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GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] 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 stage of processing. Some options control the
preprocessor and others the compiler itself. Yet other options control the assem-
bler 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 pro-
grams; 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 lan-
guages.
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:
-dr is very different from -d -r.
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.
Many options have long names starting with -f or with -W---for example,
-fstrength-reduce, -Wformat and so on. Most of these have both positive and nega-
tive forms; the negative form of -ffoo would be -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 -combine -pipe -pass-exit-codes -x language -v -###
--help --target-help --version
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fhosted -ffreestanding -fms-extensions -trigraphs
-no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -fun-
signed-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-ffriend-injection -fno-const-strings -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 -fno-nonansi-builtins -fno-operator-names -fno-optional-diags
-fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n -fno-thread-
safe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor
-Wreorder -Weffc++ -Wno-deprecated -Wstrict-null-sentinel -Wno-non-tem-
plate-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-excep-
tions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Wassign-inter-
cept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnos-
tics-show-options
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggre-
gate-return -Wno-attributes -Wc++-compat -Wcast-align -Wcast-qual -Wchar-sub-
scripts -Wcomment -Wconversion -Wno-deprecated-declarations -Wdisabled-opti-
mization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-func-
tion-declaration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-for-
mat-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import
-Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Win-
valid-pch -Wlarger-than-len -Wunsafe-loop-optimizations -Wlong-long -Wmain
-Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked
-Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredun-
dant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare
-Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef
-Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused
-Wunused-function -Wunused-label -Wunused-parameter -Wunused-value
-Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-opti-
mized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg
-fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n]
-fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw]
-fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n]
-fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n]
-fdump-tree-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vector-
izer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -felimi-
nate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fpro-
file-arcs -frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report
-fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms
-gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time
Optimization Options
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n]
-fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt
-fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks
-fcx-limited-range -fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fearly-inlining -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr -ffunction-sections -fgcse -fgcse-lm
-fgcse-sm -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping
-fif-conversion -fif-conversion2 -finline-functions -finline-func-
tions-called-once -finline-limit=n -fkeep-inline-functions -fkeep-static-con-
sts -fmerge-constants -fmerge-all-constants -fmodulo-sched
-fno-branch-count-reg -fno-default-inline -fno-defer-pop -floop-optimize2
-fmove-loop-invariants -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-opti-
mizations -funsafe-loop-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move
-foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fpro-
file-use -fregmove -frename-registers -freorder-blocks -fre-
order-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fre-
run-loop-opt -frounding-math -fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns[=n]
-fsched-stalled-insns-dep[=n] -fsched2-use-superblocks -fsched2-use-traces
-freschedule-modulo-scheduled-loops -fsignaling-nans -fsingle-precision-con-
stant -fstack-protector -fstack-protector-all -fstrength-reduce
-fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops
-fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expan-
sion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize
-ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-domina-
tor-opts -ftree-dse -ftree-copyrename -ftree-sink -ftree-ch -ftree-sra
-ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -ftree-vect-loop-version
-ftree-salias -fweb -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop
-ftree-vrp -funit-at-a-time -fwhole-program --param name=value -O -O0 -O1
-O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn]
-E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix
dir -iwithprefixbefore dir -isystem dir -isysroot dir -M -MM -MF -MG -MP
-MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro
-Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdy-
namic -s -static -static-libgcc -shared -shared-libgcc -symbolic
-Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-
section -mrodata=readonly-data-section
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check
-mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant
-mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian
-mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float
-mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -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 -mcirrus-fix-invalid-insns -mno-cir-
rus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
-mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues
-mno-tablejump -mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly
-mlow-64k -mno-low64k -mid-shared-library -mno-id-shared-library
-mshared-library-id=n -mlong-calls -mno-long-calls
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 -m16-bit
-m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux
-sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibil-
ity_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 -header-
pad_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 -nofixprebinding -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -pri-
vate_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_ta-
ble_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_ref-
erence_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version
-mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-round-
ing-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mex-
plicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmem-
ory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
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 -multi-
lib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic
-macc-4 -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
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdis-
able-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-big-switch -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
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit
-masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float
-msvr3-shlib -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-bound-
ary=num -mmmx -msse -msse2 -msse3 -mssse3 -msse4a -msse5 -m3dnow -mpopcnt
-mabm -mthreads -mno-align-stringops -minline-all-stringops -mpush-args
-maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mreg-
parm=num -msseregparm -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-thresh-
old=num -mfused-madd -mno-fused-madd
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -mno-sdata -mconstant-gp -mauto-pic
-minline-float-divide-min-latency -minline-float-divide-max-throughput -min-
line-int-divide-min-latency -minline-int-divide-max-throughput -min-
line-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-
type -mt -pthread -milp32 -mlp64
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -mis-
sue-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 -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield
-mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align -msep-data
-mno-sep-data -mshared-library-id=n -mid-shared-library
-mno-id-shared-library
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec
-minmax -mlong-calls -mshort -msoft-reg-count=count
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
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls
-mno-abicalls -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float
-msoft-float -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d
-mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -membedded-data -mno-embed-
ded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata
-msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad
-mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400
-mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mfix-sb1
-mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely
-mno-branch-likely -mfp-exceptions -mno-fp-exceptions -mvr4130-align
-mno-vr4130-align
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 -mam33 -mno-am33 -mam33-2
-mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
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 -msplit -mno-split -munix-asm -mdec-asm
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power
-mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-pow-
erpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd
-mmfpgpr -mno-mfpgpr -mnew-mnemonics -mold-mnemonics -mfull-toc -mmini-
mal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-com-
pat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mno-multiple -mstring -mno-string -mupdate -mno-update -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 -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 -mse-
cure-plt -mbss-plt -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe
-mspe=yes -mspe=no -mvrsave -mno-vrsave -mfloat-gprs=yes -mfloat-gprs=no
-mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss
-G num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float
-msoft-float -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec -mmvcle
-mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mtpf-trace
-mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamic-
stack -mstack-size -mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single
-m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media
-m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu
-mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-rene-
sas -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name -madjust-unroll -min-
dexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64
-mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mfpu -mno-fpu
-mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mimpure-text
-mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis
-mno-vis -threads -pthreads -pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm
-mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb
-mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float
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 -mv850e1 -mv850e -mv850
-mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-sec-
tion-literals -mno-text-section-literals -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-excep-
tions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive
-finstrument-functions -fno-common -fno-ident -fpcc-struct-return -fpic
-fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return -fshared-data
-fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fargument-alias -fargument-noalias -fargu-
ment-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv
-fbounds-check -fvisibility -fopenmp
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 which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which 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
Objective-C source code which 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 which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precom-
piled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which 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 which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with the traditional
preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the traditional
preprocessor).
file.ads
Ada source code file which 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 declara-
tion). 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
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recog-
nized 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 let-
ting 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 c-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
f95 f95-cpp-input
java
treelang
-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).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the com-
piler returns a non-success return code. If you specify -pass-exit-codes, the
gcc program will instead return with numerically highest error produced by any
phase that returned an error indication.
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 sim-
ply 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.
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 out-
put.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output
is being produced, whether it be an executable file, an object file, an assem-
bler 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 all command arguments are
quoted. This is useful for shell scripts to capture the driver-generated com-
mand lines.
-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.
-combine
If you are compiling multiple source files, this option tells the driver to
pass all the source files to the compiler at once (for those languages for
which the compiler can handle this). This will allow intermodule analysis
(IMA) to be performed by the compiler. Currently the only language for which
this is supported is C. If you pass source files for multiple languages to the
driver, using this option, the driver will invoke the compiler(s) that support
IMA once each, passing each compiler all the source files appropriate for it.
For those languages that do not support IMA this option will be ignored, and
the compiler will be invoked once for each source file in that language. If
you use this option in conjunction with -save-temps, the compiler will generate
multiple pre-processed files (one for each source file), but only one (com-
bined) .o or .s file.
--help
Print (on the standard output) a description of the command line options under-
stood by gcc. If the -v option is also specified then --help will also be
passed on to the various processes invoked by gcc, so that they can display the
command line options they accept. If the -Wextra option is also specified then
command line options which have no documentation associated with them will also
be displayed.
--target-help
Print (on the standard output) a description of target specific command line
options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
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 or .H; 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, C++ programs often require class libraries as well as a compiler that
understands the C++ language---and under some circumstances, you might want to com-
pile programs or header files from standard input, or otherwise without a suffix
that flags them as C++ programs. You might also like to precompile a C header file
with a .h extension to be used in C++ compilations. g++ is a program that calls
GCC with the default language set to C++, and automatically specifies linking
against the C++ library. On many systems, g++ is also installed with the name c++.
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, support all ISO C90 programs. In C++ mode, remove GNU extensions
that conflict with ISO C++.
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 undesir-
able 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, -pedantic 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 which would normally be built in but do not have semantics defined by
ISO C (such as "alloca" and "ffs") are not built-in functions with -ansi is
used.
-std=
Determine the language standard. This option is currently only supported when
compiling C or C++. A value for this option must be provided; possible values
are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
<http://gcc.gnu.org/gcc-4.1/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
this will become the default. The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++
code.
Even when this option is not specified, you can still use some of the features
of newer standards in so far as they do not conflict with previous C standards.
For example, you may use "__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as
-ansi, except that features that were not in ISO C90 but are in the specified
version (for example, // comments and the "inline" keyword in ISO C99) are not
disabled.
-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.
This option is accepted by GCC versions 4.1.3 and up. In GCC versions prior to
4.3, C99 inline semantics are not supported, and thus this option is effec-
tively assumed to be present regardless of whether or not it is specified; the
only effect of specifying it explicitly is to disable warnings about using
inline functions in C99 mode. Likewise, the option -fno-gnu89-inline is not
supported in versions of GCC before 4.3. It will be supported only in C99 or
gnu99 mode, not in C89 or gnu89 mode.
The preprocesor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be
used to check which semantics are in effect for "inline" functions.
-aux-info filename
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 dec-
laration (source file and line), whether the declaration was implicit, proto-
typed 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 dec-
laration 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.
-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 key-
word 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
that 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 those
calls, nor can you change the behavior of the functions by linking with a dif-
ferent library. In addition, when a function is recognized as a built-in func-
tion, 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 result-
ing 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
this 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))
-fhosted
Assert that compilation takes place in 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 takes place in 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.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with
this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C
conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option
allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The
user supplied compilation step can then add in an additional preprocessing step
after normal preprocessing but before compiling. The default is to use the
integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are
merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C com-
piler. They are now only supported with the -E switch. The preprocessor con-
tinues to support a pre-standard mode. See the GNU CPP manual for details.
-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 sup-
ported for C++.
-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.
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 dec-
laration 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.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++
programs; but 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 -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use
the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that
first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first
appeared in G++ 3.2. Version 0 will always be the version that conforms most
closely to the C++ ABI specification. Therefore, the ABI obtained using ver-
sion 0 will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around
bugs in the access control code.
-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" will only return 0 if it is
declared throw(), in which case the compiler will always check 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).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common seg-
ment, as C does. This saves space in the executable at the cost of not diag-
nosing duplicate definitions. If you compile with this flag and your program
mysteriously crashes after "main()" has completed, you may have an object that
is being destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been
added for putting variables into BSS without making them common.
-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, and
versions of G++ before 4.1 always worked that way. However, in ISO C++ a
friend function which is not declared in an enclosing scope can only be found
using argument dependent lookup. This option causes friends to be injected as
they were in earlier releases.
