This option implies setting the large-stack-
frame parameter to 100 and the large-stack-frame-growth
parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by
default at -O and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the
evaluation of expressions executed on all paths to the
function exit as early as possible. This is especially
useful as a code size optimization, but it often helps
for code speed as well. This flag is enabled by default
at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees.
This flag is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more
aggressive. This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is
enabled by default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The
difference between FRE and PRE is that FRE only
considers expressions that are computed on all paths
leading to the redundant computation. This analysis is
faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
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-ftree-phiprop
Perform hoisting of loads from conditional pointers on
trees. This pass is enabled by default at -O and
higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-
then-else if the loads are from adjacent locations in
the same structure and the target architecture has a
conditional move instruction. This flag is enabled by
default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by
default at -O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled
by default at -O and higher.
-fipa-reference
Discover which static variables do not escape the
compilation unit. Enabled by default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis and
interprocedural modification and reference analysis.
This option can cause excessive memory and compile-time
usage on large compilation units. It is not enabled by
default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The
functions called only from cold functions are marked as
cold. Also functions executed once (such as "cold",
"noreturn", static constructors or destructors) are
identified. Cold functions and loop less parts of
functions executed once are then optimized for size.
Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This
optimization analyzes the program to determine when
values passed to functions are constants and then
optimizes accordingly. This optimization can
substantially increase performance if the application
has constants passed to functions. This flag is enabled
by default at -O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural
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constant propagation stronger. When enabled,
interprocedural constant propagation performs function
cloning when externally visible function can be called
with constant arguments. Because this optimization can
create multiple copies of functions, it may
significantly increase code size (see --param
ipcp-unit-growth=value). This flag is enabled by
default at -O3.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant
propagation. This flag is enabled by default at -O2. It
requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of
value ranges. This flag is enabled by default at -O2. It
requires that -fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-
only variables. The optimization reduces code size and
may disturb unwind stacks by replacing a function by
equivalent one with a different name. The optimization
works more effectively with link-time optimization
enabled.
Nevertheless the behavior is similar to Gold Linker ICF
optimization, GCC ICF works on different levels and thus
the optimizations are not same - there are equivalences
that are found only by GCC and equivalences found only
by Gold.
This flag is enabled by default at -O2 and -Os.
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined
behavior due to dereferencing a null pointer. Isolate
those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a
trap. This flag is enabled by default at -O2 and higher
and depends on -fdelete-null-pointer-checks also being
enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined
behavior due a null value being used in a way forbidden
by a "returns_nonnull" or "nonnull" attribute. Isolate
those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a
trap. This is not currently enabled, but may be enabled
by -O2 in the future.
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-ftree-sink
Perform forward store motion on trees. This flag is
enabled by default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on
trees and propagate pointer alignment information. This
pass only operates on local scalar variables and is
enabled by default at -O and higher. It requires that
-ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on
trees. This pass only operates on local scalar
variables and is enabled by default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the
definition chain in order to simplify the definitions.
For example, this pass strips sign operations if the
sign of a value never matters. The flag is enabled by
default at -O and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize
conditional code. This pass is enabled by default at -O
and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch
to initializations from a scalar array. This flag is
enabled by default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace
one with a jump to the other. This optimization is
known as tail merging or cross jumping. This flag is
enabled by default at -O2 and higher. The compilation
time in this pass can be limited using max-tail-merge-
comparisons parameter and max-tail-merge-iterations
parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag
is enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for
calls to built-in functions that may set "errno" but are
otherwise side-effect free. This flag is enabled by
default at -O2 and higher if -Os is not also specified.
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-ftree-dominator-opts
Perform a variety of simple scalar cleanups
(constant/copy propagation, redundancy elimination,
range propagation and expression simplification) based
on a dominator tree traversal. This also performs jump
threading (to reduce jumps to jumps). This flag is
enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead
store is a store into a memory location that is later
overwritten by another store without any intervening
loads. In this case the earlier store can be deleted.
This flag is enabled by default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is
beneficial since it increases effectiveness of code
motion optimizations. It also saves one jump. This
flag is enabled by default at -O and higher. It is not
enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is
enabled by default at -O and higher.
-ftree-loop-linear
-floop-interchange
-floop-strip-mine
-floop-block
-floop-unroll-and-jam
Perform loop nest optimizations. Same as
-floop-nest-optimize. To use this code transformation,
GCC has to be configured with --with-isl to enable the
Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For
every SCoP we generate the polyhedral representation and
transform it back to gimple. Using -fgraphite-identity
we can check the costs or benefits of the GIMPLE ->
GRAPHITE -> GIMPLE transformation. Some minimal
optimizations are also performed by the code generator
isl, like index splitting and dead code elimination in
loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a
generic loop nest optimizer based on the Pluto
optimization algorithms. It calculates a loop structure
optimized for data-locality and parallelism. This
option is experimental.
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-floop-parallelize-all
Use the Graphite data dependence analysis to identify
loops that can be parallelized. Parallelize all the
loops that can be analyzed to not contain loop carried
dependences without checking that it is profitable to
parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA
representation, attempt to reduce copying by coalescing
versions of different user-defined variables, instead of
just compiler temporaries. This may severely limit the
ability to debug an optimized program compiled with
-fno-var-tracking-assignments. In the negated form,
this flag prevents SSA coalescing of user variables.
This option is enabled by default if optimization is
enabled, and it does very little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost
loops to branch-less equivalents. The intent is to
remove control-flow from the innermost loops in order to
improve the ability of the vectorization pass to handle
these loops. This is enabled by default if
vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache
performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to
take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code
generated with calls to a library. This flag is enabled
by default at -O3.
This pass distributes the initialization loops and
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generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call
to memset zero.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves
only invariants that are hard to handle at RTL level
(function calls, operations that expand to nontrivial
sequences of insns). With -funswitch-loops it also
moves operands of conditions that are invariant out of
the loop, so that we can use just trivial invariantness
analysis in loop unswitching. The pass also includes
store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in
loops for which determining number of iterations
requires complicated analysis. Later optimizations then
may determine the number easily. Useful especially in
connection with unrolling.
-fivopts
Perform induction variable optimizations (strength
reduction, induction variable merging and induction
variable elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to
run in n threads. This is only possible for loops whose
iterations are independent and can be arbitrarily
reordered. The optimization is only profitable on
multiprocessor machines, for loops that are
CPU-intensive, rather than constrained e.g. by memory
bandwidth. This option implies -pthread, and thus is
only supported on targets that have support for
-pthread.
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-ftree-pta
Perform function-local points-to analysis on trees.
This flag is enabled by default at -O and higher.
-ftree-sra
Perform scalar replacement of aggregates. This pass
replaces structure references with scalars to prevent
committing structures to memory too early. This flag is
enabled by default at -O and higher.
-fstore-merging
Perform merging of narrow stores to consecutive memory
addresses. This pass merges contiguous stores of
immediate values narrower than a word into fewer wider
stores to reduce the number of instructions. This is
enabled by default at -O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the
SSA->normal phase. Single use/single def temporaries
are replaced at their use location with their defining
expression. This results in non-GIMPLE code, but gives
the expanders much more complex trees to work on
resulting in better RTL generation. This is enabled by
default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This
recognizes related expressions involving multiplications
and replaces them by less expensive calculations when
possible. This is enabled by default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not
explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is
enabled by default at -O3 and when -ftree-vectorize is
enabled.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is
enabled by default at -O3 and when -ftree-vectorize is
enabled.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model
argument should be one of unlimited, dynamic or cheap.
With the unlimited model the vectorized code-path is
assumed to be profitable while with the dynamic model a
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runtime check guards the vectorized code-path to enable
it only for iteration counts that will likely execute
faster than when executing the original scalar loop.
The cheap model disables vectorization of loops where
doing so would be cost prohibitive for example due to
required runtime checks for data dependence or alignment
but otherwise is equal to the dynamic model. The
default cost model depends on other optimization flags
and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops
marked with the OpenMP or Cilk Plus simd directive. The
model argument should be one of unlimited, dynamic,
cheap. All values of model have the same meaning as
described in -fvect-cost-model and by default a cost
model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is
similar to the constant propagation pass, but instead of
values, ranges of values are propagated. This allows
the optimizers to remove unnecessary range checks like
array bound checks and null pointer checks. This is
enabled by default at -O2 and higher. Null pointer
check elimination is only done if
-fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve
dead code elimination and common subexpression
elimination. This is enabled by default at -O2 and
above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in
later iterations of the unrolled loop using the value in
the first iteration. This breaks long dependency
chains, thus improving efficiency of the scheduling
passes.
A combination of -fweb and CSE is often sufficient to
obtain the same effect. However, that is not reliable
in cases where the loop body is more complicated than a
single basic block. It also does not work at all on
some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies
of some local variables when unrolling a loop, which can
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result in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect
only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores)
performed in previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate
instructions to prefetch memory to improve the
performance of loops that access large arrays.
This option may generate better or worse code; results
are highly dependent on the structure of loops within
the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of
formatted output functions such as "sprintf",
"snprintf", "vsprintf", and "vsnprintf" (but not
"printf" of "fprintf"). This transformation allows GCC
to optimize or even eliminate branches based on the
known return value of these functions called with
arguments that are either constant, or whose values are
known to be in a range that makes determining the exact
return value possible. For example, when
-fprintf-return-value is in effect, both the branch and
the body of the "if" statement (but not the call to
"snprint") can be optimized away when "i" is a 32-bit or
smaller integer because the return value is guaranteed
to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other
optimizations and yields best results with -O2. It
works in tandem with the -Wformat-overflow and
-Wformat-truncation options. The -fprintf-return-value
option is enabled by default.
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-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations.
The difference between -fno-peephole and -fno-peephole2
is in how they are implemented in the compiler; some
targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled
at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if
they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the
control flow graph. If some branch probabilities are
specified by "__builtin_expect", then the heuristics are
used to guess branch probabilities for the rest of the
control flow graph, taking the "__builtin_expect" info
into account. The interactions between the heuristics
and "__builtin_expect" can be complex, and in some
cases, it may be useful to disable the heuristics so
that the effects of "__builtin_expect" are easier to
understand.
The default is -fguess-branch-probability at levels -O,
-O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order
to reduce number of taken branches and improve code
locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering.
The algorithm argument can be simple, which does not
increase code size (except sometimes due to secondary
effects like alignment), or stc, the "software trace
cache" algorithm, which tries to put all often executed
code together, minimizing the number of branches
executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at
levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled
function, in order to reduce number of taken branches,
partitions hot and cold basic blocks into separate
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sections of the assembly and .o files, to improve paging
and cache locality performance.
This optimization is automatically turned off in the
presence of exception handling, for linkonce sections,
for functions with a user-defined section attribute and
on any architecture that does not support named
sections.
Enabled for x86 at levels -O2, -O3.
-freorder-functions
Reorder functions in the object file in order to improve
code locality. This is implemented by using special
subsections ".text.hot" for most frequently executed
functions and ".text.unlikely" for unlikely executed
functions. Reordering is done by the linker so object
file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available to make this
option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing
rules applicable to the language being compiled. For C
(and C++), this activates optimizations based on the
type of expressions. In particular, an object of one
type is assumed never to reside at the same address as
an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can
alias an "int", but not a "void*" or a "double". A
character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member
than the one most recently written to (called
"type-punning") is common. Even with -fstrict-aliasing,
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type-punning is allowed, provided the memory is accessed
through the union type. So, the code above works as
expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the
resulting pointer and dereferencing the result has
undefined behavior, even if the cast uses a union type,
e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2,
-O3, -Os.
-fstrict-overflow
Allow the compiler to assume strict signed overflow
rules, depending on the language being compiled. For C
(and C++) this means that overflow when doing arithmetic
with signed numbers is undefined, which means that the
compiler may assume that it does not happen. This
permits various optimizations. For example, the
compiler assumes that an expression like "i + 10 > i" is
always true for signed "i". This assumption is only
valid if signed overflow is undefined, as the expression
is false if "i + 10" overflows when using twos
complement arithmetic. When this option is in effect
any attempt to determine whether an operation on signed
numbers overflows must be written carefully to not
actually involve overflow.
This option also allows the compiler to assume strict
pointer semantics: given a pointer to an object, if
adding an offset to that pointer does not produce a
pointer to the same object, the addition is undefined.
This permits the compiler to conclude that "p + u > p"
is always true for a pointer "p" and unsigned integer
"u". This assumption is only valid because pointer
wraparound is undefined, as the expression is false if
"p + u" overflows using twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that
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integer signed overflow is fully defined: it wraps.
When -fwrapv is used, there is no difference between
-fstrict-overflow and -fno-strict-overflow for integers.
With -fwrapv certain types of overflow are permitted.
For example, if the compiler gets an overflow when doing
arithmetic on constants, the overflowed value can still
be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2,
-O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two
greater than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next
32-byte boundary, but -falign-functions=24 aligns to the
next 32-byte boundary only if this can be done by
skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are
equivalent and mean that functions are not aligned.
Some assemblers only support this flag when n is a power
of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-
dependent default.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid
unnecessarily overaligning functions. It attempts to
instruct the assembler to align by the amount specified
by -falign-functions, but not to skip more bytes than
the size of the function.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary,
skipping up to n bytes like -falign-functions. This
option can easily make code slower, because it must
insert dummy operations for when the branch target is
reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent
and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are
greater than this value, then their values are used
instead.
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If n is not specified or is zero, use a machine-
dependent default which is very likely to be 1, meaning
no alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n
bytes like -falign-functions. If the loops are executed
many times, this makes up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are equivalent and
mean that loops are not aligned.
If n is not specified or is zero, use a machine-
dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for
branch targets where the targets can only be reached by
jumping, skipping up to n bytes like -falign-functions.
In this case, no dummy operations need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and
mean that loops are not aligned.
If n is not specified or is zero, use a machine-
dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons.
-funit-at-a-time has no effect, while
-fno-unit-at-a-time implies -fno-toplevel-reorder and
-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they
appear in the input file. When this option is used,
unreferenced static variables are not removed. This
option is intended to support existing code that relies
on a particular ordering. For new code, it is better to
use attributes when possible.
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Enabled at level -O0. When disabled explicitly, it also
implies -fno-section-anchors, which is otherwise enabled
at -O0 on some targets.
-fweb
Constructs webs as commonly used for register allocation
purposes and assign each web individual pseudo register.
This allows the register allocation pass to operate on
pseudos directly, but also strengthens several other
optimization passes, such as CSE, loop optimizer and
trivial dead code remover. It can, however, make
debugging impossible, since variables no longer stay in
a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the
whole program being compiled. All public functions and
variables with the exception of "main" and those merged
by attribute "externally_visible" become static
functions and in effect are optimized more aggressively
by interprocedural optimizers.
This option should not be used in combination with
-flto. Instead relying on a linker plugin should
provide safer and more precise information.
-flto[=n]
This option runs the standard link-time optimizer. When
invoked with source code, it generates GIMPLE (one of
GCC's internal representations) and writes it to special
ELF sections in the object file. When the object files
are linked together, all the function bodies are read
from these ELF sections and instantiated as if they had
been part of the same translation unit.
To use the link-time optimizer, -flto and optimization
options should be specified at compile time and during
the final link. It is recommended that you compile all
the files participating in the same link with the same
options and also specify those options at link time. For
example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode
representation of GIMPLE into special ELF sections
inside foo.o and bar.o. The final invocation reads the
GIMPLE bytecode from foo.o and bar.o, merges the two
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files into a single internal image, and compiles the
result as usual. Since both foo.o and bar.o are merged
into a single image, this causes all the interprocedural
analyses and optimizations in GCC to work across the two
files as if they were a single one. This means, for
example, that the inliner is able to inline functions in
bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization
is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges
them together into a single GIMPLE representation and
optimizes them as usual to produce myprog.
The only important thing to keep in mind is that to
enable link-time optimizations you need to use the GCC
driver to perform the link step. GCC then automatically
performs link-time optimization if any of the objects
involved were compiled with the -flto command-line
option. You generally should specify the optimization
options to be used for link-time optimization though GCC
tries to be clever at guessing an optimization level to
use from the options used at compile time if you fail to
specify one at link time. You can always override the
automatic decision to do link-time optimization by
passing -fno-lto to the link command.
To make whole program optimization effective, it is
necessary to make certain whole program assumptions.
The compiler needs to know what functions and variables
can be accessed by libraries and runtime outside of the
link-time optimized unit. When supported by the linker,
the linker plugin (see -fuse-linker-plugin) passes
information to the compiler about used and externally
visible symbols. When the linker plugin is not
available, -fwhole-program should be used to allow the
compiler to make these assumptions, which leads to more
aggressive optimization decisions.
When -fuse-linker-plugin is not enabled, when a file is
compiled with -flto, the generated object file is larger
than a regular object file because it contains GIMPLE
bytecodes and the usual final code (see
-ffat-lto-objects. This means that object files with
LTO information can be linked as normal object files; if
-fno-lto is passed to the linker, no interprocedural
optimizations are applied. Note that when
-fno-fat-lto-objects is enabled the compile stage is
faster but you cannot perform a regular, non-LTO link on
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them.
Additionally, the optimization flags used to compile
individual files are not necessarily related to those
used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized
assembler code, but the resulting binary myprog is
optimized at -O3. If, instead, the final binary is
generated with -fno-lto, then myprog is not optimized.
When producing the final binary, GCC only applies link-
time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and
libraries with GIMPLE bytecodes and final object code.
GCC automatically selects which files to optimize in LTO
mode and which files to link without further processing.
There are some code generation flags preserved by GCC
when generating bytecodes, as they need to be used
during the final link stage. Generally options
specified at link time override those specified at
compile time.
If you do not specify an optimization level option -O at
link time, then GCC uses the highest optimization level
used when compiling the object files.
Currently, the following options and their settings are
taken from the first object file that explicitly
specifies them: -fPIC, -fpic, -fpie, -fcommon,
-fexceptions, -fnon-call-exceptions, -fgnu-tm and all
the -m target flags.
Certain ABI-changing flags are required to match in all
compilation units, and trying to override this at link
time with a conflicting value is ignored. This includes
options such as -freg-struct-return and
-fpcc-struct-return.
Other options such as -ffp-contract,
-fno-strict-overflow, -fwrapv, -fno-trapv or
-fno-strict-aliasing are passed through to the link
stage and merged conservatively for conflicting
translation units. Specifically -fno-strict-overflow,
-fwrapv and -fno-trapv take precedence; and for example
-ffp-contract=off takes precedence over
-ffp-contract=fast. You can override them at link time.
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If LTO encounters objects with C linkage declared with
incompatible types in separate translation units to be
linked together (undefined behavior according to ISO C99
6.2.7), a non-fatal diagnostic may be issued. The
behavior is still undefined at run time. Similar
diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in
different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the
C++ runtime libraries and -lgfortran is added to get the
Fortran runtime libraries. In general, when mixing
languages in LTO mode, you should use the same link
command options as when mixing languages in a regular
(non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in
a library archive, say libfoo.a, it is possible to
extract and use them in an LTO link if you are using a
linker with plugin support. To create static libraries
suitable for LTO, use gcc-ar and gcc-ranlib instead of
ar and ranlib; to show the symbols of object files with
GIMPLE bytecode, use gcc-nm. Those commands require
that ar, ranlib and nm have been compiled with plugin
support. At link time, use the the flag
-fuse-linker-plugin to ensure that the library
participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the
needed GIMPLE files from libfoo.a and passes them on to
the running GCC to make them part of the aggregated
GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or
do not enable the linker plugin, then the objects inside
libfoo.a are extracted and linked as usual, but they do
not participate in the LTO optimization process. In
order to make a static library suitable for both LTO
optimization and usual linkage, compile its object files
with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of
the whole program to operate. If the program does not
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require any symbols to be exported, it is possible to
combine -flto and -fwhole-program to allow the
interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization
opportunities. Use of -fwhole-program is not needed
when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to
generate bytecode that is portable between different
types of hosts. The bytecode files are versioned and
there is a strict version check, so bytecode files
generated in one version of GCC do not work with an
older or newer version of GCC.
Link-time optimization does not work well with
generation of debugging information. Combining -flto
with -g is currently experimental and expected to
produce unexpected results.
If you specify the optional n, the optimization and code
generation done at link time is executed in parallel
using n parallel jobs by utilizing an installed make
program. The environment variable MAKE may be used to
override the program used. The default value for n is
1.
You can also specify -flto=jobserver to use GNU make's
job server mode to determine the number of parallel
jobs. This is useful when the Makefile calling GCC is
already executing in parallel. You must prepend a + to
the command recipe in the parent Makefile for this to
work. This option likely only works if MAKE is GNU
make.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time
optimizer. The value is either 1to1 to specify a
partitioning mirroring the original source files or
balanced to specify partitioning into equally sized
chunks (whenever possible) or max to create new
partition for every symbol where possible. Specifying
none as an algorithm disables partitioning and streaming
completely. The default value is balanced. While 1to1
can be used as an workaround for various code ordering
issues, the max partitioning is intended for internal
testing only. The value one specifies that exactly one
partition should be used while the value none bypasses
partitioning and executes the link-time optimization
step directly from the WPA phase.
-flto-odr-type-merging
Enable streaming of mangled types names of C++ types and
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their unification at link time. This increases size of
LTO object files, but enables diagnostics about One
Definition Rule violations.
-flto-compression-level=n
This option specifies the level of compression used for
intermediate language written to LTO object files, and
is only meaningful in conjunction with LTO mode (-flto).
Valid values are 0 (no compression) to 9 (maximum
compression). Values outside this range are clamped to
either 0 or 9. If the option is not given, a default
balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time
optimization. This option relies on plugin support in
the linker, which is available in gold or in GNU ld 2.21
or newer.
This option enables the extraction of object files with
GIMPLE bytecode out of library archives. This improves
the quality of optimization by exposing more code to the
link-time optimizer. This information specifies what
symbols can be accessed externally (by non-LTO object or
during dynamic linking). Resulting code quality
improvements on binaries (and shared libraries that use
hidden visibility) are similar to -fwhole-program. See
-flto for a description of the effect of this flag and
how to use it.
This option is enabled by default when LTO support in
GCC is enabled and GCC was configured for use with a
linker supporting plugins (GNU ld 2.21 or newer or
gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the
intermediate language and the object code. This makes
them usable for both LTO linking and normal linking.
This option is effective only when compiling with -flto
and is ignored at link time.
-fno-fat-lto-objects improves compilation time over
plain LTO, but requires the complete toolchain to be
aware of LTO. It requires a linker with linker plugin
support for basic functionality. Additionally, nm, ar
and ranlib need to support linker plugins to allow a
full-featured build environment (capable of building
static libraries etc). GCC provides the gcc-ar, gcc-nm,
gcc-ranlib wrappers to pass the right options to these
tools. With non fat LTO makefiles need to be modified to
use them.
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The default is -fno-fat-lto-objects on targets with
linker plugin support.
-fcompare-elim
After register allocation and post-register allocation
instruction splitting, identify arithmetic instructions
that compute processor flags similar to a comparison
operation based on that arithmetic. If possible,
eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot
explicitly represent the comparison operation before
register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation
instruction splitting, perform a copy-propagation pass
to try to reduce scheduling dependencies and
occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for
multi-threaded programs may be inconsistent due to
missed counter updates. When this option is specified,
GCC uses heuristics to correct or smooth out such
inconsistencies. By default, GCC emits an error message
when an inconsistent profile is detected.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the
following optimizations which are generally profitable
only with profile feedback available:
-fbranch-probabilities, -fvpt, -funroll-loops,
-fpeel-loops, -ftracer, -ftree-vectorize, and ftree-
loop-distribute-patterns.
Before you can use this option, you must first generate
profiling information.
By default, GCC emits an error message if the feedback
profiles do not match the source code. This error can
be turned into a warning by using -Wcoverage-mismatch.
Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
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-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations,
and the following optimizations which are generally
profitable only with profile feedback available:
-fbranch-probabilities, -fvpt, -funroll-loops,
-fpeel-loops, -ftracer, -ftree-vectorize,
-finline-functions, -fipa-cp, -fipa-cp-clone,
-fpredictive-commoning, -funswitch-loops,
-fgcse-after-reload, and
-ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile
information. If omitted, it defaults to fbdata.afdo in
the current directory.
Producing an AutoFDO profile data file requires running
your program with the perf utility on a supported
GNU/Linux target system. For more information, see
<https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile
data to a format that can be used by GCC. You must also
supply the unstripped binary for your program to this
tool. See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding
floating-point arithmetic. These options trade off between
speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and
inhibit other options that might change whether a
floating-point value is taken from a register or memory.
This option prevents undesirable excess precision on
machines such as the 68000 where the floating registers
(of the 68881) keep more precision than a "double" is
supposed to have. Similarly for the x86 architecture.
For most programs, the excess precision does only good,
but a few programs rely on the precise definition of
IEEE floating point. Use -ffloat-store for such
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programs, after modifying them to store all pertinent
intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision
on machines where floating-point operations occur in a
format with more precision or range than the IEEE
standard and interchange floating-point types. By
default, -fexcess-precision=fast is in effect; this
means that operations may be carried out in a wider
precision than the types specified in the source if that
would result in faster code, and it is unpredictable
when rounding to the types specified in the source code
takes place. When compiling C, if
-fexcess-precision=standard is specified then excess
precision follows the rules specified in ISO C99; in
particular, both casts and assignments cause values to
be rounded to their semantic types (whereas
-ffloat-store only affects assignments). This option is
enabled by default for C if a strict conformance option
such as -std=c99 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether
a strict conformance option is used.
-fexcess-precision=standard is not implemented for
languages other than C. On the x86, it has no effect if
-mfpmath=sse or -mfpmath=sse+387 is specified; in the
former case, IEEE semantics apply without excess
precision, and in the latter, rounding is unpredictable.
-ffast-math
Sets the options -fno-math-errno,
-funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans,
-fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro
"__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides
-Ofast since it can result in incorrect output for
programs that depend on an exact implementation of IEEE
or ISO rules/specifications for math functions. It may,
however, yield faster code for programs that do not
require the guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are
executed with a single instruction, e.g., "sqrt". A
program that relies on IEEE exceptions for math error
handling may want to use this flag for speed while
maintaining IEEE arithmetic compatibility.
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This option is not turned on by any -O option since it
can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not
require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno".
There is therefore no reason for the compiler to
consider the possibility that it might, and
-fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that
(a) assume that arguments and results are valid and (b)
may violate IEEE or ANSI standards. When used at link
time, it may include libraries or startup files that
change the default FPU control word or other similar
optimizations.
This option is not turned on by any -O option since it
can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not
require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math,
-fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-
point operations. This violates the ISO C and C++
language standard by possibly changing computation
result. NOTE: re-ordering may change the sign of zero
as well as ignore NaNs and inhibit or create underflow
or overflow (and thus cannot be used on code that relies
on rounding behavior like "(x + 2**52) - 2**52". May
also reorder floating-point comparisons and thus may not
be used when ordered comparisons are required. This
option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't
make much sense with -frounding-math. For Fortran the
option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
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Allow the reciprocal of a value to be used instead of
dividing by the value if this enables optimizations.
For example "x / y" can be replaced with "x * (1/y)",
which is useful if "(1/y)" is subject to common
subexpression elimination. Note that this loses
precision and increases the number of flops operating on
the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that
assume that arguments and results are not NaNs or
+-Infs.
This option is not turned on by any -O option since it
can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not
require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that
ignore the signedness of zero. IEEE arithmetic
specifies the behavior of distinct +0.0 and -0.0 values,
which then prohibits simplification of expressions such
as x+0.0 or 0.0*x (even with -ffinite-math-only). This
option implies that the sign of a zero result isn't
significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations
cannot generate user-visible traps. These traps include
division by zero, overflow, underflow, inexact result
and invalid operation. This option requires that
-fno-signaling-nans be in effect. Setting this option
may allow faster code if one relies on "non-stop" IEEE
arithmetic, for example.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
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Disable transformations and optimizations that assume
default floating-point rounding behavior. This is
round-to-zero for all floating point to integer
conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified
for programs that change the FP rounding mode
dynamically, or that may be executed with a non-default
rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be
affected by rounding mode) and arithmetic
transformations that are unsafe in the presence of
sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently
guarantee to disable all GCC optimizations that are
affected by rounding mode. Future versions of GCC may
provide finer control of this setting using C99's
"FENV_ACCESS" pragma. This command-line option will be
used to specify the default state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may
generate user-visible traps during floating-point
operations. Setting this option disables optimizations
that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro
"__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently
guarantee to disable all GCC optimizations that affect
signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor",
"round" and "trunc", and their "float" and "long double"
variants, to generate code that raises the "inexact"
floating-point exception for noninteger arguments. ISO
C99 and C11 allow these functions to raise the "inexact"
exception, but ISO/IEC TS 18661-1:2014, the C bindings
to IEEE 754-2008, does not allow these functions to do
so.
The default is -ffp-int-builtin-inexact, allowing the
exception to be raised. This option does nothing unless
-ftrapping-math is in effect.
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Even if -fno-fp-int-builtin-inexact is used, if the
functions generate a call to a library function then the
"inexact" exception may be raised if the library
implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision
instead of implicitly converting them to double-
precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction
step is not needed when performing complex division.
Also, there is no checking whether the result of a
complex multiplication or division is "NaN + I*NaN",
with an attempt to rescue the situation in that case.
The default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option
applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran
rules. Range reduction is done as part of complex
division, but there is no checking whether the result of
a complex multiplication or division is "NaN + I*NaN",
with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This
section includes experimental options that may produce
broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs,
you can compile it a second time using
-fbranch-probabilities, to improve optimizations based
on the number of times each branch was taken. When a
program compiled with -fprofile-arcs exits, it saves arc
execution counts to a file called sourcename.gcda for
each source file. The information in this data file is
very dependent on the structure of the generated code,
so you must use the same source code and the same
optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note
on each JUMP_INSN and CALL_INSN. These can be used to
improve optimization. Currently, they are only used in
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one place: in reorg.c, instead of guessing which path a
branch is most likely to take, the REG_BR_PROB values
are used to exactly determine which path is taken more
often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that
some data about values of expressions in the program is
gathered.
With -fbranch-probabilities, it reads back the data
gathered from profiling values of expressions for usage
in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fprofile-reorder-functions
Function reordering based on profile instrumentation
collects first time of execution of a function and
orders these functions in ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs
the compiler to add code to gather information about
values of expressions.
With -fbranch-probabilities, it reads back the data
gathered and actually performs the optimizations based
on them. Currently the optimizations include
specialization of division operations using the
knowledge about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by
making use of registers left over after register
allocation. This optimization most benefits processors
with lots of registers. Depending on the debug
information format adopted by the target, however, it
can make debugging impossible, since variables no longer
stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction
stream to schedule instructions of same type together
because target machine can execute them more efficiently
if they are adjacent to each other in the instruction
flow.
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Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size.
This transformation simplifies the control flow of the
function allowing other optimizations to do a better
job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be
determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop, -fweb and
-frename-registers. It also turns on complete loop
peeling (i.e. complete removal of loops with a small
constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is
uncertain when the loop is entered. This usually makes
programs run more slowly. -funroll-all-loops implies
the same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that
they do not roll much (from profile feedback or static
analysis). It also turns on complete loop peeling (i.e.
complete removal of loops with small constant number of
iterations).
Enabled with -O3 and/or -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop
optimizer. Enabled at level -O1
-fsplit-loops
Split a loop into two if it contains a condition that's
always true for one side of the iteration space and
false for the other.
-funswitch-loops
Move branches with loop invariant conditions out of the
loop, with duplicates of the loop on both branches
(modified according to result of the condition).
-ffunction-sections
-fdata-sections
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Place each function or data item into its own section in
the output file if the target supports arbitrary
sections. The name of the function or the name of the
data item determines the section's name in the output
file.
Use these options on systems where the linker can
perform optimizations to improve locality of reference
in the instruction space. Most systems using the ELF
object format and SPARC processors running Solaris 2
have linkers with such optimizations. AIX may have
these optimizations in the future.
Only use these options when there are significant
benefits from doing so. When you specify these options,
the assembler and linker create larger object and
executable files and are also slower. You cannot use
gprof on all systems if you specify this option, and you
may have problems with debugging if you specify both
this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before
prologue / epilogue threading. The use of target
registers can typically be exposed only during reload,
thus hoisting loads out of loops and doing inter-block
scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after
prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load
optimization, don't reuse branch target registers within
any basic block.
-fstdarg-opt
Optimize the prologue of variadic argument functions
with respect to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address
calculations by using shared "anchor" symbols to address
nearby objects. This transformation can help to reduce
the number of GOT entries and GOT accesses on some
targets.
For example, the implementation of the following
function "foo":
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static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables,
but if you compile it with -fsection-anchors, it
accesses the variables from a common anchor point
instead. The effect is similar to the following
pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control
the amount of optimization that is done. For example,
GCC does not inline functions that contain more than a
certain number of instructions. You can control some of
these constants on the command line using the --param
option.
The names of specific parameters, and the meaning of the
values, are tied to the internals of the compiler, and
are subject to change without notice in future releases.
In each case, the value is an integer. The allowable
choices for name are:
predictable-branch-outcome
When branch is predicted to be taken with
probability lower than this threshold (in percent),
then it is considered well predictable. The default
is 10.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional
branches around a block and replace them with
conditionally executed instructions. This parameter
gives the maximum number of instructions in a block
which should be considered for if-conversion. The
default is 10, though the compiler will also use
other heuristics to decide whether if-conversion is
likely to be profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional
branches around a block and replace them with
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conditionally executed instructions. These
parameters give the maximum permissible cost for the
sequence that would be generated by if-conversion
depending on whether the branch is statically
determined to be predictable or not. The units for
this parameter are the same as those for the GCC
internal seq_cost metric. The compiler will try to
provide a reasonable default for this parameter
using the BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for
cross-jumping. The algorithm used by -fcrossjumping
is O(N^2) in the number of edges incoming to each
block. Increasing values mean more aggressive
optimization, making the compilation time increase
with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be
matched at the end of two blocks before cross-
jumping is performed on them. This value is ignored
in the case where all instructions in the block
being cross-jumped from are matched. The default
value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying
basic blocks instead of jumping. The expansion is
relative to a jump instruction. The default value
is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a
block that jumps to a computed goto. To avoid
O(N^2) behavior in a number of passes, GCC factors
computed gotos early in the compilation process, and
unfactors them as late as possible. Only computed
jumps at the end of a basic blocks with no more than
max-goto-duplication-insns are unfactored. The
default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when
looking for an instruction to fill a delay slot. If
more than this arbitrary number of instructions are
searched, the time savings from filling the delay
slot are minimal, so stop searching. Increasing
values mean more aggressive optimization, making the
compilation time increase with probably small
improvement in execution time.
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max-delay-slot-live-search
When trying to fill delay slots, the maximum number
of instructions to consider when searching for a
block with valid live register information.
Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compilation
time. This parameter should be removed when the
delay slot code is rewritten to maintain the
control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that can be
allocated in order to perform the global common
subexpression elimination optimization. If more
memory than specified is required, the optimization
is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions
is larger than this value for any expression, then
RTL PRE inserts or removes the expression and thus
leaves partially redundant computations in the
instruction stream. The default value is 20.
max-pending-list-length
The maximum number of pending dependencies
scheduling allows before flushing the current state
and starting over. Large functions with few
branches or calls can create excessively large lists
which needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the
scheduler should make when modulo scheduling a loop.
Larger values can exponentially increase compilation
time.
max-inline-insns-single
Several parameters control the tree inliner used in
GCC. This number sets the maximum number of
instructions (counted in GCC's internal
representation) in a single function that the tree
inliner considers for inlining. This only affects
functions declared inline and methods implemented in
a class declaration (C++). The default value is
400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a
lot of functions that would otherwise not be
considered for inlining by the compiler are
investigated. To those functions, a different (more
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restrictive) limit compared to functions declared
inline can be applied. The default value is 40.
inline-min-speedup
When estimated performance improvement of caller +
callee runtime exceeds this threshold (in percent),
the function can be inlined regardless of the limit
on --param max-inline-insns-single and --param max-
inline-insns-auto.
large-function-insns
The limit specifying really large functions. For
functions larger than this limit after inlining,
inlining is constrained by --param large-function-
growth. This parameter is useful primarily to avoid
extreme compilation time caused by non-linear
algorithms used by the back end. The default value
is 2700.
large-function-growth
Specifies maximal growth of large function caused by
inlining in percents. The default value is 100
which limits large function growth to 2.0 times the
original size.
large-unit-insns
The limit specifying large translation unit. Growth
caused by inlining of units larger than this limit
is limited by --param inline-unit-growth. For small
units this might be too tight. For example,
consider a unit consisting of function A that is
inline and B that just calls A three times. If B is
small relative to A, the growth of unit is 300\% and
yet such inlining is very sane. For very large
units consisting of small inlineable functions,
however, the overall unit growth limit is needed to
avoid exponential explosion of code size. Thus for
smaller units, the size is increased to --param
large-unit-insns before applying --param inline-
unit-growth. The default is 10000.
inline-unit-growth
Specifies maximal overall growth of the compilation
unit caused by inlining. The default value is 20
which limits unit growth to 1.2 times the original
size. Cold functions (either marked cold via an
attribute or by profile feedback) are not accounted
into the unit size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation
unit caused by interprocedural constant propagation.
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The default value is 10 which limits unit growth to
1.1 times the original size.
large-stack-frame
The limit specifying large stack frames. While
inlining the algorithm is trying to not grow past
this limit too much. The default value is 256
bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames
caused by inlining in percents. The default value
is 1000 which limits large stack frame growth to 11
times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an
out-of-line copy of a self-recursive inline function
can grow into by performing recursive inlining.
--param max-inline-insns-recursive applies to
functions declared inline. For functions not
declared inline, recursive inlining happens only
when -finline-functions (included in -O3) is
enabled; --param max-inline-insns-recursive-auto
applies instead. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for
recursive inlining.
--param max-inline-recursive-depth applies to
functions declared inline. For functions not
declared inline, recursive inlining happens only
when -finline-functions (included in -O3) is
enabled; --param max-inline-recursive-depth-auto
applies instead. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function
having deep recursion in average and can hurt for
function having little recursion depth by increasing
the prologue size or complexity of function body to
other optimizers.
When profile feedback is available (see
-fprofile-generate) the actual recursion depth can
be guessed from the probability that function
recurses via a given call expression. This
parameter limits inlining only to call expressions
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whose probability exceeds the given threshold (in
percents). The default value is 10.
early-inlining-insns
Specify growth that the early inliner can make. In
effect it increases the amount of inlining for code
having a large abstraction penalty. The default
value is 14.
max-early-inliner-iterations
Limit of iterations of the early inliner. This
basically bounds the number of nested indirect calls
the early inliner can resolve. Deeper chains are
still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function
with comdat visibility are shared across multiple
compilation units. The default value is 20.
profile-func-internal-id
A parameter to control whether to use function
internal id in profile database lookup. If the value
is 0, the compiler uses an id that is based on
function assembler name and filename, which makes
old profile data more tolerant to source changes
such as function reordering etc. The default value
is 0.
min-vect-loop-bound
The minimum number of iterations under which loops
are not vectorized when -ftree-vectorize is used.
The number of iterations after vectorization needs
to be greater than the value specified by this
option to allow vectorization. The default value is
0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an
expression can be moved by GCSE optimizations. This
is currently supported only in the code hoisting
pass. The bigger the ratio, the more aggressive
code hoisting is with simple expressions, i.e., the
expressions that have cost less than gcse-
unrestricted-cost. Specifying 0 disables hoisting
of simple expressions. The default value is 10.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single
typical machine instruction, at which GCSE
optimizations do not constrain the distance an
expression can travel. This is currently supported
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only in the code hoisting pass. The lesser the
cost, the more aggressive code hoisting is.
Specifying 0 allows all expressions to travel
unrestricted distances. The default value is 3.
max-hoist-depth
The depth of search in the dominator tree for
expressions to hoist. This is used to avoid
quadratic behavior in hoisting algorithm. The value
of 0 does not limit on the search, but may slow down
compilation of huge functions. The default value is
30.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb
with. This is used to avoid quadratic behavior in
tree tail merging. The default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over
the function. This is used to limit compilation
time in tree tail merging. The default value is 2.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned
stores if it is legal to do so. The default value
is 1.
max-stores-to-merge
The maximum number of stores to attempt to merge
into wider stores in the store merging pass. The
minimum value is 2 and the default is 64.
max-unrolled-insns
The maximum number of instructions that a loop may
have to be unrolled. If a loop is unrolled, this
parameter also determines how many times the loop
code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by
probabilities of their execution that a loop may
have to be unrolled. If a loop is unrolled, this
parameter also determines how many times the loop
code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may
have to be peeled. If a loop is peeled, this
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parameter also determines how many times the loop
code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path
through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled
loop.
max-completely-peel-times
The maximum number of iterations of a loop to be
suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for
complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a
single loop.
max-loop-headers-insns
The maximum number of insns in loop header
duplicated by the copy loop headers pass.
lim-expensive
The minimum cost of an expensive expression in the
loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction
variables, below which all candidates are considered
for each use in induction variable optimizations.