This option is for compatibility, and may be removed in a future release of
G++.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By
default, G++ uses type "const char *" as required by the standard. Even if you
use -fno-const-strings, you cannot actually modify the value of a string con-
stant.
This option might be removed in a future release of G++. For maximum portabil-
ity, you should structure your code so that it works with string constants that
have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which 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.
-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 specifi-
cations; the compiler will still optimize based on the specifications, so
throwing an unexpected exception will result in undefined behavior.
-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 -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++.
The default if neither flag is given to follow the standard, but to allow and
give a warning for old-style code that would otherwise be invalid, or have dif-
ferent 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. -ansi implies
-fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which 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 will 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 will cause linker errors if these functions are
not inlined everywhere they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int
and getting a pointer to member function via non-standard syntax.
-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.
-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 will allow some nonconforming code to compile.
-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++ runtime 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 it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This
information is generally only useful to the G++ development team.
-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 tem-
plate class instantiation. ANSI/ISO C++ conforming programs must not rely on a
maximum depth greater than 17.
-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 will
only work if your C library supports "__cxa_atexit".
-fvisibility-inlines-hidden
Causes all inlined methods to be marked with "__attribute__ ((visibility ("hid-
den")))" 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 tem-
plates.
You may mark a method as having a visibility explicitly to negate the effect of
the switch for that method. For example, if you do want to compare pointers to
a particular inline method, or the method has local static data, you might mark
it as having default visibility.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By
default, G++ will use weak symbols if they are available. This option exists
only for testing, and should not be used by end-users; it will result in infe-
rior code and has no benefits. This option may be removed in a future release
of G++.
-nostdinc++
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:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just
won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-
neutral C++ ABI. 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 will be 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 gen-
erated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack
data into the same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other
compilers will not. You can avoid this problem by explicitly padding "A"
so that its size is a multiple of the byte size on your platform; that will
cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use
tail padding when laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other
compilers will. You can avoid this problem by explicitly padding "A" so
that its size is a multiple of its alignment (ignoring virtual base
classes); that will cause G++ and other compilers to layout "C" identi-
cally.
* Incorrect handling of bit-fields with declared widths greater than that of
their underlying types, when the bit-fields appear in a union. For exam-
ple:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too
small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be
placed at offset zero. G++ mistakenly believes that the "A" data member of
"B" is already at offset zero.
* Names of template functions whose types involve "typename" or template tem-
plate parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (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.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual
destructor, yet it declares a non-virtual one. This warning is enabled by
-Wall.
-Wreorder (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 will rearrange 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.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers'
Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator for classes
with dynamically allocated memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don't try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers'
More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of increment and
decrement operators.
* Item 7: Never overload "&&", "||", or ",".
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.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also 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 not a null pointer, it is guaranteed to of the
same size as a pointer. But this use is not portable across different compil-
ers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a
template. Since the advent of explicit template specification support in G++,
if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++
language specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit spec-
ification, unqualified-ids could be interpreted as a particular specialization
of a templatized function. Because this non-conforming behavior is no longer
the default behavior for G++, -Wnon-template-friend allows the compiler to
check existing code for potential trouble spots and is on by default. This new
compiler behavior can be turned off with -Wno-non-template-friend which keeps
the conformant compiler code but disables the helpful warning.
-Wold-style-cast (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 and much easier to search
for.
-Woverloaded-virtual (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();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a
plain pointer.
-Wsign-promo (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++ would try to preserve unsignedness, but the standard
mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while
cfront will use the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages
themselves. See
This section describes the command-line options that are only meaningful for Objec-
tive-C and Objective-C++ programs, but you can also use most of the language-inde-
pendent 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 Objec-
tive-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 compila-
tions may also use options specific to the C front-end (e.g., -Wtraditional). Sim-
ilarly, 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 "NXCon-
stantString" 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, will override the -fconstant-string-class setting and cause
"@"..."" 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_RUN-
TIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[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. Currently, this
option is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
-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 that will run non-trivial default con-
structors 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
that will run all such default destructors, in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly gen-
erated will only operate on instance variables declared in the current Objec-
tive-C class, and not those inherited from superclasses. It is the responsi-
bility of the Objective-C runtime to invoke all such methods in an object's
inheritance hierarchy. The "- (id) .cxx_construct" methods will be invoked by
the runtime immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately before the runtime deallo-
cates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has sup-
port 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, sim-
ilar to what is offered by C++ and Java. Currently, this option is only avail-
able in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++
program; when used inside of a @catch block, the @throw may appear without an
argument (as shown above), in which case the object caught by the @catch will
be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using
this scheme. When an object is thrown, it will be caught by the nearest @catch
clause capable of handling objects of that type, analogously to how "catch"
blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may
also be provided to catch any and all Objective-C exceptions not caught by pre-
vious @catch clauses (if any).
The @finally clause, if present, will be executed upon exit from the immedi-
ately preceding "@try ... @catch" section. This will happen regardless of
whether any exceptions are thrown, caught or rethrown inside the "@try ...
@catch" section, analogously to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with "NS_HANDLER"-style
idioms provided by the "NSException" class, the new exceptions can only be
used on Mac OS X 10.3 (Panther) and later systems, due to additional func-
tionality needed in the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling types other
than Objective-C objects. Furthermore, when used from Objective-C++, the
Objective-C exception model does not interoperate with C++ exceptions at
this time. This means you cannot @throw an exception from Objective-C and
"catch" it in C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchronization blocks for
thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall first check
whether a lock has been placed on the corresponding "guard" object by another
thread. If it has, the current thread shall wait until the other thread relin-
quishes its lock. Once "guard" becomes available, the current thread will
place its own lock on it, execute the code contained in the @synchronized
block, and finally relinquish the lock (thereby making "guard" available to
other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @syn-
chronized. Note that throwing exceptions out of @synchronized blocks is
allowed, and will cause the guarding object to be unlocked properly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting
object file, and allow dyld(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.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file
named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by the garbage
collector.
-Wno-protocol
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
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
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 will omit such warnings if any differences found are
confined to types which share the same size and alignment.
-Wundeclared-selector
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 sec-
tion. 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.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output
device's aspect (e.g. its width, ...). The options described below can be used to
control the diagnostic messages formatting algorithm, e.g. how many characters per
line, how often source location information should be reported. Right now, only
the C++ front end can honor these options. However it is expected, in the near
future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters.
The default is 72 characters for g++ and 0 for the rest of the front ends sup-
ported by GCC. If n is zero, then no line-wrapping will be done; each error
message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit once source location information; 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 con-
tinuation 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-show-options
This option instructs the diagnostic machinery to add text to each diagnostic
emitted, which indicates which command line option directly controls that diag-
nostic, when such an option is known to the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently
erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wim-
plicit to request warnings on implicit declarations. Each of these specific warn-
ing 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.
The following options control the amount and kinds of warnings produced by GCC; for
further, language-specific options also refer to C++ Dialect Options and Objective-
C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all pro-
grams 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 speci-
fied by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this
option (though a rare few will require -ansi or a -std option specifying the
required version of ISO C). However, without this option, certain GNU exten-
sions and traditional C and C++ features are supported as well. With this
option, they are rejected.
-pedantic 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 -pedantic 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 -pedantic. 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 gnu89 or gnu99, there is a corresponding base standard, the version of
ISO C on which the GNU extended dialect is based. Warnings from -pedantic are
given where they are required by the base standard. (It would 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
Like -pedantic, except that errors are produced rather than warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-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.
-Wcomment
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.
-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.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments sup-
plied have types appropriate to the format string specified, and that the con-
versions 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 for-
mat 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 ver-
sion 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 warn-
ing about features that go beyond a particular library's limitations. However,
if -pedantic is used with -Wformat, warnings will be given about format fea-
tures not in the selected standard version (but not for "strfmon" formats,
since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions,
-Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format
checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-for-
mat-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are
available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield
only a two-digit year.
-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.
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 will suppress
the warning if the unused arguments are all pointers, since the Single Unix
Specification says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C stan-
dard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string lit-
eral 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 repre-
sent 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 secu-
rity 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=2
Enable -Wformat plus format checks not included in -Wformat. Currently equiva-
lent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
-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.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note
this option can only be used with the -Wuninitialized option, which in turn
only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snip-
pet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is enabled by
-Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared.
The form -Wno-error-implicit-function-declaration is not supported. This warn-
ing is enabled by -Wall (as a warning, not an error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is
enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function with exter-
nal linkage, returning int, taking either zero arguments, two, or three argu-
ments of appropriate types. This warning is enabled by -Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the fol-
lowing example, the initializer for a is not fully bracketed, but that for b is
fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
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. Only the warning for
an assignment used as a truth value is supported when compiling C++; the other
warnings are only supported when compiling C.
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 mathemati-
cal notation.
Also warn about constructions where there may be confusion to which "if" state-
ment an "else" branch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In 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 will issue a warn-
ing when this flag is specified. To eliminate the warning, add explicit braces
around the innermost "if" statement so there is no way the "else" could belong
to the enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
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 standard.
The C standard defines the order in which expressions in a C program are evalu-
ated 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 expres-
sion denoting the called function), and in certain other places. Other than as
expressed by the sequence point rules, the order of evaluation of subexpres-
sions of an expression is not specified. All these rules describe only a par-
tial 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 com-
mittee 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 standard specifies that "Between the previous and next sequence
point an object shall have its stored value modified at most once by the evalu-
ation 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 present implementation of this option only works for C programs. A future
implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the
precise meaning of the sequence point rules in subtle cases. Links to discus-
sions 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.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int".
Also warn about any "return" statement with no return-value in a function whose
return-type is not "void".
For C, also warn if the return type of a function has a type qualifier such as
"const". Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. ISO C prohibits qualified "void" return types on
function definitions, so such return types always receive a warning even with-
out this option.
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.
-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 enumer-
ation range also provoke warnings when this option is used. 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.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the pro-
gram (trigraphs within comments are not warned about). This warning 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.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside
from its declaration. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. This
warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-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.
These warnings are possible only in optimizing compilation, because they
require data flow information that is computed only when optimizing. If you
don't specify -O, you simply won't get these warnings.
If you want to warn about code which uses the uninitialized value of the vari-
able 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 which are unini-
tialized 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 will depend 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.
These warnings are made optional because GCC is not smart enough to see all the
reasons why the code might be correct despite 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. Here is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
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 which would cause a
problem.
Some spurious warnings can be avoided if you declare all the functions you use
that never return as "noreturn".
This warning is enabled by -Wall.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC.
If this command line option is used, warnings will even be issued for unknown
pragmas in system header files. This is not the case if the warnings were 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 which 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.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It warns about
code which might break the strict aliasing rules that the compiler is using for
optimization. This warning catches more cases than -Wstrict-aliasing, but it
will also give a warning for some ambiguous cases that are safe.
-Wall
All of the above -W options combined. This enables all the warnings about con-
structions 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.
The following -W... options are not implied by -Wall. Some of them warn about con-
structions that users generally do not consider questionable, but which occasion-
ally you might wish to check for; others warn about constructions that are neces-
sary or hard to avoid in some cases, and there is no simple way to modify the code
to suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the
newer name is more descriptive.) Print extra warning messages for these
events:
* A function can return either with or without a value. (Falling off the end
of the function body is considered returning without a value.) For exam-
ple, this function would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma expression con-
tains no side effects. To suppress the warning, cast the unused expression
to void. For example, an expression such as x[i,j] will cause a warning,
but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things in a decla-
ration. According to the C Standard, this usage is obsolescent.