If there are more candidates than this, only the
most relevant ones are considered to avoid quadratic
time complexity.
iv-max-considered-uses
The induction variable optimizations give up on
loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller
than this value, always try to remove unnecessary
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ivs from the set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise
by dead store elimination. Larger values may result
in larger compilation times.
scev-max-expr-size
Bound on size of expressions used in the scalar
evolutions analyzer. Large expressions slow the
analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the
scalar evolutions analyzer. Complex expressions
slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by
TREE if conversion unless the loop is marked with
simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be
performed when doing loop versioning for alignment
in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be
performed when doing loop versioning for alias in
the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access
alignment for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the
brute-force algorithm for analysis of the number of
iterations of the loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it
contributes to the given permillage (i.e. 0...1000)
of the entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of
executions of basic block in function given basic
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block needs to have to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict
statically. This is useful in cases where a
function contains a single loop with known bound and
another loop with unknown bound. The known number
of iterations is predicted correctly, while the
unknown number of iterations average to roughly 10.
This means that the loop without bounds appears
artificially cold relative to the other one.
builtin-expect-probability
Control the probability of the expression having the
specified value. This parameter takes a percentage
(i.e. 0 ... 100) as input. The default probability
of 90 is obtained empirically.
align-threshold
Select fraction of the maximal frequency of
executions of a basic block in a function to align
the basic block.
align-loop-iterations
A loop expected to iterate at least the selected
number of iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation
once the given percentage of executed instructions
is covered. This limits unnecessary code size
expansion.
The tracer-dynamic-coverage-feedback parameter is
used only when profile feedback is available. The
real profiles (as opposed to statically estimated
ones) are much less balanced allowing the threshold
to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached
given percentage. This is a rather artificial
limit, as most of the duplicates are eliminated
later in cross jumping, so it may be set to much
higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of
best edge is less than this threshold (in percent).
tracer-min-branch-probability
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tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability
lower than this threshold.
Similarly to tracer-dynamic-coverage two parameters
are provided. tracer-min-branch-probability-
feedback is used for compilation with profile
feedback and tracer-min-branch-probability
compilation without. The value for compilation with
profile feedback needs to be more conservative
(higher) in order to make tracer effective.
max-cse-path-length
The maximum number of basic blocks on path that CSE
considers. The default is 10.
max-cse-insns
The maximum number of instructions CSE processes
before flushing. The default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own
memory allocation. This parameter specifies the
minimum percentage by which the garbage collector's
heap should be allowed to expand between
collections. Tuning this may improve compilation
speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper
bound of 100% when RAM >= 1GB. If "getrlimit" is
available, the notion of "RAM" is the smallest of
actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC
is not able to calculate RAM on a particular
platform, the lower bound of 30% is used. Setting
this parameter and ggc-min-heapsize to zero causes a
full collection to occur at every opportunity. This
is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before
it begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-
min-expand% beyond ggc-min-heapsize. Again, tuning
this may improve compilation speed, and has no
effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or
a limit that tries to ensure that RLIMIT_DATA or
RLIMIT_AS are not exceeded, but with a lower bound
of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to
calculate RAM on a particular platform, the lower
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bound is used. Setting this parameter very large
effectively disables garbage collection. Setting
this parameter and ggc-min-expand to zero causes a
full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look
backward for equivalent register. Increasing values
mean more aggressive optimization, making the
compilation time increase with probably slightly
better performance. The default value is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should
take into account. Increasing values mean more
aggressive optimization, making the compilation time
increase with probably slightly better performance.
The default value is 500.
max-sched-ready-insns
The maximum number of instructions ready to be
issued the scheduler should consider at any given
time during the first scheduling pass. Increasing
values mean more thorough searches, making the
compilation time increase with probably little
benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be
considered for interblock scheduling. The default
value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be
considered for pipelining in the selective
scheduler. The default value is 15.
max-sched-region-insns
The maximum number of insns in a region to be
considered for interblock scheduling. The default
value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be
considered for pipelining in the selective
scheduler. The default value is 200.
min-spec-prob
The minimum probability (in percents) of reaching a
source block for interblock speculative scheduling.
The default value is 40.
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max-sched-extend-regions-iters
The maximum number of iterations through CFG to
extend regions. A value of 0 (the default) disables
region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be
considered for speculative motion. The default
value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in
percents), so that speculative insns are scheduled.
The default value is 40.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the
scheduler to save its state across it. The default
value is 10.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and
load targeting same memory locations. The default
value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of
selective scheduling. It is a depth of search for
available instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction is
scheduled during selective scheduling. This is the
limit on the number of iterations through which the
instruction may be pipelined. The default value is
2.
selsched-insns-to-rename
The maximum number of best instructions in the ready
list that are considered for renaming in the
selective scheduler. The default value is 2.
sms-min-sc
The minimum value of stage count that swing modulo
scheduler generates. The default value is 2.
max-last-value-rtl
The maximum size measured as number of RTLs that can
be recorded in an expression in combiner for a
pseudo register as last known value of that
register. The default is 10000.
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max-combine-insns
The maximum number of instructions the RTL combiner
tries to combine. The default value is 2 at -Og and
4 otherwise.
integer-share-limit
Small integer constants can use a shared data
structure, reducing the compiler's memory usage and
increasing its speed. This sets the maximum value
of a shared integer constant. The default value is
256.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that
receive stack smashing protection when
-fstack-protection is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack
slot sharing when not optimizing. The default value
is 32.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that
needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a
field sensitive manner during pointer analysis. The
default is zero for -O0 and -O1, and 100 for -Os,
-O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are
executed before prefetch finishes. The distance
prefetched ahead is proportional to this constant.
Increasing this number may also lead to less streams
being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the
same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
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min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions
and the number of prefetches to enable prefetching
in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions
and the number of memory references to enable
prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type
system. By default, this should always be 1, which
uses a more efficient internal mechanism for
comparing types in C++ and Objective-C++. However,
if bugs in the canonical type system are causing
compilation failures, set this value to 0 to disable
canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create
arrays that are bigger than switch-conversion-max-
branch-ratio times the number of branches in the
switch.
max-partial-antic-length
Maximum length of the partial antic set computed
during the tree partial redundancy elimination
optimization (-ftree-pre) when optimizing at -O3 and
above. For some sorts of source code the enhanced
partial redundancy elimination optimization can run
away, consuming all of the memory available on the
host machine. This parameter sets a limit on the
length of the sets that are computed, which prevents
the runaway behavior. Setting a value of 0 for this
parameter allows an unlimited set length.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC)
during SCCVN processing. If this limit is hit,
SCCVN processing for the whole function is not done
and optimizations depending on it are disabled. The
default maximum SCC size is 10000.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform
when looking for redundancies for loads and stores.
If this limit is hit the search is aborted and the
load or store is not considered redundant. The
number of queries is algorithmically limited to the
number of stores on all paths from the load to the
function entry. The default maximum number of
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queries is 1000.
ira-max-loops-num
IRA uses regional register allocation by default.
If a function contains more loops than the number
given by this parameter, only at most the given
number of the most frequently-executed loops form
regions for regional register allocation. The
default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to
compress the conflict table, the table can still
require excessive amounts of memory for huge
functions. If the conflict table for a function
could be more than the size in MB given by this
parameter, the register allocator instead uses a
faster, simpler, and lower-quality algorithm that
does not require building a pseudo-register conflict
table. The default value of the parameter is 2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register
pressure in loops for decisions to move loop
invariants (see -O3). The number of available
registers reserved for some other purposes is given
by this parameter. The default value of the
parameter is 2, which is the minimal number of
registers needed by typical instructions. This
value is the best found from numerous experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in
subsequent insns. This optimization is called
inheritance. EBB is used as a region to do this
optimization. The parameter defines a minimal
fall-through edge probability in percentage used to
add BB to inheritance EBB in LRA. The default value
of the parameter is 40. The value was chosen from
numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-
time memory, with very large loops. Loops with more
basic blocks than this parameter won't have loop
invariant motion optimization performed on them.
The default value of the parameter is 1000 for -O1
and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very
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large loops. This parameter limits the number of
data references in loops that are considered for
data dependence analysis. These large loops are no
handled by the optimizations using loop data
dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use
during variable tracking dataflow analysis of any
function. If this limit is exceeded with variable
tracking at assignments enabled, analysis for that
function is retried without it, after removing all
debug insns from the function. If the limit is
exceeded even without debug insns, var tracking
analysis is completely disabled for the function.
Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when
attempting to map variable names or debug
temporaries to value expressions. This trades
compilation time for more complete debug
information. If this is set too low, value
expressions that are available and could be
represented in debug information may end up not
being used; setting this higher may enable the
compiler to find more complex debug expressions, but
compile time and memory use may grow. The default
is 12.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug
insns. The range below the parameter is reserved
exclusively for debug insns created by
-fvar-tracking-assignments, but debug insns may get
(non-overlapping) uids above it if the reserved
range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one
or more new parameters only when their cumulative
size is less or equal to ipa-sra-ptr-growth-factor
times the size of the original pointer parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA
and IPA-SRA) aim to replace scalar parts of
aggregates with uses of independent scalar
variables. These parameters control the maximum
size, in storage units, of aggregate which is
considered for replacement when compiling for speed
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(sra-max-scalarization-size-Ospeed) or size (sra-
max-scalarization-size-Osize) respectively.
tm-max-aggregate-size
When making copies of thread-local variables in a
transaction, this parameter specifies the size in
bytes after which variables are saved with the
logging functions as opposed to save/restore code
sequence pairs. This option only applies when using
-fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop
transforms, the number of parameters in a Static
Control Part (SCoP) is bounded. The default value
is 10 parameters. A variable whose value is unknown
at compilation time and defined outside a SCoP is a
parameter of the SCoP.
graphite-max-bbs-per-function
To avoid exponential effects in the detection of
SCoPs, the size of the functions analyzed by
Graphite is bounded. The default value is 100 basic
blocks.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled
with -floop-block or -floop-strip-mine, strip mine
each loop in the loop nest by a given number of
iterations. The strip length can be changed using
the loop-block-tile-size parameter. The default
value is 51 iterations.
loop-unroll-jam-size
Specify the unroll factor for the
-floop-unroll-and-jam option. The default value is
4.
loop-unroll-jam-depth
Specify the dimension to be unrolled (counting from
the most inner loop) for the -floop-unroll-and-jam.
The default value is 2.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and
types passed to a function's parameter in order to
propagate them and perform devirtualization. ipa-
cp-value-list-size is the maximum number of values
and types it stores per one formal parameter of a
function.
ipa-cp-eval-threshold
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IPA-CP calculates its own score of cloning
profitability heuristics and performs those cloning
opportunities with scores that exceed ipa-cp-eval-
threshold.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will
receive when they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single
call to another function will receive when they are
evaluated for cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of
scalar values passed in an aggregate. ipa-max-agg-
items controls the maximum number of such values per
one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate
would make the number of iterations of a loop known,
it adds a bonus of ipa-cp-loop-hint-bonus to the
profitability score of the candidate.
ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate
would make the index of an array access known, it
adds a bonus of ipa-cp-array-index-hint-bonus to the
profitability score of the candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP
employs alias analysis in order to track values
pointed to by function parameters. In order not
spend too much time analyzing huge functions, it
gives up and consider all memory clobbered after
examining ipa-max-aa-steps statements modifying
memory.
lto-partitions
Specify desired number of partitions produced during
WHOPR compilation. The number of partitions should
exceed the number of CPUs used for compilation. The
default value is 32.
lto-min-partition
Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting
very small programs into too many partitions.
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lto-max-partition
Size of max partition for WHOPR (in estimated
instructions). to provide an upper bound for
individual size of partition. Meant to be used only
with balanced partitioning.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for
suggestions when C++ name lookup fails for an
identifier. The default is 1000.
sink-frequency-threshold
The maximum relative execution frequency (in
percents) of the target block relative to a
statement's original block to allow statement
sinking of a statement. Larger numbers result in
more aggressive statement sinking. The default
value is 75. A small positive adjustment is applied
for statements with memory operands as those are
even more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that
can be sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion
(-ftree-loop-if-convert) is disabled. The default
is 2.
allow-store-data-races
Allow optimizers to introduce new data races on
stores. Set to 1 to allow, otherwise to 0. This
option is enabled by default at optimization level
-Ofast.
case-values-threshold
The smallest number of different values for which it
is best to use a jump-table instead of a tree of
conditional branches. If the value is 0, use the
default for the machine. The default is 0.
tree-reassoc-width
Set the maximum number of instructions executed in
parallel in reassociated tree. This parameter
overrides target dependent heuristics used by
default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original
implementation and is the more likely to prevent
instructions from being reordered. Algorithm 2 was
designed to be a compromise between the relatively
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conservative approach taken by algorithm 1 and the
rather aggressive approach taken by the default
scheduler. It relies more heavily on having a
regular register file and accurate register pressure
classes. See haifa-sched.c in the GCC sources for
more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that
are considered when seeking a basis for a new
straight-line strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects.
This kind of protection is enabled by default if you
are using -fsanitize=address option. To disable
global objects protection use --param
asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects.
This kind of protection is enabled by default when
using -fsanitize=address. To disable stack
protection use --param asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads.
This kind of protection is enabled by default when
using -fsanitize=address. To disable memory reads
protection use --param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes.
This kind of protection is enabled by default when
using -fsanitize=address. To disable memory writes
protection use --param asan-instrument-writes=0
option.
asan-memintrin
Enable detection for built-in functions. This kind
of protection is enabled by default when using
-fsanitize=address. To disable built-in functions
protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of
protection is enabled by default when using the
-fsanitize=address option. To disable it use
--param asan-use-after-return=0.
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Note: By default the check is disabled at run time.
To enable it, add "detect_stack_use_after_return=1"
to the environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being
instrumented is greater or equal to this number, use
callbacks instead of inline checks. E.g. to disable
inline code use --param
asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller
or equal to this number, directly poison (or
unpoison) shadow memory instead of using run-time
callbacks. The default value is 256.
chkp-max-ctor-size
Static constructors generated by Pointer Bounds
Checker may become very large and significantly
increase compile time at optimization level -O1 and
higher. This parameter is a maximum number of
statements in a single generated constructor.
Default value is 5000.
max-fsm-thread-path-insns
Maximum number of instructions to copy when
duplicating blocks on a finite state automaton jump
thread path. The default is 100.
max-fsm-thread-length
Maximum number of basic blocks on a finite state
automaton jump thread path. The default is 10.
max-fsm-thread-paths
Maximum number of new jump thread paths to create
for a finite state automaton. The default is 50.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by
parloops. The default is 0.
parloops-schedule
Schedule type of omp schedule for loops parallelized
by parloops (static, dynamic, guided, auto,
runtime). The default is static.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties
of SSA names in things like fold routines. One
level of recursion corresponds to following a use-
def chain.
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hsa-gen-debug-stores
Enable emission of special debug stores within HSA
kernels which are then read and reported by libgomp
plugin. Generation of these stores is disabled by
default, use --param hsa-gen-debug-stores=1 to
enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when
looking for a must-def specifying the dynamic type
of an object that invokes a virtual call we may be
able to devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the
default edge of a switch statement during VRP. The
default is 10.
Program Instrumentation Options
GCC supports a number of command-line options that control
adding run-time instrumentation to the code it normally
generates. For example, one purpose of instrumentation is
collect profiling statistics for use in finding program hot
spots, code coverage analysis, or profile-guided
optimizations. Another class of program instrumentation is
adding run-time checking to detect programming errors like
invalid pointer dereferences or out-of-bounds array
accesses, as well as deliberately hostile attacks such as
stack smashing or C++ vtable hijacking. There is also a
general hook which can be used to implement other forms of
tracing or function-level instrumentation for debug or
program analysis purposes.
-p Generate extra code to write profile information
suitable for the analysis program prof. You must use
this option when compiling the source files you want
data about, and you must also use it when linking.
-pg Generate extra code to write profile information
suitable for the analysis program gprof. You must use
this option when compiling the source files you want
data about, and you must also use it when linking.
-fprofile-arcs
Add code so that program flow arcs are instrumented.
During execution the program records how many times each
branch and call is executed and how many times it is
taken or returns. On targets that support constructors
with priority support, profiling properly handles
constructors, destructors and C++ constructors (and
destructors) of classes which are used as a type of a
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global variable.
When the compiled program exits it saves this data to a
file called auxname.gcda for each source file. The data
may be used for profile-directed optimizations
(-fbranch-probabilities), or for test coverage analysis
(-ftest-coverage). Each object file's auxname is
generated from the name of the output file, if
explicitly specified and it is not the final executable,
otherwise it is the basename of the source file. In
both cases any suffix is removed (e.g. foo.gcda for
input file dir/foo.c, or dir/foo.gcda for output file
specified as -o dir/foo.o).
--coverage
This option is used to compile and link code
instrumented for coverage analysis. The option is a
synonym for -fprofile-arcs -ftest-coverage (when
compiling) and -lgcov (when linking). See the
documentation for those options for more details.
* Compile the source files with -fprofile-arcs plus
optimization and code generation options. For test
coverage analysis, use the additional
-ftest-coverage option. You do not need to profile
every source file in a program.
* Link your object files with -lgcov or -fprofile-arcs
(the latter implies the former).
* Run the program on a representative workload to
generate the arc profile information. This may be
repeated any number of times. You can run
concurrent instances of your program, and provided
that the file system supports locking, the data
files will be correctly updated. Unless a strict
ISO C dialect option is in effect, "fork" calls are
detected and correctly handled without double
counting.
* For profile-directed optimizations, compile the
source files again with the same optimization and
code generation options plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce
human readable information from the .gcno and .gcda
files. Refer to the gcov documentation for further
information.
With -fprofile-arcs, for each function of your program
GCC creates a program flow graph, then finds a spanning
tree for the graph. Only arcs that are not on the
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spanning tree have to be instrumented: the compiler adds
code to count the number of times that these arcs are
executed. When an arc is the only exit or only entrance
to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to
hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility
can use to show program coverage. Each source file's
note file is called auxname.gcno. Refer to the
-fprofile-arcs option above for a description of auxname
and instructions on how to generate test coverage data.
Coverage data matches the source files more closely if
you do not optimize.
-fprofile-dir=path
Set the directory to search for the profile data files
in to path. This option affects only the profile data
generated by -fprofile-generate, -ftest-coverage,
-fprofile-arcs and used by -fprofile-use and
-fbranch-probabilities and its related options. Both
absolute and relative paths can be used. By default,
GCC uses the current directory as path, thus the profile
data file appears in the same directory as the object
file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting
application to produce profile useful for later
recompilation with profile feedback based optimization.
You must use -fprofile-generate both when compiling and
when linking your program.
The following options are enabled: -fprofile-arcs,
-fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile
information, use -fprofile-use.
-fprofile-update=method
Alter the update method for an application instrumented
for profile feedback based optimization. The method
argument should be one of single, atomic or prefer-
atomic. The first one is useful for single-threaded
applications, while the second one prevents profile
corruption by emitting thread-safe code.
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Warning: When an application does not properly join all
threads (or creates an detached thread), a profile file
can be still corrupted.
Using prefer-atomic would be transformed either to
atomic, when supported by a target, or to single
otherwise. The GCC driver automatically selects
prefer-atomic when -pthread is present in the command
line.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect
out-of-bounds and use-after-free bugs. The option
enables -fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer>
for more details. The run-time behavior can be
influenced using the ASAN_OPTIONS environment variable.
When set to "help=1", the available options are shown at
startup of the instrumented program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options. The option cannot be
combined with -fsanitize=thread and/or
-fcheck-pointer-bounds.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan/wiki> for more details.
The option cannot be combined with
-fcheck-pointer-bounds.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector.
Memory access instructions are instrumented to detect
data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer>
for more details. The run-time behavior can be
influenced using the TSAN_OPTIONS environment variable;
see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>
for a list of supported options. The option cannot be
combined with -fsanitize=address, -fsanitize=leak and/or
-fcheck-pointer-bounds.
Note that sanitized atomic builtins cannot throw
exceptions when operating on invalid memory addresses
with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This
option only matters for linking of executables and the
executable is linked against a library that overrides
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"malloc" and other allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>
for more details. The run-time behavior can be
influenced using the LSAN_OPTIONS environment variable.
The option cannot be combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined
behavior detector. Various computations are
instrumented to detect undefined behavior at runtime.
Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a
shift operation is not undefined. Note that what
exactly is considered undefined differs slightly
between C and C++, as well as between ISO C90 and
C99, etc. This option has two suboptions,
-fsanitize=shift-base and -fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second
argument of a shift operation is not negative and is
smaller than the precision of the promoted first
argument.
-fsanitize=shift-base
If the second argument of a shift operation is
within range, check that the result of a shift
operation is not undefined. Note that what exactly
is considered undefined differs slightly between C
and C++, as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN
/ -1" division.
-fsanitize=unreachable
With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics
message call instead. When reaching the
"__builtin_unreachable" call, the behavior is
undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the
size of a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly,
the application built with this option turned on
will issue an error message when it tries to
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dereference a NULL pointer, or if a reference
(possibly an rvalue reference) is bound to a NULL
pointer, or if a method is invoked on an object
pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking.
Programs built with this option turned on will issue
an error message when the end of a non-void function
is reached without actually returning a value. This
option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow
checking. We check that the result of "+", "*", and
both unary and binary "-" does not overflow in the
signed arithmetics. Note, integer promotion rules
must be taken into account. That is, the following
is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds.
Various out of bounds accesses are detected.
Flexible array members, flexible array member-like
arrays, and initializers of variables with static
storage are not instrumented. The option cannot be
combined with -fcheck-pointer-bounds.
-fsanitize=bounds-strict
This option enables strict instrumentation of array
bounds. Most out of bounds accesses are detected,
including flexible array members and flexible array
member-like arrays. Initializers of variables with
static storage are not instrumented. The option
cannot be combined with -fcheck-pointer-bounds.
-fsanitize=alignment
This option enables checking of alignment of
pointers when they are dereferenced, or when a
reference is bound to insufficiently aligned target,
or when a method or constructor is invoked on
insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory
references using the "__builtin_object_size"
function. Various out of bounds pointer accesses
are detected.
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-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike
other similar options,
-fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division
by zero can be a legitimate way of obtaining
infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer
conversion checking. We check that the result of
the conversion does not overflow. Unlike other
similar options, -fsanitize=float-cast-overflow is
not enabled by -fsanitize=undefined. This option
does not work well with "FE_INVALID" exceptions
enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls,
checking whether null values are not passed to
arguments marked as requiring a non-null value by
the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return
statements in functions marked with
"returns_nonnull" function attribute, to detect
returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from
bool. If a value other than 0/1 is loaded, a run-
time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an
enum type. If a value outside the range of values
for the enum type is loaded, a run-time error is
issued.
-fsanitize=vptr
This option enables instrumentation of C++ member
function calls, member accesses and some conversions
between pointers to base and derived classes, to
verify the referenced object has the correct dynamic
type.
While -ftrapv causes traps for signed overflows to be
emitted, -fsanitize=undefined gives a diagnostic
message. This currently works only for the C family of
languages.
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-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot
be used together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting
with different shadow memory layouts in Kernel
AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined
sections. si may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for
sanitizers mentioned in comma-separated list of opts.
Enabling this option for a sanitizer component causes it
to attempt to continue running the program as if no
error happened. This means multiple runtime errors can
be reported in a single program run, and the exit code
of the program may indicate success even when errors
have been reported. The -fno-sanitize-recover= option
can be used to alter this behavior: only the first
detected error is reported and program then exits with a
non-zero exit code.
Currently this feature only works for
-fsanitize=undefined (and its suboptions except for
-fsanitize=unreachable and -fsanitize=return),
-fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero,
-fsanitize=bounds-strict, -fsanitize=kernel-address and
-fsanitize=address. For these sanitizers error recovery
is turned on by default, except -fsanitize=address, for
which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is
also accepted, the former enables recovery for all
sanitizers that support it, the latter disables recovery
for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side,
it needs to be also enabled on the runtime library side,
otherwise the failures are still fatal. The runtime
library defaults to "halt_on_error=0" for
ThreadSanitizer and UndefinedBehaviorSanitizer, while
default value for AddressSanitizer is "halt_on_error=1".
This can be overridden through setting the
"halt_on_error" flag in the corresponding environment
variable.
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Syntax without an explicit opts parameter is deprecated.
It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-
after-scope bugs. The option sets -fstack-reuse to
none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs
the compiler to report undefined behavior using
"__builtin_trap" rather than a "libubsan" library
routine. The advantage of this is that the "libubsan"
library is not needed and is not linked in, so this is
usable even in freestanding environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation.
Inserts a call to "__sanitizer_cov_trace_pc" into every
basic block.
-fbounds-check
For front ends that support it, generate additional code
to check that indices used to access arrays are within
the declared range. This is currently only supported by
the Fortran front end, where this option defaults to
false.
-fcheck-pointer-bounds
Enable Pointer Bounds Checker instrumentation. Each
memory reference is instrumented with checks of the
pointer used for memory access against bounds associated
with that pointer.
Currently there is only an implementation for Intel MPX
available, thus x86 GNU/Linux target and -mmpx are
required to enable this feature. MPX-based
instrumentation requires a runtime library to enable MPX
in hardware and handle bounds violation signals. By
default when -fcheck-pointer-bounds and -mmpx options
are used to link a program, the GCC driver links against
the libmpx and libmpxwrappers libraries. Bounds
checking on calls to dynamic libraries requires a linker
with -z bndplt support; if GCC was configured with a
linker without support for this option (including the
Gold linker and older versions of ld), a warning is
given if you link with -mmpx without also specifying
-static, since the overall effectiveness of the bounds
checking protection is reduced. See also
-static-libmpxwrappers.
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MPX-based instrumentation may be used for debugging and
also may be included in production code to increase
program security. Depending on usage, you may have
different requirements for the runtime library. The
current version of the MPX runtime library is more
oriented for use as a debugging tool. MPX runtime
library usage implies -lpthread. See also
-static-libmpx. The runtime library behavior can be
influenced using various CHKP_RT_* environment
variables. See
<https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler>
for more details.
Generated instrumentation may be controlled by various
-fchkp-* options and by the "bnd_variable_size"
structure field attribute and "bnd_legacy", and
"bnd_instrument" function attributes. GCC also provides
a number of built-in functions for controlling the
Pointer Bounds Checker.
-fchkp-check-incomplete-type
Generate pointer bounds checks for variables with
incomplete type. Enabled by default.
-fchkp-narrow-bounds
Controls bounds used by Pointer Bounds Checker for
pointers to object fields. If narrowing is enabled then
field bounds are used. Otherwise object bounds are
used. See also -fchkp-narrow-to-innermost-array and
-fchkp-first-field-has-own-bounds. Enabled by default.
-fchkp-first-field-has-own-bounds
Forces Pointer Bounds Checker to use narrowed bounds for
the address of the first field in the structure. By
default a pointer to the first field has the same bounds
as a pointer to the whole structure.
-fchkp-flexible-struct-trailing-arrays
Forces Pointer Bounds Checker to treat all trailing
arrays in structures as possibly flexible. By default
only array fields with zero length or that are marked
with attribute bnd_variable_size are treated as
flexible.
-fchkp-narrow-to-innermost-array
Forces Pointer Bounds Checker to use bounds of the
innermost arrays in case of nested static array access.
By default this option is disabled and bounds of the
outermost array are used.
-fchkp-optimize
Enables Pointer Bounds Checker optimizations. Enabled
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by default at optimization levels -O, -O2, -O3.
-fchkp-use-fast-string-functions
Enables use of *_nobnd versions of string functions (not
copying bounds) by Pointer Bounds Checker. Disabled by
default.
-fchkp-use-nochk-string-functions
Enables use of *_nochk versions of string functions (not
checking bounds) by Pointer Bounds Checker. Disabled by
default.
-fchkp-use-static-bounds
Allow Pointer Bounds Checker to generate static bounds
holding bounds of static variables. Enabled by default.
-fchkp-use-static-const-bounds
Use statically-initialized bounds for constant bounds
instead of generating them each time they are required.
By default enabled when -fchkp-use-static-bounds is
enabled.
-fchkp-treat-zero-dynamic-size-as-infinite
With this option, objects with incomplete type whose
dynamically-obtained size is zero are treated as having
infinite size instead by Pointer Bounds Checker. This
option may be helpful if a program is linked with a
library missing size information for some symbols.
Disabled by default.
-fchkp-check-read
Instructs Pointer Bounds Checker to generate checks for
all read accesses to memory. Enabled by default.
-fchkp-check-write
Instructs Pointer Bounds Checker to generate checks for
all write accesses to memory. Enabled by default.
-fchkp-store-bounds
Instructs Pointer Bounds Checker to generate bounds
stores for pointer writes. Enabled by default.
-fchkp-instrument-calls
Instructs Pointer Bounds Checker to pass pointer bounds
to calls. Enabled by default.
-fchkp-instrument-marked-only
Instructs Pointer Bounds Checker to instrument only
functions marked with the "bnd_instrument" attribute.
Disabled by default.
-fchkp-use-wrappers
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Allows Pointer Bounds Checker to replace calls to
built-in functions with calls to wrapper functions.
When -fchkp-use-wrappers is used to link a program, the
GCC driver automatically links against libmpxwrappers.
See also -static-libmpxwrappers. Enabled by default.
-fstack-protector
Emit extra code to check for buffer overflows, such as
stack smashing attacks. This is done by adding a guard
variable to functions with vulnerable objects. This
includes functions that call "alloca", and functions
with buffers larger than 8 bytes. The guards are
initialized when a function is entered and then checked
when the function exits. If a guard check fails, an
error message is printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are
protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions
to be protected --- those that have local array
definitions, or have references to local frame
addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions
which have the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the
boundary of the stack. You should specify this flag if
you are running in an environment with multiple threads,
but you only rarely need to specify it in a single-
threaded environment since stack overflow is
automatically detected on nearly all systems if there is
only one stack.
Note that this switch does not actually cause checking
to be done; the operating system or the language runtime
must do that. The switch causes generation of code to
ensure that they see the stack being extended.
You can additionally specify a string parameter: no
means no checking, generic means force the use of old-
style checking, specific means use the best checking
method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires
no specific target support in the compiler but comes
with the following drawbacks:
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1. Modified allocation strategy for large objects: they
are always allocated dynamically if their size
exceeds a fixed threshold.
2. Fixed limit on the size of the static frame of
functions: when it is topped by a particular
function, stack checking is not reliable and a
warning is issued by the compiler.
3. Inefficiency: because of both the modified
allocation strategy and the generic implementation,
code performance is hampered.
Note that old-style stack checking is also the fallback
method for specific if no target support has been added
in the compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow
beyond a certain value, either the value of a register
or the address of a symbol. If a larger stack is
required, a signal is raised at run time. For most
targets, the signal is raised before the stack overruns
the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address
0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack
limit of 128KB. Note that this may only work with the
GNU linker.
You can locally override stack limit checking by using
the "no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it
overflows. The resulting program has a discontiguous
stack which can only overflow if the program is unable
to allocate any more memory. This is most useful when
running threaded programs, as it is no longer necessary
to calculate a good stack size to use for each thread.
This is currently only implemented for the x86 targets
running GNU/Linux.
When code compiled with -fsplit-stack calls code
compiled without -fsplit-stack, there may not be much
stack space available for the latter code to run. If
compiling all code, including library code, with
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-fsplit-stack is not an option, then the linker can fix
up these calls so that the code compiled without
-fsplit-stack always has a large stack. Support for
this is implemented in the gold linker in GNU binutils
release 2.21 and later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code.
It turns on (or off, if using -fvtable-verify=none) the
security feature that verifies at run time, for every
virtual call, that the vtable pointer through which the
call is made is valid for the type of the object, and
has not been corrupted or overwritten. If an invalid
vtable pointer is detected at run time, an error is
reported and execution of the program is immediately
halted.
This option causes run-time data structures to be built
at program startup, which are used for verifying the
vtable pointers. The options std and preinit control the
timing of when these data structures are built. In both
cases the data structures are built before execution
reaches "main". Using -fvtable-verify=std causes the
data structures to be built after shared libraries have
been loaded and initialized. -fvtable-verify=preinit
causes them to be built before shared libraries have
been loaded and initialized.
If this option appears multiple times in the command
line with different values specified, none takes highest
priority over both std and preinit; preinit takes
priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the
runtime functions for the vtable verification feature to
be called. This flag also causes the compiler to log
information about which vtable pointers it finds for
each class. This information is written to a file named
vtv_set_ptr_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or
the current working directory otherwise.
Note: This feature appends data to the log file. If you
want a fresh log file, be sure to delete any existing
one.
-fvtv-counts
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this
causes the compiler to keep track of the total number of
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virtual calls it encounters and the number of
verifications it inserts. It also counts the number of
calls to certain run-time library functions that it
inserts and logs this information for each compilation
unit. The compiler writes this information to a file
named vtv_count_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or
the current working directory otherwise. It also counts
the size of the vtable pointer sets for each class, and
writes this information to vtv_class_set_sizes.log in
the same directory.
Note: This feature appends data to the log files. To
get fresh log files, be sure to delete any existing
ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to
functions. Just after function entry and just before
function exit, the following profiling functions are
called with the address of the current function and its
call site. (On some platforms,
"__builtin_return_address" does not work beyond the
current function, so the call site information may not
be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the
current function, which may be looked up exactly in the
symbol table.
This instrumentation is also done for functions expanded
inline in other functions. The profiling calls indicate
where, conceptually, the inline function is entered and
exited. This means that addressable versions of such
functions must be available. If all your uses of a
function are expanded inline, this may mean an
additional expansion of code size. If you use "extern
inline" in your C code, an addressable version of such
functions must be provided. (This is normally the case
anyway, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away
without providing static copies.)
A function may be given the attribute
"no_instrument_function", in which case this
instrumentation is not done. This can be used, for
example, for the profiling functions listed above,
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high-priority interrupt routines, and any functions from
which the profiling functions cannot safely be called
(perhaps signal handlers, if the profiling routines
generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from
instrumentation (see the description of
-finstrument-functions). If the file that contains a
function definition matches with one of file, then that
function is not instrumented. The match is done on
substrings: if the file parameter is a substring of the
file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose
pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one
of sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note
the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to
-finstrument-functions-exclude-file-list, but this
option sets the list of function names to be excluded
from instrumentation. The function name to be matched
is its user-visible name, such as "vector<int>
blah(const vector<int> &)", not the internal mangled
name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is
done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match.
For C99 and C++ extended identifiers, the function name
must be given in UTF-8, not using universal character
names.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on
each C source file before actual compilation.
If you use the -E option, nothing is done except
preprocessing. Some of these options make sense only
together with -E because they cause the preprocessor output
to be unsuitable for actual compilation.
In addition to the options listed here, there are a number
of options to control search paths for include files
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documented in Directory Options. Options to control
preprocessor diagnostics are listed in Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed
as if they appeared during translation phase three in a
#define directive. In particular, the definition is
truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or
shell-like program you may need to use the shell's
quoting syntax to protect characters such as spaces that
have a meaning in the shell syntax.
If you wish to define a function-like macro on the
command line, write its argument list with surrounding
parentheses before the equals sign (if any).
Parentheses are meaningful to most shells, so you should
quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are
given on the command line. All -imacros file and
-include file options are processed after all -D and -U
options.
-U name
Cancel any previous definition of name, either built in
or provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the
first line of the primary source file. However, the
first directory searched for file is the preprocessor's
working directory instead of the directory containing
the main source file. If not found there, it is
searched for in the remainder of the "#include "...""
search chain as normal.
If multiple -include options are given, the files are
included in the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced
by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the
macros from a header without also processing its
declarations.
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All files specified by -imacros are processed before all
files specified by -include.
-undef
Do not predefine any system-specific or GCC-specific
macros. The standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX
threads library. You should use this option
consistently for both compilation and linking. This
option is supported on GNU/Linux targets, most other
Unix derivatives, and also on x86 Cygwin and MinGW
targets.
-M Instead of outputting the result of preprocessing,
output a rule suitable for make describing the
dependencies of the main source file. The preprocessor
outputs one make rule containing the object file name
for that source file, a colon, and the names of all the
included files, including those coming from -include or
-imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the name of the source file
with any suffix replaced with object file suffix and
with any leading directory parts removed. If there are
many included files then the rule is split into several
lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug
output, such as -dM. To avoid mixing such debug output
with the dependency rules you should explicitly specify
the dependency output file with -MF, or use an
environment variable like DEPENDENCIES_OUTPUT. Debug
output is still sent to the regular output stream as
normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MM Like -M but do not mention header files that are found
in system header directories, nor header files that are
included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double
quotes in an #include directive does not in itself
determine whether that header appears in -MM dependency
output.
-MF file
When used with -M or -MM, specifies a file to write the
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dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would
send preprocessed output.
When used with the driver options -MD or -MMD, -MF
overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files
are generated files and adds them to the dependency list
without raising an error. The dependency filename is
taken directly from the "#include" directive without
prepending any path. -MG also suppresses preprocessed
output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to
depend on nothing. These dummy rules work around errors
make gives if you remove header files without updating
the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency
generation. By default CPP takes the name of the main
input file, deletes any directory components and any
file suffix such as .c, and appends the platform's usual
object suffix. The result is the target.
An -MT option sets the target to be exactly the string
you specify. If you want multiple targets, you can
specify them as a single argument to -MT, or use
multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are
special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it
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GCC(1) GNU GCC(1)
were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not
implied. The driver determines file based on whether an
-o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it takes the
name of the input file, removes any directory components
and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if
used without -E, each -o is understood to specify a
target object file.
Since -E is not implied, -MD can be used to generate a
dependency output file as a side-effect of the
compilation process.
-MMD
Like -MD except mention only user header files, not
system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has
already been preprocessed. This suppresses things like
macro expansion, trigraph conversion, escaped newline
splicing, and processing of most directives. The
preprocessor still recognizes and removes comments, so
that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the
front ends.
-fpreprocessed is implicit if the input file has one of
the extensions .i, .ii or .mi. These are the extensions
that GCC uses for preprocessed files created by
-save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand
macros.
The option's behavior depends on the -E and
-fpreprocessed options.
With -E, preprocessing is limited to the handling of
directives such as "#define", "#ifdef", and "#error".
Other preprocessor operations, such as macro expansion
and trigraph conversion are not performed. In addition,
the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and
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most builtin macros is disabled. Macros such as
"__LINE__", which are contextually dependent, are
handled normally. This enables compilation of files
previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full
preprocessing of files previously preprocessed with "-E
-fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This
option is enabled by default for C99 (and later C
standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths
with canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the
preprocessor report correct column numbers in warnings
or errors, even if tabs appear on the line. If the
value is less than 1 or greater than 100, the option is
ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This
allows the compiler to emit diagnostic about the current
macro expansion stack when a compilation error occurs in
a macro expansion. Using this option makes the
preprocessor and the compiler consume more memory. The
level parameter can be used to choose the level of
precision of token location tracking thus decreasing the
memory consumption if necessary. Value 0 of level de-
activates this option. Value 1 tracks tokens locations
in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have
the same location. Value 2 tracks tokens locations
completely. This value is the most memory hungry. When
this option is given no argument, the default parameter
value is 2.
Note that "-ftrack-macro-expansion=2" is activated by
default.
-fexec-charset=charset
Set the execution character set, used for string and
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character constants. The default is UTF-8. charset can
be any encoding supported by the system's "iconv"
library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide
string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of
"wchar_t". As with -fexec-charset, charset can be any
encoding supported by the system's "iconv" library
routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from
the character set of the input file to the source
character set used by GCC. If the locale does not
specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by
either the locale or this command-line option.
Currently the command-line option takes precedence if
there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the
dependency-output flags to also list the files from the
precompiled header's dependencies. If not specified,
only the precompiled header are listed and not the files
that were used to create it, because those files are not
consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together
with -E. It inserts a special "#pragma", "#pragma GCC
pch_preprocess "filename"" in the output to mark the
place where the precompiled header was found, and its
filename. When -fpreprocessed is in use, GCC recognizes
this "#pragma" and loads the PCH.
This option is off by default, because the resulting
preprocessed output is only really suitable as input to
GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code,
but it is safe to edit the filename if the PCH file is
available in a different location. The filename may be
absolute or it may be relative to GCC's current
directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor
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output that let the compiler know the current working
directory at the time of preprocessing. When this
option is enabled, the preprocessor emits, after the
initial linemarker, a second linemarker with the current
working directory followed by two slashes. GCC uses
this directory, when it's present in the preprocessed
input, as the directory emitted as the current working
directory in some debugging information formats. This
option is implicitly enabled if debugging information is
enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in
the command line, this option has no effect, since no
"#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and
answer answer. This form is preferred to the older form
-A predicate(answer), which is still supported, because
it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and
answer answer.