* If -Wall or -Wunused is also specified, warn about unused arguments.
* A comparison between signed and unsigned values could produce an incorrect
result when the signed value is converted to unsigned. (But don't warn if
-Wno-sign-compare is also specified.)
* An aggregate has an initializer which does not initialize all members.
This warning can be independently controlled by -Wmissing-field-initializ-
ers.
* A function parameter is declared without a type specifier in K&R-style
functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate errors, such as
overflow, underflow, loss of precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class with-
out constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class' copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point divi-
sion by zero is not warned about, as it can be a legitimate way of obtaining
infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings
from system headers are normally suppressed, on the assumption that they usu-
ally 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 will not warn about unknown pragmas in
system headers---for that, -Wunknown-pragmas must also be used.
-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 compu-
tation introduces, and allow for it when performing comparisons (and when pro-
ducing output, but that's a different problem). In particular, instead of
testing for equality, you would check to see whether the two values have ranges
that overlap; and this is done with the relational operators, so equality com-
parisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C.
Also warn about ISO C constructs that have no traditional C equivalent, and/or
problematic constructs which 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
does not in ISO C.
* In traditional C, some preprocessor directives did not exist. Traditional
preprocessors would only consider 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 would ignore 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 would not recognize #elif, so it
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 suf-
fixes. (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 repre-
sent bit patterns, are not warned about.
* 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 omit-
ted. 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
would cause serious problems. This is a subset of the possible conversion
warnings, for the full set use -Wconversion.
* Use of ISO C style function definitions. This warning intentionally is not
issued for prototype declarations or variadic functions because these ISO C
features will 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 exten-
sion and thus not relevant to traditional C compatibility.
-Wdeclaration-after-statement (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 and was not supported by GCC versions before GCC
3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or
global variable or whenever a built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not assume any-
thing on the bounds of the loop indices. With -funsafe-loop-optimizations warn
if the compiler made such assumptions.
-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.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example,
warn if "int malloc()" is cast to "anything *".
-Wc++-compat
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.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the tar-
get type. For example, warn if a "const char *" is cast to an ordinary "char
*".
-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 will get a warn-
ing; when compiling C++, warn about the deprecated conversion from string con-
stants to "char *". These warnings will 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 will
just be a nuisance; this is why we did not make -Wall request these warnings.
-Wconversion
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 con-
versions 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.
Also, warn if a negative integer constant expression is implicitly converted to
an unsigned type. For example, warn about the assignment "x = -1" if "x" is
unsigned. But do not warn about explicit casts like "(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an
incorrect result when the signed value is converted to unsigned. This warning
is also enabled by -Wextra; to get the other warnings of -Wextra without this
warning, use -Wextra -Wno-sign-compare.
-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-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized
attributes, function attributes applied to variables, etc. This will not stop
errors for incorrect use of supported attributes.
-Wstrict-prototypes (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 which specifies the argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is given even if
there is a previous prototype.
-Wmissing-prototypes (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. The
aim is to detect global functions that fail to be declared in header files.
-Wmissing-declarations (C only)
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.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the
following code would cause 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 modi-
fication would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without
this one, use -Wextra -Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". Note
these are only possible candidates, not absolute ones. Care should be taken to
manually verify functions actually do not ever return before adding the "nore-
turn" attribute, otherwise subtle code generation bugs could be introduced.
You will not get a warning for "main" in hosted C environments.
-Wmissing-format-attribute
Warn about function pointers which might be candidates for "format" attributes.
Note these are only possible candidates, not absolute ones. GCC will guess
that function pointers with "format" attributes that are used in assignment,
initialization, parameter passing or return statements should have a corre-
sponding "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 will also warn about function definitions which might be candidates for
"format" attributes. Again, these are only possible candidates. GCC will
guess 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.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indi-
cate a typo in the user's code, as they have implementation-defined values, and
should 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 which have not been normalized; this option controls that warning.
There are four levels of warning that GCC supports. The default is -Wnormal-
ized=nfc, which warns about any identifier which is not in the ISO 10646 "C"
normalized form, NFC. NFC is the recommended form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++ allow in iden-
tifiers that when turned into NFC aren't allowable as 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 charac-
ters. 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.
You would only want to do this if you were using some other normalization
scheme (like "D"), because otherwise you can easily create bugs that are liter-
ally 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", will display just
like a regular "n" which has been placed in a superscript. ISO 10646 defines
the NFKC normalisation scheme to convert all these into a standard form as
well, and GCC will warn if your code is not in NFKC if you use -Wnormal-
ized=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 program-
ming environment is unable to be fixed to display these characters distinctly.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated
by using the "deprecated" attribute. (@pxref{Function Attributes},
@pxref{Variable Attributes}, @pxref{Type Attributes}.)
-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" will be 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;
};
-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.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole
line of source code will never be executed, because some condition is never
satisfied or because it is after a procedure that never returns.
It is possible for this option to produce a warning even though there are cir-
cumstances under which part of the affected line can be executed, so care
should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is
unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a pro-
gram there is often substantial code which checks correct functioning of the
program and is, hopefully, unreachable because the program does work. Another
common use of unreachable code is to provide behavior which is selectable at
compile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with
this option, the compiler will 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 compiler takes into account the size of the func-
tion 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++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. Accord-
ing to the 1998 ISO C++ standard, applying offsetof to a non-POD type is unde-
fined. In existing C++ implementations, however, offsetof typically gives
meaningful results even when applied to certain kinds of non-POD types. (Such
as a simple struct that fails to be a POD type only by virtue of having a con-
structor.) This flag is for users who are aware that they are writing non-
portable 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.
-Wno-int-to-pointer-cast (C only)
Suppress warnings from casts to pointer type of an integer of a different size.
-Wno-pointer-to-int-cast (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 can't be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the warning mes-
sages, use -Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into
account only when -pedantic flag is used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate
syntax when in pedantic ISO C99 mode. This is default. To inhibit the warning
messages, use -Wno-variadic-macros.
-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 regis-
ter variables.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not gen-
erally indicate that there is anything wrong with your code; it merely indi-
cates that GCC's optimizers were unable to handle the code effectively. Often,
the problem is that your code is too big or too complex; GCC will refuse to
optimize programs when the optimization itself is likely to take inordinate
amounts of time.
-Wpointer-sign
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 -pedantic, which can be disabled with -Wno-pointer-sign.
-Werror
Make all warnings into errors.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about
functions that will not be protected against stack smashing.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or
GCC:
-g Produce debugging information in the operating system's native format (stabs,
COFF, XCOFF, or DWARF 2). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging infor-
mation that only GDB can use; this extra information makes debugging work bet-
ter in GDB but will probably make 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).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may
occasionally produce surprising results: 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 were already at hand; some statements may execute in different
places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it rea-
sonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for
more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the most
expressive format available (DWARF 2, stabs, or the native format if neither of
those are supported), including GDB extensions if at all possible.
-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 out-
put which is not understood by DBX or SDB. On System V Release 4 systems this
option requires the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only
symbols that are actually used.
-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 exten-
sions 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 exten-
sions is likely to make other debuggers crash or refuse to read the program,
and may cause assemblers other than the GNU assembler (GAS) to fail with an
error.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported).
This is the format used by DBX on IRIX 6. With this option, GCC uses features
of DWARF version 3 when they are useful; version 3 is upward compatible with
version 2, but may still cause problems for older debuggers.
-gvms
Produce debugging information in VMS debug format (if that is supported). This
is the format used by DEBUG on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much informa-
tion. The default level is 2.
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 func-
tions and external variables, but no information about local variables and no
line numbers.
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-2 does not accept a concatenated debug level, because GCC used to sup-
port an option -gdwarf that meant to generate debug information in version 1 of
the DWARF format (which is very different from version 2), and it would have
been too confusing. That debug format is long obsolete, but the option cannot
be changed now. Instead use an additional -glevel option to change the debug
level for DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information
about each symbol. This option only makes sense when generating DWARF2 debug-
ging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the analysis pro-
gram 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 pro-
gram gprof. You must use this option when compiling the source files you want
data about, and you must also use it when linking.
-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.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when
it finishes.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the pro-
gram records how many times each branch and call is executed and how many times
it is taken or returns. 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 pro-
file-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 exe-
cutable, 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 analy-
sis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compil-
ing) and -lgcov (when linking). See the documentation for those options for
more details.
@bullet
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.
@cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the latter implies
the former).
@dwnngv
Run the program on a representative workload to generate the arc profile
information. This may be repeated any number of times. You can run con-
current instances of your program, and provided that the file system sup-
ports locking, the data files will be correctly updated. Also "fork" calls
are detected and correctly handled (double counting will not happen).
@exoohw
For profile-directed optimizations, compile the source files again with the
same optimization and code generation options plus -fbranch-probabilities.
@fyppix
For test coverage analysis, use gcov to produce human readable information
from the .gcno and .gcda files. Refer to the gcov documentation for fur-
ther 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 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 pro-
gram 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 will match the source
files more closely, if you do not optimize.
-dletters
-fdump-rtl-pass
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. dumpname 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.
Most debug dumps can be enabled either passing a letter to the -d option, or
with a long -fdump-rtl switch; here are the possible letters for use in letters
and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging information.
-db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file file.17.combine.
-dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the
file file.11.ce1. -dC and -fdump-rtl-ce2 enable dumping after the second
if conversion, to the file file.18.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load optimiza-
tion, to file.31.btl. -dd and -fdump-rtl-dbr enable dumping after delayed
branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.
-df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data flow analysis,
to file.08.cfg. -df and -fdump-rtl-cfg enable dumping dump after life
analysis, to file.16.life.
-dg
-fdump-rtl-greg
Dump after global register allocation, to file.23.greg.
-dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to file.05.gcse. -dG
and -fdump-rtl-bypass enable dumping after jump bypassing and control flow
optimizations, to file.07.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.
-dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to file.33.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.
-dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop optimization
pass, to file.06.loop. -dL and -fdump-rtl-loop2 enable dumping after the
second pass, to file.13.loop2.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, to
file.35.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.
-do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.
-dr
-fdump-rtl-expand
Dump after RTL generation, to file.00.expand.
-dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.32.sched2.
-ds
-fdump-rtl-cse
Dump after CSE (including the jump optimization that sometimes follows
CSE), to file.04.cse.
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization that some-
times follows CSE), to file.15.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile transforma-
tions, to file.10.vpt. -dV and -fdump-rtl-vartrack enable dumping after
variable tracking, to file.34.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to standard error.
-dp Annotate the assembler output with a comment indicating which pattern and
alternative was 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.
-dv For each of the other indicated dump files (either with -d or
-fdump-rtl-pass), dump a representation of the control flow graph suitable
for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it. Usually used
with r (-fdump-rtl-expand).
-dy Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers
and line number note output. This makes it more feasible to use diff on debug-
ging dumps for compiler invocations with different options, in particular with
and without -g.
-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. 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 lay-
out to a file. The file name is made by appending .class to the source file
name. 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 suf-
fix to the source file name. The following dumps are possible:
all Enables all inter-procedural analysis dumps; currently the only produced
dump is the cgraph dump.
cgraph
Dumps information about call-graph optimization, unused function removal,
and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options
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 suf-
fix to the source file name. If the -options form is used, options is a list
of - separated options that control the details of the dump. Not all options
are applicable to all dumps, those which are not meaningful will be 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.
slim
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. When dumping pretty-printed trees, this
option inhibits dumping the bodies of control structures.
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).
stats
Enable dumping various statistics about the pass (not honored by every dump
option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
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.
all Turn on all options, except raw, slim and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
gimple
Dump each function before and after the gimplification pass to a file. The
file name is made by appending .gimple to the source file name.
cfg Dump the control flow graph of each function to a file. The file name is
made by appending .cfg to the source file name.
vcg Dump the control flow graph of each function to a file in VCG format. The
file name is made by appending .vcg to the source file name. Note that if
the file contains more than one function, the generated file cannot be used
directly by VCG. You will need to cut and paste each function's graph into
its own separate file first.
ch Dump each function after copying loop headers. The file name is made by
appending .ch to the source file name.
ssa Dump SSA related information to a file. The file name is made by appending
.ssa to the source file name.
salias
Dump structure aliasing variable information to a file. This file name is
made by appending .salias to the source file name.
alias
Dump aliasing information for each function. The file name is made by
appending .alias to the source file name.
ccp Dump each function after CCP. The file name is made by appending .ccp to
the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending
.storeccp to the source file name.
pre Dump trees after partial redundancy elimination. The file name is made by
appending .pre to the source file name.
fre Dump trees after full redundancy elimination. The file name is made by
appending .fre to the source file name.
copyprop
Dump trees after copy propagation. The file name is made by appending
.copyprop to the source file name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by append-
ing .store_copyprop to the source file name.
dce Dump each function after dead code elimination. The file name is made by
appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is
made by appending .mudflap to the source file name.
sra Dump each function after performing scalar replacement of aggregates. The
file name is made by appending .sra to the source file name.
sink
Dump each function after performing code sinking. The file name is made by
appending .sink to the source file name.
dom Dump each function after applying dominator tree optimizations. The file
name is made by appending .dom to the source file name.
dse Dump each function after applying dead store elimination. The file name is
made by appending .dse to the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The
file name is made by appending .phiopt to the source file name.
forwprop
Dump each function after forward propagating single use variables. The
file name is made by appending .forwprop to the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file
name is made by appending .copyrename to the source file name.
nrv Dump each function after applying the named return value optimization on
generic trees. The file name is made by appending .nrv to the source file
name.
vect
Dump each function after applying vectorization of loops. The file name is
made by appending .vect to the source file name.
vrp Dump each function after Value Range Propagation (VRP). The file name is
made by appending .vrp to the source file name.
all Enable all the available tree dumps with the flags provided in this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer prints.
This information is written to standard error, unless -fdump-tree-all or
-fdump-tree-vect is specified, in which case it is output to the usual dump
listing file, .vect.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random
numbers. It is used to generate 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 -fran-
dom-seed option to produce reproducibly identical object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of
debugging output the scheduler prints. This information is written to standard
error, unless -dS or -dR is specified, in which case it is output to the usual
dump listing file, .sched or .sched2 respectively. However for n greater than
nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS.
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.
-save-temps
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 would produce 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 sensi-
ble 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.
-time
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). 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 sec-
onds.
-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.
-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 com-
pile 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 @samp{-, without spaces between multi-
ple switches. This is supposed to ease shell-processing.
-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
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 will search---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 /.
-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)---and don't do anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is
used when GCC itself is being built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for
all types declared in a compilation unit, regardless of whether or not they are
actually used in that compilation unit. Sometimes this is useful, such as 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 sig-
nificant amount of wasted space. With this option, GCC will avoid producing
debug symbol output for types that are nowhere used in the source file being
compiled.
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 com-
pilation and to make debugging produce the expected results. Statements are inde-
pendent: if you stop the program with a breakpoint between statements, you can then
assign a new value to any variable or change the program counter to any other
statement in the function and get exactly the results you would 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.
Optimization levels -O and above, in particular, enable unit-at-a-time mode, which
allows the compiler to consider information gained from later functions in the file
when compiling a function. Compiling multiple files at once to a single output
file in unit-at-a-time 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.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more mem-
ory 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: -fdefer-pop -fdelayed-branch
-fguess-branch-probability -fcprop-registers -floop-optimize -fif-conversion
-fif-conversion2 -ftree-ccp -ftree-dce -ftree-dominator-opts -ftree-dse
-ftree-ter -ftree-lrs -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch
-funit-at-a-time -fmerge-constants
-O also turns on -fomit-frame-pointer on machines where doing so does not
interfere with debugging.
-O doesn't turn on -ftree-sra for the Ada compiler. This option must be
explicitly specified on the command line to be enabled for the Ada compiler.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do
not involve a space-speed tradeoff. The compiler does not perform loop
unrolling or function inlining when you specify -O2. 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 fol-
lowing optimization flags: -fthread-jumps -fcrossjumping -foptimize-sib-
ling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse -fgcse-lm -fexpen-
sive-optimizations -fstrength-reduce -frerun-cse-after-loop -frerun-loop-opt
-fcaller-saves -fpeephole2 -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fregmove -fstrict-aliasing
-fdelete-null-pointer-checks -freorder-blocks -freorder-functions
-falign-functions -falign-jumps -falign-loops -falign-labels -ftree-vrp
-ftree-pre
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 and -fgcse-after-reload
options.
-O0 Do not optimize. 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: -falign-functions -falign-jumps
-falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition
-fprefetch-loop-arrays -ftree-vect-loop-version
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 would be -fno-foo. In the
table below, only one of the forms is listed---the one you typically will 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-default-inline
Do not make member functions inline by default merely because they are defined
inside the class scope (C++ only). Otherwise, when you specify -O, member
functions defined inside class scope are compiled inline by default; i.e., you
don't need to add inline in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function
returns. For machines which must pop arguments after a function call, the com-
piler normally lets arguments accumulate on the stack for several function
calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on
them. This produces better code by making all memory references potential com-
mon subexpressions. When they are not common subexpressions, instruction com-
bination should eliminate the separate register-load. This option is now a nop
and will be removed in 4.2.
-fforce-addr
Force memory address constants to be copied into registers before doing arith-
metic on them.
-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 debug-
ging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the stan-
dard 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.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option is used to
keep the compiler from expanding any functions inline. Note that if you are
not optimizing, no functions can be expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically
decides which functions are simple enough to be 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 if -funit-at-a-time is enabled.
-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.
Enabled by default.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag
allows the control of this limit for functions that are explicitly marked as
inline (i.e., marked with the inline keyword or defined within the class defi-
nition in c++). n is the size of functions that can be inlined in number of
pseudo instructions (not counting parameter handling). The default value of n
is 600. Increasing this value can result in more inlined code at the cost of
compilation time and memory consumption. Decreasing usually makes the compila-
tion faster and less code will be inlined (which presumably means slower pro-
grams). This option is particularly useful for programs that use inlining
heavily such as those based on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which may be speci-
fied 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 I<n>/2.
max-inline-insns-auto
is set to I<n>/2.
min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
max-inline-insns-rtl
is set to I<n>.
See below for a documentation of the individual parameters controlling inlin-
ing.
Note: pseudo instruction represents, in this particular context, an abstract
measurement of function's size. In no way does it represent a count of assem-
bly instructions and as such its exact meaning might change from one release to
an another.
-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 C. In
C++, emit any and all inline functions into the object file.
-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 the variable was 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 con-
stants) 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 vari-
ables with integral or floating point types. Languages like C or C++ require
each non-automatic variable to have distinct location, so using this option
will result 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 overlap-
ping different iterations.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead
generate a sequence of instructions that decrement a register, compare it
against zero, then branch based upon the result. This option is only meaning-
ful on architectures that support such instructions, which include x86, Pow-
erPC, IA-64 and S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-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.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array
dereferencing operations, some standard library string/heap functions, and some
other associated constructs with range/validity tests. Modules so instrumented
should be immune to buffer overflows, invalid heap use, and some other classes
of C/C++ programming errors. The instrumentation relies on a separate runtime
library (libmudflap), which will be linked into a program if -fmudflap is given
at link time. Run-time behavior of the instrumented program is controlled by
the MUDFLAP_OPTIONS environment variable. See "env MUDFLAP_OPTIONS=-help
a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is
multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if
instrumentation should ignore pointer reads. This produces less instrumenta-
tion (and therefore faster execution) and still provides some protection
against outright memory corrupting writes, but allows erroneously read data to
propagate within a program.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of itera-
tion variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we 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.
-fcse-follow-jumps
In common subexpression elimination, 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 will follow 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 which
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 has been per-
formed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also per-
forms global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may
get better runtime 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 will attempt
to move loads which 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.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common
subexpression elimination. This pass will attempt 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 per-
formed after reload. The purpose of this pass is to cleanup redundant
spilling.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify
exit test conditions and optionally do strength-reduction as well.
Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2
Perform loop optimizations using the new loop optimizer. The optimizations
(loop unrolling, peeling and unswitching, loop invariant motion) are enabled by
separate flags.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not overflow, and
that the loops with nontrivial exit condition are not infinite. This enables a
wider range of loop optimizations even if the loop optimizer itself cannot
prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the
compiler will warn you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent
code and save code size. The resulting code may or may not perform better than
without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This
include 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 "if-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.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null
pointers. The compiler assumes that dereferencing a null pointer would have
halted the program. If a pointer is checked after it has already been derefer-
enced, it cannot be null.
In some environments, this assumption is not true, and programs can safely
dereference null pointers. Use -fno-delete-null-pointer-checks to disable this
optimization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of
other simple instructions in order to maximize the amount of register tying.
This is especially helpful on machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
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 elimi-
nate 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
instruction is required.
Enabled at levels -O2, -O3, -Os.
-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-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.
-fno-fsched-stalled-insns and -fsched-stalled-insns=0 are equivalent and mean
that no insns will be moved prematurely. If n is unspecified then there is no
limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a dependency on a
stalled insn that is 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 and its value is not zero.
+-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, do use superblock scheduling algo-
rithm. Superblock scheduling allows motion across basic block boundaries
resulting on 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.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after register alloca-
tion and additionally perform code duplication in order to increase the size of
superblocks using tracer pass. See -ftracer for details on trace formation.
This mode should produce faster but significantly longer programs. Also with-
out -fbranch-probabilities the traces constructed may not match the reality and
hurt the performance. This only makes sense when scheduling after register
allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a loop was
modulo scheduled we may want to prevent the later scheduling passes from chang-
ing its schedule, we use this option to control that.
-fcaller-saves
Enable values to be allocated in registers that will be 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 than would otherwise be produced.
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.
-ftree-pre
Perform Partial Redundancy Elimination (PRE) on trees. This flag is enabled by
default at -O2 and -O3.
-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 faster than PRE, though it
exposes fewer redundancies. This flag is enabled by default at -O and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy oper-
ations. This flag is enabled by default at -O and higher.
-ftree-store-copy-prop
Perform copy propagation of memory loads and stores. This pass eliminates
unnecessary copy operations in memory references (structures, global variables,
arrays, etc). This flag is enabled by default at -O2 and higher.
-ftree-salias
Perform structural alias analysis on trees. This flag is enabled by default at
-O and higher.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O
and higher.
-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.