-C Do not discard comments. All comments are passed
through to the output file, except for comments in
processed directives, which are deleted along with the
directive.
You should be prepared for side effects when using -C;
it causes the preprocessor to treat comments as tokens
in their own right. For example, comments appearing at
the start of what would be a directive line have the
effect of turning that line into an ordinary source
line, since the first token on the line is no longer a
#.
-CC Do not discard comments, including during macro
expansion. This is like -C, except that comments
contained within macros are also passed through to the
output file where the macro is expanded.
In addition to the side-effects of the -C option, the
-CC option causes all C++-style comments inside a macro
to be converted to C-style comments. This is to prevent
later use of that macro from inadvertently commenting
out the remainder of the source line.
The -CC option is generally used to support lint
comments.
-P Inhibit generation of linemarkers in the output from the
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preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will
be sent to a program which might be confused by the
linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C
preprocessors, as opposed to ISO C preprocessors. See
the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a
pre-standard C compiler, and these options are only
supported with the -E switch, or when invoking CPP
explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character
sequences, all starting with ??, that are defined by ISO
C to stand for single characters. For example, ??/
stands for \, so '??/n' is a character constant for a
newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-
conforming modes it converts them. See the -std and
-ansi options.
-remap
Enable special code to work around file systems which
only permit very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to
other normal activities. Each name is indented to show
how deep in the #include stack it is. Precompiled
header files are also printed, even if they are found to
be invalid; an invalid precompiled header file is
printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as
specified by letters. The flags documented here are
those relevant to the preprocessor. Other letters are
interpreted by the compiler proper, or reserved for
future versions of GCC, and so are silently ignored. If
you specify letters whose behavior conflicts, the result
is undefined.
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-dM Instead of the normal output, generate a list of
#define directives for all the macros defined during
the execution of the preprocessor, including
predefined macros. This gives you a way of finding
out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the
command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is
interpreted as a synonym for -fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include
the predefined macros, and it outputs both the
#define directives and the result of preprocessing.
Both kinds of output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dI Output #include directives in addition to the result
of preprocessing.
-dU Like -dD except that only macros that are expanded,
or whose definedness is tested in preprocessor
directives, are output; the output is delayed until
the use or test of the macro; and #undef directives
are also output for macros tested but undefined at
the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used
from CPP or with -E, it dumps debugging information
about location maps. Every token in the output is
preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no
effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and
pass option directly through to the preprocessor. If
option contains commas, it is split into multiple
options at the commas. However, many options are
modified, translated or interpreted by the compiler
driver before being passed to the preprocessor, and -Wp
forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so
whenever possible you should avoid using -Wp and let the
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driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can
use this to supply system-specific preprocessor options
that GCC does not recognize.
If you want to pass an option that takes an argument,
you must use -Xpreprocessor twice, once for the option
and once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before
compilation. By default, GCC performs preprocessing as
an integrated part of input tokenization and parsing.
If this option is provided, the appropriate language
front end (cc1, cc1plus, or cc1obj for C, C++, and
Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual
compilation of the preprocessed input. This option may
be useful in conjunction with the -B or -wrapper options
to specify an alternate preprocessor or perform
additional processing of the program source between
normal preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option
contains commas, it is split into multiple options at
the commas.
-Xassembler option
Pass option as an option to the assembler. You can use
this to supply system-specific assembler options that
GCC does not recognize.
If you want to pass an option that takes an argument,
you must use -Xassembler twice, once for the option and
once for the argument.
Options for Linking
These options come into play when the compiler links object
files into an executable output file. They are meaningless
if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized
suffix is considered to name an object file or library.
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(Object files are distinguished from libraries by the
linker according to the file contents.) If linking is
done, these object files are used as input to the
linker.
-c
-S
-E If any of these options is used, then the linker is not
run, and object file names should not be used as
arguments.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The
second alternative with the library as a separate
argument is only for POSIX compliance and is not
recommended.)
It makes a difference where in the command you write
this option; the linker searches and processes libraries
and object files in the order they are specified. Thus,
foo.o -lz bar.o searches library z after file foo.o but
before bar.o. If bar.o refers to functions in z, those
functions may not be loaded.
The linker searches a standard list of directories for
the library, which is actually a file named
liblibrary.a. The linker then uses this file as if it
had been specified precisely by name.
The directories searched include several standard system
directories plus any that you specify with -L.
Normally the files found this way are library
files---archive files whose members are object files.
The linker handles an archive file by scanning through
it for members which define symbols that have so far
been referenced but not defined. But if the file that
is found is an ordinary object file, it is linked in the
usual fashion. The only difference between using an -l
option and specifying a file name is that -l surrounds
library with lib and .a and searches several
directories.
-lobjc
You need this special case of the -l option in order to
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link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when
linking. The standard system libraries are used
normally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking.
Only the libraries you specify are passed to the linker,
and options specifying linkage of the system libraries,
such as -static-libgcc or -shared-libgcc, are ignored.
The standard startup files are used normally, unless
-nostartfiles is used.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should
be supplied through some other mechanism when this
option is specified.
-nostdlib
Do not use the standard system startup files or
libraries when linking. No startup files and only the
libraries you specify are passed to the linker, and
options specifying linkage of the system libraries, such
as -static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should
be supplied through some other mechanism when this
option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal
subroutines which GCC uses to overcome shortcomings of
particular machines, or special needs for some
languages.
In most cases, you need libgcc.a even when you want to
avoid other standard libraries. In other words, when
you specify -nostdlib or -nodefaultlibs you should
usually specify -lgcc as well. This ensures that you
have no unresolved references to internal GCC library
subroutines. (An example of such an internal subroutine
is "__main", used to ensure C++ constructors are
called.)
-pie
Produce a position independent executable on targets
that support it. For predictable results, you must also
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specify the same set of options used for compilation
(-fpie, -fPIE, or model suboptions) when you specify
this linker option.
-no-pie
Don't produce a position independent executable.
-pthread
Link with the POSIX threads library. This option is
supported on GNU/Linux targets, most other Unix
derivatives, and also on x86 Cygwin and MinGW targets.
On some targets this option also sets flags for the
preprocessor, so it should be used consistently for both
compilation and linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on
targets that support it. This instructs the linker to
add all symbols, not only used ones, to the dynamic
symbol table. This option is needed for some uses of
"dlopen" or to allow obtaining backtraces from within a
program.
-s Remove all symbol table and relocation information from
the executable.
-static
On systems that support dynamic linking, this prevents
linking with the shared libraries. On other systems,
this option has no effect.
-shared
Produce a shared object which can then be linked with
other objects to form an executable. Not all systems
support this option. For predictable results, you must
also specify the same set of options used for
compilation (-fpic, -fPIC, or model suboptions) when you
specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library,
these options force the use of either the shared or
static version, respectively. If no shared version of
libgcc was built when the compiler was configured, these
options have no effect.
There are several situations in which an application
should use the shared libgcc instead of the static
version. The most common of these is when the
application wishes to throw and catch exceptions across
different shared libraries. In that case, each of the
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libraries as well as the application itself should use
the shared libgcc.
Therefore, the G++ and driver automatically adds
-shared-libgcc
whenever you build a shared library or a main
executable, because C++
programs typically use exceptions, so this is the right
thing to do.
If, instead, you use the GCC driver to create shared
libraries, you may find that they are not always linked
with the shared libgcc. If GCC finds, at its
configuration time, that you have a non-GNU linker or a
GNU linker that does not support option --eh-frame-hdr,
it links the shared version of libgcc into shared
libraries by default. Otherwise, it takes advantage of
the linker and optimizes away the linking with the
shared version of libgcc, linking with the static
version of libgcc by default. This allows exceptions to
propagate through such shared libraries, without
incurring relocation costs at library load time.
However, if a library or main executable is supposed to
throw or catch exceptions, you must link it using the
G++ driver, as appropriate for the languages used in the
program, or using the option -shared-libgcc, such that
it is linked with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a
program, the GCC driver automatically links against
libasan. If libasan is available as a shared library,
and the -static option is not used, then this links
against the shared version of libasan. The
-static-libasan option directs the GCC driver to link
libasan statically, without necessarily linking other
libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a
program, the GCC driver automatically links against
libtsan. If libtsan is available as a shared library,
and the -static option is not used, then this links
against the shared version of libtsan. The
-static-libtsan option directs the GCC driver to link
libtsan statically, without necessarily linking other
libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a
program, the GCC driver automatically links against
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liblsan. If liblsan is available as a shared library,
and the -static option is not used, then this links
against the shared version of liblsan. The
-static-liblsan option directs the GCC driver to link
liblsan statically, without necessarily linking other
libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a
program, the GCC driver automatically links against
libubsan. If libubsan is available as a shared library,
and the -static option is not used, then this links
against the shared version of libubsan. The
-static-libubsan option directs the GCC driver to link
libubsan statically, without necessarily linking other
libraries statically.
-static-libmpx
When the -fcheck-pointer bounds and -mmpx options are
used to link a program, the GCC driver automatically
links against libmpx. If libmpx is available as a
shared library, and the -static option is not used, then
this links against the shared version of libmpx. The
-static-libmpx option directs the GCC driver to link
libmpx statically, without necessarily linking other
libraries statically.
-static-libmpxwrappers
When the -fcheck-pointer bounds and -mmpx options are
used to link a program without also using
-fno-chkp-use-wrappers, the GCC driver automatically
links against libmpxwrappers. If libmpxwrappers is
available as a shared library, and the -static option is
not used, then this links against the shared version of
libmpxwrappers. The -static-libmpxwrappers option
directs the GCC driver to link libmpxwrappers
statically, without necessarily linking other libraries
statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it
normally automatically links against libstdc++. If
libstdc++ is available as a shared library, and the
-static option is not used, then this links against the
shared version of libstdc++. That is normally fine.
However, it is sometimes useful to freeze the version of
libstdc++ used by the program without going all the way
to a fully static link. The -static-libstdc++ option
directs the g++ driver to link libstdc++ statically,
without necessarily linking other libraries statically.
-symbolic
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Bind references to global symbols when building a shared
object. Warn about any unresolved references (unless
overridden by the link editor option -Xlinker -z
-Xlinker defs). Only a few systems support this option.
-T script
Use script as the linker script. This option is
supported by most systems using the GNU linker. On some
targets, such as bare-board targets without an operating
system, the -T option may be required when linking to
avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use
this to supply system-specific linker options that GCC
does not recognize.
If you want to pass an option that takes a separate
argument, you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass
-assert definitions, you must write -Xlinker -assert
-Xlinker definitions. It does not work to write
-Xlinker "-assert definitions", because this passes the
entire string as a single argument, which is not what
the linker expects.
When using the GNU linker, it is usually more convenient
to pass arguments to linker options using the
option=value syntax than as separate arguments. For
example, you can specify -Xlinker -Map=output.map rather
than -Xlinker -Map -Xlinker output.map. Other linkers
may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option
contains commas, it is split into multiple options at
the commas. You can use this syntax to pass an argument
to the option. For example, -Wl,-Map,output.map passes
-Map output.map to the linker. When using the GNU
linker, you can also get the same effect with
-Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking
of library modules to define it. You can use -u
multiple times with different symbols to force loading
of additional library modules.
-z keyword
-z is passed directly on to the linker along with the
keyword keyword. See the section in the documentation of
your linker for permitted values and their meanings.
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Options for Directory Search
These options specify directories to search for header
files, for libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be
searched for header files during preprocessing. If dir
begins with =, then the = is replaced by the sysroot
prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the
quote form of the directive, "#include "file"".
Directories specified with -I, -isystem, or -idirafter
apply to lookup for both the "#include "file"" and
"#include <file>" directives.
You can specify any number or combination of these
options on the command line to search for header files
in several directories. The lookup order is as follows:
1. For the quote form of the include directive, the
directory of the current file is searched first.
2. For the quote form of the include directive, the
directories specified by -iquote options are
searched in left-to-right order, as they appear on
the command line.
3. Directories specified with -I options are scanned in
left-to-right order.
4. Directories specified with -isystem options are
scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are
scanned in left-to-right order.
You can use -I to override a system header file,
substituting your own version, since these directories
are searched before the standard system header file
directories. However, you should not use this option to
add directories that contain vendor-supplied system
header files; use -isystem for that.
The -isystem and -idirafter options also mark the
directory as a system directory, so that it gets the
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same special treatment that is applied to the standard
system directories.
If a standard system include directory, or a directory
specified with -isystem, is also specified with -I, the
-I option is ignored. The directory is still searched
but as a system directory at its normal position in the
system include chain. This is to ensure that GCC's
procedure to fix buggy system headers and the ordering
for the "#include_next" directive are not inadvertently
changed. If you really need to change the search order
for system directories, use the -nostdinc and/or
-isystem options.
-I- Split the include path. This option has been
deprecated. Please use -iquote instead for -I
directories before the -I- and remove the -I- option.
Any directories specified with -I options before -I- are
searched only for headers requested with
"#include "file""; they are not searched for
"#include <file>". If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of
the current file directory as the first search directory
for "#include "file"". There is no way to override this
effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you
should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include
search path. -iwithprefixbefore puts it in the same
place -I would; -iwithprefix puts it where -idirafter
would.
-isysroot dir
This option is like the --sysroot option, but applies
only to header files (except for Darwin targets, where
it applies to both header files and libraries). See the
--sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing
target-specific C++ headers.
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-nostdinc
Do not search the standard system directories for header
files. Only the directories explicitly specified with
-I, -iquote, -isystem, and/or -idirafter options (and
the directory of the current file, if appropriate) are
searched.
-nostdinc++
Do not search for header files in the C++-specific
standard directories, but do still search the other
standard directories. (This option is used when
building the C++ library.)
-iplugindir=dir
Set the directory to search for plugins that are passed
by -fplugin=name instead of -fplugin=path/name.so. This
option is not meant to be used by the user, but only
passed by the driver.
-Ldir
Add directory dir to the list of directories to be
searched for -l.
-Bprefix
This option specifies where to find the executables,
libraries, include files, and data files of the compiler
itself.
The compiler driver program runs one or more of the
subprograms cpp, cc1, as and ld. It tries prefix as a
prefix for each program it tries to run, both with and
without machine/version/ for the corresponding target
machine and compiler version.
For each subprogram to be run, the compiler driver first
tries the -B prefix, if any. If that name is not found,
or if -B is not specified, the driver tries two standard
prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If
neither of those results in a file name that is found,
the unmodified program name is searched for using the
directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B
refers to a directory, and if necessary it adds a
directory separator character at the end of the path.
-B prefixes that effectively specify directory names
also apply to libraries in the linker, because the
compiler translates these options into -L options for
the linker. They also apply to include files in the
preprocessor, because the compiler translates these
options into -isystem options for the preprocessor. In
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this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched
for using the -B prefix, if needed. If it is not found
there, the two standard prefixes above are tried, and
that is all. The file is left out of the link if it is
not found by those means.
Another way to specify a prefix much like the -B prefix
is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the range 0 to 9,
then it is replaced by [dir/]include. This is to help
with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to
/../ or /./, or make the path absolute when generating a
relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and
libraries. For example, if the compiler normally
searches for headers in /usr/include and libraries in
/usr/lib, it instead searches dir/usr/include and
dir/usr/lib.
If you use both this option and the -isysroot option,
then the --sysroot option applies to libraries, but the
-isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the
necessary support for this option. If your linker does
not support this option, the header file aspect of
--sysroot still works, but the library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root
directory specified with --sysroot, depending on the
other options used, so that headers may for example be
found in dir/suffix/usr/include instead of
dir/usr/include. This option disables the addition of
such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface
conventions used in code generation.
Most of them have both positive and negative forms; the
negative form of -ffoo is -fno-foo. In the table below,
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only one of the forms is listed---the one that is not the
default. You can figure out the other form by either
removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared
local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none. all enables
stack reuse for all local variables and temporaries,
named_vars enables the reuse only for user defined local
variables with names, and none disables stack reuse
completely. The default value is all. The option is
needed when the program extends the lifetime of a scoped
local variable or a compiler generated temporary beyond
the end point defined by the language. When a lifetime
of a variable ends, and if the variable lives in memory,
the optimizing compiler has the freedom to reuse its
stack space with other temporaries or scoped local
variables whose live range does not overlap with it.
Legacy code extending local lifetime is likely to break
with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
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A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well
defined by the C++ standard. When a lifetime of a
temporary ends, and if the temporary lives in memory,
the optimizing compiler has the freedom to reuse its
stack space with other temporaries or scoped local
variables whose live range does not overlap with it.
However some of the legacy code relies on the behavior
of older compilers in which temporaries' stack space is
not reused, the aggressive stack reuse can lead to
runtime errors. This option is used to control the
temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on
addition, subtraction, multiplication operations. The
options -ftrapv and -fwrapv override each other, so
using -ftrapv -fwrapv on the command-line results in
-fwrapv being effective. Note that only active options
override, so using -ftrapv -fwrapv -fno-wrapv on the
command-line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement
representation. This flag enables some optimizations
and disables others. The options -ftrapv and -fwrapv
override each other, so using -ftrapv -fwrapv on the
command-line results in -fwrapv being effective. Note
that only active options override, so using -ftrapv
-fwrapv -fno-wrapv on the command-line results in
-ftrapv being effective.
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-fexceptions
Enable exception handling. Generates extra code needed
to propagate exceptions. For some targets, this implies
GCC generates frame unwind information for all
functions, which can produce significant data size
overhead, although it does not affect execution. If you
do not specify this option, GCC enables it by default
for languages like C++ that normally require exception
handling, and disables it for languages like C that do
not normally require it. However, you may need to
enable this option when compiling C code that needs to
interoperate properly with exception handlers written in
C++. You may also wish to disable this option if you
are compiling older C++ programs that don't use
exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific
runtime support that does not exist everywhere.
Moreover, it only allows trapping instructions to throw
exceptions, i.e. memory references or floating-point
instructions. It does not allow exceptions to be thrown
from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but
don't otherwise contribute to the execution of the
program can be optimized away. This option is enabled
by default for the Ada front end, as permitted by the
Ada language specification. Optimization passes that
cause dead exceptions to be removed are enabled
independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates
any needed static data, but does not affect the
generated code in any other way. You normally do not
need to enable this option; instead, a language
processor that needs this handling enables it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by
target machine. The table is exact at each instruction
boundary, so it can be used for stack unwinding from
asynchronous events (such as debugger or garbage
collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the
C++ compiler uses the "STB_GNU_UNIQUE" binding to make
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sure that definitions of template static data members
and static local variables in inline functions are
unique even in the presence of "RTLD_LOCAL"; this is
necessary to avoid problems with a library used by two
different "RTLD_LOCAL" plugins depending on a definition
in one of them and therefore disagreeing with the other
one about the binding of the symbol. But this causes
"dlclose" to be ignored for affected DSOs; if your
program relies on reinitialization of a DSO via
"dlclose" and "dlopen", you can use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory
like longer ones, rather than in registers. This
convention is less efficient, but it has the advantage
of allowing intercallability between GCC-compiled files
and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in
memory depends on the target configuration macros.
Short structures and unions are those whose size and
alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return
switch is not binary compatible with code compiled with
the -freg-struct-return switch. Use it to conform to a
non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when
possible. This is more efficient for small structures
than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever
convention is standard for the target. If there is no
standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the
principal compiler. In those cases, we can choose the
standard, and we chose the more efficient register
return alternative.
Warning: code compiled with the -freg-struct-return
switch is not binary compatible with code compiled with
the -fpcc-struct-return switch. Use it to conform to a
non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it
needs for the declared range of possible values.
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Specifically, the "enum" type is equivalent to the
smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate
code that is not binary compatible with code generated
without that switch. Use it to conform to a non-default
application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short
unsigned int" instead of the default for the target.
This option is useful for building programs to run under
WINE.
Warning: the -fshort-wchar switch causes GCC to generate
code that is not binary compatible with code generated
without that switch. Use it to conform to a non-default
application binary interface.
-fno-common
In C code, this option controls the placement of global
variables defined without an initializer, known as
tentative definitions in the C standard. Tentative
definitions are distinct from declarations of a variable
with the "extern" keyword, which do not allocate
storage.
Unix C compilers have traditionally allocated storage
for uninitialized global variables in a common block.
This allows the linker to resolve all tentative
definitions of the same variable in different
compilation units to the same object, or to a non-
tentative definition. This is the behavior specified by
-fcommon, and is the default for GCC on most targets. On
the other hand, this behavior is not required by ISO C,
and on some targets may carry a speed or code size
penalty on variable references.
The -fno-common option specifies that the compiler
should instead place uninitialized global variables in
the data section of the object file. This inhibits the
merging of tentative definitions by the linker so you
get a multiple-definition error if the same variable is
defined in more than one compilation unit. Compiling
with -fno-common is useful on targets for which it
provides better performance, or if you wish to verify
that the program will work on other systems that always
treat uninitialized variable definitions this way.
-fno-ident
Ignore the "#ident" directive.
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-finhibit-size-directive
Don't output a ".size" assembler directive, or anything
else that would cause trouble if the function is split
in the middle, and the two halves are placed at
locations far apart in memory. This option is used when
compiling crtstuff.c; you should not need to use it for
anything else.
-fverbose-asm
Put extra commentary information in the generated
assembly code to make it more readable. This option is
generally only of use to those who actually need to read
the generated assembly code (perhaps while debugging the
compiler itself).
-fno-verbose-asm, the default, causes the extra
information to be omitted and is useful when comparing
two assembler files.
The added comments include:
* information on the compiler version and command-line
options,
* the source code lines associated with the assembly
instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to
the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the
result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
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.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than
machines and hence the precise format of the comments is
subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the
compiler to be recorded into the object file that is
being created. This switch is only implemented on some
targets and the exact format of the recording is target
and binary file format dependent, but it usually takes
the form of a section containing ASCII text. This
switch is related to the -fverbose-asm switch, but that
switch only records information in the assembler output
file as comments, so it never reaches the object file.
See also -grecord-gcc-switches for another way of
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storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for
use in a shared library, if supported for the target
machine. Such code accesses all constant addresses
through a global offset table (GOT). The dynamic loader
resolves the GOT entries when the program starts (the
dynamic loader is not part of GCC; it is part of the
operating system). If the GOT size for the linked
executable exceeds a machine-specific maximum size, you
get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC, 28k on
AArch64 and 32k on the m68k and RS/6000. The x86 has no
such limit.)
Position-independent code requires special support, and
therefore works only on certain machines. For the x86,
GCC supports PIC for System V but not for the Sun 386i.
Code generated for the IBM RS/6000 is always
position-independent.
When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 1.
-fPIC
If supported for the target machine, emit position-
independent code, suitable for dynamic linking and
avoiding any limit on the size of the global offset
table. This option makes a difference on AArch64, m68k,
PowerPC and SPARC.
Position-independent code requires special support, and
therefore works only on certain machines.
When this flag is set, the macros "__pic__" and
"__PIC__" are defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but
generated position independent code can be only linked
into executables. Usually these options are used when
-pie GCC option is used during linking.
-fpie and -fPIE both define the macros "__pie__" and
"__PIE__". The macros have the value 1 for -fpie and 2
for -fPIE.
-fno-plt
Do not use the PLT for external function calls in
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position-independent code. Instead, load the callee
address at call sites from the GOT and branch to it.
This leads to more efficient code by eliminating PLT
stubs and exposing GOT loads to optimizations. On
architectures such as 32-bit x86 where PLT stubs expect
the GOT pointer in a specific register, this gives more
register allocation freedom to the compiler. Lazy
binding requires use of the PLT; with -fno-plt all
external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be
used to avoid calls through the PLT for specific
external functions.
In position-dependent code, a few targets also convert
calls to functions that are marked to not use the PLT to
use the GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where
it would be more efficient than other code generation
strategies. This option is of use in conjunction with
-fpic or -fPIC for building code that forms part of a
dynamic linker and cannot reference the address of a
jump table. On some targets, jump tables do not require
a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register;
generated code should never refer to it (except perhaps
as a stack pointer, frame pointer or in some other fixed
role).
reg must be the name of a register. The register names
accepted are machine-specific and are defined in the
"REGISTER_NAMES" macro in the machine description macro
file.
This flag does not have a negative form, because it
specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register
that is clobbered by function calls. It may be
allocated for temporaries or variables that do not live
across a call. Functions compiled this way do not save
and restore the register reg.
It is an error to use this flag with the frame pointer
or stack pointer. Use of this flag for other registers
that have fixed pervasive roles in the machine's
execution model produces disastrous results.
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This flag does not have a negative form, because it
specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register
saved by functions. It may be allocated even for
temporaries or variables that live across a call.
Functions compiled this way save and restore the
register reg if they use it.
It is an error to use this flag with the frame pointer
or stack pointer. Use of this flag for other registers
that have fixed pervasive roles in the machine's
execution model produces disastrous results.
A different sort of disaster results from the use of
this flag for a register in which function values may be
returned.
This flag does not have a negative form, because it
specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members
together without holes. When a value is specified
(which must be a small power of two), pack structure
members according to this value, representing the
maximum alignment (that is, objects with default
alignment requirements larger than this are output
potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate
code that is not binary compatible with code generated
without that switch. Additionally, it makes the code
suboptimal. Use it to conform to a non-default
application binary interface.
-fleading-underscore
This option and its counterpart,
-fno-leading-underscore, forcibly change the way C
symbols are represented in the object file. One use is
to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code
generated without that switch. Use it to conform to a
non-default application binary interface. Not all
targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The
model argument should be one of global-dynamic, local-
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dynamic, initial-exec or local-exec. Note that the
choice is subject to optimization: the compiler may use
a more efficient model for symbols not visible outside
of the translation unit, or if -fpic is not given on the
command line.
The default without -fpic is initial-exec; with -fpic
the default is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested
functions, always generate them instead of using
descriptors. Otherwise, for targets that do not need
them, like for example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at
run time on the stack when the address of a nested
function is taken, and is used to call the nested
function indirectly. Therefore, it requires the stack
to be made executable in order for the program to work
properly.
-fno-trampolines is enabled by default on a language by
language basis to let the compiler avoid generating
them, if it computes that this is safe, and replace them
with descriptors. Descriptors are made up of data only,
but the generated code must be prepared to deal with
them. As of this writing, -fno-trampolines is enabled
by default only for Ada.
Moreover, code compiled with -ftrampolines and code
compiled with -fno-trampolines are not binary compatible
if nested functions are present. This option must
therefore be used on a program-wide basis and be
manipulated with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the
specified option---all symbols are marked with this
unless overridden within the code. Using this feature
can very substantially improve linking and load times of
shared object libraries, produce more optimized code,
provide near-perfect API export and prevent symbol
clashes. It is strongly recommended that you use this
in any shared objects you distribute.
Despite the nomenclature, default always means public;
i.e., available to be linked against from outside the
shared object. protected and internal are pretty
useless in real-world usage so the only other commonly
used option is hidden. The default if -fvisibility
isn't specified is default, i.e., make every symbol
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public.
A good explanation of the benefits offered by ensuring
ELF symbols have the correct visibility is given by "How
To Write Shared Libraries" by Ulrich Drepper (which can
be found at
<https://www.akkadia.org/drepper/>)---however a superior
solution made possible by this option to marking things
hidden when the default is public is to make the default
hidden and mark things public. This is the norm with
DLLs on Windows and with -fvisibility=hidden and
"__attribute__ ((visibility("default")))" instead of
"__declspec(dllexport)" you get almost identical
semantics with identical syntax. This is a great boon
to those working with cross-platform projects.
For those adding visibility support to existing code,
you may find "#pragma GCC visibility" of use. This
works by you enclosing the declarations you wish to set
visibility for with (for example) "#pragma GCC
visibility push(hidden)" and "#pragma GCC visibility
pop". Bear in mind that symbol visibility should be
viewed as part of the API interface contract and thus
all new code should always specify visibility when it is
not the default; i.e., declarations only for use within
the local DSO should always be marked explicitly as
hidden as so to avoid PLT indirection overheads---making
this abundantly clear also aids readability and self-
documentation of the code. Note that due to ISO C++
specification requirements, "operator new" and "operator
delete" must always be of default visibility.
Be aware that headers from outside your project, in
particular system headers and headers from any other
library you use, may not be expecting to be compiled
with visibility other than the default. You may need to
explicitly say "#pragma GCC visibility push(default)"
before including any such headers.
"extern" declarations are not affected by -fvisibility,
so a lot of code can be recompiled with
-fvisibility=hidden with no modifications. However,
this means that calls to "extern" functions with no
explicit visibility use the PLT, so it is more effective
to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern"
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage
entities. This means that, for instance, an exception
class that is be thrown between DSOs must be explicitly
marked with default visibility so that the type_info
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nodes are unified between the DSOs.
An overview of these techniques, their benefits and how
to use them is at <http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-
fields (or other structure fields, although the compiler
usually honors those types anyway) should use a single
access of the width of the field's type, aligned to a
natural alignment if possible. For example, targets
with memory-mapped peripheral registers might require
all such accesses to be 16 bits wide; with this flag you
can declare all peripheral bit-fields as "unsigned
short" (assuming short is 16 bits on these targets) to
force GCC to use 16-bit accesses instead of, perhaps, a
more efficient 32-bit access.
If this option is disabled, the compiler uses the most
efficient instruction. In the previous example, that
might be a 32-bit load instruction, even though that
accesses bytes that do not contain any portion of the
bit-field, or memory-mapped registers unrelated to the
one being updated.
In some cases, such as when the "packed" attribute is
applied to a structure field, it may not be possible to
access the field with a single read or write that is
correctly aligned for the target machine. In this case
GCC falls back to generating multiple accesses rather
than code that will fault or truncate the result at run
time.
Note: Due to restrictions of the C/C++11 memory model,
write accesses are not allowed to touch non bit-field
members. It is therefore recommended to define all bits
of the field's type as bit-field members.
The default value of this option is determined by the
application binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of
the "__sync" family of functions may be used to
implement the C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the
only useful form of the option is -fno-sync-libcalls.
This option is used in the implementation of the
libatomic runtime library.
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GCC Developer Options
This section describes command-line options that are
primarily of interest to GCC developers, including options
to support compiler testing and investigation of compiler
bugs and compile-time performance problems. This includes
options that produce debug dumps at various points in the
compilation; that print statistics such as memory use and
execution time; and that print information about GCC's
configuration, such as where it searches for libraries. You
should rarely need to use any of these options for ordinary
compilation and linking tasks.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times
specified by letters. This is used for debugging the
RTL-based passes of the compiler. The file names for
most of the dumps are made by appending a pass number
and a word to the dumpname, and the files are created in
the directory of the output file. In case of =filename
option, the dump is output on the given file instead of
the pass numbered dump files. Note that the pass number
is assigned as passes are registered into the pass
manager. Most passes are registered in the order that
they will execute and for these passes the number
corresponds to the pass execution order. However,
passes registered by plugins, passes specific to
compilation targets, or passes that are otherwise
registered after all the other passes are numbered
higher than a pass named "final", even if they are
executed earlier. dumpname is generated from the name
of the output file if explicitly specified and not an
executable, otherwise it is the basename of the source
file.
Some -dletters switches have different meaning when -E
is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or
some -d option letters. Here are the possible letters
for use in pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have
unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
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Dump after auto-inc-dec discovery. This pass is
only run on architectures that have auto inc or auto
dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping
after the two branch target load optimization
passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow
optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3
enable dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping
after the two common subexpression elimination
passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination
passes.
-fdump-rtl-dbr
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Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping
after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable
dumping after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping
after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value
sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent
reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
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-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and
epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable
dumping after the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of
instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run
on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register
file" registers to the x87's stack-like registers.
This pass is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
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-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable
dumping after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard
registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty
files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous
debugging information.
-dD Dump all macro definitions, at the end of
preprocessing, in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment
indicating which pattern and alternative is used.
The length of each instruction is also printed.
-dP Dump the RTL in the assembler output as a comment
before each instruction. Also turns on -dp
annotation.
-dx Just generate RTL for a function instead of
compiling it. Usually used with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output.
This makes it more feasible to use diff on debugging
dumps for compiler invocations with different compiler
binaries and/or different text / bss / data / heap /
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stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file
if an internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers
and address output. This makes it more feasible to use
diff on debugging dumps for compiler invocations with
different options, in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above),
suppress instruction numbers for the links to the
previous and next instructions in a sequence.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the
entire translation unit to a file. The file name is
made by appending .tu to the source file name, and the
file is created in the same directory as the output
file. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree
options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and
virtual function table layout to a file. The file name
is made by appending .class to the source file name, and
the file is created in the same directory as the output
file. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree
options.
-fdump-ipa-switch
Control the dumping at various stages of inter-
procedural analysis language tree to a file. The file
name is generated by appending a switch specific suffix
to the source file name, and the file is created in the
same directory as the output file. The following dumps
are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization,
unused function removal, and inlining decisions.
inline
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Dump after function inlining.
-fdump-passes
Print on stderr the list of optimization passes that are
turned on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a
separate file. The file name is generated by appending
a suffix ending in .statistics to the source file name,
and the file is created in the same directory as the
output file. If the -option form is used, -stats causes
counters to be summed over the whole compilation unit
while -details dumps every event as the passes generate
them. The default with no option is to sum counters for
each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is
generated by appending a switch-specific suffix to the
source file name, and the file is created in the same
directory as the output file. In case of =filename
option, the dump is output on the given file instead of
the auto named dump files. If the -options form is
used, options is a list of - separated options which
control the details of the dump. Not all options are
applicable to all dumps; those that are not meaningful
are ignored. The following options are available
address
Print the address of each node. Usually this is not
meaningful as it changes according to the
environment and source file. Its primary use is for
tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given
decl, use that in the dump instead of "DECL_NAME".
Its primary use is ease of use working backward from
mangled names in the assembly file.
slim
When dumping front-end intermediate representations,
inhibit dumping of members of a scope or body of a
function merely because that scope has been reached.
Only dump such items when they are directly
reachable by some other path.
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When dumping pretty-printed trees, this option
inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed)
form instead of the default LISP-like
representation.
raw Print a raw representation of the tree. By default,
trees are pretty-printed into a C-like
representation.
details
Enable more detailed dumps (not honored by every
dump option). Also include information from the
optimization passes.
stats
Enable dumping various statistics about the pass
(not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in
raw dumps).
graph
For each of the other indicated dump files
(-fdump-rtl-pass), dump a representation of the
control flow graph suitable for viewing with
GraphViz to file.passid.pass.dot. Each function in
the file is pretty-printed as a subgraph, so that
GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and
the RTL is always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each
variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each
statement.
scev
Enable showing scalar evolution analysis details.
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optimized
Enable showing optimization information (only
available in certain passes).
missed
Enable showing missed optimization information (only
available in certain passes).
note
Enable other detailed optimization information (only
available in certain passes).
=filename
Instead of an auto named dump file, output into the
given file name. The file names stdout and stderr
are treated specially and are considered already
open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=/dev/stderr file.c
outputs vectorizer dump into foo.dump, while the PRE
dump is output on to stderr. If two conflicting dump
filenames are given for the same pass, then the
latter option overrides the earlier one.
all Turn on all options, except raw, slim, verbose and
lineno.
optall
Turn on all optimization options, i.e., optimized,
missed, and note.
To determine what tree dumps are available or find the
dump for a pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr
output look for a code that corresponds to the pass
you are interested in. For example, the codes
"tree-evrp", "tree-vrp1", and "tree-vrp2" correspond
to the three Value Range Propagation passes. The
number at the end distinguishes distinct invocations
of the same pass.
2. To enable the creation of the dump file, append the
pass code to the -fdump- option prefix and invoke
GCC with it. For example, to enable the dump from
the Early Value Range Propagation pass, invoke GCC
with the -fdump-tree-evrp option. Optionally, you
may specify the name of the dump file. If you don't
specify one, GCC creates as described below.
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3. Find the pass dump in a file whose name is composed
of three components separated by a period: the name
of the source file GCC was invoked to compile, a
numeric suffix indicating the pass number followed
by the letter t for tree passes (and the letter r
for RTL passes), and finally the pass code. For
example, the Early VRP pass dump might be in a file
named myfile.c.038t.evrp in the current working
directory. Note that the numeric codes are not
stable and may change from one version of GCC to
another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization
passes. If the -options form is used, options is a list
of - separated option keywords to select the dump
details and optimizations.
The options can be divided into two groups: options
describing the verbosity of the dump, and options
describing which optimizations should be included. The
options from both the groups can be freely mixed as they
are non-overlapping. However, in case of any conflicts,
the later options override the earlier options on the
command line.
The following options control the dump verbosity:
optimized
Print information when an optimization is
successfully applied. It is up to a pass to decide
which information is relevant. For example, the
vectorizer passes print the source location of loops
which are successfully vectorized.
missed
Print information about missed optimizations.
Individual passes control which information to
include in the output.
note
Print verbose information about optimizations, such
as certain transformations, more detailed messages
about decisions etc.
all Print detailed optimization information. This
includes optimized, missed, and note.
One or more of the following option keywords can be used
to describe a group of optimizations:
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ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi
Processing) optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a
superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall,
which means to dump all info about successful
optimizations from all the passes.
If the filename is provided, then the dumps from all the
applicable optimizations are concatenated into the
filename. Otherwise the dump is output onto stderr.
Though multiple -fopt-info options are accepted, only
one of them can include a filename. If other filenames
are provided then all but the first such option are
ignored.
Note that the output filename is overwritten in case of
multiple translation units. If a combined output from
multiple translation units is desired, stderr should be
used instead.
In the following example, the optimization info is
output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes
into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization
opportunities from vectorization passes on stderr. Note
that -fopt-info-vec-missed is equivalent to
-fopt-info-missed-vec.
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As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well
as optimized locations from all the inlining passes into
inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are
in conflict since only one output file is allowed. In
this case, only the first option takes effect and the
subsequent options are ignored. Thus only vec.miss is
produced which contains dumps from the vectorizer about
missed opportunities.
-fsched-verbose=n
On targets that use instruction scheduling, this option
controls the amount of debugging output the scheduler
prints to the dump files.
For n greater than zero, -fsched-verbose outputs the
same information as -fdump-rtl-sched1 and
-fdump-rtl-sched2. For n greater than one, it also
output basic block probabilities, detailed ready list
information and unit/insn info. For n greater than two,
it includes RTL at abort point, control-flow and regions
info. And for n over four, -fsched-verbose also
includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly
disable/enable optimization passes. These options are
intended for use for debugging GCC. Compiler users
should use regular options for enabling/disabling passes
instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If
the same pass is statically invoked in the compiler
multiple times, the pass name should be appended
with a sequential number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If
the same pass is statically invoked in the compiler
multiple times, the pass name should be appended
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with a sequential number starting from 1. range-
list is a comma-separated list of function ranges or
assembler names. Each range is a number pair
separated by a colon. The range is inclusive in
both ends. If the range is trivial, the number pair
can be simplified as a single number. If the
function's call graph node's uid falls within one of
the specified ranges, the pass is disabled for that
function. The uid is shown in the function header
of a dump file, and the pass names can be dumped by
using option -fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the
description of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If
the same pass is statically invoked in the compiler
multiple times, the pass name should be appended
with a sequential number starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option
argument description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the
description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
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Enable internal consistency checking. The default
depends on the compiler configuration. -fchecking=2
enables further internal consistency checking that might
affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of
random numbers in generating certain symbol names that
have to be different in every compiled file. It is also
used to place unique stamps in coverage data files and
the object files that produce them. You can use the
-frandom-seed option to produce reproducibly identical
object files.
The string can either be a number (decimal, octal or
hex) or an arbitrary string (in which case it's
converted to a number by computing CRC32).
The string should be different for every file you
compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files
permanently; place them in the current directory and
name them based on the source file. Thus, compiling
foo.c with -c -save-temps produces files foo.i and
foo.s, as well as foo.o. This creates a preprocessed
foo.i output file even though the compiler now normally
uses an integrated preprocessor.
When used in combination with the -x command-line
option, -save-temps is sensible enough to avoid over
writing an input source file with the same extension as
an intermediate file. The corresponding intermediate
file may be obtained by renaming the source file before
using -save-temps.
If you invoke GCC in parallel, compiling several
different source files that share a common base name in
different subdirectories or the same source file
compiled for multiple output destinations, it is likely
that the different parallel compilers will interfere
with each other, and overwrite the temporary files. For
instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to
simultaneously by both compilers.
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-save-temps=obj
Store the usual "temporary" intermediate files
permanently. If the -o option is used, the temporary
files are based on the object file. If the -o option is
not used, the -save-temps=obj switch behaves like
-save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s,
dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the
compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if
linking is done).
Without the specification of an output file, the output
looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that
is time spent executing the program itself. The second
number is "system time", time spent executing operating
system routines on behalf of the program. Both numbers
are in seconds.
With the specification of an output file, the output is
appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before
the program name, and the options passed to the program
are displayed, so that one can later tell what file was
being compiled, and with which options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file.