-ftree-store-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass
operates on both local scalar variables and memory stores and loads (global
variables, structures, arrays, etc). This flag is enabled by default at -O2
and higher.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default
at -O and higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redun-
dancy 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-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
Perform linear loop transformations on tree. This flag can improve cache per-
formance and allow further loop optimizations to take place.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that
would be 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 triv-
ial invariantness analysis in loop unswitching. The pass also includes store
motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in the loop for that deter-
mining 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 vari-
able merging and induction variable elimination) on trees.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure refer-
ences with scalars to prevent committing structures to memory too early. This
flag is enabled by default at -O and higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler tempo-
raries to other variables at copy locations, usually resulting in variable
names which more closely resemble the original variables. This flag is enabled
by default at -O and higher.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single
use/single def temporaries are replaced at their use location with their defin-
ing 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-lrs
Perform live range splitting during the SSA->normal phase. Distinct live
ranges of a variable are split into unique variables, allowing for better opti-
mization later. This is enabled by default at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop
appears to be vectorizable except that data alignment or data dependence cannot
be determined at compile time then vectorized and non-vectorized versions of
the loop are generated along with runtime checks for alignment or dependence to
control which version is executed. This option is enabled by default except at
level -Os where it is disabled.
-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.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation sim-
plifies the control flow of the function allowing other optimizations to do
better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop. -funroll-loops implies both -fstrength-reduce and
-frerun-cse-after-loop. This option makes code larger, and may or may not make
it run faster.
-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,
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later iterations of the
unrolled loop using the value in the first iteration. This breaks long depen-
dency chains, thus improving efficiency of the scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect.
However in cases the loop body is more complicated than a single basic block,
this is not reliable. It also does not work at all on some of the architec-
tures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some local vari-
ables when unrolling a loop which can result in superior code.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to
improve the performance of loops that access large arrays.
These options may generate better or worse code; results are highly dependent
on the structure of loops within the source code.
-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 com-
piler; 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 will use heuristics to guess branch probabilities if they are not provided
by profiling feedback (-fprofile-arcs). These heuristics are based on the con-
trol flow graph. If some branch probabilities are specified by
__builtin_expect, then the heuristics will be used to guess branch probabili-
ties 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 -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 sep-
arate sections of the assembly and .o files, to improve paging and cache local-
ity performance.
This optimization is automatically turned off in the presence of exception han-
dling, for linkonce sections, for functions with a user-defined section
attribute and on any architecture that does not support named sections.
-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 sec-
tions and linker must place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See
-fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows 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() {
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, type-punning is allowed, provided the memory is accessed
through the union type. So, the code above will work as expected. However,
this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis should
define a function that computes, given an "tree" node, an alias set for the
node. Nodes in different alias sets are not allowed to alias. For an example,
see the C front-end function "c_get_alias_set".
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 would align 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 func-
tions will not be 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.
-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 will
not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this
value, then their values are used instead.
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. The hope is that the loop will be executed many times,
which will make up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will
not be 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 will
not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code. This allows
some extra optimizations to take place but consumes more memory (in general).
There are some compatibility issues with unit-at-at-time mode:
* enabling unit-at-a-time mode may change the order in which functions, vari-
ables, and top-level "asm" statements are emitted, and will likely break
code relying on some particular ordering. The majority of such top-level
"asm" statements, though, can be replaced by "section" attributes.
* unit-at-a-time mode removes unreferenced static variables and functions.
This may result in undefined references when an "asm" statement refers
directly to variables or functions that are otherwise unused. In that case
either the variable/function shall be listed as an operand of the "asm"
statement operand or, in the case of top-level "asm" statements the
attribute "used" shall be used on the declaration.
* Static functions now can use non-standard passing conventions that may
break "asm" statements calling functions directly. Again, attribute "used"
will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may
not be supported by future releases of GCC.
Enabled at levels -O, -O2, -O3, -Os.
-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 will no longer stay in a
"home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents whole program being com-
piled. All public functions and variables with the exception of "main" and
those merged by attribute "externally_visible" become static functions and in a
affect gets more aggressively optimized by interprocedural optimizers. While
this option is equivalent to proper use of "static" keyword for programs con-
sisting of single file, in combination with option --combine this flag can be
used to compile most of smaller scale C programs since the functions and vari-
ables become local for the whole combined compilation unit, not for the single
source file itself.
-fcprop-registers
After register allocation and post-register allocation instruction splitting,
we perform a copy-propagation pass to try to reduce scheduling dependencies and
occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-generate
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".
-fprofile-use
Enable profile feedback directed optimizations, and optimizations generally
profitable only with profile feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-fun-
roll-loops", "-fpeel-loops", "-ftracer", "-fno-loop-optimize".
The following options control compiler behavior regarding floating point arith-
metic. These options trade off between speed and correctness. All must be specif-
ically 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 programs, after
modifying them to store all pertinent intermediate computations into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffi-
nite-math-only, -fno-rounding-math, -fno-signaling-nans and fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option should never be turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation of IEEE
or ISO rules/specifications for math functions.
-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.
This option should never be turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation of IEEE
or ISO rules/specifications for math functions.
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 argu-
ments 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 should never be turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation of IEEE
or ISO rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments
and results are not NaNs or +-Infs.
This option should never be turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation of IEEE
or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visi-
ble traps. These traps include division by zero, overflow, underflow, inexact
result and invalid operation. This option implies -fno-signaling-nans. Set-
ting this option may allow faster code if one relies on "non-stop" IEEE arith-
metic, for example.
This option should never be turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation of IEEE
or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
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 dynam-
ically, 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.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implic-
itly converting it to double precision constant.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when
performing complex division. 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.
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 sec-
ond time using -fbranch-probabilities, to improve optimizations based on the
number of times each branch was taken. When the program compiled with -fpro-
file-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 one place: in reorg.c, instead of guessing which path a branch is
mostly 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 and adds REG_VALUE_PROFILE notes to instructions for
their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to
gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually per-
forms the optimizations based on them. Currently the optimizations include
specialization of division operation using the knowledge about the value of the
denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of regis-
ters left over after register allocation. This optimization will most benefit
processors with lots of registers. Depending on the debug information format
adopted by the target, however, it can make debugging impossible, since vari-
ables will no longer stay in a "home register".
Enabled by default with -funroll-loops.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation sim-
plifies the control flow of the function allowing other optimizations to do
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 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 the loops for that there is enough information that they do not roll much
(from profile feedback). It also turns on complete loop peeling (i.e. complete
removal of loops with small constant number of iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the new loop optimizer. Enabled at
level -O1
-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).
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to
improve the performance of loops that access large arrays.
Disabled at level -Os.
-ffunction-sections
-fdata-sections
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 will create larger object
and executable files and will also be slower. You will not be able to 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 in within any basic block.
-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.
--param name=value
In some places, GCC uses various constants to control the amount of optimiza-
tion that is done. For example, GCC will not inline functions that contain
more that a certain number of instructions. You can control some of these con-
stants 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
given in the following table:
salias-max-implicit-fields
The maximum number of fields in a variable without direct structure
accesses for which structure aliasing will consider trying to track each
field. The default is 5
sra-max-structure-size
The maximum structure size, in bytes, at which the scalar replacement of
aggregates (SRA) optimization will perform block copies. The default
value, 0, implies that GCC will select the most appropriate size itself.
sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated fields and the
complete structure size. We say that if the ratio of the number of bytes
in instantiated fields to the number of bytes in the complete structure
exceeds this parameter, then block copies are not used. The default is 75.
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. 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 compile time increase with probably small improvement in executable
size.
min-crossjump-insns
The minimum number of instructions which must be matched at the end of two
blocks before crossjumping will be performed on them. This value is
ignored in the case where all instructions in the block being crossjumped
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 instruc-
tion to fill a delay slot. If more than this arbitrary number of instruc-
tions is searched, the time savings from filling the delay slot will be
minimal so stop searching. Increasing values mean more aggressive opti-
mization, making the compile time increase with probably small improvement
in executable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to con-
sider when searching for a block with valid live register information.
Increasing this arbitrarily chosen value means more aggressive optimiza-
tion, increasing the compile 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 will be allocated in order to
perform the global common subexpression elimination optimization. If more
memory than specified is required, the optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run. The default is 1.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before
flushing the current state and starting over. Large functions with few
branches or calls can create excessively large lists which needlessly con-
sume memory and resources.
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 representa-
tion) in a single function that the tree inliner will consider for inlin-
ing. This only affects functions declared inline and methods implemented
in a class declaration (C++). The default value is 450.
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 will be
investigated. To those functions, a different (more restrictive) limit
compared to functions declared inline can be applied. The default value is
90.
large-function-insns
The limit specifying really large functions. For functions larger than
this limit after inlining inlining is constrained by --param large-func-
tion-growth. This parameter is useful primarily to avoid extreme compila-
tion time caused by non-linear algorithms used by the backend. This param-
eter is ignored when -funit-at-a-time is not used. The default value is
2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents.
This parameter is ignored when -funit-at-a-time is not used. 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 (consider unit consisting of function A
that is inline and B that just calls A three time. 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 inlininable 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 aplying --param inline-unit-growth. The default is 10000
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlin-
ing. This parameter is ignored when -funit-at-a-time is not used. The
default value is 50 which limits unit growth to 1.5 times the original
size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of self recursive
inline function can grow into by performing recursive inlining.
For functions declared inline --param max-inline-insns-recursive is taken
into account. For function not declared inline, recursive inlining happens
only when -finline-functions (included in -O3) is enabled and --param max-
inline-insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive inlining.
For functions declared inline --param max-inline-recursive-depth is taken
into account. For function not declared inline, recursive inlining happens
only when -finline-functions (included in -O3) is enabled and --param max-
inline-recursive-depth-auto is used. The default value is 450.
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 increas-
ing 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 probability that function will recurse
via given call expression. This parameter limits inlining only to call
expression whose probability exceeds given threshold (in percents). The
default value is 10.
inline-call-cost
Specify cost of call instruction relative to simple arithmetics operations
(having cost of 1). Increasing this cost disqualifies inlining of non-leaf
functions and at the same time increases size of leaf function that is
believed to reduce function size by being inlined. In effect it increases
amount of inlining for code having large abstraction penalty (many func-
tions that just pass the arguments to other functions) and decrease inlin-
ing for code with low abstraction penalty. The default value is 16.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is
unrolled, and if the loop is unrolled, it determines how many times the
loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execu-
tion that a loop should have if that loop is unrolled, and if the loop is
unrolled, it 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 should have if that loop is
peeled, and if the loop is peeled, it determines how many times the loop
code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
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-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
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 that all candi-
dates are considered for each use in induction variable optimizations.
Only the most relevant candidates are considered if there are more candi-
dates, 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 number of candidates in the set is smaller than this value, we always
try to remove unnecessary ivs from the set during its optimization when a
new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large
expressions slow the analyzer.
vect-max-version-checks
The maximum number of runtime checks that can be performed when doing loop
versioning in the vectorizer. See option ftree-vect-loop-version for more
information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force algorithm for
analysis of # of iterations of the loop tries to evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in pro-
gram given basic block needs to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic block in
function given basic block needs to have to be considered hot
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is use-
ful in cases where function contain single loop with known bound and other
loop with unknown. We predict the known number of iterations correctly,
while the unknown number of iterations average to roughly 10. This means
that the loop without bounds would appear artificially cold relative to the
other one.
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 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 rather hokey argument, as most of the duplicates will be 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-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this
threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compi-
lation for profile feedback and one for 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
Maximum number of basic blocks on path that cse considers. The default is
10.
max-cse-insns
The maximum instructions CSE process before flushing. The default is 1000.
global-var-threshold
Counts the number of function calls (n) and the number of call-clobbered
variables (v). If nxv is larger than this limit, a single artificial vari-
able will be created to represent all the call-clobbered variables at func-
tion call sites. This artificial variable will then be made to alias every
call-clobbered variable. (done as "int * size_t" on the host machine;
beware overflow).
max-aliased-vops
Maximum number of virtual operands allowed to represent aliases before
triggering the alias grouping heuristic. Alias grouping reduces compile
times and memory consumption needed for aliasing at the expense of preci-
sion loss in alias information.
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 cal-
culate RAM on a particular platform, the lower bound of 30% is used. Set-
ting 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 which 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 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 equiva-
lent register. Increasing values mean more aggressive optimization, making
the compile time increase with probably slightly better performance. The
default value is 100.
max-cselib-memory-location
The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compile
time increase with probably slightly better performance. The default value
is 500.
max-flow-memory-location
Similar as max-cselib-memory-location but for dataflow liveness. The
default value is 100.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional
branch or duplicate the code on its destination. Code is duplicated when
its estimated size is smaller than this value multiplied by the estimated
size of unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is
available and may be set to higher values than reorder-block-duplicate
since information about the hot spots is more accurate.
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-sched-region-insns
The maximum number of insns in a region to be considered for interblock
scheduling. The default value is 100.
min-sched-prob
The minimum probability of reaching a source block for interblock specula-
tive scheduling. The default value is 40.
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.
integer-share-limit
Small integer constants can use a shared data structure, reducing the com-
piler's memory usage and increasing its speed. This sets the maximum value
of a shared integer constant's. The default value is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the incremental SSA
updater that should be registered to trigger the virtual mappings heuristic
defined by virtual-mappings-ratio. The default value is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio bigger than the
number of virtual symbols to be updated, then the incremental SSA updater
switches to a full update for those symbols. The default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive stack smashing
protection when -fstack-protection is used.
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 we will treat in a field sensitive
manner during pointer analysis.
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.
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 mul-
tiple 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 driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply sys-
tem-specific preprocessor options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreproces-
sor twice, once for the option and once for the argument.
-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 dur-
ing translation phase three in a #define directive. In particular, the defini-
tion will be 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 will need to 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.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard pre-
defined macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header
files. Directories named by -I are searched before the standard system include
directories. If the directory dir is a standard system include directory, the
option is ignored to ensure that the default search order for system directo-
ries and the special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the second non-
option argument to cpp. gcc has a different interpretation of a second non-
option argument, so you must use -o to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present
this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promo-
tion causing a change of sign in "#if" expressions. Note that many of the pre-
processor's warnings are on by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever
a backslash-newline appears in a // comment. (Both forms have the same
effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However,
a trigraph that would form an escaped newline (??/ at the end of a line) can,
by changing where the comment begins or ends. Therefore, only trigraphs that
would form escaped newlines produce warnings inside a comment.
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 warn-
ings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C.
Also warn about ISO C constructs that have no traditional C equivalent, and
problematic constructs which should be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if
directive, outside of defined. Such identifiers are replaced with zero.
-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 will
also warn if the macro has not been used at the time it is redefined or unde-
fined.
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 CPP will report 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
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens
in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older
programs. This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will
be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in
finding bugs in your own code, therefore suppressed. If you are responsible
for the system library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are
left out by default, since they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into
errors. This includes mandatory diagnostics that GCC issues without -pedantic
but treats as warnings.
-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 basename of the source file with any suffix replaced with object file suf-
fix. 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 will still be sent to the regular out-
put 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 direc-
tories, 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 will appear in -MM
dependency output. This is a slight change in semantics from GCC versions 3.0
and earlier.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no
-MF switch is given the preprocessor sends the rules to the same place it would
have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default depen-
dency 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 depen-
dency 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 Make-
file 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, including any path, deletes any file
suffix such as .c, and appends the platform's usual object suffix. The result
is the target.
An -MT option will set 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 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 take the basename of
the input file 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 @pxref{dashMF,,-MF}), 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.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output
flags to also list the files from the precompiled header's dependencies. If
not specified only the precompiled header would be listed and not the files
that were used to create it because those files are not consulted when a pre-
compiled 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 file-
name may be absolute or it may be relative to GCC's current directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has noth-
ing to do with standards conformance or extensions; it merely selects which
base syntax to expect. If you give none of these options, cpp will deduce the
language from the extension of the source file: .c, .cc, .m, or .S. Some other
common extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most generic
mode.
Note: Previous versions of cpp accepted a -lang option which selected both the
language and the standards conformance level. This option has been removed,
because it conflicts with the -l option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows
about C and C++ standards; others may be added in the future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this ver-
sion of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publica-
tion, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++
code.
-I- Split the include path. 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 direc-
tory as the first search directory for "#include "file"". This option has been
deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the
directories you have specified with -I options (and the directory of the cur-
rent 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 build-
ing the C++ library.)
-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 preproces-
sor'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.
All files specified by -imacros are processed before all files specified by
-include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I
and the standard system directories have been exhausted. dir is treated as a
system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the pre-
fix 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.
See the --sysroot option for more information.
-isystem dir
Search dir for header files, after all directories specified by -I but before
the standard system directories. Mark it as a system directory, so that it
gets the same special treatment as is applied to the standard system directo-
ries.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not
searched for "#include <file>", before all directories specified by -I and
before the standard system directories.
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental;
in a future version of GCC, it will be enabled by default for C99 and C++.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed.
This suppresses things like macro expansion, trigraph conversion, escaped new-
line splicing, and processing of most directives. The preprocessor still rec-
ognizes 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.
-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.
-fexec-charset=charset
Set the execution character set, used for string and 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 con-
stants. 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 con-
flict. charset can be any encoding supported by the system's "iconv" library
routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the
compiler know the current working directory at the time of preprocessing. When
this option is enabled, the preprocessor will emit, after the initial line-
marker, a second linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it's present in the preprocessed
input, as the directory emitted as the current working directory in some debug-
ging 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.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnos-
tics are being scanned by a program that does not understand the column num-
bers, such as dejagnu.
-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.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be
preceded by a space. Other characters are interpreted by the compiler proper,
or reserved for future versions of GCC, and so are silently ignored. If you
specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define directives for all
the macros defined during the execution of the preprocessor, including pre-
defined 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
will show all the predefined macros.
D Like M 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.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
-P Inhibit generation of linemarkers in the output from the 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.
-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 preproces-
sor 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.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO
C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting
with ??, that are defined by ISO C to stand for single characters. For exam-
ple, ??/ stands for \, so '??/n' is a character constant for a newline. By
default, GCC ignores trigraphs, but in standard-conforming modes it converts
them. See the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit very short
file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing
anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execu-
tion, and report the final form of the include path.
-H Print the name of each header file used, in addition to other normal activi-
ties. 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 ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as
normal. With two dashes, exit immediately.
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 which GCC does not know how to 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 exe-
cutable 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. (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.
-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 rec-
ommended.)
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 speci-
fied. 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 mem-
bers 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 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 will be passed to the linker. The standard startup files are used nor-
mally, unless -nostartfiles is used. The compiler may generate calls to "mem-
cmp", "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 will be passed to the linker.
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 that GCC uses to overcome shortcom-
ings 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. (For example, __main, used to
ensure C++ constructors will be called.)
-pie
Produce a position independent executable on targets which support it. For
predictable results, you must also specify the same set of options that were
used to generate code (-fpie, -fPIE, or model suboptions) when you specify this
option.
-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 that were used to generate code
(-fpic, -fPIC, or model suboptions) when you specify this 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 libraries as well as the application
itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever
you build a shared library or a main executable, because C++ and Java 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 will not always be 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 will link the shared version of
libgcc into shared libraries by default. Otherwise, it will take advantage of
the linker and optimize 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 excep-
tions, you must link it using the G++ or GCJ 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.
-symbolic
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.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-spe-
cific linker options which GCC does not know how to recognize.
If you want to pass an option that takes an 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.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split
into multiple options at the commas.
-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 load-
ing of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for
parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for
header files. This can be used to override a system header file, substituting
your own version, since these directories are searched before the 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). If
you use more than one -I option, the directories are scanned in left-to-right
order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem,
is also specified with -I, the -I option will be ignored. The directory will
still be searched but as a system directory at its normal position in the sys-
tem 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.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for
header files only for the case of #include "file"; they are not searched for
#include <file>, otherwise just like -I.
-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 each subprogram to be run, the compiler driver first tries the -B prefix,
if any. If that name is not found, or if -B was not specified, the driver
tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc/.
If neither of those results in a file name that is found, the unmodified pro-
gram name is searched for using the directories specified in your PATH environ-
ment variable.
The compiler will check to see if the path provided by the -B refers to a
directory, and if necessary it will add 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 includes files in the preprocessor, because the
compiler translates these options into -isystem options for the preprocessor.
In this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B pre-
fix, 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 environ-
ment 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 will be replaced by [dir/]include. This is
to help with boot-strapping the compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to
override the defaults that 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.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example,
if the compiler would normally search for headers in /usr/include and libraries
in /usr/lib, it will instead search dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option
will apply to libraries, but the -isysroot option will apply 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 will still work, but the library aspect will not.
-I- This option has been deprecated. Please use -iquote instead for -I directories
before the -I- and remove the -I-. Any directories you specify with -I options
before the -I- option are searched only for the case of #include "file"; they
are not searched for #include <file>.
If additional directories are specified with -I options after the -I-, these
directories are searched for all #include directives. (Ordinarily all -I
directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where
the current input file came from) as the first search directory for #include
"file". There is no way to override this effect of -I-. With -I. you can
specify searching the directory which was current when the compiler was
invoked. That is not exactly the same as what the preprocessor does by
default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header
files. Thus, -I- and -nostdinc are independent.
Specifying Target Machine and Compiler Version
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 version other than the one
that was installed last. Sometimes this is inconvenient, so GCC provides options
that will switch to another cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type
when configuring GCC as a cross-compiler. For example, if a cross-compiler was
configured with configure arm-elf, meaning to compile for an arm processor with
elf binaries, then you would specify -b arm-elf to run that cross compiler.
Because there are other options beginning with -b, the configuration must con-
tain a hyphen.
-V version
The argument version specifies which version of GCC to run. This is useful
when multiple versions are installed. For example, version might be 4.0, mean-
ing to run GCC version 4.0.
The -V and -b options work by running the <machine>-gcc-<version> executable, so
there's no real reason to use them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed
compilers for completely different target machines, such as VAX vs. 68000 vs.
80386.
In addition, each of these target machine types can have its own special options,
starting with -m, to choose among various hardware models or configurations---for
example, 68010 vs 68020, floating coprocessor or none. A single installed version
of the compiler can compile for any model or configuration, according to the
options specified.
Some configurations of the compiler also support additional special options, usu-
ally for compatibility with other compilers on the same platform.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu
Prepend the name of the cpu to all public symbol names. In multiple-processor
systems, there are many ARC variants with different instruction and register
set characteristics. This flag prevents code compiled for one cpu to be linked
with code compiled for another. No facility exists for handling variants that
are "almost identical". This is an all or nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are supported depend on the
configuration. All variants support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section, data-section, and read-
only-data-section respectively by default. This can be overridden with the
"section" attribute.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-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 will cause the
stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb instruction
sets. Without this option 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.
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or the merging
of those instruction with the instructions in the function's body. This means
that all functions will 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 if functions inside an exe-
cutable piece of code. The default is -msched-prolog.
-mhard-float
Generate output containing floating point instructions. This is the default.