If the optional argument is omitted (or if file is "."),
the name of the dump file is determined by appending
".gkd" to the compilation output file name.
-fcompare-debug[=opts]
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If no error occurs during compilation, run the compiler
a second time, adding opts and -fcompare-debug-second to
the arguments passed to the second compilation. Dump
the final internal representation in both compilations,
and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is
used.
The environment variable GCC_COMPARE_DEBUG, if defined,
non-empty and nonzero, implicitly enables
-fcompare-debug. If GCC_COMPARE_DEBUG is defined to a
string starting with a dash, then it is used for opts,
otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts,
is equivalent to -fno-compare-debug, which disables the
dumping of the final representation and the second
compilation, preventing even GCC_COMPARE_DEBUG from
taking effect.
To verify full coverage during -fcompare-debug testing,
set GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden, which GCC rejects as an
invalid option in any actual compilation (rather than
preprocessing, assembly or linking). To get just a
warning, setting GCC_COMPARE_DEBUG to
-w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the
second compilation requested by -fcompare-debug, along
with options to silence warnings, and omitting other
options that would cause side-effect compiler outputs to
files or to the standard output. Dump files and
preserved temporary files are renamed so as to contain
the ".gk" additional extension during the second
compilation, to avoid overwriting those generated by the
first.
When this option is passed to the compiler driver, it
causes the first compilation to be skipped, which makes
it useful for little other than debugging the compiler
proper.
-gtoggle
Turn off generation of debug info, if leaving out this
option generates it, or turn it on at level 2 otherwise.
The position of this argument in the command line does
not matter; it takes effect after all other options are
processed, and it does so only once, no matter how many
times it is given. This is mainly intended to be used
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with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that
-gtoggle toggles -g.
-Q Makes the compiler print out each function name as it is
compiled, and print some statistics about each pass when
it finishes.
-ftime-report
Makes the compiler print some statistics about the time
consumed by each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts
separately for each pass.
-fira-verbose=n
Control the verbosity of the dump file for the
integrated register allocator. The default value is 5.
If the value n is greater or equal to 10, the dump
output is sent to stderr using the same format as n
minus 10.
-flto-report
Prints a report with internal details on the workings of
the link-time optimizer. The contents of this report
vary from version to version. It is meant to be useful
to GCC developers when processing object files in LTO
mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of
Link Time Optimization.
-fmem-report
Makes the compiler print some statistics about permanent
memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent
memory allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent
memory allocation before or after interprocedural
optimization.
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-fprofile-report
Makes the compiler print some statistics about
consistency of the (estimated) profile and effect of
individual passes.
-fstack-usage
Makes the compiler output stack usage information for
the program, on a per-function basis. The filename for
the dump is made by appending .su to the auxname.
auxname is generated from the name of the output file,
if explicitly specified and it is not an executable,
otherwise it is the basename of the source file. An
entry is made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic",
"bounded".
The qualifier "static" means that the function
manipulates the stack statically: a fixed number of
bytes are allocated for the frame on function entry and
released on function exit; no stack adjustments are
otherwise made in the function. The second field is
this fixed number of bytes.
The qualifier "dynamic" means that the function
manipulates the stack dynamically: in addition to the
static allocation described above, stack adjustments are
made in the body of the function, for example to
push/pop arguments around function calls. If the
qualifier "bounded" is also present, the amount of these
adjustments is bounded at compile time and the second
field is an upper bound of the total amount of stack
used by the function. If it is not present, the amount
of these adjustments is not bounded at compile time and
the second field only represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of
the compilation. This option is supported only by the
C++ front end, and the information is generally only
useful to the G++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug
counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-
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value-list is a comma-separated list of name:value pairs
which sets the upper bound of each debug counter name to
value. All debug counters have the initial upper bound
of "UINT_MAX"; thus "dbg_cnt" returns true always unless
the upper bound is set by this option. For example,
with -fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)"
returns true only for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library
that would be used when linking---and don't do anything
else. With this option, GCC does not compile or link
anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib
selected by any other switches present in the command
line. This directory is supposed to exist in
GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to
compiler switches that enable them. The directory name
is separated from the switches by ;, and each switch
starts with an @ instead of the -, without spaces
between multiple switches. This is supposed to ease
shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected
multilib, relative to some lib subdirectory. If OS
libraries are present in the lib subdirectory and no
multilibs are used, this is usually just ., if OS
libraries are present in libsuffix sibling directories
this prints e.g. ../lib64, ../lib or ../lib32, or if OS
libraries are present in lib/subdir subdirectories it
prints e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected
multiarch, relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such
as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs
but you do want to link with libgcc.a. You can do:
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gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory
and a list of program and library directories gcc
searches---and don't do anything else.
This is useful when gcc prints the error message
installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put cpp0
and the other compiler components where gcc expects to
find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed
them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during
compilation. This is the target sysroot specified
either at configure time or using the --sysroot option,
possibly with an extra suffix that depends on
compilation options. If no target sysroot is specified,
the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when
searching for headers, or give an error if the compiler
is not configured with such a suffix---and don't do
anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or
7)---and don't do anything else. This is the compiler
version used in filesystem paths, specs, can be
depending on how the compiler has been configured just a
single number (major version), two numbers separated by
dot (major and minor version) or three numbers separated
by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version, always 3 numbers
separated by dots, major, minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do
anything else. (This is used when GCC itself is being
built.)
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Machine-Dependent Options
Each target machine supported by GCC can have its own
options---for example, to allow you to compile for a
particular processor variant or ABI, or to control
optimizations specific to that machine. By convention, the
names of machine-specific options start with -m.
Some configurations of the compiler also support additional
target-specific options, usually for compatibility with
other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible
values are ilp32 for SysV-like data model where int,
long int and pointers are 32 bits, and lp64 for SysV-
like data model where int is 32 bits, but long int and
pointers are 64 bits.
The default depends on the specific target
configuration. Note that the LP64 and ILP32 ABIs are
not link-compatible; you must compile your entire
program with the same ABI, and link with a compatible
set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC
is configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose
registers. This will prevent the compiler from using
floating-point and Advanced SIMD registers but will not
impose any restrictions on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when
GCC is configured for an aarch64-*-* but not an
aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and
its statically defined symbols must be within 1MB of
each other. Programs can be statically or dynamically
linked.
-mcmodel=small
Generate code for the small code model. The program and
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its statically defined symbols must be within 4GB of
each other. Programs can be statically or dynamically
linked. This is the default code model.
-mcmodel=large
Generate code for the large code model. This makes no
assumptions about addresses and sizes of sections.
Programs can be statically linked only.
-mstrict-align
Avoid generating memory accesses that may not be aligned
on a natural object boundary as described in the
architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The
former behavior is the default.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage
mechanism for dynamic accesses of TLS variables. This
is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage
mechanism for dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values
are 12, 24, 32, 48. This option requires binutils 2.26
or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53
erratum number 835769. This involves inserting a NOP
instruction between memory instructions and 64-bit
integer multiply-accumulate instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53
erratum number 843419. This erratum workaround is made
at link time and this will only pass the corresponding
flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root
approximation. This option only has an effect if
-ffast-math or -funsafe-math-optimizations is used as
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well. Enabling this reduces precision of reciprocal
square root results to about 16 bits for single
precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This
option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling
this reduces precision of square root results to about
16 bits for single precision and to 32 bits for double
precision. If enabled, it implies
-mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This
option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling
this reduces precision of division results to about 16
bits for single precision and to 32 bits for double
precision.
-march=name
Specify the name of the target architecture and,
optionally, one or more feature modifiers. This option
has the form -march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a,
armv8.2-a, armv8.3-a or native.
The value armv8.3-a implies armv8.2-a and enables
compiler support for the ARMv8.3-A architecture
extensions.
The value armv8.2-a implies armv8.1-a and enables
compiler support for the ARMv8.2-A architecture
extensions.
The value armv8.1-a implies armv8-a and enables compiler
support for the ARMv8.1-A architecture extension. In
particular, it enables the +crc and +lse features.
The value native is available on native AArch64
GNU/Linux and causes the compiler to pick the
architecture of the host system. This option has no
effect if the compiler is unable to recognize the
architecture of the host system,
The permissible values for feature are listed in the
sub-section on aarch64-feature-modifiers,,-march and
-mcpu Feature Modifiers. Where conflicting feature
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modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it
can emit when generating assembly code. If -march is
specified without either of -mtune or -mcpu also being
specified, the code is tuned to perform well across a
range of target processors implementing the target
architecture.
-mtune=name
Specify the name of the target processor for which GCC
should tune the performance of the code. Permissible
values for this option are: generic, cortex-a35,
cortex-a53, cortex-a57, cortex-a72, cortex-a73,
exynos-m1, falkor, qdf24xx, xgene1, vulcan, thunderx,
thunderxt88, thunderxt88p1, thunderxt81, thunderxt83,
thunderx2t99, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 specify
that GCC should tune for a big.LITTLE system.
Additionally on native AArch64 GNU/Linux systems the
value native tunes performance to the host system. This
option has no effect if the compiler is unable to
recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified,
the code is tuned to perform well across a range of
target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally
suffixed by one or more feature modifiers. This option
has the form -mcpu=cpu{+[no]feature}*, where the
permissible values for cpu are the same as those
available for -mtune. The permissible values for
feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature
Modifiers. Where conflicting feature modifiers are
specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it
can emit when generating assembly code (as if by -march)
and to determine the target processor for which to tune
for performance (as if by -mtune). Where this option is
used in conjunction with -march or -mtune, those options
take precedence over the appropriate part of this
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option.
-moverride=string
Override tuning decisions made by the back-end in
response to a -mtune= switch. The syntax, semantics,
and accepted values for string in this option are not
guaranteed to be consistent across releases.
This option is only intended to be useful when
developing GCC.
-mpc-relative-literal-loads
Enable PC-relative literal loads. With this option
literal pools are accessed using a single instruction
and emitted after each function. This limits the
maximum size of functions to 1MB. This is enabled by
default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address
signing will be applied. Permissible values are none,
which disables return address signing, non-leaf, which
enables pointer signing for functions which are not leaf
functions, and all, which enables pointer signing for
all functions. The default value is none.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of
the following and their inverses nofeature:
crc Enable CRC extension. This is on by default for
-march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced
SIMD and floating-point instructions.
fp Enable floating-point instructions. This is on by
default for all possible values for options -march and
-mcpu.
simd
Enable Advanced SIMD instructions. This also enables
floating-point instructions. This is on by default for
all possible values for options -march and -mcpu.
lse Enable Large System Extension instructions. This is on
by default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point
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instructions.
Feature crypto implies simd, which implies fp. Conversely,
nofp implies nosimd, which implies nocrypto.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63".
That allows code to run on hardware variants that lack
these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short
instruction generation. This can result in increased
instruction count, so this may either reduce or increase
overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple"
instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across
releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an
"fsub" instruction and test the flags. This is faster
than a software comparison, but can get incorrect
results in the presence of NaNs, or when two different
small numbers are compared such that their difference is
calculated as zero. The default is -msoft-cmpsf, which
uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the
stack pointer. E.g., a value of 8 means that the eight
bytes in the range "sp+0...sp+7" can be used by leaf
functions without stack allocation. Values other than 8
or 16 are untested and unlikely to work. Note also that
this option changes the ABI; compiling a program with a
different stack offset than the libraries have been
compiled with generally does not work. This option can
be useful if you want to evaluate if a different stack
offset would give you better code, but to actually use a
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different stack offset to build working programs, it is
recommended to configure the toolchain with the
appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has
been set to truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all
calls might be beyond the offset range of the "b" / "bl"
instructions, and therefore load the function address
into a register before performing a (otherwise direct)
call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all
direct calls are in the range of the "b" / "bl"
instructions, so use these instructions for direct
calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned
values. This does not apply to function addresses for
which -mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit.
This determines the floating-point mode that is provided
and expected at function call and return time. Making
this mode match the mode you predominantly need at
function start can make your programs smaller and faster
by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or
restored when the function returns, and when it
calls other functions. This mode is useful for
compiling libraries or other compilation units you
might want to incorporate into different programs
with different prevailing FPU modes, and the
convenience of being able to use a single object
file outweighs the size and speed overhead for any
extra mode switching that might be needed, compared
with what would be needed with a more specific
choice of prevailing FPU mode.
truncate
This is the mode used for floating-point
calculations with truncating (i.e. round towards
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zero) rounding mode. That includes conversion from
floating point to integer.
round-nearest
This is the mode used for floating-point
calculations with round-to-nearest-or-even rounding
mode.
int This is the mode used to perform integer
calculations in the FPU, e.g. integer multiply, or
integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively,
splitting of 32-bit loads, generation of post-increment
addresses, and generation of post-modify addresses. The
defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default
is -mvect-double, which uses DImode as preferred SIMD
mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num
may be 4 or 8. The default is 8. Note that this is an
ABI change, even though many library function interfaces
are unaffected if they don't use SIMD vector modes in
places that affect size and/or alignment of relevant
types.
-msplit-vecmove-early
Split vector moves into single word moves before reload.
In theory this can give better register allocation, but
so far the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes
loading small negative constants and certain bitmasks
faster. Allowable values for reg are r43 and r63, which
specify use of that register as a fixed register, and
none, which means that no register is used for this
purpose. The default is -m1reg-none.
ARC Options
The following options control the architecture variant for
which code is being compiled:
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-mbarrel-shifter
Generate instructions supported by barrel shifter. This
is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in
effect.
-mcpu=cpu
Set architecture type, register usage, and instruction
scheduling parameters for cpu. There are also shortcut
alias options available for backward compatibility and
convenience. Supported values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This
is the default when configured with
--with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-
precision floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision
floating-point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions
except the atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
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hs38_linux
Compile for ARC HS38 CPU with all hardware
extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions
enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit
multiply instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and
"mul64"-family instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions
enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit
multiply instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and
"mul64"-family instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for
the compact implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for
the fast implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX
extension aux registers.
-mea
Generate extended arithmetic instructions. Currently
only "divaw", "adds", "subs", and "sat16" are supported.
This is always enabled for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700.
This option is deprecated.
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-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate
instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid
for -mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if
-mcpu=ARC700 is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for
the compact implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for
the fast implementation.
-msimd
Enable generation of ARC SIMD instructions via target-
specific builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility
purposes only. Software floating-point code is emitted
by default, and this default can overridden by FPX
options; -mspfp, -mspfp-compact, or -mspfp-fast for
single precision, and -mdpfp, -mdpfp-compact, or
-mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional
extension to implement atomic memory built-in functions.
Not available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option
is on by default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
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Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You
can specify the option using either a string or numeric
value for multo. wlh1 is the default value. The
recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following
instructions are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The
following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The
following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The
following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The
following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The
following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
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ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware
extensions for ARCv2 cores. Supported values for fpu
are:
fpus
Enables support for single-precision floating-point
hardware extensions.
fpud
Enables support for double-precision floating-point
hardware extensions. The single-precision
floating-point extension is also enabled. Not
available for ARC EM.
fpuda
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point
extension is also enabled. This option is only
available for ARC EM.
fpuda_div
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point,
square-root, and divide extensions are also enabled.
This option is only available for ARC EM.
fpuda_fma
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point
and fused multiply and add hardware extensions are
also enabled. This option is only available for ARC
EM.
fpuda_all
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. All single-precision floating-point
hardware extensions are also enabled. This option
is only available for ARC EM.
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fpus_div
Enables support for single-precision floating-point,
square-root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point,
square-root and divide hardware extensions. This
option includes option fpus_div. Not available for
ARC EM.
fpus_fma
Enables support for single-precision floating-point
and fused multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point
and fused multiply and add hardware extensions.
This option includes option fpus_fma. Not available
for ARC EM.
fpus_all
Enables support for all single-precision floating-
point hardware extensions.
fpud_all
Enables support for all single- and double-precision
floating-point hardware extensions. Not available
for ARC EM.
The following options are passed through to the assembler,
and also define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A
extensions. Also sets the preprocessor symbol
"__Xdsp_packa". This option is deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi
butterfly extension. Also sets the preprocessor symbol
"__Xdvbf". This option is deprecated.
-mlock
Passed down to the assembler to enable the locked
load/store conditional extension. Also sets the
preprocessor symbol "__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the
preprocessor symbol "__Xxmac_d16". This option is
deprecated.
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-mmac-24
Passed down to the assembler. Also sets the
preprocessor symbol "__Xxmac_24". This option is
deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-
stamp counter extension instruction. Also sets the
preprocessor symbol "__Xrtsc". This option is
deprecated.
-mswape
Passed down to the assembler to enable the swap byte
ordering extension instruction. Also sets the
preprocessor symbol "__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and
single-operand instructions for telephony. Also sets
the preprocessor symbol "__Xtelephony". This option is
deprecated.
-mxy
Passed down to the assembler to enable the XY memory
extension. Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is
annotated:
-misize
Annotate assembler instructions with estimated
addresses.
-mannotate-align
Explain what alignment considerations lead to the
decision to make an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the
"arclinux" emulation. This option is enabled by default
in tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is not
requested.
-marclinux_prof
Passed through to the linker, to specify use of the
"arclinux_prof" emulation. This option is enabled by
default in tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is
requested.
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The following options control the semantics of generated
code:
-mlong-calls
Generate calls as register indirect calls, thus
providing access to the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls,
which is the offset available for an unconditional
branch-and-link instruction. Conditional execution of
function calls is suppressed, to allow use of the 25-bit
range, rather than the 21-bit range with conditional
branch-and-link. This is the default for tool chains
built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
-mno-sdata
Do not generate sdata references. This is the default
for tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile
references. This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register
displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg
to generate compare-and-branch ("brcc") instructions. It
has no effect on generation of these instructions driven
by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table
shortening. This is the default for -Os.
-mcompact-casesi
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Enable compact "casesi" pattern. This is the default
for -Os, and only available for ARCv1 cores.
-mno-cond-exec
Disable the ARCompact-specific pass to generate
conditional execution instructions.
Due to delay slot scheduling and interactions between
operand numbers, literal sizes, instruction lengths, and
the support for conditional execution, the target-
independent pass to generate conditional execution is
often lacking, so the ARC port has kept a special pass
around that tries to find more conditional execution
generation opportunities after register allocation,
branch shortening, and delay slot scheduling have been
done. This pass generally, but not always, improves
performance and code size, at the cost of extra
compilation time, which is why there is an option to
switch it off. If you have a problem with call
instructions exceeding their allowable offset range
because they are conditionalized, you should consider
using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into
"add.f", "adc" etc.
-mindexed-loads
Enable the use of indexed loads. This can be
problematic because some optimizers then assume that
indexed stores exist, which is not the case.
Enable Local Register Allocation. This is still
experimental for ARC, so by default the compiler uses
standard reload (i.e. -mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mno-millicode
When optimizing for size (using -Os), prologues and
epilogues that have to save or restore a large number of
registers are often shortened by using call to a special
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function in libgcc; this is referred to as a millicode
call. As these calls can pose performance issues,
and/or cause linking issues when linking in a
nonstandard way, this option is provided to turn off
millicode call generation.
-mmixed-code
Tweak register allocation to help 16-bit instruction
generation. This generally has the effect of decreasing
the average instruction size while increasing the
instruction count.
-mq-class
Enable q instruction alternatives. This is the default
for -Os.
-mRcq
Enable Rcq constraint handling. Most short code
generation depends on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec
mostly depends on this. This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction
lengths and alignment. The recognized values for level
are:
0 No size optimization. This level is deprecated and
treated like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after
barriers are dropped.
3 In addition, optional data alignment is dropped, and
the option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise,
the behavior when this is not set is equivalent to level
1.
-mtune=cpu
Set instruction scheduling parameters for cpu,
overriding any implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
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ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being
equal to a normal instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When
tuning for ARC700 and optimizing for speed, branches
without filled delay slot are preferably emitted
unaligned and long, unless profiling indicates that the
probability for the branch to be taken is below
probability. The default is (REG_BR_PROB_BASE/2), i.e.
5000.
The following options are maintained for backward
compatibility, but are now deprecated and will be removed in
a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these
options is now deprecated. Big-endian code is supported
by configuring GCC to build "arceb-elf32" and
"arceb-linux-uclibc" targets, for which big endian is
the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these
options is now deprecated. Little-endian code is
supported by configuring GCC to build "arc-elf32" and
"arc-linux-uclibc" targets, for which little endian is
the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
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-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu
are replaced by ARC600, ARC601, ARC700 and ARC700-xmac
respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values
are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM
Procedure Call Standard for all functions, even if this
is not strictly necessary for correct execution of the
code. Specifying -fomit-frame-pointer with this option
causes the stack frames not to be generated for leaf
functions. The default is -mno-apcs-frame. This option
is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
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-mthumb-interwork
Generate code that supports calling between the ARM and
Thumb instruction sets. Without this option, on pre-v5
architectures, the two instruction sets cannot be
reliably used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is
generated when -mthumb-interwork is specified. In AAPCS
configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function
prologue, or the merging of those instruction with the
instructions in the function's body. This means that
all functions start with a recognizable set of
instructions (or in fact one of a choice from a small
set of different function prologues), and this
information can be used to locate the start of functions
inside an executable piece of code. The default is
-msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible
values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing
library calls for floating-point operations. softfp
allows the generation of code using hardware floating-
point instructions, but still uses the soft-float
calling conventions. hard allows generation of
floating-point instructions and uses FPU-specific
calling conventions.
The default depends on the specific target
configuration. Note that the hard-float and soft-float
ABIs are not link-compatible; you must compile your
entire program with the same ABI, and link with a
compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian
mode. This is the default for all standard
configurations.
-mbig-endian
Generate code for a processor running in big-endian
mode; the default is to compile code for a little-endian
processor.
-march=name
This specifies the name of the target ARM architecture.
GCC uses this name to determine what kind of
instructions it can emit when generating assembly code.
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This option can be used in conjunction with or instead
of the -mcpu= option. Permissible names are: armv2,
armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5e,
armv5t, armv5te, armv6, armv6-m, armv6j, armv6k,
armv6kz, armv6s-m, armv6t2, armv6z, armv6zk, armv7,
armv7-a, armv7-m, armv7-r, armv7e-m, armv7ve, armv8-a,
armv8-a+crc, armv8.1-a, armv8.1-a+crc, armv8-m.base,
armv8-m.main, armv8-m.main+dsp, iwmmxt, iwmmxt2.
Architecture revisions older than armv4t are deprecated.
-march=armv6s-m is the armv6-m architecture with support
for the (now mandatory) SVC instruction.
-march=armv6zk is an alias for armv6kz, existing for
backwards compatibility.
-march=armv7ve is the armv7-a architecture with
virtualization extensions.
-march=armv8-a+crc enables code generation for the
ARMv8-A architecture together with the optional CRC32
extensions.
-march=armv8.1-a enables compiler support for the
ARMv8.1-A architecture. This also enables the features
provided by -march=armv8-a+crc.
-march=armv8.2-a enables compiler support for the
ARMv8.2-A architecture. This also enables the features
provided by -march=armv8.1-a.
-march=armv8.2-a+fp16 enables compiler support for the
ARMv8.2-A architecture with the optional FP16
instructions extension. This also enables the features
provided by -march=armv8.1-a and implies
-mfp16-format=ieee.
-march=native causes the compiler to auto-detect the
architecture of the build computer. At present, this
feature is only supported on GNU/Linux, and not all
architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM
processor for which GCC should tune the performance of
the code. For some ARM implementations better
performance can be obtained by using this option.
Permissible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm,
arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
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arm710c, arm7100, arm720, arm7500, arm7500fe, arm7tdmi,
arm7tdmi-s, arm710t, arm720t, arm740t, strongarm,
strongarm110, strongarm1100, strongarm1110, arm8,
arm810, arm9, arm9e, arm920, arm920t, arm922t,
arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e,
arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore,
mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7,
cortex-a8, cortex-a9, cortex-a12, cortex-a15,
cortex-a17, cortex-a32, cortex-a35, cortex-a53,
cortex-a57, cortex-a72, cortex-a73, cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-m33,
cortex-m23, cortex-m7, cortex-m4, cortex-m3, cortex-m1,
cortex-m0, cortex-m0plus, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, marvell-pj4, xscale, iwmmxt, iwmmxt2, ep9312,
fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should
tune the performance of the code for a big.LITTLE
system. Permissible names are: cortex-a15.cortex-a7,
cortex-a17.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a72.cortex-a35,
cortex-a73.cortex-a53.
-mtune=generic-arch specifies that GCC should tune the
performance for a blend of processors within
architecture arch. The aim is to generate code that run
well on the current most popular processors, balancing
between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs.
The effects of this option may change in future GCC
versions as CPU models come and go.
-mtune=native causes the compiler to auto-detect the CPU
of the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the
option has no effect.
-mcpu=name
This specifies the name of the target ARM processor.
GCC uses this name to derive the name of the target ARM
architecture (as if specified by -march) and the ARM
processor type for which to tune for performance (as if
specified by -mtune). Where this option is used in
conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Permissible names for this option are the same as those
for -mtune.
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-mcpu=generic-arch is also permissible, and is
equivalent to -march=arch -mtune=generic-arch. See
-mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU
of the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the
option has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware
emulation) is available on the target. Permissible
names are: vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16,
vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3,
neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for
neon-vfpv3 and vfp is an alias for vfpv2.
If -msoft-float is specified this specifies the format
of floating-point values.
If the selected floating-point hardware includes the
NEON extension (e.g. -mfpu=neon), note that floating-
point operations are not generated by GCC's auto-
vectorization pass unless -funsafe-math-optimizations is
also specified. This is because NEON hardware does not
fully implement the IEEE 754 standard for floating-point
arithmetic (in particular denormal values are treated as
zero), so the use of NEON instructions may lead to a
loss of precision.
You can also set the fpu name at function level by using
the "target("fpu=")" function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision
floating-point type. Permissible names are none, ieee,
and alternative; the default is none, in which case the
"__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to
a multiple of the number of bits set by this option.
Permissible values are 8, 32 and 64. The default value
varies for different toolchains. For the COFF targeted
toolchain the default value is 8. A value of 64 is only
allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more
efficient code, but can also increase the size of the
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program. Different values are potentially incompatible.
Code compiled with one value cannot necessarily expect
to work with code or libraries compiled with another
value, if they exchange information using structures or
unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a
"noreturn" function. It is executed if the function
tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first
loading the address of the function into a register and
then performing a subroutine call on this register.
This switch is needed if the target function lies
outside of the 64-megabyte addressing range of the
offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls
are turned into long calls. The heuristic is that
static functions, functions that have the "short_call"
attribute, functions that are inside the scope of a
"#pragma no_long_calls" directive, and functions whose
definitions have already been compiled within the
current compilation unit are not turned into long calls.
The exceptions to this rule are that weak function
definitions, functions with the "long_call" attribute or
the "section" attribute, and functions that are within
the scope of a "#pragma long_calls" directive are always
turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior, as does
placing the function calls within the scope of a
"#pragma long_calls_off" directive. Note these switches
have no effect on how the compiler generates code to
handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each
function. The runtime system is responsible for
initializing this register with an appropriate value
before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For
standard PIC base case, the default is any suitable
register determined by compiler. For single PIC base
case, the default is R9 if target is EABI based or
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stack-checking is enabled, otherwise the default is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data
segments is fixed at static link time. This permits
using PC-relative addressing operations to access data
known to be in the data segment. For non-VxWorks RTP
targets, this option is enabled by default. When
disabled on such targets, it will enable
-msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section,
directly preceding the function prologue. The generated
code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the
value of "pc" stored at "fp + 0". If the trace function
then looks at location "pc - 12" and the top 8 bits are
set, then we know that there is a function name embedded
immediately preceding this location and has length
"((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and
Thumb states. The default for most configurations is to
generate code that executes in ARM state, but the
default can be changed by configuring GCC with the
--with-mode=state configure option.
You can also override the ARM and Thumb mode for each
function by using the "target("thumb")" and
"target("arm")" function attributes or pragmas.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all non-leaf functions. (A
leaf function is one that does not call any other
functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
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Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all leaf functions. (A leaf
function is one that does not call any other functions.)
The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being
compiled an ARM instruction set header which switches to
Thumb mode before executing the rest of the function.
This allows these functions to be called from non-
interworking code. This option is not valid in AAPCS
configurations because interworking is enabled by
default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual
functions) to execute correctly regardless of whether
the target code has been compiled for interworking or
not. There is a small overhead in the cost of executing
a function pointer if this option is enabled. This
option is not valid in AAPCS configurations because
interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage
pointer. The valid models are soft, which generates
calls to "__aeabi_read_tp", cp15, which fetches the
thread pointer from "cp15" directly (supported in the
arm6k architecture), and auto, which uses the best
available method for the selected processor. The
default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local
storage. Two dialects are supported---gnu and gnu2.
The gnu dialect selects the original GNU scheme for
supporting local and global dynamic TLS models. The
gnu2 dialect selects the GNU descriptor scheme, which
provides better performance for shared libraries. The
GNU descriptor scheme is compatible with the original
scheme, but does require new assembler, linker and
library support. Initial and local exec TLS models are
unaffected by this option and always use the original
scheme.
-mword-relocations
Only generate absolute relocations on word-sized values
(i.e. R_ARM_ABS32). This is enabled by default on
targets (uClinux, SymbianOS) where the runtime loader
imposes this restriction, and when -fpic or -fPIC is
specified.
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-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when
"ldrd" instructions with overlapping destination and
base registers are used. This option avoids generating
these instructions. This option is enabled by default
when -mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32-
bit values from addresses that are not 16- or 32- bit
aligned. By default unaligned access is disabled for
all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline
architectures, and enabled for all other architectures.
If unaligned access is not enabled then words in packed
data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in
the generated object file to either true or false,
depending upon the setting of this option. If unaligned
access is enabled then the preprocessor symbol
"__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
Enables using Neon to handle scalar 64-bits operations.
This is disabled by default since the cost of moving
data from core registers to Neon is high.
-mslow-flash-data
Assume loading data from flash is slower than fetching
instruction. Therefore literal load is minimized for
better performance. This option is only supported when
compiling for ARMv7 M-profile and off by default.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax.
The default is currently off which implies divided
syntax. This option has no impact on Thumb2. However,
this may change in future releases of GCC. Divided
syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the
rules of ARMv8. IT blocks can only contain a single
16-bit instruction from a select set of instructions.
This option is on by default for ARMv8 Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler
file. This is an option used only for regression
testing of the compiler and not intended for ordinary
use in compiling code. This option is disabled by
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default.
-mpure-code
Do not allow constant data to be placed in code
sections. Additionally, when compiling for ELF object
format give all text sections the ELF processor-specific
section attribute "SHF_ARM_PURECODE". This option is
only available when generating non-pic code for ARMv7-M
targets.
-mcmse
Generate secure code as per the "ARMv8-M Security
Extensions: Requirements on Development Tools
Engineering Specification", which can be found on
<http://infocenter.arm.com/help/topic/com.arm.doc.ecm0359818/ECM0359818_armv8m_security_extensions_reqs_on_dev_tools_1_0.pdf>.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or
MCU type.
The default for this option is@tie{}avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program
memory. mcu@tie{}= "attiny22", "attiny26",
"at90c8534", "at90s2313", "at90s2323", "at90s2333",
"at90s2343", "at90s4414", "at90s4433", "at90s4434",
"at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program
memory and with the "MOVW" instruction. mcu@tie{}=
"ata5272", "ata6616c", "attiny13", "attiny13a",
"attiny2313", "attiny2313a", "attiny24",
"attiny24a", "attiny25", "attiny261", "attiny261a",
"attiny43u", "attiny4313", "attiny44", "attiny44a",
"attiny441", "attiny45", "attiny461", "attiny461a",
"attiny48", "attiny828", "attiny84", "attiny84a",
"attiny841", "attiny85", "attiny861", "attiny861a",
"attiny87", "attiny88", "at86rf401".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB
of program memory. mcu@tie{}= "at43usb355",
"at76c711".
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"avr31"
"Classic" devices with 128@tie{}KiB of program
memory. mcu@tie{}= "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB
of program memory and with the "MOVW" instruction.
mcu@tie{}= "ata5505", "ata6617c", "ata664251",
"atmega16u2", "atmega32u2", "atmega8u2",
"attiny1634", "attiny167", "at90usb162",
"at90usb82".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program
memory. mcu@tie{}= "ata6285", "ata6286", "ata6289",
"ata6612c", "atmega48", "atmega48a", "atmega48p",
"atmega48pa", "atmega48pb", "atmega8", "atmega8a",
"atmega8hva", "atmega8515", "atmega8535",
"atmega88", "atmega88a", "atmega88p", "atmega88pa",
"atmega88pb", "at90pwm1", "at90pwm2", "at90pwm2b",
"at90pwm3", "at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16@tie{}KiB up to
64@tie{}KiB of program memory. mcu@tie{}=
"ata5702m322", "ata5782", "ata5790", "ata5790n",
"ata5791", "ata5795", "ata5831", "ata6613c",
"ata6614q", "ata8210", "ata8510", "atmega16",
"atmega16a", "atmega16hva", "atmega16hva2",
"atmega16hvb", "atmega16hvbrevb", "atmega16m1",
"atmega16u4", "atmega161", "atmega162", "atmega163",
"atmega164a", "atmega164p", "atmega164pa",
"atmega165", "atmega165a", "atmega165p",
"atmega165pa", "atmega168", "atmega168a",
"atmega168p", "atmega168pa", "atmega168pb",
"atmega169", "atmega169a", "atmega169p",
"atmega169pa", "atmega32", "atmega32a",
"atmega32c1", "atmega32hvb", "atmega32hvbrevb",
"atmega32m1", "atmega32u4", "atmega32u6",
"atmega323", "atmega324a", "atmega324p",
"atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega325pa", "atmega3250",
"atmega3250a", "atmega3250p", "atmega3250pa",
"atmega328", "atmega328p", "atmega328pb",
"atmega329", "atmega329a", "atmega329p",
"atmega329pa", "atmega3290", "atmega3290a",
"atmega3290p", "atmega3290pa", "atmega406",
"atmega64", "atmega64a", "atmega64c1",
"atmega64hve", "atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644",
"atmega644a", "atmega644p", "atmega644pa",
"atmega644rfr2", "atmega645", "atmega645a",
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"atmega645p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega649", "atmega649a",
"atmega649p", "atmega6490", "atmega6490a",
"atmega6490p", "at90can32", "at90can64",
"at90pwm161", "at90pwm216", "at90pwm316",
"at90scr100", "at90usb646", "at90usb647", "at94k",
"m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program
memory. mcu@tie{}= "atmega128", "atmega128a",
"atmega128rfa1", "atmega128rfr2", "atmega1280",
"atmega1281", "atmega1284", "atmega1284p",
"atmega1284rfr2", "at90can128", "at90usb1286",
"at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more
than 128@tie{}KiB of program memory. mcu@tie{}=
"atmega256rfr2", "atmega2560", "atmega2561",
"atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to
64@tie{}KiB of program memory. mcu@tie{}=
"atxmega16a4", "atxmega16a4u", "atxmega16c4",
"atxmega16d4", "atxmega16e5", "atxmega32a4",
"atxmega32a4u", "atxmega32c3", "atxmega32c4",
"atxmega32d3", "atxmega32d4", "atxmega32e5",
"atxmega8e5".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory. mcu@tie{}=
"atxmega64a3", "atxmega64a3u", "atxmega64a4u",
"atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory and more than
64@tie{}KiB of RAM. mcu@tie{}= "atxmega64a1",
"atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of
program memory. mcu@tie{}= "atxmega128a3",
"atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4",
"atxmega192a3", "atxmega192a3u", "atxmega192c3",
"atxmega192d3", "atxmega256a3", "atxmega256a3b",
"atxmega256a3bu", "atxmega256a3u", "atxmega256c3",
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"atxmega256d3", "atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128@tie{}KiB of
program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega128a1", "atxmega128a1u",
"atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512@tie{}B up to
4@tie{}KiB of program memory. mcu@tie{}=
"attiny10", "attiny20", "attiny4", "attiny40",
"attiny5", "attiny9".
"avr1"
This ISA is implemented by the minimal AVR core and
supported for assembler only. mcu@tie{}=
"attiny11", "attiny12", "attiny15", "attiny28",
"at90s1200".
-mabsdata
Assume that all data in static storage can be accessed
by LDS / STS instructions. This option has only an
effect on reduced Tiny devices like ATtiny40. See also
the "absdata" AVR Variable Attributes,variable
attribute.
-maccumulate-args
Accumulate outgoing function arguments and
acquire/release the needed stack space for outgoing
function arguments once in function prologue/epilogue.
Without this option, outgoing arguments are pushed
before calling a function and popped afterwards.
Popping the arguments after the function call can be
expensive on AVR so that accumulating the stack space
might lead to smaller executables because arguments need
not be removed from the stack after such a function
call.
This option can lead to reduced code size for functions
that perform several calls to functions that get their
arguments on the stack like calls to printf-like
functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions
to cost. Reasonable values for cost are small, non-
negative integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to
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appropriate subroutines. Code size is smaller.
-mint8
Assume "int" to be 8-bit integer. This affects the
sizes of all types: a "char" is 1 byte, an "int" is 1
byte, a "long" is 2 bytes, and "long long" is 4 bytes.
Please note that this option does not conform to the C
standards, but it results in smaller code size.
-mn-flash=num
Assume that the flash memory has a size of num times
64@tie{}KiB.
-mno-interrupts
Generated code is not compatible with hardware
interrupts. Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the
shorter "RCALL" resp. "RJMP" instruction if applicable.
Setting -mrelax just adds the --mlink-relax option to
the assembler's command line and the --relax option to
the linker's command line.
Jump relaxing is performed by the linker because jump
offsets are not known before code is located. Therefore,
the assembler code generated by the compiler is the
same, but the instructions in the executable may differ
from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed,
see the section on "EIND" and linker stubs below.
-mrmw
Assume that the device supports the Read-Modify-Write
instructions "XCH", "LAC", "LAS" and "LAT".
-msp8
Treat the stack pointer register as an 8-bit register,
i.e. assume the high byte of the stack pointer is zero.
In general, you don't need to set this option by hand.
This option is used internally by the compiler to select
and build multilibs for architectures "avr2" and
"avr25". These architectures mix devices with and
without "SPH". For any setting other than -mmcu=avr2 or
-mmcu=avr25 the compiler driver adds or removes this
option from the compiler proper's command line, because
the compiler then knows if the device or architecture
has an 8-bit stack pointer and thus no "SPH" register or
not.
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-mstrict-X
Use address register "X" in a way proposed by the
hardware. This means that "X" is only used in indirect,
post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the
same way as "Y" or "Z" which then is emulated by
additional instructions. For example, loading a value
with "X+const" addressing with a small non-negative
"const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8@tie{}bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero
for fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library
"lib<mcu>.a".
-Waddr-space-convert
Warn about conversions between address spaces in the
case where the resulting address space is not contained
in the incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector
prefix. Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The
address of a function or label is represented as word
address so that indirect jumps and calls can target any code
address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more
than 128@tie{}Ki bytes of program memory space, there is a
special function register called "EIND" that serves as most
significant part of the target address when "EICALL" or
"EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as
follows by the compiler and are subject to some limitations:
* The compiler never sets "EIND".
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* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to
emulate an indirect call/jump by means of a "RET"
instruction.
* The compiler assumes that "EIND" never changes during
the startup code or during the application. In
particular, "EIND" is not saved/restored in function or
interrupt service routine prologue/epilogue.
* For indirect calls to functions and computed goto, the
linker generates stubs. Stubs are jump pads sometimes
also called trampolines. Thus, the indirect call/jump
jumps to such a stub. The stub contains a direct jump
to the desired address.
* Linker relaxation must be turned on so that the linker
generates the stubs correctly in all situations. See the
compiler option -mrelax and the linker option --relax.
There are corner cases where the linker is supposed to
generate stubs but aborts without relaxation and without
a helpful error message.
* The default linker script is arranged for code with
"EIND = 0". If code is supposed to work for a setup
with "EIND != 0", a custom linker script has to be used
in order to place the sections whose name start with
".trampolines" into the segment where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice
that startup code is a blend of code from libgcc and
AVR-LibC. For the impact of AVR-LibC on "EIND", see the
AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set
up "EIND" early, for example by means of initialization
code located in section ".init3". Such code runs prior
to general startup code that initializes RAM and calls
constructors, but after the bit of startup code from
AVR-LibC that sets "EIND" to the segment where the
vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
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The "__trampolines_start" symbol is defined in the
linker script.
* Stubs are generated automatically by the linker if the
following two conditions are met:
modifier>
-<The address of a label is taken by means of the "gs"
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels
in the following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-
save function is used, see -mcall-prologues>
command-line option.
dispatch>
-<Switch/case dispatch tables. If you do not want such
tables you can specify the -fno-jump-tables
command-line option.
startup/shutdown.>
-<C and C++ constructors/destructors called during
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not
supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has
to be called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
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and the application be linked with
-Wl,--defsym,func_4=0x4. Alternatively, "func_4" can be
defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ"
Special Function Registers
Some AVR devices support memories larger than the
64@tie{}KiB range that can be accessed with 16-bit pointers.
To access memory locations outside this 64@tie{}KiB range,
the content of a "RAMP" register is used as high part of the
address: The "X", "Y", "Z" address register is concatenated
with the "RAMPX", "RAMPY", "RAMPZ" special function
register, respectively, to get a wide address. Similarly,
"RAMPD" is used together with direct addressing.
* The startup code initializes the "RAMP" special function
registers with zero.
* If a AVR Named Address Spaces,named address space other
than generic or "__flash" is used, then "RAMPZ" is set
as needed before the operation.
* If the device supports RAM larger than 64@tie{}KiB and
the compiler needs to change "RAMPZ" to accomplish an
operation, "RAMPZ" is reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the
ISR prologue/epilogue saves/restores that SFR and
initializes it with zero in case the ISR code might
(implicitly) use it.
* RAM larger than 64@tie{}KiB is not supported by GCC for
AVR targets. If you use inline assembler to read from
locations outside the 16-bit address range and change
one of the "RAMP" registers, you must reset it to zero
after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code
can test for the presence or absence of features. Almost
any of the following built-in macros are deduced from device
capabilities and thus triggered by the -mmcu= command-line
option.
For even more AVR-specific built-in macros see AVR Named
Address Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that
identifies the architecture and depends on the -mmcu=mcu
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option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35",
"avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively. If
mcu specifies a device, this built-in macro is set
accordingly. For example, with -mmcu=atmega8 the macro
is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which
reflects the device's name. For example, -mmcu=atmega8
defines the built-in macro "__AVR_ATmega8__",
-mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme
"__AVR_Device__" where Device is the device name as from
the AVR user manual. The difference between Device in
the built-in macro and device in -mmcu=device is that
the latter is always lowercase.
If device is not a device but only a core architecture
like avr51, this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the
device's name. For example, with -mmcu=atmega8 the macro
is defined to "atmega8".
If device is not a device but only a core architecture
like avr51, this macro is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of
devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+"
instructions.
"__AVR_HAVE_MOVW__"
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The device has the "MOVW" instruction to perform 16-bit
register-register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+"
instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This
is the case for devices with at least 16@tie{}KiB of
program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions.
This is the case for devices with more than 128@tie{}KiB
of program memory. This also means that the program
counter (PC) is 3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is
the case for devices with up to 128@tie{}KiB of program
memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit
respectively 16-bit register by the compiler. The
definition of these macros is affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer)
special function register or has an 8-bit stack pointer,
respectively. The definition of these macros is
affected by -mmcu= and in the cases of -mmcu=avr2 and
-mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ"
special function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line
option.
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"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip
32-bit instructions because of a hardware erratum. Skip
instructions are "SBRS", "SBRC", "SBIS", "SBIC" and
"CPSE". The second macro is only defined if
"__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC,
LAS and LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function
registers directly like "IN", "OUT", "SBI", etc. may use
a different address as if addressed by an instruction to
access RAM like "LD" or "STS". This offset depends on
the device architecture and has to be subtracted from
the RAM address in order to get the respective
I/O@tie{}address.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with
AVR-Libc. See the --with-avrlibc configure option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor.
Currently, cpu can be one of bf512, bf514, bf516, bf518,
bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532,
bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544,
bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m,
bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision
of the target Blackfin processor. Any workarounds
available for the targeted silicon revision are enabled.
If sirevision is none, no workarounds are enabled. If
sirevision is any, all workarounds for the targeted
processor are enabled. The "__SILICON_REVISION__" macro
is defined to two hexadecimal digits representing the
major and minor numbers in the silicon revision. If
sirevision is none, the "__SILICON_REVISION__" is not
defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this
optional sirevision is not used, GCC assumes the latest
known silicon revision of the targeted Blackfin
processor.
GCC defines a preprocessor macro for the specified cpu.
For the bfin-elf toolchain, this option causes the
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hardware BSP provided by libgloss to be linked in if
-msim is not given.
Without this option, bf532 is used as the processor by
default.
Note that support for bf561 is incomplete. For bf561,
only the preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator.
This causes the simulator BSP provided by libgloss to be
linked in. This option has effect only for bfin-elf
toolchain. Certain other options, such as
-mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up
and restore frame pointers and makes an extra register
available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all
functions, which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler ensures that the generated
code does not contain speculative loads after jump
instructions. If this option is used,
"__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads
from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated
code does not contain CSYNC or SSYNC instructions too
soon after conditional branches. If this option is
used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC
instructions from occurring too soon after a conditional
branch.
-mlow-64k
When enabled, the compiler is free to take advantage of
the knowledge that the entire program fits into the low
64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is
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the default.
-mstack-check-l1
Do stack checking using information placed into L1
scratchpad memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the
library ID method. This allows for execute in place and
shared libraries in an environment without virtual
memory management. This option implies -fPIC. With a
bfin-elf target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared
libraries are being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the
library ID method, but assumes that this library or
executable won't link against any other ID shared
libraries. That allows the compiler to use faster code
for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link
against any ID shared libraries. Slower code is
generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based
shared library being compiled. Specifying a value of 0
generates more compact code; specifying other values
forces the allocation of that number to the current
library but is no more space- or time-efficient than
omitting this option.
-msep-data
Generate code that allows the data segment to be located
in a different area of memory from the text segment.
This allows for execute in place in an environment
without virtual memory management by eliminating
relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows
the text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first
loading the address of the function into a register and
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then performing a subroutine call on this register.
This switch is needed if the target function lies
outside of the 24-bit addressing range of the offset-
based version of subroutine call instruction.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior. Note
these switches have no effect on how the compiler
generates code to handle function calls via function
pointers.
-mfast-fp
Link with the fast floating-point library. This library
relaxes some of the IEEE floating-point standard's rules
for checking inputs against Not-a-Number (NAN), in the
interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no
effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin
processors. This option causes proper start files and
link scripts supporting multicore to be used, and
defines the macro "__BFIN_MULTICORE". It can only be
used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which
selects the one-application-per-core programming model.
Without -mcorea or -mcoreb, the
single-application/dual-core programming model is used.
In this model, the main function of Core B should be
named as "coreb_main".
If this option is not used, the single-core application
programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when
using the one-application-per-core programming model.
Proper start files and link scripts are used to support
Core A, and the macro "__BFIN_COREA" is defined. This
option can only be used in conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when
using the one-application-per-core programming model.
Proper start files and link scripts are used to support
Core B, and the macro "__BFIN_COREB" is defined. When
this option is used, "coreb_main" should be used instead
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of "main". This option can only be used in conjunction
with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start
files and link scripts are used to put the application
into SDRAM, and the macro "__BFIN_SDRAM" is defined.
The loader should initialize SDRAM before loading the
application.
-micplb
Assume that ICPLBs are enabled at run time. This has an
effect on certain anomaly workarounds. For Linux
targets, the default is to assume ICPLBs are enabled;
for standalone applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC
uses this name to determine what kind of instructions it
can emit when generating assembly code. Permissible
names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the
default.
-msim
Choose startup files and linker script suitable for the
simulator.
-msdata=default
Put small global and static data in the ".neardata"
section, which is pointed to by register "B14". Put
small uninitialized global and static data in the ".bss"
section, which is adjacent to the ".neardata" section.
Put small read-only data into the ".rodata" section.
The corresponding sections used for large pieces of data
are ".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections
reserved for small data, and use addressing relative to
the "B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and
use absolute addresses to access all data. Put all
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initialized global and static data in the ".fardata"
section, and all uninitialized data in the ".far"
section. Put all constant data into the ".const"
section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The
choices for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX.
Default is v0 except for cris-axis-linux-gnu, where the
default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about
the generated code, except for the ABI and the set of
available instructions. The choices for architecture-
type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for
-march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions
for CPU models where it applies. This option is active
by default.
-mpdebug
Enable CRIS-specific verbose debug-related information
in the assembly code. This option also has the effect
of turning off the #NO_APP formatted-code indicator to
the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous
instruction; always emit compare and test instructions
before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing
modes other than post-increment.
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-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate
arrangements) for the stack frame, individual data and
constants to be aligned for the maximum single data
access size for the chosen CPU model. The default is to
arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options
above, these options arrange for stack frame, writable
data and constants to all be 32-bit, 16-bit or 8-bit
aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function
prologue and epilogue which set up the stack frame are
omitted and no return instructions or return sequences
are generated in the code. Use this option only
together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved
registers must be saved, or storage for local variables
needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate)
instruction sequences that load addresses for functions
from the PLT part of the GOT rather than (traditional on
other architectures) calls to the PLT. The default is
-mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-
elf and cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-
linux-gnu target.
-sim
This option, recognized for the cris-axis-elf, arranges
to link with input-output functions from a simulator
library. Code, initialized data and zero-initialized
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data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized
data at 0x40000000 and zero-initialized data at
0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions.
Disabled by default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+
architecture is default.
-msim
Links the library libsim.a which is in compatible with
simulator. Applicable to ELF compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates "sbit"/"cbit" instructions for bit
manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far
or medium. medium is default. However, far is not valid
with -mcr16c, as the CR16C architecture does not support
the far data model.
Darwin Options
These options are defined for all architectures running the
Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it
creates an object file for the single architecture that GCC
was built to target. Apple's GCC on Darwin does create
"fat" files if multiple -arch options are used; it does so
by running the compiler or linker multiple times and joining
the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or
i686) is determined by the flags that specify the ISA that
GCC is targeting, like -mcpu or -march. The
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-force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with
an ISA mismatch. The assembler, as, only permits
instructions to be used that are valid for the subtype of
the file it is generating, so you cannot put 64-bit
instructions in a ppc750 object file. The linker for shared
libraries, /usr/bin/libtool, fails and prints an error if
asked to create a shared library with a less restrictive
subtype than its input files (for instance, trying to put a
ppc970 object file in a ppc7400 library). The linker for
executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list
of directories to be searched for header files. These
directories are interleaved with those specified by -I
options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in
it. A framework is a directory with a Headers and/or
PrivateHeaders directory contained directly in it that
ends in .framework. The name of a framework is the name
of this directory excluding the .framework. Headers
associated with the framework are found in one of those
two directories, with Headers being searched first. A
subframework is a framework directory that is in a
framework's Frameworks directory. Includes of
subframework headers can only appear in a header of a
framework that contains the subframework, or in a
sibling subframework header. Two subframeworks are
siblings if they occur in the same framework. A
subframework should not have the same name as a
framework; a warning is issued if this is violated.
Currently a subframework cannot have subframeworks; in
the future, the mechanism may be extended to support
this. The standard frameworks can be found in
/System/Library/Frameworks and /Library/Frameworks. An
example include looks like "#include
<Framework/header.h>", where Framework denotes the name
of the framework and header.h is found in the
PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system
directory. The main difference between this -iframework
and -F is that with -iframework the compiler does not
warn about constructs contained within header files
found via dir. This option is valid only for the C
family of languages.
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-gused
Emit debugging information for symbols that are used.
For stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default
ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable
will run on is version. Typical values of version
include 10.1, 10.2, and 10.3.9.
If the compiler was built to use the system's headers by
default, then the default for this option is the system
version on which the compiler is running, otherwise the
default is to make choices that are compatible with as
many systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option
sets -static, -fno-common, -fno-use-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions, -fapple-kext,
-fno-weak and -fno-rtti where applicable. This mode
also sets -mno-altivec, -msoft-float, -fno-builtin and
-mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that
"sizeof(bool)==1". By default "sizeof(bool)" is 4 when
compiling for Darwin/PowerPC and 1 when compiling for
Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to
generate code that is not binary compatible with code
generated without that switch. Using this switch may
require recompiling all other modules in a program,
including system libraries. Use this switch to conform
to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development,
such as to allow GDB to dynamically load .o files into
already-running programs. -findirect-data and
-ffix-and-continue are provided for backwards
compatibility.
-all_load
Loads all members of static archive libraries. See man
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ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the
wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the
dynamic linker will bind all undefined references when
the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for
more information.
-bundle_loader executable
This option specifies the executable that will load the
build output file being linked. See man ld(1) for more
information.
-dynamiclib
When passed this option, GCC produces a dynamic library
instead of an executable when linking, using the Darwin
libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype,
instead of one controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
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-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The
Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha
implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point
instructions for floating-point operations. When
-msoft-float is specified, functions in libgcc.a are
used to perform floating-point operations. Unless they
are replaced by routines that emulate the floating-point
operations, or compiled in such a way as to call such
emulations routines, these routines issue floating-point
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operations. If you are compiling for an Alpha without
floating-point operations, you must ensure that the
library is built so as not to call them.
Note that Alpha implementations without floating-point
operations are required to have floating-point
registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-
point register set. -mno-fp-regs implies -msoft-float.
If the floating-point register set is not used,
floating-point operands are passed in integer registers
as if they were integers and floating-point results are
passed in $0 instead of $f0. This is a non-standard
calling sequence, so any function with a floating-point
argument or return value called by code compiled with
-mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that
does not use, and hence need not save and restore, any
floating-point registers.
-mieee
The Alpha architecture implements floating-point
hardware optimized for maximum performance. It is
mostly compliant with the IEEE floating-point standard.
However, for full compliance, software assistance is
required. This option generates code fully IEEE-
compliant code except that the inexact-flag is not
maintained (see below). If this option is turned on,
the preprocessor macro "_IEEE_FP" is defined during
compilation. The resulting code is less efficient but
is able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also
maintains the IEEE inexact-flag. Turning on this option
causes the generated code to implement fully-compliant
IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT"
is defined as a preprocessor macro. On some Alpha
implementations the resulting code may execute
significantly slower than the code generated by default.
Since there is very little code that depends on the
inexact-flag, you should normally not specify this
option. Other Alpha compilers call this option
-ieee_with_inexact.
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-mfp-trap-mode=trap-mode
This option controls what floating-point related traps
are enabled. Other Alpha compilers call this option
-fptm trap-mode. The trap mode can be set to one of
four values:
n This is the default (normal) setting. The only
traps that are enabled are the ones that cannot be
disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow
traps are enabled as well.
su Like u, but the instructions are marked to be safe
for software completion (see Alpha architecture
manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers
call this option -fprm rounding-mode. The rounding-mode
can be one of:
n Normal IEEE rounding mode. Floating-point numbers
are rounded towards the nearest machine number or
towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are
rounded towards zero.
d Dynamic rounding mode. A field in the floating-
point control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in
effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your
program modifies the fpcr, d corresponds to round
towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are
imprecise. This means without software assistance it is
impossible to recover from a floating trap and program
execution normally needs to be terminated. GCC can
generate code that can assist operating system trap
handlers in determining the exact location that caused a
floating-point trap. Depending on the requirements of
an application, different levels of precisions can be
selected:
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p Program precision. This option is the default and
means a trap handler can only identify which program
caused a floating-point exception.
f Function precision. The trap handler can determine
the function that caused a floating-point exception.
i Instruction precision. The trap handler can
determine the exact instruction that caused a
floating-point exception.
Other Alpha compilers provide the equivalent options
called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant.
You must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line
.eflag 48 in the function prologue of the generated
assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant
to see if it can construct it from smaller constants in
two or three instructions. If it cannot, it outputs the
constant as a literal and generates code to load it from
the data segment at run time.
Use this option to require GCC to construct all integer
constants using code, even if it takes more instructions
(the maximum is six).
You typically use this option to build a shared library
dynamic loader. Itself a shared library, it must
relocate itself in memory before it can find the
variables and constants in its own data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the
optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by the
CPU type specified via -mcpu= option or that of the CPU
on which GCC was built if none is specified.
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-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G
floating-point arithmetic instead of IEEE single and
double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate
symbol relocations except via assembler macros. Use of
these macros does not allow optimal instruction
scheduling. GNU binutils as of version 2.12 supports a
new syntax that allows the compiler to explicitly mark
which relocations should apply to which instructions.
This option is mostly useful for debugging, as GCC
detects the capabilities of the assembler when it is
built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is
accessed via gp-relative relocations. When -msmall-data
is used, objects 8 bytes long or smaller are placed in a
small data area (the ".sdata" and ".sbss" sections) and
are accessed via 16-bit relocations off of the $gp
register. This limits the size of the small data area
to 64KB, but allows the variables to be directly
accessed via a single instruction.
The default is -mlarge-data. With this option the data
area is limited to just below 2GB. Programs that
require more than 2GB of data must use "malloc" or
"mmap" to allocate the data in the heap instead of in
the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the
code of the entire program (or shared library) fits in
4MB, and is thus reachable with a branch instruction.
When -msmall-data is used, the compiler can assume that
all local symbols share the same $gp value, and thus
reduce the number of instructions required for a
function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling
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parameters for machine type cpu_type. You can specify
either the EV style name or the corresponding chip
number. GCC supports scheduling parameters for the EV4,
EV5 and EV6 family of processors and chooses the default
values for the instruction set from the processor you
specify. If you do not specify a processor type, GCC
defaults to the processor on which the compiler was
built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set
extensions.
ev5
21164
Schedules as an EV5 and has no instruction set
extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX
extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and
MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX,
and MAX extensions.
Native toolchains also support the value native, which
selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not
recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for
machine type cpu_type. The instruction set is not
changed.
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Native toolchains also support the value native, which
selects the best architecture option for the host
processor. -mtune=native has no effect if GCC does not
recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical
memory references as seen by the application. This
number is highly dependent on the memory access patterns
used by the application and the size of the external
cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of
clock cycles for "typical" EV4 & EV5 hardware for
the Level 1, 2 & 3 caches (also called Dcache,
Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce
smaller code, but it does assume that all symbolic
values and addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so
there is no need to include the simulator library
(libsim.a) on the linker command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator.
This causes an alternate runtime startup and library to
be linked. You must not use this option when generating
programs that will run on real hardware; you must
provide your own runtime library for whatever I/O
functions are needed.
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-mlra
Enable Local Register Allocation. This is still
experimental for FT32, so by default the compiler uses
standard reload.
-mnodiv
Do not use div and mod instructions.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code
registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
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-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to
represent pointers to functions. Without any
PIC/PIE-related options, it implies -fPIE. With -fpic
or -fpie, it assumes GOT entries and small data are
within a 12-bit range from the GOT base address; with
-fPIC or -fPIE, GOT offsets are computed with 32 bits.
With a bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no
effect without -mfdpic. It's enabled by default if
optimizing for speed and compiling for shared libraries
(i.e., -fPIC or -fpic), or when an optimization option
such as -O3 or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local
code.
-mtls
Do not assume a large TLS segment when generating
thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI
for data that is known to be in read-only sections.
It's enabled by default, except for -fpic or -fpie: even
though it may help make the global offset table smaller,
it trades 1 instruction for 4. With -fPIC or -fPIE, it
trades 3 instructions for 4, one of which may be shared
by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to
be a win. If it is not, -mno-gprel-ro can be used to
disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's
implied by -mlibrary-pic, as well as by -fPIC and -fpic
without -mfdpic. You should never have to use it
explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame
pointer whenever a stack frame is allocated. This
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option is enabled by default and can be disabled with
-mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the
current compilation unit. This allows the functions to
be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting
NOPs into the previous packet. This option only has an
effect when VLIW packing is enabled. It doesn't create
new packets; it merely adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions
(default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions
(default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
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-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions
(default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW
instructions.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional
execution (default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional
execution.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations
(default).
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
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-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and
will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from
the compiler-generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant
"membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code.
Possible values are frv, fr550, tomcat, fr500, fr450,
fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on
*-*-linux-*uclibc*, *-*-linux-*musl* and
*-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on
*-*-linux-*uclibc* targets.
-mmusl
Use the musl C library. This is the default on
*-*-linux-*musl* targets.
-mbionic
Use Bionic C library. This is the default on
*-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is
the default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking,
this option makes the GCC driver pass Android-specific
options to the linker. Finally, this option causes the
preprocessor macro "__ANDROID__" to be defined.
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-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not
enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by
default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass
standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when
possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal
mode. This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be
used with -ms.
-mexr
Extended registers are stored on stack before execution
of function with monitor attribute. Default option is
-mexr. This option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before
execution of function with monitor attribute. Default
option is -mno-exr. This option is valid only for H8S
targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as
for the H8/300. The default for the H8/300H and H8S is
to align longs and floats on 4-byte boundaries.
-malign-300 causes them to be aligned on 2-byte
boundaries. This option has no effect on the H8/300.
HPPA Options
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These -m options are defined for the HPPA family of
computers:
-march=architecture-type
Generate code for the specified architecture. The
choices for architecture-type are 1.0 for PA 1.0, 1.1
for PA 1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine
the proper architecture option for your machine. Code
compiled for lower numbered architectures runs on higher
numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0
respectively.
-mcaller-copies
The caller copies function arguments passed by hidden
reference. This option should be used with care as it
is not compatible with the default 32-bit runtime.
However, only aggregates larger than eight bytes are
passed by hidden reference and the option provides
better compatibility with OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility
purposes only.
-mdisable-fpregs
Prevent floating-point registers from being used in any
manner. This is necessary for compiling kernels that
perform lazy context switching of floating-point
registers. If you use this option and attempt to
perform floating-point operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes.
This avoids some rather obscure problems when compiling
MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space
registers. This allows GCC to generate faster indirect
calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and
kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space
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boundaries. This allows GCC to emit code that performs
faster indirect calls.
This option does not work in the presence of shared
libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling
kernel code. A register range is specified as two
registers separated by a dash. Multiple register ranges
can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as
sometimes required by the HP-UX 10 linker. This is
equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for
ELF systems.
-mgas
Enable the use of assembler directives only GAS
understands.
-mschedule=cpu-type
Schedule code according to the constraints for the
machine type cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and 8000. Refer to
/usr/lib/sched.models on an HP-UX system to determine
the proper scheduling option for your machine. The
default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note
this makes symbolic debugging impossible. It also
triggers a bug in the HP-UX 8 and HP-UX 9 linkers in
which they give bogus error messages when linking some
programs.
-msoft-float
Generate output containing library calls for floating
point. Warning: the requisite libraries are not
available for all HPPA targets. Normally the facilities
of the machine's usual C compiler are used, but this
cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library
functions for cross-compilation.
-msoft-float changes the calling convention in the
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output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you
need to compile libgcc.a, the library that comes with
GCC, with -msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The
default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and "_WSIO", for
workstation IO. These options are available under HP-UX
and HI-UX.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to
ld when building a shared library. It is the default
when GCC is configured, explicitly or implicitly, with
the GNU linker. This option does not affect which ld is
called; it only changes what parameters are passed to
that ld. The ld that is called is determined by the
--with-ld configure option, GCC's program search path,
and finally by the user's PATH. The linker used by GCC
can be printed using which `gcc -print-prog-name=ld`.
This option is only available on the 64-bit HP-UX GCC,
i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld
when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when
GCC is configured, explicitly or implicitly, with the HP
linker. This option does not affect which ld is called;
it only changes what parameters are passed to that ld.
The ld that is called is determined by the --with-ld
configure option, GCC's program search path, and finally
by the user's PATH. The linker used by GCC can be
printed using which `gcc -print-prog-name=ld`. This
option is only available on the 64-bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This
ensures that a call is always able to reach linker
generated stubs. The default is to generate long calls
only when the distance from the call site to the
beginning of the function or translation unit, as the
case may be, exceeds a predefined limit set by the
branch type being used. The limits for normal calls are
7,600,000 and 240,000 bytes, respectively for the PA 2.0
and PA 1.X architectures. Sibcalls are always limited
at 240,000 bytes.
Distances are measured from the beginning of functions
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when using the -ffunction-sections option, or when using
the -mgas and -mno-portable-runtime options together
under HP-UX with the SOM linker.
It is normally not desirable to use this option as it
degrades performance. However, it may be useful in
large applications, particularly when partial linking is
used to build the application.
The types of long calls used depends on the capabilities
of the assembler and linker, and the type of code being
generated. The impact on systems that support long
absolute calls, and long pic symbol-difference or pc-
relative calls should be relatively small. However, an
indirect call is used on 32-bit ELF systems in pic code
and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for
the specified UNIX standard. The choices for unix-std
are 93, 95 and 98. 93 is supported on all HP-UX
versions. 95 is available on HP-UX 10.10 and later. 98
is available on HP-UX 11.11 and later. The default
values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though
to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and
3.4. -munix=95 provides additional predefines for
"XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the
startfile unix95.o. -munix=98 provides additional
predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED",
"_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the
interfaces for various library routines. It also
affects the operational behavior of the C library.
Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than
one UNIX standard must test, set and restore the
variable "__xpg4_extended_mask" as appropriate. Most
GNU software doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search
libdld.sl when the -static option is specified on HP-UX
10 and later.
-static
The HP-UX implementation of setlocale in libc has a
dependency on libdld.sl. There isn't an archive version
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of libdld.sl. Thus, when the -static option is
specified, special link options are needed to resolve
this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary
options to link with libdld.sl when the -static option
is specified. This causes the resulting binary to be
dynamic. On the 64-bit port, the linkers generate
dynamic binaries by default in any case. The -nolibdld
option can be used to prevent the GCC driver from adding
these link options.
-threads
Add support for multithreading with the dce thread
library under HP-UX. This option sets flags for both
the preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64
architecture.
-mbig-endian
Generate code for a big-endian target. This is the
default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the
default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is
the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is
the default.
-mno-pic
Generate code that does not use a global pointer
register. The result is not position independent code,
and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and
after volatile asm statements.
-mregister-names
-mno-register-names
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Generate (or don't) in, loc, and out register names for
the stacked registers. This may make assembler output
more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small
data section. This may be useful for working around
optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer
value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware
code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point
values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point
values using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-
point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using
the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using
the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer
values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum
latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum
throughput algorithm.
-mno-inline-sqrt
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Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused
multiply/add or multiply/subtract instructions. The
default is to use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line
number debugging info. This may be useful when not
using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately
preceding the instruction that triggered the stop bit.
This can improve instruction scheduling, but does not
always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling
kernel code. A register range is specified as two
registers separated by a dash. Multiple register ranges
can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values
are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU,
Valid values are itanium, itanium1, merced, itanium2,
and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits. These are HP-UX specific
flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload.
This results in generation of "ld.a" instructions and
the corresponding check instructions ("ld.c" / "chk.a").
The default setting is disabled.
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-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload.
This results in generation of "ld.a" instructions and
the corresponding check instructions ("ld.c" / "chk.a").
The default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This
feature is available only during region scheduling (i.e.
before reload). This results in generation of the
"ld.s" instructions and the corresponding check
instructions "chk.s". The default setting is disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads before
reload. This is effective only with
-msched-br-data-spec enabled. The default setting is
enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions
that are dependent on the data speculative loads after
reload. This is effective only with
-msched-ar-data-spec enabled. The default setting is
enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions
that are dependent on the control speculative loads.
This is effective only with -msched-control-spec
enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for
schedule only if there are no other choices at the
moment. This makes the use of the data speculation much
more conservative. The default setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen
for schedule only if there are no other choices at the
moment. This makes the use of the control speculation
much more conservative. The default setting is
disabled.
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-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered
during computation of the instructions priorities. This
makes the use of the speculation a bit more
conservative. The default setting is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on
by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option
is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling.
This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not
likely to cause a conflict when placed into the same
instruction group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective
scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction
group, giving lower priority to subsequent memory insns
attempting to schedule in the same instruction group.
Frequently useful to prevent cache bank conflicts. The
default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a
hard limit, disallowing more than that number in an
instruction group. Otherwise, the limit is "soft",
meaning that non-memory operations are preferred when
the limit is reached, but memory operations may still be
scheduled.
LM32 Options
These -m options are defined for the LatticeMico32
architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
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Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be
one of r8c for the R8C/Tiny series, m16c for the M16C
(up to /60) series, m32cm for the M16C/80 series, or
m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator.
This causes an alternate runtime library to be linked in
which supports, for example, file I/O. You must not use
this option when generating programs that will run on
real hardware; you must provide your own runtime library
for whatever I/O functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers
GCC uses during code generation. These pseudo-registers
are used like real registers, so there is a tradeoff
between GCC's ability to fit the code into available
registers, and the performance penalty of using memory
instead of registers. Note that all modules in a
program must be compiled with the same value for this
option. Because of that, you must not use this option
with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D
architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
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-mmodel=small
Assume all objects live in the lower 16MB of memory (so
that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable
with the "bl" instruction. This is the default.
The addressability of a particular object can be set
with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address
space (the compiler generates "seth/add3" instructions
to load their addresses), and assume all subroutines are
reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address
space (the compiler generates "seth/add3" instructions
to load their addresses), and assume subroutines may not
be reachable with the "bl" instruction (the compiler
generates the much slower "seth/add3/jl" instruction
sequence).
-msdata=none
Disable use of the small data area. Variables are put
into one of ".data", ".bss", or ".rodata" (unless the
"section" attribute has been specified). This is the
default.
The small data area consists of sections ".sdata" and
".sbss". Objects may be explicitly put in the small
data area with the "section" attribute using one of
these sections.
-msdata=sdata
Put small global and static data in the small data area,
but do not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area,
and generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num
bytes into the small data or BSS sections instead of the
normal data or BSS sections. The default value of num
is 8. The -msdata option must be set to one of sdata or
use for this option to have any effect.
All modules should be compiled with the same -G num
value. Compiling with different values of num may or
may not work; if it doesn't the linker gives an error
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message---incorrect code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display
some statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is
the default.
-missue-rate=number
Issue number instructions per cycle. number can only be
1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are
preferred over conditional code, if it is 2, then the
opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache.
The default is 12. Valid numbers are between 0 and 15
inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a
trap.
-mflush-func=name
Specifies the name of the operating system function to
call to flush the cache. The default is _flush_cache,
but a function call is only used if a trap is not
available.
-mno-flush-func
Indicates that there is no OS function for flushing the
cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire
processors. The default settings depend on which
architecture was selected when the compiler was configured;
the defaults for the most common choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire
instruction set architecture. Permissible values of
arch for M680x0 architectures are: 68000, 68010, 68020,
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68030, 68040, 68060 and cpu32. ColdFire architectures
are selected according to Freescale's ISA classification
and the permissible values are: isaa, isaaplus, isab and
isac.
GCC defines a macro "__mcfarch__" whenever it is
generating code for a ColdFire target. The arch in this
macro is one of the -march arguments given above.
When used together, -march and -mtune select code that
runs on a family of similar processors but that is
optimized for a particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire
processor. The M680x0 cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302, 68332 and cpu32. The
ColdFire cpus are given by the table below, which also
classifies the CPUs into families:
Family : -mcpu arguments
51qm
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5484 5485
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483
-mcpu=cpu overrides -march=arch if arch is compatible
with cpu. Other combinations of -mcpu and -march are
rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire
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target cpu is selected. It also defines
"__mcf_family_family", where the value of family is
given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within
the constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020, 68030,
68040, 68060 and cpu32. The ColdFire microarchitectures
are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to
run relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets as
well. These two options select the same tuning
decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when
tuning for 680x0 architecture arch. It also defines
"mcarch" unless either -ansi or a non-GNU -std option is
used. If GCC is tuning for a range of architectures, as
selected by -mtune=68020-40 or -mtune=68020-60, it
defines the macros for every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for
ColdFire microarchitecture uarch, where uarch is one of
the arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when
the compiler is configured for 68000-based systems. It
is equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or
EC000 core, including the 68008, 68302, 68306, 68307,
68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when
the compiler is configured for 68010-based systems. It
is equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when
the compiler is configured for 68020-based systems. It
is equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when
the compiler is configured for 68030-based systems. It
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is equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when
the compiler is configured for 68040-based systems. It
is equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions
that have to be emulated by software on the 68040. Use
this option if your 68040 does not have code to emulate
those instructions.
-m68060
Generate output for a 68060. This is the default when
the compiler is configured for 68060-based systems. It
is equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882
instructions that have to be emulated by software on the
68060. Use this option if your 68060 does not have code
to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when
the compiler is configured for CPU32-based systems. It
is equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or
CPU32+ core, including the 68330, 68331, 68332, 68333,
68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the
default when the compiler is configured for 520X-based
systems. It is equivalent to -mcpu=5206, and is now
deprecated in favor of that option.
Use this option for microcontroller with a 5200 core,
including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is
now deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X
family. The option is now deprecated in favor of the
equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is
now deprecated in favor of the equivalent -mcpu=5307.
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-m5407
Generate output for a ColdFire 5407 CPU. The option is
now deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g.
547x/548x). This includes use of hardware floating-
point instructions. The option is equivalent to
-mcpu=547x, and is now deprecated in favor of that
option.
-m68020-40
Generate output for a 68040, without using any of the
new instructions. This results in code that can run
relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the 68881
instructions that are emulated on the 68040.
The option is equivalent to -march=68020
-mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the
new instructions. This results in code that can run
relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the 68881
instructions that are emulated on the 68060.
The option is equivalent to -march=68020
-mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the
default for 68020 and above, and for ColdFire devices
that have an FPU. It defines the macro "__HAVE_68881__"
on M680x0 targets and "__mcffpu__" on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library
calls instead. This is the default for 68000, 68010,
and 68832 targets. It is also the default for ColdFire
devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and
remainder instructions. If -march is used without
-mcpu, the default is "on" for ColdFire architectures
and "off" for M680x0 architectures. Otherwise, the
default is taken from the target CPU (either the default
CPU, or the one specified by -mcpu). For example, the
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default is "off" for -mcpu=5206 and "on" for
-mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is
enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short
int". Additionally, parameters passed on the stack are
also aligned to a 16-bit boundary even on targets whose
API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is
the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000,
-mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option
implies -mbitfield. This is the default if you use a
configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return
with the "rtd" instruction, which pops their arguments
while returning. This saves one instruction in the
caller since there is no need to pop the arguments
there.
This calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need
to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all
functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is
generated for calls to those functions.
In addition, seriously incorrect code results if you
call a function with too many arguments. (Normally,
extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020,
68030, 68040, 68060 and CPU32 processors, but not by the
68000 or 5200.
-mno-rtd
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Do not use the calling conventions selected by -mrtd.
This is the default.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long",
"float", "double", and "long double" variables on a
32-bit boundary (-malign-int) or a 16-bit boundary
(-mno-align-int). Aligning variables on 32-bit
boundaries produces code that runs somewhat faster on
processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC aligns
structures containing the above types differently than
most published application binary interface
specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000
directly, instead of using a global offset table. At
present, this option implies -fpic, allowing at most a
16-bit offset for pc-relative addressing. -fPIC is not
presently supported with -mpcrel, though this could be
supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are
handled by the system.
-msep-data
Generate code that allows the data segment to be located
in a different area of memory from the text segment.
This allows for execute-in-place in an environment
without virtual memory management. This option implies
-fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows
the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the
library ID method. This allows for execute-in-place and
shared libraries in an environment without virtual
memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared
libraries are being used. This is the default.
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-mshared-library-id=n
Specifies the identification number of the ID-based
shared library being compiled. Specifying a value of 0
generates more compact code; specifying other values
forces the allocation of that number to the current
library, but is no more space- or time-efficient than
omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire,
generate code that works if the GOT has more than 8192
entries. This code is larger and slower than code
generated without this option. On M680x0 processors,
this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values
from the GOT. While this is relatively efficient, it
only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error
such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with
-mxgot. It should then work with very large GOTs.
However, code generated with -mxgot is less efficient,
since it takes 4 instructions to fetch the value of a
global symbol.
Note that some linkers, including newer versions of the
GNU linker, can create multiple GOTs and sort GOT
entries. If you have such a linker, you should only
need to use -mxgot when compiling a single object file
that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating
position-independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is
to use 16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core
processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done
in two instructions or less.
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-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so
omit the simulator library (libsim.a) from the linker
command line.
-mstack-increment=size
Set the maximum amount for a single stack increment
operation. Large values can increase the speed of
programs that contain functions that need a large amount
of stack space, but they can also trigger a segmentation
fault if the stack is extended too much. The default
value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute
difference between two registers.
-mall-opts
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Enables all the optional instructions---average,
multiply, divide, bit operations, leading zero, absolute
difference, min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the
average of two registers.
-mbased=n
Variables of size n bytes or smaller are placed in the
".based" section by default. Based variables use the
$tp register as a base register, and there is a 128-byte
limit to the ".based" section.
-mbitops
Enables the bit operation instructions---bit test
("btstm"), set ("bsetm"), clear ("bclrm"), invert
("bnotm"), and test-and-set ("tas").
-mc=name
Selects which section constant data is placed in. name
may be tiny, near, or far.
-mclip
Enables the "clip" instruction. Note that -mclip is not
useful unless you also provide -mminmax.
-mconfig=name
Selects one of the built-in core configurations. Each
MeP chip has one or more modules in it; each module has
a core CPU and a variety of coprocessors, optional
instructions, and peripherals. The "MeP-Integrator"
tool, not part of GCC, provides these configurations
through this option; using this option is the same as
using all the corresponding command-line options. The
default configuration is default.
-mcop
Enables the coprocessor instructions. By default, this
is a 32-bit coprocessor. Note that the coprocessor is
normally enabled via the -mconfig= option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW
coprocessor.
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-mdc
Causes constant variables to be placed in the ".near"
section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the
"io" attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by
default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section
by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate
instructions.
-mno-opts
Disables all the optional instructions enabled by
-mall-opts.
-mrepeat
Enables the "repeat" and "erepeat" instructions, used
for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section.
Note that there is a 65536-byte limit to this section.
Accesses to these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the
compiler does not currently generate these itself, but
this option is included for compatibility with other
tools, like "as".
-msdram
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Link the SDRAM-based runtime instead of the default
ROM-based runtime.
-msim
Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in
support for reset and exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section.
Without this option, functions default to the ".near"
section.
-mtiny=n
Variables that are n bytes or smaller are allocated to
the ".tiny" section. These variables use the $gp base
register. The default for this option is 4, but note
that there's a 65536-byte limit to the ".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use
-fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU.
Supported values are in the format vX.YY.Z, where X is a
major version, YY is the minor version, and Z is
compatibility code. Example values are v3.00.a,
v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
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-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer
divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32
multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed
load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code
crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD)
based software intrusive debug agent called xmdstub.
This uses startup file crt1.o and sets the start
address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader.
This model uses startup file crt2.o which does not
contain a processor reset vector handler. This is
suitable for transferring control on a processor
reset to the bootloader rather than the application.
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novectors
for applications that do not require any of the
MicroBlaze vectors. This option may be useful for
applications running within a monitoring
application. This model uses crt3.o as a startup
file.
Option -xl-mode-app-model is a deprecated alias for
-mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for
mips*el-*-* configurations.
-march=arch
Generate code that runs on arch, which can be the name
of a generic MIPS ISA, or the name of a particular
processor. The ISA names are: mips1, mips2, mips3,
mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6,
mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The
processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem,
4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1,
24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn,
74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1,
1004kf1_1, i6400, interaptiv, loongson2e, loongson2f,
loongson3a, m4k, m14k, m14kc, m14ke, m14kec, m5100,
m5101, octeon, octeon+, octeon2, octeon3, orion, p5600,
r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700,
r6000, r8000, rm7000, rm9000, r10000, r12000, r14000,
r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130,
vr4300, vr5000, vr5400, vr5500, xlr and xlp. The
special value from-abi selects the most compatible
architecture for the selected ABI (that is, mips1 for
32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value
native, which selects the best architecture option for
the host processor. -march=native has no effect if GCC
does not recognize the processor.
In processor names, a final 000 can be abbreviated as k
(for example, -march=r2k). Prefixes are optional, and
vr may be written r.
Names of the form nf2_1 refer to processors with FPUs
clocked at half the rate of the core, names of the form
nf1_1 refer to processors with FPUs clocked at the same
rate as the core, and names of the form nf3_2 refer to
processors with FPUs clocked a ratio of 3:2 with respect
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to the core. For compatibility reasons, nf is accepted
as a synonym for nf2_1 while nx and bfx are accepted as
synonyms for nf1_1.
GCC defines two macros based on the value of this
option. The first is "_MIPS_ARCH", which gives the name
of target architecture, as a string. The second has the
form "_MIPS_ARCH_foo", where foo is the capitalized
value of "_MIPS_ARCH". For example, -march=r2000 sets
"_MIPS_ARCH" to "r2000" and defines the macro
"_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor
names given above. In other words, it has the full
prefix and does not abbreviate 000 as k. In the case of
from-abi, the macro names the resolved architecture
(either "mips1" or "mips3"). It names the default
architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option
controls the way instructions are scheduled, and the
perceived cost of arithmetic operations. The list of
arch values is the same as for -march.