-msoft-float
Generate output containing library calls for floating point. Warning: the req-
uisite libraries are not available for all ARM 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 output file; therefore, it
is only useful if you compile all of a program with this option. In particu-
lar, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-mfloat-abi=name
Specifies which ABI to use for floating point values. Permissible values are:
soft, softfp and hard.
soft and hard are equivalent to -msoft-float and -mhard-float respectively.
softfp allows the generation of floating point instructions, but still uses the
soft-float calling conventions.
-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.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Gen-
erate code for a little-endian word order but a big-endian byte order. That
is, a byte order of the form 32107654. Note: this option should only be used
if you require compatibility with code for big-endian ARM processors generated
by versions of the compiler prior to 2.8.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this name to
determine what kind of instructions it can emit when generating assembly code.
Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620,
arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm,
strongarm110, strongarm1100, 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, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt,
ep9312.
-mtune=name
This option is very similar to the -mcpu= option, except that instead of speci-
fying the actual target processor type, and hence restricting which instruc-
tions can be used, it specifies that GCC should tune the performance of the
code as if the target were of the type specified in this option, but still
choosing the instructions that it will generate based on the cpu specified by a
-mcpu= option. For some ARM implementations better performance can be obtained
by using this option.
-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.
This option can be used in conjunction with or instead of the -mcpu= option.
Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5,
armv5t, armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating point hardware (or hardware emulation) is avail-
able on the target. Permissible names are: fpa, fpe2, fpe3, maverick, vfp.
-mfp and -mfpe are synonyms for -mfpu=fpenumber, for compatibility with older
versions of GCC.
If -msoft-float is specified this specifies the format of floating point val-
ues.
-mstructure-size-boundary=n
The size of all structures and unions will be 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 the larger number can produce faster, more efficient code, but can
also increase the size of the program. Different values are potentially incom-
patible. 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 will be 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 reg-
ister. This switch is needed if the target function will lie 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 will be turned into long
calls. The heuristic is that static functions, functions which 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, will not be turned into long calls. The exception to
this rule is 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, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will
restore the default behavior, as will 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.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it
in the prologue for each function. The run-time system is responsible for ini-
tializing this register with an appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10 unless
stack-checking is enabled, when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around problems
with invalid Maverick instruction combinations. This option is only valid if
the -mcpu=ep9312 option has been used to enable generation of instructions for
the Cirrus Maverick floating point co-processor. This option is not enabled by
default, since the problem is only present in older Maverick implementations.
The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns
switch.
-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
Generate code for the 16-bit Thumb instruction set. The default is to use the
32-bit ARM instruction set.
-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
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-interwork-
ing code.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute
correctly regardless of whether the target code has been compiled for inter-
working or not. There is a small overhead in the cost of executing a function
pointer if this option is enabled.
-mtp=name
Specify the access model for the thread local storage pointer. The valid mod-
els 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.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C com-
piler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11,
attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K
program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333,
at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory
space (MCU types: atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory
space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program mem-
ory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value,
__stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size will be
smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate subroutines.
Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
-mint8
Assume int to be 8 bit integer. This affects the sizes of all types: A char
will be 1 byte, an int will be 1 byte, an long will be 2 bytes and long long
will be 4 bytes. Please note that this option does not comply to the C stan-
dards, but it will provide you with smaller code size.
Blackfin Options
-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 reg-
ister 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 will ensure that the generated code does not contain
speculative loads after jump instructions. This option is enabled by default.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain
CSYNC or SSYNC instructions too soon after conditional branches. This option
is enabled by default.
-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 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 vir-
tual 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.
-mshared-library-id=n
Specified the identification number of the ID based shared library being com-
piled. Specifying a value of 0 will generate more compact code, specifying
other values will force the allocation of that number to the current library
but is no more space or time efficient than omitting this option.
-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 reg-
ister. This switch is needed if the target function will lie outside of the 24
bit addressing range of the offset based version of subroutine call instruc-
tion.
This feature is not enabled by default. Specifying -mno-long-calls will
restore the default behavior. Note these switches have no effect on how the
compiler generates code to handle function calls via function pointers.
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.
-melinux-stacksize=n
Only available with the cris-axis-aout target. Arranges for indications in the
program to the kernel loader that the stack of the program should be set to 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 to turn off the #NO_APP formatted-code indica-
tor to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit com-
pare 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.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no-options) arranges (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 that
sets 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 variable needs to be allo-
cated.
-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 (tradi-
tional on other architectures) calls to the PLT. The default is -mgotplt.
-maout
Legacy no-op option only recognized with the cris-axis-aout target.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-
gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where it selects a
GNU/linux-like multilib, include files and instruction set for -march=v8.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-aout and cris-axis-elf arranges to
link with input-output functions from a simulator library. Code, initialized
data and zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and
zero-initialized data at 0x80000000.
CRX Options
These options are defined specifically for the CRX ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mpush-args
Push instructions will be used to pass outgoing arguments when functions are
called. Enabled by default.
Darwin Options
These options are defined for all architectures running the Darwin operating sys-
tem.
FSF GCC on Darwin does not create "fat" object files; it will create an object file
for the single architecture that it 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 targetting, like -mcpu or -march. The
-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, will only permit instructions to be used that are valid for the sub-
type of the file it is generating, so you cannot put 64-bit instructions in an
ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail
and print an error if asked to create a shared library with a less restrictive sub-
type than its input files (for instance, trying to put a ppc970 object file in a
ppc7400 library). The linker for executables, ld, will quietly give 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 speci-
fied 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 sub-
framework should not have the same name as a framework, a warning will be
issued if this is violated. Currently a subframework cannot have subframe-
works, in the future, the mechanism may be extended to support this. The stan-
dard frameworks can be found in "/System/Library/Frameworks" and
"/Library/Frameworks". An example include looks like "#include <Frame-
work/header.h>", where Framework denotes the name of the framework and header.h
is found in the "PrivateHeaders" or "Headers" directory.
-gused
Emit debugging information for symbols that are used. For STABS debugging for-
mat, 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.
The default for this option is to make choices that seem to be most useful.
-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 turn around development. Needed to enable 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 ld(1) for more informa-
tion.
-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 be loading the build output file
being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead of an exe-
cutable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one con-
trolled 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
-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 will be 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 will issue floating-point opera-
tions. 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 val-
ues 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-com-
pliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the resulting code may exe-
cute 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_inex-
act.
-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 su, 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 loca-
tion that caused a floating point trap. Depending on the requirements of an
application, different levels of precisions can be selected:
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 instruc-
tion 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. Under DEC Unix, this has the
effect that IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can con-
struct it from smaller constants in two or three instructions. If it cannot,
it will output the constant as a literal and generate code to load it from the
data segment at runtime.
Use this option to require GCC to construct all integer constants using code,
even if it takes more instructions (the maximum is six).
You would 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.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied assem-
bler (-malpha-as) or by the GNU assembler -mgas.
-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 was specified.
-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 accord-
ingly.
-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 seg-
ment.
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 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 will choose the default values for the instruction set from the
processor you specify. If you do not specify a processor type, GCC will
default 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.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The
instruction set is not changed.
-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.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style
condition (e.g. error) codes.
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.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, that 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.
-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 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.
-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.
-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.
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.
-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 bound-
aries. -malign-300 causes them to be aligned on 2 byte boundaries. This
option has no effect on the H8/300.
HPPA Options
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 architec-
ture option for your machine. Code compiled for lower numbered architectures
will run 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.
-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.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by mod-
ifying the return pointer for the function call to be the target of the condi-
tional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner. This is neces-
sary for compiling kernels which perform lazy context switching of floating
point registers. If you use this option and attempt to perform floating point
operations, the compiler will abort.
-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 boundaries. This allows GCC
to emit code which performs faster indirect calls.
This option will not work in the presence of shared libraries or nested func-
tions.
-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 sepa-
rated 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 link-
ers in which they give bogus error messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the req-
uisite libraries are not available for all HPPA targets. Normally the facili-
ties 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 target
hppa1.1-*-pro does provide software floating point support.
-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 particu-
lar, 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 GNU ld specific options. 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 have any affect on 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 HP ld specific options. 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 have any affect on 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 func-
tion 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 when using the -ffunc-
tion-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 will degrade 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 sup-
port 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 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 bina-
ries 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.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the
ABI and the set of available instructions. The choices for cpu-type are:
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 cor-
responding -mtune 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,
the code generated option will change to reflect the processors that were
most common when that version of GCC was released.
There is no -march=generic option because -march indicates the instruction
set the compiler can use, and there is no generic instruction set applica-
ble to all processors. In contrast, -mtune indicates the processor (or, in
this case, collection of processors) for which the code is optimized.
i386
Original Intel's i386 CPU.
i486
Intel's i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction set sup-
port.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro instruction
set will be used, so the code will run on all i686 familly chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX instruction set sup-
port.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction
set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction
set support. Used by Centrino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3
instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
SSE2 and SSE3 instruction set support.
core2
Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3
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 sup-
port.
athlon, athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instruc-
tions 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
AMD K8 core based CPUs with x86-64 instruction set support. (This super-
sets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set
extensions.)
amdfam10
AMD Family 10 core based CPUs with x86-64 instruction set support. (This
supersets MMX, SSE, SSE2, SSE3, SSE4A, SSE5, 3dNOW!, enhanced 3dNOW!, ABM
and 64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruc-
tion set support.
winchip2
IDT Winchip2 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 CPU with MMX and SSE instruction set support. (No scheduling is
implemented for this chip.)
While picking a specific cpu-type will schedule things appropriately for that
particular chip, the compiler will not generate any code that does not run on
the i386 without the -march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type
are the same as for -mtune. Moreover, specifying -march=cpu-type implies
-mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and
-mtune=pentiumpro respectively. These synonyms are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The choices for
unit are:
387 Use the standard 387 floating point coprocessor present majority of chips
and emulated otherwise. Code compiled with this option will run almost
everywhere. The temporary results are computed in 80bit precision instead
of 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 i386 compiler.
sse Use scalar floating point instructions present in the SSE instruction set.
This instruction set is supported by Pentium3 and newer chips, in the AMD
line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of
SSE instruction set supports only single precision arithmetics, thus the
double and extended precision arithmetics is still done using 387. Later
version, present only in Pentium4 and the future AMD x86-64 chips supports
double precision arithmetics too.
For the i386 compiler, you need to 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 80bit.
This is the default choice for the x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This effectively double
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 instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or
att (the default one). Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point comparisons.
These handle correctly the case where the result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warning: the req-
uisite libraries are not part of GCC. Normally the facilities of the machine's
usual C compiler are used, but this can't be done directly in cross-compila-
tion. You must make your own arrangements to provide suitable library func-
tions for cross-compilation.
On machines where a function returns floating point results in the 80387 regis-
ter 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 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 FreeBSD, OpenBSD and NetBSD. This option is overrid-
den when -march indicates that the target cpu will always have an FPU and so
the instruction will not need emulation. As of revision 2.6.1, these instruc-
tions 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 will produce 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 will be aligned differently than the published application binary inter-
face specifications for the 386 and will not be binary compatible with struc-
tures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386 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) would prefer "long double" to be
aligned to an 8 or 16 byte boundary. In arrays or structures conforming to the
ABI, this would not be possible. So specifying a -m128bit-long-double will
align "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 to be 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 target ABI, the structures
and arrays containing "long double" variables will change their size as well as
function calling convention for function taking "long double" will be modified.
Hence they will not be binary compatible with arrays or structures in code com-
piled without that switch.
-mmlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold are placed
in large data section. This value must be the same across all object linked
into the binary and defaults to 65535.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the "bss" or
"data" segments. -msvr3-shlib places them into "bss". These options are mean-
ingful only on System V Release 3.