When this option is not used, GCC optimizes for the
processor specified by -march. By using -march and
-mtune together, it is possible to generate code that
runs on a family of processors, but optimize the code
for one particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and
"_MIPS_TUNE_foo", which work in the same way as the
-march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
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-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is
targeting a MIPS32 or MIPS64 architecture, it makes use
of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-
function basis by means of "mips16" and "nomips16"
attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This
option is provided for regression testing of mixed
MIPS16/non-MIPS16 code generation, and is not intended
for ordinary use in compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard
(uncompressed) MIPS ISA be link-compatible with MIPS16
and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot
jump directly to MIPS16 or microMIPS code; it must
either use a call or an indirect jump.
-minterlink-compressed therefore disables direct jumps
unless GCC knows that the target of the jump is not
compressed.
-minterlink-mips16
-mno-interlink-mips16
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Aliases of -minterlink-compressed and
-mno-interlink-compressed. These options predate the
microMIPS ASE and are retained for backwards
compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant.
GCC normally generates 64-bit code when you select a
64-bit architecture, but you can use -mgp32 to get
32-bit code instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which
floating-point registers are 64 rather than 32 bits
wide. You can select this combination with -mabi=32
-mfp64. This ABI relies on the "mthc1" and "mfhc1"
instructions and is therefore only supported for
MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values
remain the same, but each scalar value is passed in a
single 64-bit register rather than a pair of 32-bit
registers. For example, scalar floating-point values
are returned in $f0 only, not a $f0/$f1 pair. The set
of call-saved registers also remains the same in that
the even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to
enable a transition from 32-bit to 64-bit registers.
These are FPXX (-mfpxx) and FP64A (-mfp64
-mno-odd-spreg). The FPXX extension mandates that all
code must execute correctly when run using 32-bit or
64-bit registers. The code can be interlinked with
either FP32 or FP64, but not both. The FP64A extension
is similar to the FP64 extension but forbids the use of
odd-numbered single-precision registers. This can be
used in conjunction with the "FRE" mode of FPUs in
MIPS32R5 processors and allows both FP32 and FP64A code
to interlink and run in the same process without
changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for
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SVR4-style dynamic objects. -mabicalls is the default
for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully
position-independent, and that can therefore be linked
into shared libraries. This option only affects
-mabicalls.
All -mabicalls code has traditionally been
position-independent, regardless of options like -fPIC
and -fpic. However, as an extension, the GNU toolchain
allows executables to use absolute accesses for
locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to
locally-defined functions. This mode is selected by
-mno-shared.
-mno-shared depends on binutils 2.16 or higher and
generates objects that can only be linked by the GNU
linker. However, the option does not affect the ABI of
the final executable; it only affects the ABI of
relocatable objects. Using -mno-shared generally makes
executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic
linkers support PLTs and copy relocations. This option
only affects -mno-shared -mabicalls. For the n64 ABI,
this option has no effect without -msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of
the global offset table.
GCC normally uses a single instruction to load values
from the GOT. While this is relatively efficient, it
only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error
such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with
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-mxgot. This works with very large GOTs, although the
code is also less efficient, since it takes three
instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you
have such a linker, you should only need to use -mxgot
when a single object file accesses more than 64k's worth
of GOT entries. Very few do.
These options have no effect unless GCC is generating
position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions.
Implement floating-point calculations using library
calls instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts
that the program being compiled does not perform any
floating-point operations. This option is presently
supported only by some bare-metal MIPS configurations,
where it may select a special set of libraries that lack
all floating-point support (including, for example, the
floating-point "printf" formats). If code compiled with
-mno-float accidentally contains floating-point
operations, it is likely to suffer a link-time or run-
time failure.
-msingle-float
Assume that the floating-point coprocessor only supports
single-precision operations.
-mdouble-float
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Assume that the floating-point coprocessor supports
double-precision operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision
floating-point registers for the o32 ABI. This is the
default for processors that are known to support these
registers. When using the o32 FPXX ABI, -mno-odd-spreg
is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-
a-number (NaN) IEEE 754 floating-point data with the
"abs.fmt" and "neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy
treatment is selected. In this case these instructions
are considered arithmetic and avoided where correct
operation is required and the input operand might be a
NaN. A longer sequence of instructions that manipulate
the sign bit of floating-point datum manually is used
instead unless the -ffinite-math-only option has also
been specified.
The -mabs=2008 option selects the IEEE 754-2008
treatment. In this case these instructions are
considered non-arithmetic and therefore operating
correctly in all cases, including in particular where
the input operand is a NaN. These instructions are
therefore always used for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-
a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In
this case quiet NaNs (qNaNs) are denoted by the first
bit of their trailing significand field being 0, whereas
signaling NaNs (sNaNs) are denoted by the first bit of
their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008
encoding. In this case qNaNs are denoted by the first
bit of their trailing significand field being 1, whereas
sNaNs are denoted by the first bit of their trailing
significand field being 0.
The default is -mnan=legacy unless GCC has been
configured with --with-nan=2008.
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-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to
implement atomic memory built-in functions. When
neither option is specified, GCC uses the instructions
if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate
the instructions and -mno-llsc can be useful when
compiling for nonstandard ISAs. You can make either
option the default by configuring GCC with --with-llsc
and --without-llsc respectively. --with-llsc is the
default for some configurations; see the installation
documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro
"__mips_dsp". It also defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros
"__mips_dsp" and "__mips_dspr2". It also defines
"__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point
instructions.
This option requires hardware floating-point support
to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension
instructions. This option can only be used when
generating 64-bit code and requires hardware floating-
point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d
implies -mpaired-single.
-mmicromips
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-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a
per-function basis by means of "micromips" and
"nomicromips" attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing
instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ)
instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address
(XPA) instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for
an explanation of the default and the way that the
pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits
wide.
The default size of "int"s, "long"s and pointers depends
on the ABI. All the supported ABIs use 32-bit "int"s.
The n64 ABI uses 64-bit "long"s, as does the 64-bit
EABI; the others use 32-bit "long"s. Pointers are the
same size as "long"s, or the same size as integer
registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit
values, regardless of the selected ABI. This option is
useful in combination with -mabi=64 and -mno-abicalls
because it allows GCC to generate shorter and faster
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references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small
data section if that data is no bigger than num bytes.
GCC can then generate more efficient accesses to the
data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data
too, such as to static variables in C. -mlocal-sdata is
the default for all configurations.
If the linker complains that an application is using too
much small data, you might want to try rebuilding the
less performance-critical parts with -mno-local-sdata.
You might also want to build large libraries with
-mno-local-sdata, so that the libraries leave more room
for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is
in a small data section if the size of that data is
within the -G limit. -mextern-sdata is the default for
all configurations.
If you compile a module Mod with -mextern-sdata -G num
-mgpopt, and Mod references a variable Var that is no
bigger than num bytes, you must make sure that Var is
placed in a small data section. If Var is defined by
another module, you must either compile that module with
a high-enough -G setting or attach a "section" attribute
to Var's definition. If Var is common, you must link
the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to
compile and link every module with the same -G option.
However, you may wish to build a library that supports
several different small data limits. You can do this by
compiling the library with the highest supported -G
setting and additionally using -mno-extern-sdata to stop
the library from making assumptions about externally-
defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that
are known to be in a small data section; see -G,
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-mlocal-sdata and -mextern-sdata. -mgpopt is the
default for all configurations.
-mno-gpopt is useful for cases where the $gp register
might not hold the value of "_gp". For example, if the
code is part of a library that might be used in a boot
monitor, programs that call boot monitor routines pass
an unknown value in $gp. (In such situations, the boot
monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and
-mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first
if possible, then next in the small data section if
possible, otherwise in data. This gives slightly slower
code than the default, but reduces the amount of RAM
required when executing, and thus may be preferred for
some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only
data section. This option is only meaningful in
conjunction with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from
executable sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections.
This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access
executable sections, but other instructions must not
do so. This option is useful on 4KSc and 4KSd
processors when the code TLBs have the Read Inhibit
bit set. It is also useful on processors that can
be configured to have a dual instruction/data SRAM
interface and that, like the M4K, automatically
redirect PC-relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections.
This option can be useful on targets that are
configured to have a dual instruction/data SRAM
interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the
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instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()"
assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for
backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when
dealing with symbolic addresses. The alternative,
selected by -mno-explicit-relocs, is to use assembler
macros instead.
-mexplicit-relocs is the default if GCC was configured
to use an assembler that supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating
either a conditional trap or a break instruction. Using
traps results in smaller code, but is only supported on
MIPS II and later. Also, some versions of the Linux
kernel have a bug that prevents trap from generating the
proper signal ("SIGFPE"). Use -mdivide-traps to allow
conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be
overridden at configure time using --with-divide=breaks.
Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive
load or store instructions to enable load/store bonding.
This option is enabled by default but only takes effect
when the selected architecture is known to support
bonding.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial
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block moves. The default is -mno-memcpy, which allows
GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction.
Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256
megabyte segment.
This option has no effect on abicalls code. The default
is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul"
instructions, as provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer
instructions. The default is -mimadd on architectures
that support "madd" and "msub" except for the 74k
architecture where it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-
accumulate instructions, when they are available. The
default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions
are used, the intermediate product is calculated to
infinite precision and is not subject to the FCSR Flush
to Zero bit. This may be undesirable in some
circumstances. On other processors the result is
numerically identical to the equivalent computation
using separate multiply, add, subtract and negate
instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over
user assembler files (with a .s suffix) when assembling
them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during
refill) errata. The workarounds are implemented by the
assembler rather than by GCC.
-mfix-r4000
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-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an
incorrect result if executed immediately after
starting an integer division.
- A double-word or a variable shift may give an
incorrect result if executed while an integer
multiplication is in progress.
- An integer division may give an incorrect result if
started in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an
incorrect result if executed immediately after
starting an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on
revisions prior to 3.0. They may deadlock on
revisions 2.6 and earlier.
This option can only be used if the target architecture
supports branch-likely instructions. -mfix-r10000 is
the default when -march=r10000 is used; -mno-fix-r10000
is the default otherwise.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The
workarounds are implemented by the assembler rather than
by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct
result if one of the operands is negative.
The workarounds for the division errata rely on special
functions in libgcc.a. At present, these functions are
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only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between
certain pairs of instructions. These errata are handled
by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The
workarounds are implemented by the assembler rather than
by GCC, although GCC avoids using "mflo" and "mfhi" if
the VR4130 "macc", "macchi", "dmacc" and "dmacchi"
instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag
currently works around the SB-1 revision 2 "F1" and "F2"
floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to
avoid the side-effects of speculation on R10K
processors.
In common with many processors, the R10K tries to
predict the outcome of a conditional branch and
speculatively executes instructions from the "taken"
branch. It later aborts these instructions if the
predicted outcome is wrong. However, on the R10K, even
aborted instructions can have side effects.
This problem only affects kernel stores and, depending
on the system, kernel loads. As an example, a
speculatively-executed store may load the target memory
into cache and mark the cache line as dirty, even if the
store itself is later aborted. If a DMA operation
writes to the same area of memory before the "dirty"
line is flushed, the cached data overwrites the DMA-ed
data. See the R10K processor manual for a full
description, including other potential problems.
One workaround is to insert cache barrier instructions
before every memory access that might be speculatively
executed and that might have side effects even if
aborted. -mr10k-cache-barrier=setting controls GCC's
implementation of this workaround. It assumes that
aborted accesses to any byte in the following regions
does not have side effects:
1. the memory occupied by the current function's stack
frame;
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2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-
constant address.
It is the kernel's responsibility to ensure that
speculative accesses to these regions are indeed safe.
If the input program contains a function declaration
such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and
"jal foo" to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It
expects non-GCC functions (such as hand-written assembly
code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that
might be speculatively executed and that might have
side effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be
speculatively executed and that might have side
effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is
the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D
caches, or to not call any such function. If called,
the function must take the same arguments as the common
"_flush_func", that is, the address of the memory range
for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches).
The default depends on the target GCC was configured
for, but commonly is either "_flush_func" or
"__cpu_flush".
mbranch-cost=num
Set the cost of branches to roughly num "simple"
instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across
releases. A zero cost redundantly selects the default,
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which is based on the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions,
regardless of the default for the selected architecture.
By default, Branch Likely instructions may be generated
if they are supported by the selected architecture. An
exception is for the MIPS32 and MIPS64 architectures and
processors that implement those architectures; for
those, Branch Likely instructions are not be generated
by default because the MIPS32 and MIPS64 architectures
specifically deprecate their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be
generated. The default is -mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact
branch instructions will never be generated.
The -mcompact-branches=always option ensures that a
compact branch instruction will be generated if
available. If a compact branch instruction is not
available, a delay slot form of the branch will be used
instead.
This option is supported from MIPS Release 6 onwards.
The -mcompact-branches=optimal option will cause a delay
slot branch to be used if one is available in the
current ISA and the delay slot is successfully filled.
If the delay slot is not filled, a compact branch will
be chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This
affects how FP instructions are scheduled for some
processors. The default is that FP exceptions are
enabled.
For instance, on the SB-1, if FP exceptions are
disabled, and we are emitting 64-bit code, then we can
use both FP pipes. Otherwise, we can only use one FP
pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only
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issue two instructions together if the first one is
8-byte aligned. When this option is enabled, GCC aligns
pairs of instructions that it thinks should execute in
parallel.
This option only has an effect when optimizing for the
VR4130. It normally makes code faster, but at the
expense of making it bigger. It is enabled by default
at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on
architectures that support it. The "synci" instructions
(if enabled) are generated when
"__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can
be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is
generally safe to use "synci". However, on many multi-
core (SMP) systems, it does not invalidate the
instruction caches on all cores and may lead to
undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via
register $25 into direct calls. This is only possible
if the linker can resolve the destination at link time
and if the destination is within range for a direct
call.
-mrelax-pic-calls is the default if GCC was configured
to use an assembler and a linker that support the
".reloc" assembly directive and -mexplicit-relocs is in
effect. With -mno-explicit-relocs, this optimization
can be performed by the assembler and the linker alone
without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify
the calling function's return address. When enabled,
this option extends the usual "_mcount" interface with a
new ra-address parameter, which has type "intptr_t *"
and is passed in register $12. "_mcount" can then
modify the return address by doing both of the
following:
* Returning the new address in register $31.
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* Storing the new address in "*ra-address", if ra-
address is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32
ABI. When using the o32 ABI, calling functions will
allocate 16 bytes on the stack for the called function
to write out register arguments. When enabled, this
optimization will suppress the allocation of the frame
header if it can be determined that it is unused.
This optimization is off by default at all optimization
levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of
"lwxc1", "swxc1", "ldxc1", "sdxc1" instructions.
Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of
4-operand "madd.s", "madd.d" and related instructions.
Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being
compiled, passing all values in registers, no matter the
size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that
compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return
values that (in the called function) are seen as
registers $0 and up, as opposed to the GNU ABI which
uses global registers $231 and up.
-mzero-extend
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-mno-zero-extend
When reading data from memory in sizes shorter than 64
bits, use (do not use) zero-extending load instructions
by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have
the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the
sign of the dividend. Both methods are arithmetically
valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so
the assembly code can be used with the "PREFIX" assembly
directive.
-melf
Generate an executable in the ELF format, rather than
the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when
static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base
addresses. Using a base address automatically generates
a request (handled by the assembler and the linker) for
a constant to be set up in a global register. The
register is used for one or more base address requests
within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code,
but the number of different data items that can be
addressed is limited. This means that a program that
uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single
exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300
architectures:
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-mmult-bug
Generate code to avoid bugs in the multiply instructions
for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply
instructions for the MN10300 processors.
-mam33
Generate code using features specific to the AM33
processor.
-mno-am33
Do not generate code using features specific to the AM33
processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0
processor.
-mam34
Generate code using features specific to the AM34
processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type
when scheduling instructions. This does not change the
targeted processor type. The CPU type must be one of
mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer,
return the pointer in both "a0" and "d0". Otherwise,
the pointer is returned only in "a0", and attempts to
call such functions without a prototype result in
errors. Note that this option is on by default; use
-mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object
file.
-mrelax
Indicate to the linker that it should perform a
relaxation optimization pass to shorten branches, calls
and absolute memory addresses. This option only has an
effect when used on the command line for the final link
step.
This option makes symbolic debugging impossible.
-mliw
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Allow the compiler to generate Long Instruction Word
instructions if the target is the AM33 or later. This
is the default. This option defines the preprocessor
macro "__LIW__".
-mnoliw
Do not allow the compiler to generate Long Instruction
Word instructions. This option defines the preprocessor
macro "__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc
instructions if the target is the AM33 or later. This
is the default. This option defines the preprocessor
macro "__SETLB__".
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc
instructions. This option defines the preprocessor
macro "__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for
moxie-*-* configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the
default for moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object
file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants.
Normally such constants are signed decimals, but this
option is available for testsuite and/or aesthetic
purposes.
-mmcu=
Select the MCU to target. This is used to create a C
preprocessor symbol based upon the MCU name, converted
to upper case and pre- and post-fixed with __. This in
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turn is used by the msp430.h header file to select an
MCU-specific supplementary header file.
The option also sets the ISA to use. If the MCU name is
one that is known to only support the 430 ISA then that
is selected, otherwise the 430X ISA is selected. A
generic MCU name of msp430 can also be used to select
the 430 ISA. Similarly the generic msp430x MCU name
selects the 430X ISA.
In addition an MCU-specific linker script is added to
the linker command line. The script's name is the name
of the MCU with .ld appended. Thus specifying -mmcu=xxx
on the gcc command line defines the C preprocessor
symbol "__XXX__" and cause the linker to search for a
script called xxx.ld.
This option is also passed on to the assembler.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts
between the MCU name specified by the -mmcu option and
the ISA set by the -mcpu option and/or the hardware
multiply support set by the -mhwmult option. It also
toggles warnings about unrecognized MCU names. This
option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430,
msp430x and msp430xv2. This option is deprecated. The
-mmcu= option should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker
script. Overrides any scripts that would be selected by
the -mmcu= option.
-mlarge
Use large-model addressing (20-bit pointers, 32-bit
"size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit
"size_t").
-mrelax
This option is passed to the assembler and linker, and
allows the linker to perform certain optimizations that
cannot be done until the final link.
mhwmult=
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Describes the type of hardware multiply supported by the
target. Accepted values are none for no hardware
multiply, 16bit for the original 16-bit-only multiply
supported by early MCUs. 32bit for the 16/32-bit
multiply supported by later MCUs and f5series for the
16/32-bit multiply supported by F5-series MCUs. A value
of auto can also be given. This tells GCC to deduce the
hardware multiply support based upon the MCU name
provided by the -mmcu option. If no -mmcu option is
specified or if the MCU name is not recognized then no
hardware multiply support is assumed. "auto" is the
default setting.
Hardware multiplies are normally performed by calling a
library routine. This saves space in the generated
code. When compiling at -O3 or higher however the
hardware multiplier is invoked inline. This makes for
bigger, but faster code.
The hardware multiply routines disable interrupts whilst
running and restore the previous interrupt state when
they finish. This makes them safe to use inside
interrupt handlers as well as in normal code.
-minrt
Enable the use of a minimum runtime environment - no
static initializers or constructors. This is intended
for memory-constrained devices. The compiler includes
special symbols in some objects that tell the linker and
runtime which code fragments are required.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions
and data that do not have one of the "lower", "upper",
"either" or "section" attributes. Possible values are
"lower", "upper", "either" or "any". The first three
behave like the corresponding attribute. The fourth
possible value - "any" - is the default. It leaves
placement entirely up to the linker script and how it
assigns the standard sections (".text", ".data", etc) to
the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable
the fixes for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to
enable warning messages when a silicon errata might need
to be applied.
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NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mperf-ext
Generate performance extension instructions.
-mno-perf-ext
Do not generate performance extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be
4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a
power of 2 between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
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Set the code model to one of
small
All the data and read-only data segments must be
within 512KB addressing space. The text segment
must be within 16MB addressing space.
medium
The data segment must be within 512KB while the
read-only data segment can be within 4GB addressing
space. The text segment should be still within 16MB
addressing space.
large
All the text and data segments can be within 4GB
addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II
processor.
-G num
Put global and static objects less than or equal to num
bytes into the small data or BSS sections instead of the
normal data or BSS sections. The default value of num
is 8.
-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The
following option names are recognized:
none
Do not generate GP-relative accesses.
local
Generate GP-relative accesses for small data objects
that are not external, weak, or uninitialized common
symbols. Also use GP-relative addressing for objects
that have been explicitly placed in a small data
section via a "section" attribute.
global
As for local, but also generate GP-relative accesses
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for small data objects that are external, weak, or
common. If you use this option, you must ensure
that all parts of your program (including libraries)
are compiled with the same -G setting.
data
Generate GP-relative accesses for all data objects
in the program. If you use this option, the entire
data and BSS segments of your program must fit in
64K of memory and you must use an appropriate linker
script to allocate them within the addressable range
of the global pointer.
all Generate GP-relative addresses for function pointers
as well as data pointers. If you use this option,
the entire text, data, and BSS segments of your
program must fit in 64K of memory and you must use
an appropriate linker script to allocate them within
the addressable range of the global pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt
is equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is
specified to generate position-independent code. Note
that the Nios II ABI does not permit GP-relative
accesses from shared libraries.
You may need to specify -mno-gpopt explicitly when
building programs that include large amounts of small
data, including large GOT data sections. In this case,
the 16-bit offset for GP-relative addressing may not be
large enough to allow access to the entire small data
section.
-mel
-meb
Generate little-endian (default) or big-endian
(experimental) code, respectively.
-march=arch
This specifies the name of the target Nios II
architecture. GCC uses this name to determine what kind
of instructions it can emit when generating assembly
code. Permissible names are: r1, r2.
The preprocessor macro "__nios2_arch__" is available to
programs, with value 1 or 2, indicating the targeted ISA
level.
-mbypass-cache
-mno-bypass-cache
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Force all load and store instructions to always bypass
cache by using I/O variants of the instructions. The
default is not to bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O
variants of the load and store instructions. The default
is not to bypass the cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers.
The default is to use the fast divide at -O3 and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div"
family of instructions by the compiler. The default is
to emit "mul" and not emit "div" and "mulx".
-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit
manipulation) and CDX (code density) instructions.
Enabling these instructions also requires -march=r2.
Since these instructions are optional extensions to the
R2 architecture, the default is not to emit them.
-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom
instruction with encoding N when generating code that
uses insn. For example, -mcustom-fadds=253 generates
custom instruction 253 for single-precision floating-
point add operations instead of the default behavior of
using a library call.
The following values of insn are supported. Except as
otherwise noted, floating-point operations are expected
to be implemented with normal IEEE 754 semantics and
correspond directly to the C operators or the equivalent
GCC built-in functions.
Single-precision floating point:
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fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.
fnegs
Unary negation.
fabss
Unary absolute value.
fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.
fmins, fmaxs
Floating-point minimum and maximum. These
instructions are only generated if
-ffinite-math-only is specified.
fsqrts
Unary square root operation.
fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential
functions. These instructions are only generated if
-funsafe-math-optimizations is also specified.
Double-precision floating point:
faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.
fnegd
Unary negation.
fabsd
Unary absolute value.
fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.
fmind, fmaxd
Double-precision minimum and maximum. These
instructions are only generated if
-ffinite-math-only is specified.
fsqrtd
Unary square root operation.
fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential
functions. These instructions are only generated if
-funsafe-math-optimizations is also specified.
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Conversions:
fextsd
Conversion from single precision to double
precision.
ftruncds
Conversion from double precision to single
precision.
fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned
integer types, with truncation towards zero.
round
Conversion from single-precision floating point to
signed integer, rounding to the nearest integer and
ties away from zero. This corresponds to the
"__builtin_lroundf" function when -fno-math-errno is
used.
floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to
floating-point types.
In addition, all of the following transfer instructions
for internal registers X and Y must be provided to use
any of the double-precision floating-point instructions.
Custom instructions taking two double-precision source
operands expect the first operand in the 64-bit register
X. The other operand (or only operand of a unary
operation) is given to the custom arithmetic instruction
with the least significant half in source register src1
and the most significant half in src2. A custom
instruction that returns a double-precision result
returns the most significant 32 bits in the destination
register and the other half in 32-bit register Y. GCC
automatically generates the necessary code sequences to
write register X and/or read register Y when double-
precision floating-point instructions are used.
fwrx
Write src1 into the least significant half of X and
src2 into the most significant half of X.
fwry
Write src1 into Y.
frdxhi, frdxlo
Read the most or least (respectively) significant
half of X and store it in dest.
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frdy
Read the value of Y and store it into dest.
Note that you can gain more local control over
generation of Nios II custom instructions by using the
"target("custom-insn=N")" and "target("no-custom-insn")"
function attributes or pragmas.
-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom
instruction encodings (see -mcustom-insn above).
Currently, the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to:
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to:
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to:
-mcustom-floatus=243 -mcustom-fixsi=244
-mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250
-mcustom-fcmpnes=251 -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
Custom instruction assignments given by individual
-mcustom-insn= options override those given by
-mcustom-fpu-cfg=, regardless of the order of the
options on the command line.
Note that you can gain more local control over selection
of a FPU configuration by using the
"target("custom-fpu-cfg=name")" function attribute or
pragma.
These additional -m options are available for the Altera
Nios II ELF (bare-metal) target:
-mhal
Link with HAL BSP. This suppresses linking with the
GCC-provided C runtime startup and termination code, and
is typically used in conjunction with -msys-crt0= to
specify the location of the alternate startup code
provided by the HAL BSP.
-msmallc
Link with a limited version of the C library, -lsmallc,
rather than Newlib.
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-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to
use when linking. This option is only useful in
conjunction with -mhal.
-msys-lib=systemlib
systemlib is the library name of the library that
provides low-level system calls required by the C
library, e.g. "read" and "write". This option is
typically used to link with a library provided by a HAL
BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m32
-m64
Generate code for 32-bit or 64-bit ABI.
-mmainkernel
Link in code for a __main kernel. This is for stand-
alone instead of offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the
default when any level of optimization is selected.
-msoft-stack
Generate code that does not use ".local" memory directly
for stack storage. Instead, a per-warp stack pointer is
maintained explicitly. This enables variable-length
stack allocation (with variable-length arrays or
"alloca"), and when global memory is used for underlying
storage, makes it possible to access automatic variables
from other threads, or with atomic instructions. This
code generation variant is used for OpenMP offloading,
but the option is exposed on its own for the purpose of
testing the compiler; to generate code suitable for
linking into programs using OpenMP offloading, use
option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute
all threads in each warp, while maintaining memory state
and side effects as if only one thread in each warp was
active outside of OpenMP SIMD regions. All atomic
operations and calls to runtime (malloc, free, vprintf)
are conditionally executed (iff current lane index
equals the master lane index), and the register being
assigned is copied via a shuffle instruction from the
master lane. Outside of SIMD regions lane 0 is the
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master; inside, each thread sees itself as the master.
Shared memory array "int __nvptx_uni[]" stores all-zeros
or all-ones bitmasks for each warp, indicating current
mode (0 outside of SIMD regions). Each thread can
bitwise-and the bitmask at position "tid.y" with current
lane index to compute the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables
-msoft-stack and -muniform-simt options, and selects
corresponding multilib variant.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default.
(FIS floating point on the PDP-11/40 is not supported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix
assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the
default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This
is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying
memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
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-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for
experimenting with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the
default.
-munix-asm
Use Unix assembler syntax. This is the default when
configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when
configured for any PDP-11 target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction
scheduling parameters for array element type ae_type.
Supported values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code
generated with this option runs on any of the other AE
types. The code is not as efficient as it would be if
compiled for a specific AE type, and some types of
operation (e.g., multiplication) do not work properly on
all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful
AE type for compiled code, and is the default.
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-mae=MAC selects a DSP-style MAC AE. Code compiled with
this option may suffer from poor performance of byte
(char) manipulation, since the DSP AE does not provide
hardware support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an
address in a load/store instruction, without first
loading it into a register. Typically, the use of this
option generates larger programs, which run faster than
when the option isn't used. However, the results vary
from program to program, so it is left as a user option,
rather than being permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient
code. These warnings can be generated, for example,
when compiling code that performs byte-level memory
operations on the MAC AE type. The MAC AE has no
hardware support for byte-level memory operations, so
all byte load/stores must be synthesized from word
load/store operations. This is inefficient and a
warning is generated to indicate that you should rewrite
the code to avoid byte operations, or to target an AE
type that has the necessary hardware support. This
option disables these warnings.
PowerPC Options
These are listed under
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of
PLTs. Ignored for non-PIC. The default is -mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention.
ABI-string contains two parts: the size of integer types
and the registers used for floating-point types. For
example -march=rv64ifd -mabi=lp64d means that long and
pointers are 64-bit (implicitly defining int to be
32-bit), and that floating-point values up to 64 bits
wide are passed in F registers. Contrast this with
-march=rv64ifd -mabi=lp64f, which still allows the
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compiler to generate code that uses the F and D
extensions but only allows floating-point values up to
32 bits long to be passed in registers; or
-march=rv64ifd -mabi=lp64, in which no floating-point
arguments will be passed in registers.
The default for this argument is system dependent, users
who want a specific calling convention should specify
one explicitly. The valid calling conventions are:
ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some
calling conventions are impossible to implement on some
ISAs: for example, -march=rv32if -mabi=ilp32d is invalid
because the ABI requires 64-bit values be passed in F
registers, but F registers are only 32 bits wide.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and
square root instructions. This requires the F or D
extensions for floating-point registers. The default is
to use them if the specified architecture has these
instructions.
-mdiv
-mno-div
Do or don't use hardware instructions for integer
division. This requires the M extension. The default
is to use them if the specified architecture has these
instructions.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA
strings must be lower-case. Examples include rv64i,
rv32g, and rv32imaf.
-mtune=processor-string
Optimize the output for the given processor, specified
by microarchitecture name.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a
special section (on some targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue
code that uses library function calls. The default is
to use fast inline prologues and epilogues.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The
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default is set depending on whether the processor we are
optimizing for supports fast unaligned access or not.
-mcmodel=medlow
Generate code for the medium-low code model. The program
and its statically defined symbols must lie within a
single 2 GiB address range and must lie between absolute
addresses -2 GiB and +2 GiB. Programs can be statically
or dynamically linked. This is the default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program
and its statically defined symbols must be within any
single 2 GiB address range. Programs can be statically
or dynamically linked.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when
dealing with symbolic addresses. The alternative is to
use assembler macros instead, which may limit
optimization.
RL78 Options
-msim
Links in additional target libraries to support
operation within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and
division support to be used. The simplest is "none",
which uses software for both multiplication and
division. This is the default. The "g13" value is for
the hardware multiply/divide peripheral found on the
RL78/G13 (S2 core) targets. The "g14" value selects the
use of the multiplication and division instructions
supported by the RL78/G14 (S3 core) parts. The value
"rl78" is an alias for "g14" and the value "mg10" is an
alias for "none".
In addition a C preprocessor macro is defined, based
upon the setting of this option. Possible values are:
"__RL78_MUL_NONE__", "__RL78_MUL_G13__" or
"__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
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-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the
G14 core, also known as an S3 core or just RL78. The
G13 or S2 core does not have multiply or divide
instructions, instead it uses a hardware peripheral for
these operations. The G10 or S1 core does not have
register banks, so it uses a different calling
convention.
If this option is set it also selects the type of
hardware multiply support to use, unless this is
overridden by an explicit -mmul=none option on the
command line. Thus specifying -mcpu=g13 enables the use
of the G13 hardware multiply peripheral and specifying
-mcpu=g10 disables the use of hardware multiplications
altogether.
Note, although the RL78/G14 core is the default target,
specifying -mcpu=g14 or -mcpu=rl78 on the command line
does change the behavior of the toolchain since it also
enables G14 hardware multiply support. If these options
are not specified on the command line then software
multiplication routines will be used even though the
code targets the RL78 core. This is for backwards
compatibility with older toolchains which did not have
hardware multiply and divide support.
In addition a C preprocessor macro is defined, based
upon the setting of this option. Possible values are:
"__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option.
They are provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available
registers. By default registers "r24..r31" are reserved
for use in interrupt handlers. With this option enabled
these registers can be used in ordinary functions as
well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles)
or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles.
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-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should
preserve the MDUC registers. This is only necessary if
normal code might use the MDUC registers, for example
because it performs multiplication and division
operations. The default is to ignore the MDUC registers
as this makes the interrupt handlers faster. The target
option -mg13 needs to be passed for this to work as this
feature is only available on the G13 target (S2 core).
The MDUC registers will only be saved if the interrupt
handler performs a multiplication or division operation
or it calls another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and
PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are
available on the processor you are using. The default
value of these options is determined when configuring
GCC. Specifying the -mcpu=cpu_type overrides the
specification of these options. We recommend you use
the -mcpu=cpu_type option rather than the options listed
above.
Specifying -mpowerpc-gpopt allows GCC to use the
optional PowerPC architecture instructions in the
General Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt allows GCC to use the
optional PowerPC architecture instructions in the
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Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from
condition register field instruction implemented on the
POWER4 processor and other processors that support the
PowerPC V2.01 architecture. The -mpopcntb option allows
GCC to generate the popcount and double-precision FP
reciprocal estimate instruction implemented on the
POWER5 processor and other processors that support the
PowerPC V2.02 architecture. The -mpopcntd option allows
GCC to generate the popcount instruction implemented on
the POWER7 processor and other processors that support
the PowerPC V2.06 architecture. The -mfprnd option
allows GCC to generate the FP round to integer
instructions implemented on the POWER5+ processor and
other processors that support the PowerPC V2.03
architecture. The -mcmpb option allows GCC to generate
the compare bytes instruction implemented on the POWER6
processor and other processors that support the PowerPC
V2.05 architecture. The -mmfpgpr option allows GCC to
generate the FP move to/from general-purpose register
instructions implemented on the POWER6X processor and
other processors that support the extended PowerPC V2.05
architecture. The -mhard-dfp option allows GCC to
generate the decimal floating-point instructions
implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the
additional 64-bit instructions that are found in the
full PowerPC64 architecture and to treat GPRs as 64-bit,
doubleword quantities. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction
scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp,
440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603,
603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc,
e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan,
power3, power4, power5, power5+, power6, power6x,
power7, power8, power9, powerpc, powerpc64, powerpc64le,
and rs64.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le
specify pure 32-bit PowerPC (either endian), 64-bit big
endian PowerPC and 64-bit little endian PowerPC
architecture machine types, with an appropriate, generic
processor model assumed for scheduling purposes.
The other options specify a specific processor. Code
generated under those options runs best on that
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processor, and may not run at all on others.
The -mcpu options automatically enable or disable the
following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple
-mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt
-mpowerpc-gfxopt -msingle-float -mdouble-float
-msimple-fpu -mstring -mmulhw -mdlmzb -mmfpgpr -mvsx
-mcrypto -mdirect-move -mhtm -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic
-mfloat128 -mfloat128-hardware
The particular options set for any particular CPU varies
between compiler versions, depending on what setting
seems to produce optimal code for that CPU; it doesn't
necessarily reflect the actual hardware's capabilities.
If you wish to set an individual option to a particular
value, you may specify it after the -mcpu option, like
-mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not
enabled or disabled by the -mcpu option at present
because AIX does not have full support for these
options. You may still enable or disable them
individually if you're sure it'll work in your
environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine
type cpu_type, but do not set the architecture type or
register usage, as -mcpu=cpu_type does. The same values
for cpu_type are used for -mtune as for -mcpu. If both
are specified, the code generated uses the architecture
and registers set by -mcpu, but the scheduling
parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is
limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC
and other static data may be up to a total of 4G in
size. This is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may
be up to 4G in size. Other data and code is only
limited by the 64-bit address space.
-maltivec
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-mno-altivec
Generate code that uses (does not use) AltiVec
instructions, and also enable the use of built-in
functions that allow more direct access to the AltiVec
instruction set. You may also need to set -mabi=altivec
to adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, rather than -maltivec=le or
-maltivec=be, the element order for AltiVec intrinsics
such as "vec_splat", "vec_extract", and "vec_insert"
match array element order corresponding to the
endianness of the target. That is, element zero
identifies the leftmost element in a vector register
when targeting a big-endian platform, and identifies the
rightmost element in a vector register when targeting a
little-endian platform.
-maltivec=be
Generate AltiVec instructions using big-endian element
order, regardless of whether the target is big- or
little-endian. This is the default when targeting a
big-endian platform.
The element order is used to interpret element numbers
in AltiVec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert". By default, these
match array element order corresponding to the
endianness for the target.
-maltivec=le
Generate AltiVec instructions using little-endian
element order, regardless of whether the target is big-
or little-endian. This is the default when targeting a
little-endian platform. This option is currently
ignored when targeting a big-endian platform.
The element order is used to interpret element numbers
in AltiVec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert". By default, these
match array element order corresponding to the
endianness for the target.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec
code.
-mgen-cell-microcode
Generate Cell microcode instructions.
-mwarn-cell-microcode
Warn when a Cell microcode instruction is emitted. An
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example of a Cell microcode instruction is a variable
shift.
-msecure-plt
Generate code that allows ld and ld.so to build
executables and shared libraries with non-executable
".plt" and ".got" sections. This is a PowerPC 32-bit
SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so
fills in, and requires ".plt" and ".got" sections that
are both writable and executable. This is a PowerPC
32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL
instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and
-mno-isel instead.
-mlra
Enable Local Register Allocation. By default the port
uses LRA. (i.e. -mno-lra).
-mspe
-mno-spe
This switch enables or disables the generation of SPE
simd instructions.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED
simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe
instead.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar
(VSX) instructions, and also enable the use of built-in
functions that allow more direct access to the VSX
instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that
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allow direct access to the cryptographic instructions
that were added in version 2.07 of the PowerPC ISA.
-mdirect-move
-mno-direct-move
Generate code that uses (does not use) the instructions
to move data between the general purpose registers and
the vector/scalar (VSX) registers that were added in
version 2.07 of the PowerPC ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that
allow direct access to the Hardware Transactional Memory
(HTM) instructions that were added in version 2.07 of
the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer
operations adjacent so that the instructions can be
fused together on power8 and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and
scalar instructions that were added in version 2.07 of
the PowerPC ISA. Also enable the use of built-in
functions that allow more direct access to the vector
instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic
quad word memory instructions. The -mquad-memory option
requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad
word memory instructions. The -mquad-memory-atomic
option requires use of 64-bit mode.
-mupper-regs-di
-mno-upper-regs-di
Generate code that uses (does not use) the scalar
instructions that target all 64 registers in the
vector/scalar floating point register set that were
added in version 2.06 of the PowerPC ISA when processing
integers. -mupper-regs-di is turned on by default if
you use any of the -mcpu=power7, -mcpu=power8,
-mcpu=power9, or -mvsx options.
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-mupper-regs-df
-mno-upper-regs-df
Generate code that uses (does not use) the scalar double
precision instructions that target all 64 registers in
the vector/scalar floating point register set that were
added in version 2.06 of the PowerPC ISA.
-mupper-regs-df is turned on by default if you use any
of the -mcpu=power7, -mcpu=power8, -mcpu=power9, or
-mvsx options.
-mupper-regs-sf
-mno-upper-regs-sf
Generate code that uses (does not use) the scalar single
precision instructions that target all 64 registers in
the vector/scalar floating point register set that were
added in version 2.07 of the PowerPC ISA.
-mupper-regs-sf is turned on by default if you use
either of the -mcpu=power8, -mpower8-vector, or
-mcpu=power9 options.
-mupper-regs
-mno-upper-regs
Generate code that uses (does not use) the scalar
instructions that target all 64 registers in the
vector/scalar floating point register set, depending on
the model of the machine.
If the -mno-upper-regs option is used, it turns off both
-mupper-regs-sf and -mupper-regs-df options.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit
floating point and use either software emulation for
IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx, -mcpu=power7, or
-mcpu=power8) must be enabled to use the -mfloat128
option. The -mfloat128 option only works on PowerPC
64-bit Linux systems.
If you use the ISA 3.0 instruction set (-mcpu=power9),
the -mfloat128 option will also enable the generation of
ISA 3.0 IEEE 128-bit floating point instructions.
Otherwise, IEEE 128-bit floating point will be done with
software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to
support the __float128 data type.
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If you use -mfloat128-hardware, it will enable the
option -mfloat128 as well.
If you select ISA 3.0 instructions with -mcpu=power9,
but do not use either -mfloat128 or -mfloat128-hardware,
the IEEE 128-bit floating point support will not be
enabled.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of
floating-point operations on the general-purpose
registers for architectures that support it.
The argument yes or single enables the use of single-
precision floating-point operations.
The argument double enables the use of single and
double-precision floating-point operations.
The argument no disables floating-point operations on
the general-purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of
Darwin and SVR4 targets (including GNU/Linux). The
32-bit environment sets int, long and pointer to 32 bits
and generates code that runs on any PowerPC variant.
The 64-bit environment sets int to 32 bits and long and
pointer to 64 bits, and generates code for PowerPC64, as
for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which
is created for every executable file. The -mfull-toc
option is selected by default. In that case, GCC
allocates at least one TOC entry for each unique non-
automatic variable reference in your program. GCC also
places floating-point constants in the TOC. However,
only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you
have overflowed the available TOC space, you can reduce
the amount of TOC space used with the -mno-fp-in-toc and
-mno-sum-in-toc options. -mno-fp-in-toc prevents GCC
from putting floating-point constants in the TOC and
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-mno-sum-in-toc forces GCC to generate code to calculate
the sum of an address and a constant at run time instead
of putting that sum into the TOC. You may specify one
or both of these options. Each causes GCC to produce
very slightly slower and larger code at the expense of
conserving TOC space.
If you still run out of space in the TOC even when you
specify both of these options, specify -mminimal-toc
instead. This option causes GCC to make only one TOC
entry for every file. When you specify this option, GCC
produces code that is slower and larger but which uses
extremely little TOC space. You may wish to use this
option only on files that contain less frequently-
executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit
pointers, 64-bit "long" type, and the infrastructure
needed to support them. Specifying -maix64 implies
-mpowerpc64, while -maix32 disables the 64-bit ABI and
implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL
compiler semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond
the register save area (RSA) on the stack in addition to
argument FPRs. Do not assume that most significant
double in 128-bit long double value is properly rounded
when comparing values and converting to double. Use XL
symbol names for long double support routines.
The AIX calling convention was extended but not
initially documented to handle an obscure K&R C case of
calling a function that takes the address of its
arguments with fewer arguments than declared. IBM XL
compilers access floating-point arguments that do not
fit in the RSA from the stack when a subroutine is
compiled without optimization. Because always storing
floating-point arguments on the stack is inefficient and
rarely needed, this option is not enabled by default and
only is necessary when calling subroutines compiled by
IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link
an application written to use message passing with
special startup code to enable the application to run.
The system must have PE installed in the standard
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location (/usr/lpp/ppe.poe/), or the specs file must be
overridden with the -specs= option to specify the
appropriate directory location. The Parallel
Environment does not support threads, so the -mpe option
and the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the
option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point
doubles, on their natural size-based boundary. The
option -malign-power instructs GCC to follow the ABI-
specified alignment rules. GCC defaults to the standard
alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and
-malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-
point register set. Software floating-point emulation
is provided if you use the -msoft-float option, and pass
the option to GCC when linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-
point operations. -mdouble-float implies
-msingle-float.
-msimple-fpu
Do not generate "sqrt" and "div" instructions for
hardware floating-point unit.
-mfpu=name
Specify type of floating-point unit. Valid values for
name are sp_lite (equivalent to -msingle-float
-msimple-fpu), dp_lite (equivalent to -mdouble-float
-msimple-fpu), sp_full (equivalent to -msingle-float),
and dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on
Xilinx PPC 405/440.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple
word instructions and the store multiple word
instructions. These instructions are generated by
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default on POWER systems, and not generated on PowerPC
systems. Do not use -mmultiple on little-endian PowerPC
systems, since those instructions do not work when the
processor is in little-endian mode. The exceptions are
PPC740 and PPC750 which permit these instructions in
little-endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string
instructions and the store string word instructions to
save multiple registers and do small block moves. These
instructions are generated by default on POWER systems,
and not generated on PowerPC systems. Do not use
-mstring on little-endian PowerPC systems, since those
instructions do not work when the processor is in
little-endian mode. The exceptions are PPC740 and
PPC750 which permit these instructions in little-endian
mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the
address of the calculated memory location. These
instructions are generated by default. If you use
-mno-update, there is a small window between the time
that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks
the stack frame across interrupts or signals may get
corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of
indexed load or store instructions. These instructions
can incur a performance penalty on Power6 processors in
certain situations, such as when stepping through large
arrays that cross a 16M boundary. This option is
enabled by default when targeting Power6 and disabled
otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-
point multiply and accumulate instructions. These
instructions are generated by default if hardware
floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-
independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
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-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word
multiply and multiply-accumulate instructions on the IBM
405, 440, 464 and 476 processors. These instructions
are generated by default when targeting those
processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search
dlmzb instruction on the IBM 405, 440, 464 and 476
processors. This instruction is generated by default
when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do)
force structures and unions that contain bit-fields to
be aligned to the base type of the bit-field.
For example, by default a structure containing nothing
but 8 "unsigned" bit-fields of length 1 is aligned to a
4-byte boundary and has a size of 4 bytes. By using
-mno-bit-align, the structure is aligned to a 1-byte
boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do)
assume that unaligned memory references are handled by
the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static
executable to be relocated to a different address at run
time. A simple embedded PowerPC system loader should
relocate the entire contents of ".got2" and 4-byte
locations listed in the ".fixup" section, a table of
32-bit addresses generated by this option. For this to
work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib. -mrelocatable code
aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a
".fixup" section to allow static executables to be
relocated at run time, but -mrelocatable-lib does not
use the smaller stack alignment of -mrelocatable.
Objects compiled with -mrelocatable-lib may be linked
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with objects compiled with any combination of the
-mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do)
assume that register 2 contains a pointer to a global
area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code
for the processor in little-endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code
for the processor in big-endian mode. The -mbig-endian
option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it
is not relocatable, but that its external references are
relocatable. The resulting code is suitable for
applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each
function. The runtime system is responsible for
initializing this register with an appropriate value
before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to
dispatch-slot restricted instructions during the second
scheduling pass. The argument priority takes the value
0, 1, or 2 to assign no, highest, or second-highest
(respectively) priority to dispatch-slot restricted
instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered
costly by the target during instruction scheduling. The
argument dependence_type takes one of the following
values:
no No dependence is costly.
all All dependences are costly.
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true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than
or equal to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used
during the second scheduling pass. The argument scheme
takes one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant
issue slots, according to the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into
separate groups. Insert exactly as many NOPs as
needed to force an insn to a new group, according to
the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into
separate groups. Insert number NOPs to force an
insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code
using calling conventions that adhere to the March 1995
draft of the System V Application Binary Interface,
PowerPC processor supplement. This is the default
unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code
for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code
for the Linux-based GNU system.
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-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code
for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code
for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code
for the OpenBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX
ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as
specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or
remove such extension. Valid values are altivec, no-
altivec, spe, no-spe, ibmlongdouble, ieeelongdouble,
elfv1, elfv2.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This
does not change the default ABI, instead it adds the SPE
ABI extensions to the current ABI.
-mabi=no-spe
Disable Book-E SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision
long double. This is a PowerPC 32-bit SYSV ABI option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision
long double. This is a PowerPC 32-bit Linux ABI option.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is
the default ABI for big-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system
support and is likely to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is
the default ABI for little-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system
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support and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value
pairs in a .gnu.attributes section that specify ABI
variations in function parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that
all calls to variable argument functions are properly
prototyped. Otherwise, the compiler must insert an
instruction before every non-prototyped call to set or
clear bit 6 of the condition code register ("CR") to
indicate whether floating-point values are passed in the
floating-point registers in case the function takes
variable arguments. With -mprototype, only calls to
prototyped variable argument functions set or clear the
bit.
-msim
On embedded PowerPC systems, assume that the startup
module is called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the default
for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup
module is called crt0.o and the standard C libraries are
libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup
module is called crt0.o and the standard C libraries are
libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup
module is called crt0.o and the standard C libraries are
libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that
you are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in
the ELF flags header to indicate that eabi extended
relocations are used.
-meabi
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-mno-eabi
On System V.4 and embedded PowerPC systems do (do not)
adhere to the Embedded Applications Binary Interface
(EABI), which is a set of modifications to the System
V.4 specifications. Selecting -meabi means that the
stack is aligned to an 8-byte boundary, a function
"__eabi" is called from "main" to set up the EABI
environment, and the -msdata option can use both "r2"
and "r13" to point to two separate small data areas.
Selecting -mno-eabi means that the stack is aligned to a
16-byte boundary, no EABI initialization function is
called from "main", and the -msdata option only uses
"r13" to point to a single small data area. The -meabi
option is on by default if you configured GCC using one
of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small
initialized "const" global and static data in the
".sdata2" section, which is pointed to by register "r2".
Put small initialized non-"const" global and static data
in the ".sdata" section, which is pointed to by register
"r13". Put small uninitialized global and static data
in the ".sbss" section, which is adjacent to the
".sdata" section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The
-msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small
global and static data in the ".sdata" section, which is
pointed to by register "r13". Put small uninitialized
global and static data in the ".sbss" section, which is
adjacent to the ".sdata" section. The -msdata=sysv
option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is
used, compile code the same as -msdata=eabi, otherwise
compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small
global data in the ".sdata" section. Put small
uninitialized global data in the ".sbss" section. Do
not use register "r13" to address small data however.
This is the default behavior unless other -msdata
options are used.
-msdata=none
-mno-sdata
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On embedded PowerPC systems, put all initialized global
and static data in the ".data" section, and all
uninitialized data in the ".bss" section.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or
structure copies) less than or equal to num bytes. The
minimum value for num is 32 bytes on 32-bit targets and
64 bytes on 64-bit targets. The default value is
target-specific.
-G num
On embedded PowerPC systems, put global and static items
less than or equal to num bytes into the small data or
BSS sections instead of the normal data or BSS section.
By default, num is 8. The -G num switch is also passed
to the linker. All modules should be compiled with the
same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not)
emit register names in the assembly language output
using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a
longer and more expensive calling sequence is required.
This is required for calls farther than 32 megabytes
(33,554,432 bytes) from the current location. A short
call is generated if the compiler knows the call cannot
be that far away. This setting can be overridden by the
"shortcall" function attribute, or by "#pragma
longcall(0)".
Some linkers are capable of detecting out-of-range calls
and generating glue code on the fly. On these systems,
long calls are unnecessary and generate slower code. As
of this writing, the AIX linker can do this, as can the
GNU linker for PowerPC/64. It is planned to add this
feature to the GNU linker for 32-bit PowerPC systems as
well.
On Darwin/PPC systems, "#pragma longcall" generates
"jbsr callee, L42", plus a branch island (glue code).
The two target addresses represent the callee and the
branch island. The Darwin/PPC linker prefers the first
address and generates a "bl callee" if the PPC "bl"
instruction reaches the callee directly; otherwise, the
linker generates "bl L42" to call the branch island.
The branch island is appended to the body of the calling
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function; it computes the full 32-bit address of the
callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the
compiler emit to the glue for every direct call, and the
Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall
specifications when the linker is known to generate
glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a
relocation specifying the function argument. The
relocation allows the linker to reliably associate
function call with argument setup instructions for TLS
optimization, which in turn allows GCC to better
schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and
reciprocal square root estimate instructions with
additional Newton-Raphson steps to increase precision
instead of doing a divide or square root and divide for
floating-point arguments. You should use the
-ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -ffinite-math-only,
-freciprocal-math and -fno-trapping-math). Note that
while the throughput of the sequence is generally higher
than the throughput of the non-reciprocal instruction,
the precision of the sequence can be decreased by up to
2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for
reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate
instructions may be used. opt is a comma-separated list
of options, which may be preceded by a "!" to invert the
option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to
-mrecip.
none
Disable all estimate instructions, equivalent to
-mno-recip.
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div Enable the reciprocal approximation instructions for
both single and double precision.
divf
Enable the single-precision reciprocal approximation
instructions.
divd
Enable the double-precision reciprocal approximation
instructions.
rsqrt
Enable the reciprocal square root approximation
instructions for both single and double precision.
rsqrtf
Enable the single-precision reciprocal square root
approximation instructions.
rsqrtd
Enable the double-precision reciprocal square root
approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the
reciprocal estimate instructions, except for the
"FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions
which handle the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate
instructions provide higher-precision estimates than is
mandated by the PowerPC ABI. Selecting -mcpu=power6,
-mcpu=power7 or -mcpu=power8 automatically selects
-mrecip-precision. The double-precision square root
estimate instructions are not generated by default on
low-precision machines, since they do not provide an
estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics
using an external library. The only type supported at
present is mass, which specifies to use IBM's
Mathematical Acceleration Subsystem (MASS) libraries for
vectorizing intrinsics using external libraries. GCC
currently emits calls to "acosd2", "acosf4", "acoshd2",
"acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4",
"atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2",
"atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4",
"coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2",
"erff4", "exp2d2", "exp2f4", "expd2", "expf4",
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"expm1d2", "expm1f4", "hypotd2", "hypotf4", "lgammad2",
"lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4",
"sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2",
"sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when
generating code for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The
MASS libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when
the -funsafe-math-optimizations option is used to
optimize rounding of floating-point values to 64-bit
integer and back to floating point. The "friz"
instruction does not return the same value if the
floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static
chain register ("r11") when calling through a pointer on
AIX and 64-bit Linux systems where a function pointer
points to a 3-word descriptor giving the function
address, TOC value to be loaded in register "r2", and
static chain value to be loaded in register "r11". The
-mpointers-to-nested-functions is on by default. You
cannot call through pointers to nested functions or
pointers to functions compiled in other languages that
use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in
the reserved stack location in the function prologue if
the function calls through a pointer on AIX and 64-bit
Linux systems. If the TOC value is not saved in the
prologue, it is saved just before the call through the
pointer. The -mno-save-toc-indirect option is the
default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure
parameters with a maximum alignment of 64 bits, for
compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did
not align a structure parameter on a 128-bit boundary
when that structure contained a member requiring 128-bit
alignment. This is corrected in more recent versions of
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GCC. This option may be used to generate code that is
compatible with functions compiled with older versions
of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard.
Supported locations are global for global canary or tls
for per-thread canary in the TLS block (the default with
GNU libc version 2.4 or later).
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore
specify which register to use as base register for
reading the canary, and from what offset from that base
register. The default for those is as specified in the
relevant ABI.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles)
or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX floating-point hardware only
works on 32-bit values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX
floating-point hardware. The default is enabled for the
RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for
32-bit floating-point values, however, so the FPU
hardware is not used for doubles if the -m64bit-doubles
option is used.
Note If the -fpu option is enabled then
-funsafe-math-optimizations is also enabled
automatically. This is because the RX FPU instructions
are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently
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three types are supported, the generic RX600 and RX200
series hardware and the specific RX610 CPU. The default
is RX600.
The only difference between RX600 and RX610 is that the
RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point
unit and so -nofpu is enabled by default when this type
is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The
default is -mlittle-endian-data, i.e. to store data in
the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static
variables which can be placed into the small data area.
Using the small data area can lead to smaller and faster
code, but the size of area is limited and it is up to
the programmer to ensure that the area does not
overflow. Also when the small data area is used one of
the RX's registers (usually "r13") is reserved for use
pointing to this area, so it is no longer available for
use by the compiler. This could result in slower and/or
larger code if variables are pushed onto the stack
instead of being held in this register.
Note, common variables (variables that have not been
initialized) and constants are not placed into the small
data area as they are assigned to other sections in the
output executable.
The default value is zero, which disables this feature.
Note, this feature is not enabled by default with higher
optimization levels (-O2 etc) because of the potentially
detrimental effects of reserving a register. It is up
to the programmer to experiment and discover whether
this feature is of benefit to their program. See the
description of the -mpid option for a description of how
the actual register to hold the small data area pointer
is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the
libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
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When generating assembler output use a syntax that is
compatible with Renesas's AS100 assembler. This syntax
can also be handled by the GAS assembler, but it has
some restrictions so it is not generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that
can be used as an operand in a RX instruction. Although
the RX instruction set does allow constants of up to 4
bytes in length to be used in instructions, a longer
value equates to a longer instruction. Thus in some
circumstances it can be beneficial to restrict the size
of constants that are used in instructions. Constants
that are too big are instead placed into a constant pool
and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the
default) or 4 means that constants of any size are
allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a
process whereby the linker attempts to reduce the size
of a program by finding shorter versions of various
instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast
interrupt handler functions. The value N can be between
0 and 4. A value of 1 means that register "r13" is
reserved for the exclusive use of fast interrupt
handlers. A value of 2 reserves "r13" and "r12". A
value of 3 reserves "r13", "r12" and "r11", and a value
of 4 reserves "r13" through "r10". A value of 0, the
default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should
preserve the accumulator register. This is only
necessary if normal code might use the accumulator
register, for example because it performs 64-bit
multiplications. The default is to ignore the
accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data.
When enabled any access to constant data is done via an
offset from a base address held in a register. This
allows the location of constant data to be determined at
run time without requiring the executable to be
relocated, which is a benefit to embedded applications
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with tight memory constraints. Data that can be
modified is not affected by this option.
Note, using this feature reserves a register, usually
"r13", for the constant data base address. This can
result in slower and/or larger code, especially in
complicated functions.
The actual register chosen to hold the constant data
base address depends upon whether the -msmall-data-limit
and/or the -mint-register command-line options are
enabled. Starting with register "r13" and proceeding
downwards, registers are allocated first to satisfy the
requirements of -mint-register, then -mpid and finally
-msmall-data-limit. Thus it is possible for the small
data area register to be "r8" if both -mint-register=4
and -mpid are specified on the command line.
By default this feature is not enabled. The default can
be restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds
more than one fast interrupt handler when it is
compiling a file. The default is to issue a warning for
each extra fast interrupt handler found, as the RX only
supports one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation
instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU",
"SUNTIL" "SWHILE" and also the "RMPA" instruction.
These instructions may prefetch data, which is not safe
to do if accessing an I/O register. (See section 12.2.7
of the RX62N Group User's Manual for more information).
The default is to allow these instructions, but it is
not possible for GCC to reliably detect all
circumstances where a string instruction might be used
to access an I/O register, so their use cannot be
disabled automatically. Instead it is reliant upon the
programmer to use the -mno-allow-string-insns option if
their program accesses I/O space.
When the instructions are enabled GCC defines the C
preprocessor symbol "__RX_ALLOW_STRING_INSNS__",
otherwise it defines the symbol
"__RX_DISALLOW_STRING_INSNS__".
-mjsr
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-mno-jsr
Use only (or not only) "JSR" instructions to access
functions. This option can be used when code size
exceeds the range of "BSR" instructions. Note that
-mno-jsr does not mean to not use "JSR" but instead
means that any type of branch may be used.
Note: The generic GCC command-line option -ffixed-reg has
special significance to the RX port when used with the
"interrupt" function attribute. This attribute indicates a
function intended to process fast interrupts. GCC ensures
that it only uses the registers "r10", "r11", "r12" and/or
"r13" and only provided that the normal use of the
corresponding registers have been restricted via the
-ffixed-reg or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries
architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point
instructions and registers for floating-point
operations. When -msoft-float is specified, functions
in libgcc.a are used to perform floating-point
operations. When -mhard-float is specified, the
compiler generates IEEE floating-point instructions.
This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point
instructions for decimal-floating-point operations.
When -mno-hard-dfp is specified, functions in libgcc.a
are used to perform decimal-floating-point operations.
When -mhard-dfp is specified, the compiler generates
decimal-floating-point hardware instructions. This is
the default for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A
size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame
as backchain pointer into the callee's stack frame. A
backchain may be needed to allow debugging using tools
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that do not understand DWARF call frame information.
When -mno-packed-stack is in effect, the backchain
pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed
into the topmost word of the 96/160 byte register save
area.
In general, code compiled with -mbackchain is call-
compatible with code compiled with -mmo-backchain;
however, use of the backchain for debugging purposes
usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In
order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When
-mno-packed-stack is specified, the compiler uses the
all fields of the 96/160 byte register save area only
for their default purpose; unused fields still take up
stack space. When -mpacked-stack is specified, register
save slots are densely packed at the top of the register
save area; unused space is reused for other purposes,
allowing for more efficient use of the available stack
space. However, when -mbackchain is also in effect, the
topmost word of the save area is always used to store
the backchain, and the return address register is always
saved two words below the backchain.
As long as the stack frame backchain is not used, code
generated with -mpacked-stack is call-compatible with
code generated with -mno-packed-stack. Note that some
non-FSF releases of GCC 2.95 for S/390 or zSeries
generated code that uses the stack frame backchain at
run time, not just for debugging purposes. Such code is
not call-compatible with code compiled with
-mpacked-stack. Also, note that the combination of
-mbackchain, -mpacked-stack and -mhard-float is not
supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras"
instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed
64k. The default is to use the "basr" instruction
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instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the
GNU/Linux for S/390 ABI. When -m64 is specified,
generate code compliant to the GNU/Linux for zSeries
ABI. This allows GCC in particular to generate 64-bit
instructions. For the s390 targets, the default is
-m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the
instructions available on z/Architecture. When -mesa is
specified, generate code using the instructions
available on ESA/390. Note that -mesa is not possible
with -m64. When generating code compliant to the
GNU/Linux for S/390 ABI, the default is -mesa. When
generating code compliant to the GNU/Linux for zSeries
ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of
instructions available with the transactional execution
facility introduced with the IBM zEnterprise EC12
machine generation S/390 System z Built-in Functions.
-mhtm is enabled by default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the
instructions available with the vector extension
facility introduced with the IBM z13 machine generation.
This option changes the ABI for some vector type values
with regard to alignment and calling conventions. In
case vector type values are being used in an ABI-
relevant context a GAS .gnu_attribute command will be
added to mark the resulting binary with the ABI used.
-mvx is enabled by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions
and builtins using instructions available with the
vector extension facility introduced with the IBM z13
machine generation. This option adds support for vector
to be used as a keyword to define vector type variables
and arguments. vector is only available when GNU
extensions are enabled. It will not be expanded when
requesting strict standard compliance e.g. with
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-std=c99. In addition to the GCC low-level builtins
-mzvector enables a set of builtins added for
compatibility with AltiVec-style implementations like
Power and Cell. In order to make use of these builtins
the header file vecintrin.h needs to be included.
-mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle"
instruction to perform block moves. When -mno-mvcle is
specified, use a "mvc" loop instead. This is the
default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information
when compiling. The default is to not print debug
information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name
of a system representing a certain processor type.
Possible values for cpu-type are z900/arch5, z990/arch6,
z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12,
z13/arch11, and native.
The default is -march=z900. g5/arch3 and g6 are
deprecated and will be removed with future releases.
Specifying native as cpu type can be used to select the
best architecture option for the host processor.
-march=native has no effect if GCC does not recognize
the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of
available instructions. The list of cpu-type values is
the same as for -march. The default is the value used
for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS
specific branches to trace routines in the operating
system. This option is off by default, even when
compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-
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point multiply and accumulate instructions. These
instructions are generated by default if hardware
floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given
frame size. Because this is a compile-time check it
doesn't need to be a real problem when the program runs.
It is intended to identify functions that most probably
cause a stack overflow. It is useful to be used in an
environment with limited stack size e.g. the linux
kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses
dynamically-sized arrays. This is generally a bad idea
with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits
additional instructions in the function prologue that
trigger a trap if the stack size is stack-guard bytes
above the stack-size (remember that the stack on S/390
grows downward). If the stack-guard option is omitted
the smallest power of 2 larger than the frame size of
the compiled function is chosen. These options are
intended to be used to help debugging stack overflow
problems. The additionally emitted code causes only
little overhead and hence can also be used in
production-like systems without greater performance
degradation. The given values have to be exact powers
of 2 and stack-size has to be greater than stack-guard
without exceeding 64k. In order to be efficient the
extra code makes the assumption that the stack starts at
an address aligned to the value given by stack-size.
The stack-guard option can only be used in conjunction
with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching"
function prologue is generated for all functions in the
compilation unit. The funtion label is prepended with
the given number of two-byte NOP instructions (pre-
halfwords, maximum 1000000). After the label, 2 *
post-halfwords bytes are appended, using the largest NOP
like instructions the architecture allows (maximum
1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions
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with the "hotpatch" attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store
instructions.
-mmac
Enable the use of multiply-accumulate instructions.
Disabled by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is
the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a
SH2a-FPU in such a way that the floating-point unit is
not used.
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-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no
double-precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-
point unit is in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-
point unit is in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit
that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point
unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the
floating-point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point
unit is in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-
precision floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the
floating-point unit is not used.
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-m4-200-single
Generate code for SH4-200 assuming the floating-point
unit is in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-
precision floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the
floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-
precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-
precision floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes
-isa=sh4-nofpu to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a
way that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no
double-precision floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point
unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes
-dsp to the assembler. GCC doesn't generate any DSP
instructions at the moment.
-mb Compile code for the processor in big-endian mode.
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-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this
changes the calling conventions, and thus some functions
from the standard C library do not work unless you
recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when
possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is
to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check
-mdalign for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC
before the Renesas conventions were available. This
option is the default for all targets of the SH
toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if
-mrenesas is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point
comparisons, which affects the handling of cases where
the result of a comparison is unordered. By default
-mieee is implicitly enabled. If -ffinite-math-only is
enabled -mno-ieee is implicitly set, which results in
faster floating-point greater-equal and less-equal
comparisons. The implicit settings can be overridden by
specifying either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries
after setting up nested function trampolines. This
option has no effect if -musermode is in effect and the
selected code generation option (e.g. -m4) does not
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allow the use of the "icbi" instruction. If the
selected code generation option does not allow the use
of the "icbi" instruction, and -musermode is not in
effect, the inlined code manipulates the instruction
cache address array directly with an associative write.
This not only requires privileged mode at run time, but
it also fails if the cache line had been mapped via the
TLB and has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to
multiple of 4 bytes, which is incompatible with the SH
ABI.
-matomic-model=model
Sets the model of atomic operations and additional
parameters as a comma separated list. For details on
the atomic built-in functions see __atomic Builtins.
The following models and parameters are supported:
none
Disable compiler generated atomic sequences and emit
library calls for atomic operations. This is the
default if the target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic
sequences for the atomic built-in functions. The
generated atomic sequences require additional
support from the interrupt/exception handling code
of the system and are only suitable for SH3* and
SH4* single-core systems. This option is enabled by
default when the target is "sh*-*-linux*" and SH3*
or SH4*. When the target is SH4A, this option also
partially utilizes the hardware atomic instructions
"movli.l" and "movco.l" to create more efficient
code, unless strict is specified.
soft-tcb
Generate software atomic sequences that use a
variable in the thread control block. This is a
variation of the gUSA sequences which can also be
used on SH1* and SH2* targets. The generated atomic
sequences require additional support from the
interrupt/exception handling code of the system and
are only suitable for single-core systems. When
using this model, the gbr-offset= parameter has to
be specified as well.
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soft-imask
Generate software atomic sequences that temporarily
disable interrupts by setting "SR.IMASK = 1111".
This model works only when the program runs in
privileged mode and is only suitable for single-core
systems. Additional support from the
interrupt/exception handling code of the system is
not required. This model is enabled by default when
the target is "sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the
"movli.l" and "movco.l" instructions only. This is
only available on SH4A and is suitable for multi-
core systems. Since the hardware instructions
support only 32 bit atomic variables access to 8 or
16 bit variables is emulated with 32 bit accesses.
Code compiled with this option is also compatible
with other software atomic model interrupt/exception
handling systems if executed on an SH4A system.
Additional support from the interrupt/exception
handling code of the system is not required for this
model.
gbr-offset=
This parameter specifies the offset in bytes of the
variable in the thread control block structure that
should be used by the generated atomic sequences
when the soft-tcb model has been selected. For
other models this parameter is ignored. The
specified value must be an integer multiple of four
and in the range 0-1020.
strict
This parameter prevents mixed usage of multiple
atomic models, even if they are compatible, and
makes the compiler generate atomic sequences of the
specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set".
Notice that depending on the particular hardware and
software configuration this can degrade overall
performance due to the operand cache line flushes that
are implied by the "tas.b" instruction. On multi-core
SH4A processors the "tas.b" instruction must be used
with caution since it can result in data corruption for
certain cache configurations.
-mprefergot
When generating position-independent code, emit function
calls using the Global Offset Table instead of the
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Procedure Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged
mode code. Specifying -musermode also implies
-mno-inline-ic_invalidate if the inlined code would not
work in user mode. -musermode is the default when the
target is "sh*-*-linux*". If the target is SH1* or SH2*
-musermode has no effect, since there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer
division operations. strategy can be one of:
call-div1
Calls a library function that uses the single-step
division instruction "div1" to perform the
operation. Division by zero calculates an
unspecified result and does not trap. This is the
default except for SH4, SH2A and SHcompact.
call-fp
Calls a library function that performs the operation
in double precision floating point. Division by
zero causes a floating-point exception. This is the
default for SHcompact with FPU. Specifying this for
targets that do not have a double precision FPU
defaults to "call-div1".
call-table
Calls a library function that uses a lookup table
for small divisors and the "div1" instruction with
case distinction for larger divisors. Division by
zero calculates an unspecified result and does not
trap. This is the default for SH4. Specifying this
for targets that do not have dynamic shift
instructions defaults to "call-div1".
When a division strategy has not been specified the
default strategy is selected based on the current
target. For SH2A the default strategy is to use the
"divs" and "divu" instructions instead of library
function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the
function prologue rather than around each call.
Generally beneficial for performance and size. Also
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needed for unwinding to avoid changing the stack frame
around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit
signed division to name. This only affects the name
used in the call division strategies, and the compiler
still expects the same sets of input/output/clobbered
registers as if this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator can not use. This is useful when compiling
kernel code. A register range is specified as two
registers separated by a dash. Multiple register ranges
can be specified separated by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction.
Higher numbers make the compiler try to generate more
branch-free code if possible. If not specified the value
is selected depending on the processor type that is
being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement
conditional branch instructions "bt" and "bf" are fast.
If -mzdcbranch is specified, the compiler prefers zero
displacement branch code sequences. This is enabled by
default when generating code for SH4 and SH4A. It can
be explicitly disabled by specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches,
which stuffs the delay slot with a "nop" if a suitable
instruction cannot be found. By default this option is
disabled. It can be enabled to work around hardware
bugs as found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-
point multiply and accumulate instructions. These
instructions are generated by default if hardware
floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-
independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
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-mno-fsca
Allow or disallow the compiler to emit the "fsca"
instruction for sine and cosine approximations. The
option -mfsca must be used in combination with
-funsafe-math-optimizations. It is enabled by default
when generating code for SH4A. Using -mno-fsca disables
sine and cosine approximations even if
-funsafe-math-optimizations is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra"
instruction for reciprocal square root approximations.
The option -mfsrra must be used in combination with
-funsafe-math-optimizations and -ffinite-math-only. It
is enabled by default when generating code for SH4A.
Using -mno-fsrra disables reciprocal square root
approximations even if -funsafe-math-optimizations and
-ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for
conditional move instruction patterns. This can result
in faster code on the SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware
capabilities generated by the Solaris assembler. This
is only necessary when object files use ISA extensions
not supported by the current machine, but check at
runtime whether or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the
compiler to not pass -z text to the linker when linking
a shared object. Using this option, you can link
position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error
message. However, the necessary relocations trigger
copy-on-write, and the shared object is not actually
shared across processes. Instead of using
-mimpure-text, you should compile all source code with
-fpic or -fPIC.
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These switches are supported in addition to the above on
Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global
registers 2 through 4, which the SPARC SVR4 ABI reserves
for applications. Like the global register 1, each
global register 2 through 4 is then treated as an
allocable register that is clobbered by function calls.
This is the default.
To be fully SVR4 ABI-compliant at the cost of some
performance loss, specify -mno-app-regs. You should
compile libraries and system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore
instructions and uses a "flat" or single register window
model. This model is compatible with the regular
register window model. The local registers and the
input registers (0--5) are still treated as "call-saved"
registers and are saved on the stack as needed.
With -mno-flat (the default), the compiler generates
save/restore instructions (except for leaf functions).
This is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions.
This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating
point. Warning: the requisite libraries are not
available for all SPARC targets. Normally the
facilities of the machine's usual C compiler are used,
but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable
library functions for cross-compilation. The embedded
targets sparc-*-aout and sparclite-*-* do provide
software floating-point support.
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-msoft-float changes the calling convention in the
output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you
need to compile libgcc.a, the library that comes with
GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double)
floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word
(long double) floating-point instructions. The
functions called are those specified in the SPARC ABI.
This is the default.
As of this writing, there are no SPARC implementations
that have hardware support for the quad-word floating-
point instructions. They all invoke a trap handler for
one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the
trap handler overhead, this is much slower than calling
the ABI library routines. Thus the -msoft-quad-float
option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the
default.
With -munaligned-doubles, GCC assumes that doubles have
8-byte alignment only if they are contained in another
type, or if they have an absolute address. Otherwise,
it assumes they have 4-byte alignment. Specifying this
option avoids some rare compatibility problems with code
generated by other compilers. It is not the default
because it results in a performance loss, especially for
floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor
mode. This is relevant only for the "casa" instruction
emitted for the LEON3 processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that
structures should have 8-byte alignment. This enables
the use of pairs of "ldd" and "std" instructions for
copies in structure assignment, in place of twice as
many "ld" and "st" pairs. However, the use of this
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changed alignment directly violates the SPARC ABI.
Thus, it's intended only for use on targets where the
developer acknowledges that their resulting code is not
directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates
checking code in functions returning structures or
unions to detect size mismatches between the two sides
of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has
no effect in 64-bit mode.
-mlra
-mno-lra
Enable Local Register Allocation. This is the default
for SPARC since GCC 7 so -mno-lra needs to be passed to
get old Reload.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type.
Supported values for cpu_type are v7, cypress, v8,
supersparc, hypersparc, leon, leon3, leon3v7, sparclite,
f930, f934, sparclite86x, sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the
value native, which selects the best architecture option
for the host processor. -mcpu=native has no effect if
GCC does not recognize the processor.
Default instruction scheduling parameters are used for
values that select an architecture and not an
implementation. These are v7, v8, sparclite, sparclet,
v9.
Here is a list of each supported architecture and their
supported implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
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v9 ultrasparc, ultrasparc3, niagara, niagara2,
niagara3, niagara4, niagara7, m8
By default (unless configured otherwise), GCC generates
code for the V7 variant of the SPARC architecture. With
-mcpu=cypress, the compiler additionally optimizes it
for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also
appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of
the SPARC architecture. The only difference from V7
code is that the compiler emits the integer multiply and
integer divide instructions which exist in SPARC-V8 but
not in SPARC-V7. With -mcpu=supersparc, the compiler
additionally optimizes it for the SuperSPARC chip, as
used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the
SPARClite variant of the SPARC architecture. This adds
the integer multiply, integer divide step and scan
("ffs") instructions which exist in SPARClite but not in
SPARC-V7. With -mcpu=f930, the compiler additionally
optimizes it for the Fujitsu MB86930 chip, which is the
original SPARClite, with no FPU. With -mcpu=f934, the
compiler additionally optimizes it for the Fujitsu
MB86934 chip, which is the more recent SPARClite with
FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet
variant of the SPARC architecture. This adds the
integer multiply, multiply/accumulate, integer divide
step and scan ("ffs") instructions which exist in
SPARClet but not in SPARC-V7. With -mcpu=tsc701, the
compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of
the SPARC architecture. This adds 64-bit integer and
floating-point move instructions, 3 additional
floating-point condition code registers and conditional
move instructions. With -mcpu=ultrasparc, the compiler
additionally optimizes it for the Sun UltraSPARC
I/II/IIi chips. With -mcpu=ultrasparc3, the compiler
additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara,
the compiler additionally optimizes it for Sun
UltraSPARC T1 chips. With -mcpu=niagara2, the compiler
additionally optimizes it for Sun UltraSPARC T2 chips.
With -mcpu=niagara3, the compiler additionally optimizes
it for Sun UltraSPARC T3 chips. With -mcpu=niagara4,
the compiler additionally optimizes it for Sun
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UltraSPARC T4 chips. With -mcpu=niagara7, the compiler
additionally optimizes it for Oracle SPARC M7 chips.
With -mcpu=m8, the compiler additionally optimizes it
for Oracle M8 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine
type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those
that select a particular CPU implementation. Those are
cypress, supersparc, hypersparc, leon, leon3, leon3v7,
f930, f934, sparclite86x, tsc701, ultrasparc,
ultrasparc3, niagara, niagara2, niagara3, niagara4,
niagara7 and m8. With native Solaris and GNU/Linux
toolchains, native can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI.
The difference from the V8 ABI is that the global and
out registers are considered 64 bits wide. This is
enabled by default on Solaris in 32-bit mode for all
SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of
the UltraSPARC Visual Instruction Set extensions. The
default is -mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of
version 2.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis2 when targeting a cpu
that supports such instructions, such as UltraSPARC-III
and later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of
version 3.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis3 when targeting a cpu
that supports such instructions, such as niagara-3 and
later. Setting -mvis3 also sets -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of
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version 4.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis4 when targeting a cpu
that supports such instructions, such as niagara-7 and
later. Setting -mvis4 also sets -mvis3, -mvis2 and
-mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of
version 4.0 of the UltraSPARC Visual Instruction Set
extensions, plus the additional VIS instructions
introduced in the Oracle SPARC Architecture 2017. The
default is -mvis4b when targeting a cpu that supports
such instructions, such as m8 and later. Setting
-mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage
of the UltraSPARC Compare-and-Branch-on-Condition
instructions. The default is -mcbcond when targeting a
CPU that supports such instructions, such as Niagara-4
and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of
the UltraSPARC Fused Multiply-Add Floating-point
instructions. The default is -mfmaf when targeting a
CPU that supports such instructions, such as Niagara-3
and later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage
of the Floating-point Multiply Single to Double (FsMULd)
instruction. The default is -mfsmuld when targeting a
CPU supporting the architecture versions V8 or V9 with
FPU except -mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of
the UltraSPARC Population Count instruction. The
default is -mpopc when targeting a CPU that supports
such an instruction, such as Niagara-2 and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of
the UltraSPARC Subtract-Extended-with-Carry instruction.
The default is -msubxc when targeting a CPU that
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supports such an instruction, such as Niagara-7 and
later.
-mfix-at697f
Enable the documented workaround for the single erratum
of the Atmel AT697F processor (which corresponds to
erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point
errata and the data cache nullify errata of the UT699
processor.
-mfix-ut700
Enable the documented workaround for the back-to-back
store errata of the UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back
store errata of the GR712RC processor.
These -m options are supported in addition to the above on
SPARC-V9 processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses,
programs must be linked in the low 32 bits of
memory. Programs can be statically or dynamically
linked.
medmid
The Medium/Middle code model: 64-bit addresses,
programs must be linked in the low 44 bits of
memory, the text and data segments must be less than
2GB in size and the data segment must be located
within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses,
programs may be linked anywhere in memory, the text
and data segments must be less than 2GB in size and
the data segment must be located within 2GB of the
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text segment.
embmedany
The Medium/Anywhere code model for embedded systems:
64-bit addresses, the text and data segments must be
less than 2GB in size, both starting anywhere in
memory (determined at link time). The global
register %g4 points to the base of the data segment.
Programs are statically linked and PIC is not
supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and
operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D
of the SPARC-V9 architecture manual, as set in the
processor's "PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer,
and frame pointer if present, are offset by -2047 which
must be added back when making stack frame references.
This is the default in 64-bit mode. Otherwise, assume
no such offset is present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations.
By default, GCC gives an error when it generates code
that requires a dynamic relocation. -mno-error-reloc
disables the error, -mwarn-reloc generates a warning
instead.
-msafe-dma
-munsafe-dma
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Instructions that initiate or test completion of DMA
must not be reordered with respect to loads and stores
of the memory that is being accessed. With -munsafe-dma
you must use the "volatile" keyword to protect memory
accesses, but that can lead to inefficient code in
places where the memory is known to not change. Rather
than mark the memory as volatile, you can use -msafe-dma
to tell the compiler to treat the DMA instructions as
potentially affecting all memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to
avoid pipeline stalls for always-taken or probably-taken
branches. A hint is not generated closer than 8
instructions away from its branch. There is little
reason to disable them, except for debugging purposes,
or to make an object a little bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses
are never larger than 18 bits. With -mlarge-mem code is
generated that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes
the SPU-style main function interface (which has an
unconventional parameter list). With -mstdmain, GCC
links your program against startup code that assumes a
C99-style interface to "main", including a local copy of
"argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling
kernel code. A register range is specified as two
registers separated by a dash. Multiple register ranges
can be specified separated by a comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address
space accessed via the "__ea" named address space
qualifier are either 32 or 64 bits wide. The default is
32 bits. As this is an ABI-changing option, all object
code in an executable must be compiled with the same
setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as
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superset of the generic address space. This enables
explicit type casts between "__ea" and generic pointer
as well as implicit conversions of generic pointers to
"__ea" pointers. The default is to allow address space
pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the
compiler links to an executable and selects a software-
managed cache for accessing variables in the "__ea"
address space with a particular cache size. Possible
options for cache-size are 8, 16, 32, 64 and 128. The
default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the
compiler links to an executable and selects whether
atomic updates to the software-managed cache of PPU-side
variables are used. If you use atomic updates, changes
to a PPU variable from SPU code using the "__ea" named
address space qualifier do not interfere with changes to
other PPU variables residing in the same cache line from
PPU code. If you do not use atomic updates, such
interference may occur; however, writing back cache
lines is more efficient. The default behavior is to use
atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts NOPs to increase dual issue when
it expects it to increase performance. n can be a value
from 0 to 10. A smaller n inserts fewer NOPs. 10 is
the default, 0 is the same as -mno-dual-nops. Disabled
with -Os.
-mhint-max-nops=n
Maximum number of NOPs to insert for a branch hint. A
branch hint must be at least 8 instructions away from
the branch it is affecting. GCC inserts up to n NOPs to
enforce this, otherwise it does not generate the branch
hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the
hint to be within 256 instructions of the branch it is
affecting. By default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall
indefinitely. By default, GCC inserts the "hbrp"
instruction to make sure this stall won't happen.
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Options for System V
These additional options are available on System V Release 4
for compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that
-symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler,
in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output
file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for
libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor.
The assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for
direct calls is limited to 500M in either direction.
PC-relative addresses are 32 bits. Absolute addresses
support the full address range.
-mcmodel=large
Generate code for the large model. There is no
limitation on call distance, pc-relative addresses, or
absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the
only supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long, and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
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TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the
only supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int,
long, and pointer to 32 bits. This is the only
supported behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are
assumed to be far away, the compiler always loads the
function's address into a register, and calls indirect
through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the
same index pointer 4 or more times to copy pointer into
the "ep" register, and use the shorter "sld" and "sst"
instructions. The -mep option is on by default if you
optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and
restore registers at the prologue and epilogue of a
function. The external functions are slower, but use
less code space if more than one function saves the same
number of registers. The -mprolog-function option is on
by default if you optimize.
-mspace
Try to make the code as small as possible. At present,
this just turns on the -mep and -mprolog-function
options.
-mtda=n
Put static or global variables whose size is n bytes or
less into the tiny data area that register "ep" points
to. The tiny data area can hold up to 256 bytes in
total (128 bytes for byte references).
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-msda=n
Put static or global variables whose size is n bytes or
less into the small data area that register "gp" points
to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or
less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The
preprocessor constant "__v850e3v5__" is defined if this
option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This
is an alias for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The
preprocessor constant "__v850e2v3__" is defined if this
option is used.
-mv850e2
Specify that the target processor is the V850E2. The
preprocessor constant "__v850e2__" is defined if this
option is used.
-mv850e1
Specify that the target processor is the V850E1. The
preprocessor constants "__v850e1__" and "__v850e__" are
defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This
is an alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The
preprocessor constant "__v850e__" is defined if this
option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2
nor -mv850e2v3 nor -mv850e3v5 are defined then a default
target processor is chosen and the relevant __v850*__
preprocessor constant is defined.
The preprocessor constants "__v850" and "__v851__" are
always defined, regardless of which processor variant is
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the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT"
instruction for the v850e, v850e1, v850e2, v850e2v3 and
v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is
in use (see -mrh850-abi), and disabled by default when
the GCC ABI is in use. If "CALLT" instructions are
being generated then the C preprocessor symbol
"__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line
option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative
jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware
floating point instructions. This option is only
significant when the target architecture is V850E2V3 or
higher. If hardware floating point instructions are
being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol
"__NO_FPU__" is defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use
of this instruction is not enabled by default when the
e3v5 architecture is selected because its use is still
experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI.
This is the default. With this version of the ABI the
following rules apply:
* Integer sized structures and unions are returned via
a memory pointer rather than a register.
* Large structures and unions (more than 8 bytes in
size) are passed by value.
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* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled
by default. The -mno-disable-callt command-line
option is not supported.
When this version of the ABI is enabled the C
preprocessor symbol "__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI.
With this version of the ABI the following rules apply:
* Integer sized structures and unions are returned in
register "r10".
* Large structures and unions (more than 8 bytes in
size) are passed by reference.
* Functions are aligned to 32-bit boundaries, unless
optimizing for size.
* The -m8byte-align command-line option is not
supported.
* The -mdisable-callt command-line option is supported
but not enabled by default.
When this version of the ABI is enabled the C
preprocessor symbol "__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be
aligned on 8-byte boundaries. The default is to
restrict the alignment of all objects to at most
4-bytes. When -m8byte-align is in effect the C
preprocessor symbol "__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this
option only if the assembler/linker complain about out
of range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code
generated by the compiler. This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed
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registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so
on) that the Unix assembler for the VAX cannot handle
across long ranges.
-mgnu
Do output those jump instructions, on the assumption
that the GNU assembler is being used.
-mg Output code for G-format floating-point numbers instead
of D-format.
Visium Options
-mdebug
A program which performs file I/O and is destined to run
on an MCM target should be linked with this option. It
causes the libraries libc.a and libdebug.a to be linked.
The program should be run on the target under the
control of the GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run
on the simulator should be linked with option. This
causes libraries libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions.
This is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for
floating-point.
-msoft-float changes the calling convention in the
output file; therefore, it is only useful if you compile
all of a program with this option. In particular, you
need to compile libgcc.a, the library that comes with
GCC, with -msoft-float in order for this to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type.
Supported values for cpu_type are mcm, gr5 and gr6.
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mcm is a synonym of gr5 present for backward
compatibility.
By default (unless configured otherwise), GCC generates
code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant
of the Visium architecture. The only difference from
GR5 code is that the compiler will generate block move
instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine
type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are
no restrictions on the access to general registers.
This is the default.
-muser-mode
Generate code for the user mode, where the access to
some general registers is forbidden: on the GR5,
registers r24 to r31 cannot be accessed in this mode; on
the GR6, only registers r29 to r31 are affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is
to return POSIX-style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as
the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for
size are 32 or short for 32 bit pointers, 64 or long for
64 bit pointers, and no for supporting only 32 bit
pointers. The later option disables "pragma
pointer_size".
VxWorks Options
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The options in this section are defined for all VxWorks
targets. Options specific to the target hardware are listed
with the other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real
time processes (RTPs). This option switches from the
former to the latter. It also defines the preprocessor
macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather
than static libraries. The options -static and -shared
can also be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are
defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is
equivalent to -Wl,-z,now and is defined for
compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is
the default and is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of
computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In
contrast to -mtune=cpu-type, which merely tunes the
generated code for the specified cpu-type, -march=cpu-
type allows GCC to generate code that may not run at all
on processors other than the one indicated. Specifying
-march=cpu-type implies -mtune=cpu-type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at
compilation time by determining the processor type
of the compiling machine. Using -march=native
enables all instruction subsets supported by the
local machine (hence the result might not run on
different machines). Using -mtune=native produces
code optimized for the local machine under the
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constraints of the selected instruction set.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for
this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with
MMX instruction set support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction
set is used, so the code runs on all i686 family
chips. When used with -mtune, it has the same
meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with
MMX instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core
with MMX and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium
III CPU with MMX, SSE and SSE2 instruction set
support. Used by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2
instruction set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX,
SSE, SSE2 and SSE3 instruction set support.
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nocona
Improved version of Intel Pentium 4 CPU with 64-bit
extensions, MMX, SSE, SSE2 and SSE3 instruction set
support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3 and SSSE3 instruction set support.
nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT
instruction set support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and
PCLMUL instruction set support.
sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AES and PCLMUL instruction set support.
ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AES, PCLMUL, FSGSBASE, RDRND and F16C instruction
set support.
haswell
Intel Haswell CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2 and F16C instruction set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction
set support.
skylake
Intel Skylake CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC and XSAVES instruction set support.
bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE,
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MMX, SSE, SSE2, SSE3 and SSSE3 instruction set
support.
silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES, PCLMUL and RDRND instruction set support.
knl Intel Knight's Landing CPU with 64-bit extensions,
MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW,
AVX512F, AVX512PF, AVX512ER and AVX512CD instruction
set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions,
MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F,
AVX512VL, AVX512BW, AVX512DQ and AVX512CD
instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow!
instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and
SSE prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced
3DNow! and full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64
instruction set support, including the AMD Opteron,
Athlon 64, and Athlon 64 FX processors. (This
supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow!
and 64-bit instruction set extensions.)
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k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3
instruction set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64
instruction set support. (This supersets MMX, SSE,
SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and
64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64
instruction set support. (This supersets FMA4, AVX,
XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64
instruction set support. (This supersets BMI, TBM,
F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16,
MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2,
ABM and 64-bit instruction set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64
instruction set support. (This supersets BMI, TBM,
F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES,
PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.
bdver4
AMD Family 15h core based CPUs with x86-64
instruction set support. (This supersets BMI, BMI2,
TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP,
AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.
znver1
AMD Family 17h core based CPUs with x86-64
instruction set support. (This supersets BMI, BMI2,
F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED,
MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX,
SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit
instruction set extensions.
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btver1
CPUs based on AMD Family 14h cores with x86-64
instruction set support. (This supersets MMX, SSE,
SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit
instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64
instruction set support. This includes MOVBE, F16C,
BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM,
SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit
instruction set extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with
additional MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with
additional MMX and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set
support. (No scheduling is implemented for this
chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE
instruction set support. (No scheduling is
implemented for this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is
implemented for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow!
instruction set support. (No scheduling is
implemented for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction
set support. (No scheduling is implemented for this
chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is
implemented for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is
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implemented for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set
support. (No scheduling is implemented for this
chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2,
SSE3 and SSSE3 instruction set support. (No
scheduling is implemented for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3
and SSSE3 instruction set support. (No scheduling
is implemented for this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3
and SSSE3 instruction set support. (No scheduling
is implemented for this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2,
SSE3, SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2,
SSE3, SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow!
instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the
generated code, except for the ABI and the set of
available instructions. While picking a specific cpu-
type schedules things appropriately for that particular
chip, the compiler does not generate any code that
cannot run on the default machine type unless you use a
-march=cpu-type option. For example, if GCC is
configured for i686-pc-linux-gnu then -mtune=pentium4
generates code that is tuned for Pentium 4 but still
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runs on i686 machines.
The choices for cpu-type are the same as for -march. In
addition, -mtune supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common
IA32/AMD64/EM64T processors. If you know the CPU on
which your code will run, then you should use the
corresponding -mtune or -march option instead of
-mtune=generic. But, if you do not know exactly
what CPU users of your application will have, then
you should use this option.
As new processors are deployed in the marketplace,
the behavior of this option will change. Therefore,
if you upgrade to a newer version of GCC, code
generation controlled by this option will change to
reflect the processors that are most common at the
time that version of GCC is released.
There is no -march=generic option because -march
indicates the instruction set the compiler can use,
and there is no generic instruction set applicable
to all processors. In contrast, -mtune indicates
the processor (or, in this case, collection of
processors) for which the code is optimized.
intel
Produce code optimized for the most current Intel
processors, which are Haswell and Silvermont for
this version of GCC. If you know the CPU on which
your code will run, then you should use the
corresponding -mtune or -march option instead of
-mtune=intel. But, if you want your application
performs better on both Haswell and Silvermont, then
you should use this option.
As new Intel processors are deployed in the
marketplace, the behavior of this option will
change. Therefore, if you upgrade to a newer
version of GCC, code generation controlled by this
option will change to reflect the most current Intel
processors at the time that version of GCC is
released.
There is no -march=intel option because -march
indicates the instruction set the compiler can use,
and there is no common instruction set applicable to
all processors. In contrast, -mtune indicates the
processor (or, in this case, collection of
processors) for which the code is optimized.
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-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit
unit. The choices for unit are:
387 Use the standard 387 floating-point coprocessor
present on the majority of chips and emulated
otherwise. Code compiled with this option runs
almost everywhere. The temporary results are
computed in 80-bit precision instead of the
precision specified by the type, resulting in
slightly different results compared to most of other
chips. See -ffloat-store for more detailed
description.
This is the default choice for non-Darwin x86-32
targets.
sse Use scalar floating-point instructions present in
the SSE instruction set. This instruction set is
supported by Pentium III and newer chips, and in the
AMD line by Athlon-4, Athlon XP and Athlon MP chips.
The earlier version of the SSE instruction set
supports only single-precision arithmetic, thus the
double and extended-precision arithmetic are still
done using 387. A later version, present only in
Pentium 4 and AMD x86-64 chips, supports double-
precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-
type, -msse or -msse2 switches to enable SSE
extensions and make this option effective. For the
x86-64 compiler, these extensions are enabled by
default.
The resulting code should be considerably faster in
the majority of cases and avoid the numerical
instability problems of 387 code, but may break some
existing code that expects temporaries to be 80
bits.
This is the default choice for the x86-64 compiler,
Darwin x86-32 targets, and the default choice for
x86-32 targets with the SSE2 instruction set when
-ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once.
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This effectively doubles the amount of available
registers, and on chips with separate execution
units for 387 and SSE the execution resources too.
Use this option with care, as it is still
experimental, because the GCC register allocator
does not model separate functional units well,
resulting in unstable performance.
-masm=dialect
Output assembly instructions using selected dialect.
Also affects which dialect is used for basic "asm" and
extended "asm". Supported choices (in dialect order) are
att or intel. The default is att. Darwin does not
support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-
point comparisons. These correctly handle the case
where the result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for
floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating
point.
Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C
compiler are used, but this cannot be done directly in
cross-compilation. You must make your own arrangements
to provide suitable library functions for
cross-compilation.
On machines where a function returns floating-point
results in the 80387 register stack, some floating-point
opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of
functions.
The usual calling convention has functions return values
of types "float" and "double" in an FPU register, even
if there is no FPU. The idea is that the operating
system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be
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returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and
"sqrt" instructions for the 387. Specify this option to
avoid generating those instructions. This option is the
default on OpenBSD and NetBSD. This option is
overridden when -march indicates that the target CPU
always has an FPU and so the instruction does not need
emulation. These instructions are not generated unless
you also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and
"long long" variables on a two-word boundary or a one-
word boundary. Aligning "double" variables on a two-
word boundary produces code that runs somewhat faster on
a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch,
structures containing the above types are aligned
differently than the published application binary
interface specifications for the x86-32 and are not
binary compatible with structures in code compiled
without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type.
The x86-32 application binary interface specifies the
size to be 96 bits, so -m96bit-long-double is the
default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long
double" to be aligned to an 8- or 16-byte boundary. In
arrays or structures conforming to the ABI, this is not
possible. So specifying -m128bit-long-double aligns
"long double" to a 16-byte boundary by padding the "long
double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the
default choice as its ABI specifies that "long double"
is aligned on 16-byte boundary.
Notice that neither of these options enable any extra
precision over the x87 standard of 80 bits for a "long
double".
Warning: if you override the default value for your
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target ABI, this changes the size of structures and
arrays containing "long double" variables, as well as
modifying the function calling convention for functions
taking "long double". Hence they are not binary-
compatible with code compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A
size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default for 32-bit
Bionic C library. A size of 128 bits makes the "long
double" type equivalent to the "__float128" type. This
is the default for 64-bit Bionic C library.
Warning: if you override the default value for your
target ABI, this changes the size of structures and
arrays containing "long double" variables, as well as
modifying the function calling convention for functions
taking "long double". Hence they are not binary-
compatible with code compiled without that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for
type are compat uses increased alignment value
compatible uses GCC 4.8 and earlier, abi uses alignment
value as specified by the psABI, and cacheline uses
increased alignment value to match the cache line size.
compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger
than threshold are placed in the large data section.
This value must be the same across all objects linked
into the binary, and defaults to 65535.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return
with the "ret num" instruction, which pops their
arguments while returning. This saves one instruction
in the caller since there is no need to pop the
arguments there.
You can specify that an individual function is called
with this calling sequence with the function attribute
"stdcall". You can also override the -mrtd option by
using the function attribute "cdecl".
Warning: this calling convention is incompatible with
the one normally used on Unix, so you cannot use it if
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you need to call libraries compiled with the Unix
compiler.
Also, you must provide function prototypes for all
functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is
generated for calls to those functions.
In addition, seriously incorrect code results if you
call a function with too many arguments. (Normally,
extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer
arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can
control this behavior for a specific function by using
the function attribute "regparm".
Warning: if you use this switch, and num is nonzero,
then you must build all modules with the same value,
including any libraries. This includes the system
libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and
double arguments and return values. You can control
this behavior for a specific function by using the
function attribute "sseregparm".
Warning: if you use this switch then you must build all
modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX
registers. This is the default on Solaris@tie{}8 and 9
and VxWorks to match the ABI of the Sun Studio compilers
until version 12. Later compiler versions (starting
with Studio 12 Update@tie{}1) follow the ABI used by
other x86 targets, which is the default on
Solaris@tie{}10 and later. Only use this option if you
need to remain compatible with existing code produced by
those previous compiler versions or older versions of
GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits.
When -mpc32 is specified, the significands of results of
floating-point operations are rounded to 24 bits (single
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precision); -mpc64 rounds the significands of results of
floating-point operations to 53 bits (double precision)
and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double
precision), which is the default. When this option is
used, floating-point operations in higher precisions are
not available to the programmer without setting the FPU
control word explicitly.
Setting the rounding of floating-point operations to
less than the default 80 bits can speed some programs by
2% or more. Note that some mathematical libraries
assume that extended-precision (80-bit) floating-point
operations are enabled by default; routines in such
libraries could suffer significant loss of accuracy,
typically through so-called "catastrophic cancellation",
when this option is used to set the precision to less
than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the
-mstackrealign option generates an alternate prologue
and epilogue that realigns the run-time stack if
necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep
16-byte stack alignment for SSE compatibility. See also
the attribute "force_align_arg_pointer", applicable to
individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised
to num byte boundary. If -mpreferred-stack-boundary is
not specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64
architecture with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be used to keep the
stack boundary aligned to 8 byte boundary. Since x86-64
ABI require 16 byte stack alignment, this is ABI
incompatible and intended to be used in controlled
environment where stack space is important limitation.
This option leads to wrong code when functions compiled
with 16 byte stack alignment (such as functions from a
standard library) are called with misaligned stack. In
this case, SSE instructions may lead to misaligned
memory access traps. In addition, variable arguments
are handled incorrectly for 16 byte aligned objects
(including x87 long double and __int128), leading to
wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries.
This includes the system libraries and startup modules.
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-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to
num byte boundary. If -mincoming-stack-boundary is not
specified, the one specified by
-mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double"
values should be aligned to an 8-byte boundary (see
-malign-double) or suffer significant run time
performance penalties. On Pentium III, the Streaming
SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack,
the stack boundary must be as aligned as that required
by any value stored on the stack. Further, every
function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher
preferred stack boundary from a function compiled with a
lower preferred stack boundary most likely misaligns the
stack. It is recommended that libraries that use
callbacks always use the default setting.
This extra alignment does consume extra stack space, and
generally increases code size. Code that is sensitive
to stack space usage, such as embedded systems and
operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
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-mclfushopt
-mfsgsbase
-mrdrnd
-mf16c
-mfma
-mfma4
-mprefetchwt1
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mtbm
-mmpx
-mmwaitx
-mclzero
-mpku
These switches enable the use of instructions in the
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F,
AVX512PF, AVX512ER, AVX512CD, SHA, AES, PCLMUL,
FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM,
AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA AVX512VBMI,
BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT, RTM, MPX,
MWAITX, PKU, 3DNow! or enhanced 3DNow! extended
instruction sets. Each has a corresponding -mno- option
to disable use of these instructions.
These extensions are also available as built-in
functions: see x86 Built-in Functions, for details of
the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from
floating-point code (as opposed to 387 instructions),
see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used.
Instead, it generates new AVX instructions or AVX
equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended
instructions in generated code, even without
-mfpmath=sse. Applications that perform run-time CPU
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detection must compile separate files for each supported
architecture, using the appropriate flags. In
particular, the file containing the CPU detection code
should be compiled without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86
performance tuning features and default settings. The
names can be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code
generation features. feature-list is a comma separated
list of feature names. See also -mdump-tune-features.
When specified, the feature is turned on if it is not
preceded with ^, otherwise, it is turned off.
-mtune-ctrl=feature-list is intended to be used by GCC
developers. Using it may lead to code paths not covered
by testing and can potentially result in compiler ICEs
or runtime errors.
-mno-default
This option instructs GCC to turn off all tunable
features. See also -mtune-ctrl=feature-list and
-mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in
the prologue of functions that use string instructions.
String instructions depend on the DF flag to select
between autoincrement or autodecrement mode. While the
ABI specifies the DF flag to be cleared on function
entry, some operating systems violate this specification
by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with
the DF flag set, which leads to wrong direction mode
when string instructions are used. This option can be
enabled by default on 32-bit x86 targets by configuring
GCC with the --enable-cld configure option. Generation
of "cld" instructions can be suppressed with the
-mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper"
instruction before a transfer of control flow out of the
function to minimize the AVX to SSE transition penalty
as well as remove unnecessary "zeroupper" intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX
instructions instead of 256-bit AVX instructions in the
auto-vectorizer.
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-mcx16
This option enables GCC to generate "CMPXCHG16B"
instructions in 64-bit code to implement compare-and-
exchange operations on 16-byte aligned 128-bit objects.
This is useful for atomic updates of data structures
exceeding one machine word in size. The compiler uses
this instruction to implement __sync Builtins. However,
for __atomic Builtins operating on 128-bit integers, a
library call is always used.
-msahf
This option enables generation of "SAHF" instructions in
64-bit code. Early Intel Pentium 4 CPUs with Intel 64
support, prior to the introduction of Pentium 4 G1 step
in December 2005, lacked the "LAHF" and "SAHF"
instructions which are supported by AMD64. These are
load and store instructions, respectively, for certain
status flags. In 64-bit mode, the "SAHF" instruction is
used to optimize "fmod", "drem", and "remainder" built-
in functions; see Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to
implement "__builtin_bswap32" and "__builtin_bswap64".
-mcrc32
This option enables built-in functions
"__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to
generate the "crc32" machine instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS"
instructions (and their vectorized variants "RCPPS" and
"RSQRTPS") with an additional Newton-Raphson step to
increase precision instead of "DIVSS" and "SQRTSS" (and
their vectorized variants) for single-precision
floating-point arguments. These instructions are
generated only when -funsafe-math-optimizations is
enabled together with -ffinite-math-only and
-fno-trapping-math. Note that while the throughput of
the sequence is higher than the throughput of the non-
reciprocal instruction, the precision of the sequence
can be decreased by up to 2 ulp (i.e. the inverse of 1.0
equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of
"RSQRTSS" (or "RSQRTPS") already with -ffast-math (or
the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with
additional Newton-Raphson step for vectorized single-
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float division and vectorized "sqrtf(x)" already with
-ffast-math (or the above option combination), and
doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate
instructions may be used. opt is a comma-separated list
of options, which may be preceded by a ! to invert the
option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to
-mrecip.
none
Disable all estimate instructions, equivalent to
-mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the
reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics
using an external library. Supported values for type
are svml for the Intel short vector math library and
acml for the AMD math core library. To use this option,
both -ftree-vectorize and -funsafe-math-optimizations
have to be enabled, and an SVML or ACML ABI-compatible
library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2",
"vmldLog102", "vmldLog102", "vmldPow2", "vmldTanh2",
"vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2",
"vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2",
"vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2",
"vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsLog104",
"vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4",
"vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4",
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"vmlsAcosh4" and "vmlsAcos4" for corresponding function
type when -mveclibabi=svml is used, and "__vrd2_sin",
"__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2",
"__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf",
"__vrs4_expf", "__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for the corresponding
function type when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention.
Permissible values are sysv for the ABI used on
GNU/Linux and other systems, and ms for the Microsoft
ABI. The default is to use the Microsoft ABI when
targeting Microsoft Windows and the SysV ABI on all
other systems. You can control this behavior for
specific functions by using the function attributes
"ms_abi" and "sysv_abi".
-mtls-dialect=type
Generate code to access thread-local storage using the
gnu or gnu2 conventions. gnu is the conservative
default; gnu2 is more efficient, but it may add compile-
and run-time requirements that cannot be satisfied on
all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This
method is shorter and usually equally fast as method
using SUB/MOV operations and is enabled by default. In
some cases disabling it may improve performance because
of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for
outgoing arguments is computed in the function prologue.
This is faster on most modern CPUs because of reduced
dependencies, improved scheduling and reduced stack
usage when the preferred stack boundary is not equal to
2. The drawback is a notable increase in code size.
This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW.
Programs that rely on thread-safe exception handling
must compile and link all code with the -mthreads
option. When compiling, -mthreads defines -D_MT; when
linking, it links in a special thread helper library
-lmingwthrd which cleans up per-thread exception-
handling data.
-mms-bitfields
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-mno-ms-bitfields
Enable/disable bit-field layout compatible with the
native Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are
used, it may be that the Microsoft ABI lays out the
structure differently than the way GCC normally does.
Particularly when moving packed data between functions
compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may
be necessary to access either format.
This option is enabled by default for Microsoft Windows
targets. This behavior can also be controlled locally
by use of variable or type attributes. For more
information, see x86 Variable Attributes and x86 Type
Attributes.
The Microsoft structure layout algorithm is fairly
simple with the exception of the bit-field packing. The
padding and alignment of members of structures and
whether a bit-field can straddle a storage-unit boundary
are determine by these rules:
which they are
1. Structure members are stored sequentially in the order in
declared: the first member has the lowest memory
address and the last member the highest.
alignment requirement
2. Every data object has an alignment requirement. The
for all data except structures, unions, and arrays
is either the size of the object or the current
packing size (specified with either the "aligned"
attribute or the "pack" pragma), whichever is less.
For structures, unions, and arrays, the alignment
requirement is the largest alignment requirement of
its members. Every object is allocated an offset so
that:
offset % alignment_requirement == 0
4-byte allocation
3. Adjacent bit-
fields are packed into the same 1-, 2-, or
unit if the integral types are the same size and if
the next bit-field fits into the current allocation
unit without crossing the boundary imposed by the
common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following
ways:
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fields that
1. If a zero-length bit-
field is inserted between two bit-
are normally coalesced, the bit-fields are not
coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length
bit-field. If the zero-length bit-field were
removed, "t1"'s size would be 4 bytes.
"foo", and the
2. If a zero-length bit-
field is inserted after a bit-field,
alignment of the zero-length bit-field is greater
than the member that follows it, "bar", "bar" is
aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than
offset 1. Accordingly, the size of "t2" is 4. For
"t3", the zero-length bit-field does not affect the
alignment of "bar" or, as a result, the size of the
structure.
Taking this into account, it is important to note
the following:
the type of the
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1. If a zero-length bit-
field follows a normal bit-field,
zero-length bit-field may affect the alignment
of the structure as whole. For example, "t2" has
a size of 4 bytes, since the zero-length bit-
field follows a normal bit-field, and is of type
short.
normal bit-field, it may
2. Even if a zero-length bit-
field is not followed by a
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
ignored:
3. Zero-length bit-fields following non-bit-
field members are
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string
operations. This switch reduces code size and improves
performance in case the destination is already aligned,
but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the
destination is known to be aligned to least a 4-byte
boundary. This enables more inlining and increases code
size, but may improve performance of code that depends
on fast "memcpy", "strlen", and "memset" for short
lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time
checks with inline code for small blocks and a library
call for large blocks.
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-mstringop-strategy=alg
Override the internal decision heuristic for the
particular algorithm to use for inlining string
operations. The allowed values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified
size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if
"__builtin_memcpy" should be inlined and what inline
algorithm to use when the expected size of the copy
operation is known. strategy is a comma-separated list
of alg:max_size:dest_align triplets. alg is specified in
-mstringop-strategy, max_size specifies the max byte
size with which inline algorithm alg is allowed. For
the last triplet, the max_size must be "-1". The
max_size of the triplets in the list must be specified
in increasing order. The minimal byte size for alg is 0
for the first triplet and "max_size + 1" of the
preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that
it is to control "__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set
up, and restore frame pointers and makes an extra
register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for
leaf functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with
offsets from the TLS segment register (%gs for 32-bit,
%fs for 64-bit), or whether the thread base pointer must
be added. Whether or not this is valid depends on the
operating system, and whether it maps the segment to
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cover the entire TLS area.
For systems that use the GNU C Library, the default is
on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE
instructions with VEX prefix. The option -mavx turns
this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter
call before the prologue. Note: On x86 architectures
the attribute "ms_hook_prologue" isn't possible at the
moment for -mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc
section that contains pointers to each profiling call.
This is useful for automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the
profiling functions as NOPs. This is useful when they
should be patched in later dynamically. This is likely
only useful together with -mrecord-mcount.
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with
SSE extensions disabled, -mskip-rax-setup can be used to
skip setting up RAX register when there are no variable
arguments passed in vector registers.
Warning: Since RAX register is used to avoid
unnecessarily saving vector registers on stack when
passing variable arguments, the impacts of this option
are callees may waste some stack space, misbehave or
jump to a random location. GCC 4.4 or newer don't have
those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned
integer divide is much faster than 32-bit/64-bit integer
divide. This option generates a run-time check. If
both dividend and divisor are within range of 0 to 255,
8-bit unsigned integer divide is used instead of
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32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
Generate stack protection code using canary at guard.
Supported locations are global for global canary or tls
for per-thread canary in the TLS block (the default).
This option has effect only when -fstack-protector or
-fstack-protector-all is specified.
-mmitigate-rop
Try to avoid generating code sequences that contain
unintended return opcodes, to mitigate against certain
forms of attack. At the moment, this option is limited
in what it can do and should not be relied on to provide
serious protection.
-mgeneral-regs-only
Generate code that uses only the general-purpose
registers. This prevents the compiler from using
floating-point, vector, mask and bound registers.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default
is keep, which keeps indirect call and jump unmodified.
thunk converts indirect call and jump to call and return
thunk. thunk-inline converts indirect call and jump to
inlined call and return thunk. thunk-extern converts
indirect call and jump to external call and return thunk
provided in a separate object file. You can control
this behavior for a specific function by using the
function attribute "indirect_branch".
Note that -mcmodel=large is incompatible with
-mindirect-branch=thunk nor
-mindirect-branch=thunk-extern since the thunk function
may not be reachable in large code model.
-mfunction-return=choice
Convert function return with choice. The default is
keep, which keeps function return unmodified. thunk
converts function return to call and return thunk.
thunk-inline converts function return to inlined call
and return thunk. thunk-extern converts function return
to external call and return thunk provided in a separate
object file. You can control this behavior for a
specific function by using the function attribute
"function_return".
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Note that -mcmodel=large is incompatible with
-mfunction-return=thunk nor
-mfunction-return=thunk-extern since the thunk function
may not be reachable in large code model.
-mindirect-branch-register
Force indirect call and jump via register.
These -m switches are supported in addition to the above on
x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit
environment. The -m32 option sets "int", "long", and
pointer types to 32 bits, and generates code that runs
on any i386 system.
The -m64 option sets "int" to 32 bits and "long" and
pointer types to 64 bits, and generates code for the
x86-64 architecture. For Darwin only the -m64 option
also turns off the -fno-pic and -mdynamic-no-pic
options.
The -mx32 option sets "int", "long", and pointer types
to 32 bits, and generates code for the x86-64
architecture.
The -m16 option is the same as -m32, except for that it
outputs the ".code16gcc" assembly directive at the
beginning of the assembly output so that the binary can
run in 16-bit mode.
The -miamcu option generates code which conforms to
Intel MCU psABI. It requires the -m32 option to be
turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The
red zone is mandated by the x86-64 ABI; it is a 128-byte
area beyond the location of the stack pointer that is
not modified by signal or interrupt handlers and
therefore can be used for temporary data without
adjusting the stack pointer. The flag -mno-red-zone
disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and
its symbols must be linked in the lower 2 GB of the
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address space. Pointers are 64 bits. Programs can be
statically or dynamically linked. This is the default
code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel
runs in the negative 2 GB of the address space. This
model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is
linked in the lower 2 GB of the address space. Small
symbols are also placed there. Symbols with sizes
larger than -mlarge-data-threshold are put into large
data or BSS sections and can be located above 2GB.
Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no
assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only
supported for 64-bit and x32 environments. It is the
default address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only
supported for 32-bit and x32 environments. It is the
default address mode for 32-bit and x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows
targets:
-mconsole
This option specifies that a console application is to
be generated, by instructing the linker to set the PE
header subsystem type required for console applications.
This option is available for Cygwin and MinGW targets
and is enabled by default on those targets.
-mdll
This option is available for Cygwin and MinGW targets.
It specifies that a DLL---a dynamic link library---is to
be generated, enabling the selection of the required
runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets.
It specifies that the "dllimport" attribute should be
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ignored.
-mthread
This option is available for MinGW targets. It specifies
that MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It
causes the "UNICODE" preprocessor macro to be
predefined, and chooses Unicode-capable runtime startup
code.
-mwin32
This option is available for Cygwin and MinGW targets.
It specifies that the typical Microsoft Windows
predefined macros are to be set in the pre-processor,
but does not influence the choice of runtime
library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets.
It specifies that a GUI application is to be generated
by instructing the linker to set the PE header subsystem
type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies
that the executable flag for the stack used by nested
functions isn't set. This is necessary for binaries
running in kernel mode of Microsoft Windows, as there
the User32 API, which is used to set executable
privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets.
It specifies that relocated-data in read-only section is
put into the ".data" section. This is a necessary for
older runtimes not supporting modification of ".rdata"
sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets.
It specifies that the GNU extension to the PE file
format that permits the correct alignment of COMMON
variables should be used when generating code. It is
enabled by default if GCC detects that the target
assembler found during configuration supports the
feature.
See also under x86 Options for standard options.
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Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the
simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for
loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When
enabled, "CONST16" instructions are always used in place
of the standard "L32R" instructions. The use of
"CONST16" is enabled by default only if the "L32R"
instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and
multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option
is not also enabled. Disabling fused multiply/add and
multiply/subtract instructions forces the compiler to
use separate instructions for the multiply and
add/subtract operations. This may be desirable in some
cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions
do not round the intermediate result, thereby producing
results with more bits of precision than specified by
the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program
output is not sensitive to the compiler's ability to
combine multiply and add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW"
instructions before "volatile" memory references to
guarantee sequential consistency. The default is
-mserialize-volatile. Use -mno-serialize-volatile to
omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa
code must be position-independent code (PIC), this
option disables PIC for compiling kernel code.
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-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools.
The default is -mno-text-section-literals, which places
literals in a separate section in the output file. This
allows the literal pool to be placed in a data RAM/ROM,
and it also allows the linker to combine literal pools
from separate object files to remove redundant literals
and improve code size. With -mtext-section-literals,
the literals are interspersed in the text section in
order to keep them as close as possible to their
references. This may be necessary for large assembly
files. Literals for each function are placed right
before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools.
The default is -mno-auto-litpools, which places literals
in a separate section in the output file unless
-mtext-section-literals is used. With -mauto-litpools
the literals are interspersed in the text section by the
assembler. Compiler does not produce explicit
".literal" directives and loads literals into registers
with "MOVI" instructions instead of "L32R" to let the
assembler do relaxation and place literals as necessary.
This option allows assembler to create several literal
pools per function and assemble very big functions,
which may not be possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler
to automatically align instructions to reduce branch
penalties at the expense of some code density. The
assembler attempts to widen density instructions to
align branch targets and the instructions following call
instructions. If there are not enough preceding safe
density instructions to align a target, no widening is
performed. The default is -mtarget-align. These
options do not affect the treatment of auto-aligned
instructions like "LOOP", which the assembler always
aligns, either by widening density instructions or by
inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler
to translate direct calls to indirect calls unless it
can determine that the target of a direct call is in the
range allowed by the call instruction. This translation
typically occurs for calls to functions in other source
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GCC(1) GNU GCC(1)
files. Specifically, the assembler translates a direct
"CALL" instruction into an "L32R" followed by a "CALLX"
instruction. The default is -mno-longcalls. This
option should be used in programs where the call target
can potentially be out of range. This option is
implemented in the assembler, not the compiler, so the
assembly code generated by GCC still shows direct call
instructions---look at the disassembled object code to
see the actual instructions. Note that the assembler
uses an indirect call for every cross-file call, not
just those that really are out of range.
zSeries Options
These are listed under
ENVIRONMENT
This section describes several environment variables that
affect how GCC operates. Some of them work by specifying
directories or prefixes to use when searching for various
kinds of files. Some are used to specify other aspects of
the compilation environment.
Note that you can also specify places to search using
options such as -B, -I and -L. These take precedence over
places specified using environment variables, which in turn
take precedence over those specified by the configuration of
GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC
uses localization information which allows GCC to work
with different national conventions. GCC inspects the
locale categories LC_CTYPE and LC_MESSAGES if it has
been configured to do so. These locale categories can
be set to any value supported by your installation. A
typical value is en_GB.UTF-8 for English in the United
Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character
boundaries in a string; this is needed for some
multibyte encodings that contain quote and escape
characters that are otherwise interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the
language to use in diagnostic messages.
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If the LC_ALL environment variable is set, it overrides
the value of LC_CTYPE and LC_MESSAGES; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of the
LANG environment variable. If none of these variables
are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for
temporary files. GCC uses temporary files to hold the
output of one stage of compilation which is to be used
as input to the next stage: for example, the output of
the preprocessor, which is the input to the compiler
proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to
passing -fcompare-debug to the compiler driver. See the
documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use
in the names of the subprograms executed by the
compiler. No slash is added when this prefix is
combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure
out an appropriate prefix to use based on the pathname
it is invoked with.
If GCC cannot find the subprogram using the specified
prefix, it tries looking in the usual places for the
subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/
where prefix is the prefix to the installed compiler. In
many cases prefix is the value of "prefix" when you ran
the configure script.
Other prefixes specified with -B take precedence over
this prefix.
This prefix is also used for finding files such as
crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in
finding the directories to search for header files. For
each of the standard directories whose name normally
begins with /usr/local/lib/gcc (more precisely, with the
value of GCC_INCLUDE_DIR), GCC tries replacing that
beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC
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GCC(1) GNU GCC(1)
searches foo/bar just before it searches the standard
directory /usr/local/lib/bar. If a standard directory
begins with the configured prefix then the value of
prefix is replaced by GCC_EXEC_PREFIX when looking for
header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories
thus specified when searching for subprograms, if it
cannot find the subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a
native compiler, GCC tries the directories thus
specified when searching for special linker files, if it
cannot find them using GCC_EXEC_PREFIX. Linking using
GCC also uses these directories when searching for
ordinary libraries for the -l option (but directories
specified with -L come first).
LANG
This variable is used to pass locale information to the
compiler. One way in which this information is used is
to determine the character set to be used when character
literals, string literals and comments are parsed in C
and C++. When the compiler is configured to allow
multibyte characters, the following values for LANG are
recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value,
then the compiler uses "mblen" and "mbtowc" as defined
by the default locale to recognize and translate
multibyte characters.
Some additional environment variables affect the behavior of
the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
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Each variable's value is a list of directories separated
by a special character, much like PATH, in which to look
for header files. The special character,
"PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it
is a semicolon, and for almost all other targets it is a
colon.
CPATH specifies a list of directories to be searched as
if specified with -I, but after any paths given with -I
options on the command line. This environment variable
is used regardless of which language is being
preprocessed.
The remaining environment variables apply only when
preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if
specified with -isystem, but after any paths given with
-isystem options on the command line.
In all these variables, an empty element instructs the
compiler to search its current working directory. Empty
elements can appear at the beginning or end of a path.
For instance, if the value of CPATH is
":/special/include", that has the same effect as
-I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to
output dependencies for Make based on the non-system
header files processed by the compiler. System header
files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file
name, in which case the Make rules are written to that
file, guessing the target name from the source file
name. Or the value can have the form file target, in
which case the rules are written to file file using
target as the target name.
In other words, this environment variable is equivalent
to combining the options -MM and -MF, with an optional
-MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see
above), except that system header files are not ignored,
so it implies -M rather than -MM. However, the
dependence on the main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX
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GCC(1) GNU GCC(1)
timestamp to be used in replacement of the current date
and time in the "__DATE__" and "__TIME__" macros, so
that the embedded timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,
defined as the number of seconds (excluding leap
seconds) since 01 Jan 1970 00:00:00 represented in
ASCII; identical to the output of @command{date +%s} on
GNU/Linux and other systems that support the %s
extension in the "date" command.
The value should be a known timestamp such as the last
modification time of the source or package and it should
be set by the build process.
BUGS
For instructions on reporting bugs, see
<https://gcc.gnu.org/bugs/>.
FOOTNOTES
1. On some systems, gcc -shared needs to build
supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the
correct support libraries to link against. Failing to
supply the correct flags may lead to subtle defects.
Supplying them in cases where they are not necessary is
innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1),
ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries
for gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for
contributors to GCC.
COPYRIGHT
Copyright (c) 1988-2017 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation
License, Version 1.3 or any later version published by the
Free Software Foundation; with the Invariant Sections being
"GNU General Public License" and "Funding Free Software",
the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below). A copy of the
license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
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GCC(1) GNU GCC(1)
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
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