-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 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 will be
generated for calls to those functions.
In addition, seriously incorrect code will result 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.
-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).
On Pentium and PentiumPro, "double" and "long double" values should be aligned
to an 8 byte boundary (see -malign-double) or suffer significant run time per-
formance 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 func-
tion compiled with a lower preferred stack boundary will most likely misalign
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 sys-
tems and operating system kernels, may want to reduce the preferred alignment
to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4a
-mno-sse4a
-msse5
-mno-sse5
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2
or 3DNow! extended instruction sets. 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 have SSE/SSE2 instructions generated automatically from floating-point code
(as opposed to 387 instructions), see -mfpmath=sse.
These options will enable GCC to use these extended instructions in generated
code, even without -mfpmath=sse. Applications which perform runtime CPU detec-
tion 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.
-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 will be
computed in the function prologue. This is faster on most modern CPUs because
of reduced dependencies, improved scheduling and reduced stack usage when pre-
ferred 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 Mingw32. Code that relies 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.
-mno-align-stringops
Do not align 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 destination is known to be
aligned at least to 4 byte boundary. This enables more inlining, increase code
size, but may improve performance of code that depends on fast memcpy, strlen
and memset for short lengths.
-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 reg-
ister available in leaf functions. The option -fomit-frame-pointer removes the
frame pointer for all 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 seg-
ment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base
pointer must be added. Whether or not this is legal depends on the operating
system, and whether it maps the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64 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 and generates code that runs on any i386
system. The 64-bit environment sets int to 32 bits and long and pointer to 64
bits and generates code for AMD's x86-64 architecture.
-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 will not be 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 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 but symbols can be located anywhere in the address space.
Programs can be statically or dynamically linked, but building of shared
libraries are not supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about
addresses and sizes of sections. Currently GCC does not implement this model.
-mfused-madd
-mno-fused-madd
Enable automatic generation of fused floating point multiply-add instructions
if the ISA supports such instructions. The -mfused-madd option is on by
default.
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 state-
ments.
-mregister-names
-mno-register-names
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.
-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.
-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-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 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 can not use. This is useful when
compiling kernel code. A register range is specified as two registers sepa-
rated 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.
-mt
-pthread
Add support for multithreading using the POSIX threads library. This option
sets flags for both the preprocessor and linker. It does not affect the thread
safety of object code produced by the compiler or that of libraries supplied
with it. These are HP-UX specific flags.
-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.
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 alter-
nate 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 will use during code
generation. These pseudo-registers will be 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 the default runtime
libraries gcc builds.
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.
-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 will
generate "seth/add3" instructions to load their addresses), and assume all sub-
routines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler will
generate "seth/add3" instructions to load their addresses), and assume subrou-
tines may not be reachable with the "bl" instruction (the compiler will gener-
ate the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be 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 dif-
ferent values of num may or may not work; if it doesn't the linker will give an
error message---incorrect code will not be 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 will be preferred over
conditional code, if it is 2, then the opposite will apply.
-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 will only be 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 the 68000 series. The default values for
these options depends on which style of 68000 was selected when the compiler was
configured; the defaults for the most common choices are given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is config-
ured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the
68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is config-
ured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point. This is the
default for most 68020 systems unless --nfp was specified when the compiler was
configured.
-m68030
Generate output for a 68030. This is the default when the compiler is config-
ured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the compiler is config-
ured for 68040-based systems.
This option inhibits the use of 68881/68882 instructions that have to be emu-
lated 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 config-
ured for 68060-based systems.
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 config-
ured for CPU32-based systems.
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" family cpu. This is the default when the
compiler is configured for 520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202,
MCF5203, MCF5204 and MCF5202.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This
results in code which 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.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This
results in code which 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.
-msoft-float
Generate output containing library calls for floating point. Warning: the req-
uisite libraries are not available for all m68k targets. Normally the facili-
ties of the machine's usual C compiler are used, but this can't be done
directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation. The embedded targets
m68k-*-aout and m68k-*-coff do provide software floating point support.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, param-
eters passed on the stack are also aligned to a 16-bit boundary even on targets
whose API mandates promotion to 32-bit.
-mnobitfield
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 com-
piler.
Also, you must provide function prototypes for all functions that take variable
numbers of arguments (including "printf"); otherwise incorrect code will be
generated for calls to those functions.
In addition, seriously incorrect code will result 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.
-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 will align structures contain-
ing the above types differently than most published application binary inter-
face 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 will be handled by the sys-
tem.
-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 environ-
ment 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 vir-
tual 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.
-mshared-library-id=n
Specified the identification number of the ID based shared library being com-
piled. Specifying a value of 0 will generate more compact code, specifying
other values will force the allocation of that number to the current library
but is no more space or time efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The
default values for these options depends on which style of microcontroller was
selected when the compiler was configured; the defaults for the most common choices
are given below.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default when the compiler is config-
ured for 68HC11-based systems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default when the compiler is config-
ured for 68HC12-based systems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-decrement
addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away,
the compiler will use the "call" instruction to call a function and the "rtc"
instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the code genera-
tion. The maximum number is 32. Using more pseudo-soft register may or may
not result in better code depending on the program. The default is 4 for
68HC11 and 2 for 68HC12.
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.
-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 four 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.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configura-
tions.
-march=arch
Generate code that will run 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, and mips64. The processor names are: 4kc, 4km,
4kp, 5kc, 5kf, 20kc, 24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900,
r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000, vr4100,
vr4111, vr4120, vr4130, vr4300, vr5000, vr5400 and vr5500. 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).
In processor names, a final 000 can be abbreviated as k (for example,
-march=r2k). Prefixes are optional, and vr may be written r.
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 sec-
ond has the form _MIPS_ARCH_foo, where foo is the capitalized value of
_MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and
define the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other
words, it will have the full prefix and will 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 instruc-
tions 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 will optimize for the processor specified by
-march. By using -march and -mtune together, it is possible to generate code
that will run on a family of processors, but optimize the code for one particu-
lar 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.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or
MIPS64 architecture, it will make use of the MIPS16e ASE.
-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>.
-mabicalls
-mno-abicalls
Generate (do not generate) SVR4-style position-independent code. -mabicalls is
the default for SVR4-based systems.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset ta-
ble.
GCC normally uses a single instruction to load values from the GOT. While this
is relatively efficient, it will only work if the GOT is smaller than about
64k. Anything larger will cause 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 -mxgot. It should then
work with very large GOTs, although it will also be less efficient, since it
will take 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.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point
calculations using library calls instead.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision oper-
ations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision opera-
tions. This is the default.
-mdsp
-mno-dsp
Use (do not use) the MIPS DSP ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option can only be used when generating 64-bit code and requires hard-
ware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-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-abi-
calls because it allows GCC to generate shorter and faster references to sym-
bolic addresses.
-G num
Put global and static items less than or equal to num bytes into the small data
or bss section instead of the normal data or bss section. This allows the data
to be accessed using a single instruction.
All modules should be compiled with the same -G num value.
-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 exe-
cuting, 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.
-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 back-
wards 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 assem-
bler 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 sup-
ported 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 -mdi-
vide-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 dis-
abled using -mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial 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.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate instructions,
when they are available. The default is -mfused-madd.
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.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files
(with a .s suffix) when assembling them.
-mfix-r4000
-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-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 only provided by the "mips64vr*-elf" configura-
tions.
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 will avoid 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.)
-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-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 which implement those
architectures; for those, Branch Likely instructions will not be generated by
default because the MIPS32 and MIPS64 architectures specifically deprecate
their use.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we schedule FP
instructions 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 issue two instructions
together if the first one is 8-byte aligned. When this option is enabled, GCC
will align 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.
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
-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 predic-
tion 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 dif-
ferent 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 func-
tion.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 pro-
cessors. 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 which uses features specific to the AM33 processor.
-mno-am33
Do not generate code which uses features specific to the AM33 processor. This
is the default.
-mreturn-pointer-on-d0
When generating a function which 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 would 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.
MT Options
These -m options are defined for Morpho MT architectures:
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system repre-
senting a certain processor type. Possible values for cpu-type are ms1-64-001,
ms1-16-002, ms1-16-003 and ms2.
When this option is not used, the default is -march=ms1-16-002.
-mbacc
Use byte loads and stores when generating code.
-mno-bacc
Do not use byte loads and stores when generating code.
-msim
Use simulator runtime
-mno-crt0
Do not link in the C run-time initialization object file crti.o. Other run-
time initialization and termination files such as startup.o and exit.o are
still included on the linker command line.
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.
-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 gen-
eration only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. 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.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mfprnd
-mno-fprnd
-mmfpgpr
-mno-mfpgpr
GCC supports two related instruction set architectures for the RS/6000 and Pow-
erPC. The POWER instruction set are those instructions supported by the rios
chip set used in the original RS/6000 systems and the PowerPC instruction set
is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors,
and the IBM 4xx, 6xx, and follow-on microprocessors.
Neither architecture is a subset of the other. However there is a large common
subset of instructions supported by both. An MQ register is included in pro-
cessors supporting the POWER architecture.
You use these options to specify which instructions are available on the pro-
cessor 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.
The -mpower option allows GCC to generate instructions that are found only in
the POWER architecture and to use the MQ register. Specifying -mpower2 implies
-power and also allows GCC to generate instructions that are present in the
POWER2 architecture but not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in
the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt
implies -mpowerpc and also allows GCC to use the optional PowerPC architecture
instructions in the General Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use
the optional PowerPC architecture instructions in the 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 Pow-
erPC V2.02 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 -mmfpgpr option
allows GCC to generate the FP move to/from general purpose register instruc-
tions implemented on the POWER6X processor and other processors that support
the extended PowerPC V2.05 architecture.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instruc-
tions that are found in the full PowerPC64 architecture and to treat GPRs as
64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instruc-
tions in the common subset of both architectures plus some special AIX common-
mode calls, and will not use the MQ register. Specifying both -mpower and
-mpowerpc permits GCC to use any instruction from either architecture and to
allow use of the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With
-mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC
architecture. With -mold-mnemonics it uses the assembler mnemonics defined for
the POWER architecture. Instructions defined in only one architecture have
only one mnemonic; GCC uses that mnemonic irrespective of which of these
options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Speci-
fying -mcpu=cpu_type sometimes overrides the value of these option. Unless you
are building a cross-compiler, you should normally not specify either
-mnew-mnemonics or -mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction
scheduling parameters for machine type cpu_type. Supported values for cpu_type
are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620,
630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5,
power, power2, power3, power4, power5, power5+, power6, power6x, common, pow-
erpc, powerpc64, rios, rios1, rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code generated under this
option will run on any POWER or PowerPC processor. GCC will use only the
instructions in the common subset of both architectures, and will not use the
MQ register. GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic
POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit 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 will run best on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options: -mal-
tivec, -mfprnd, -mhard-float, -mmfcrf, -mmultiple, -mnew-mnemonics, -mpopcntb,
-mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring,
-mmfpgpr. The particular options set for any particular CPU will vary 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, register usage, or choice of mnemonics, as
-mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for
-mcpu. If both are specified, the code generated will use the architecture,
registers, and mnemonics set by -mcpu, but the scheduling parameters set by
-mtune.
-mswdiv
-mno-swdiv
Generate code to compute division as reciprocal estimate and iterative refine-
ment, creating opportunities for increased throughput. This feature requires:
optional PowerPC Graphics instruction set for single precision and FRE instruc-
tion for double precision, assuming divides cannot generate user-visible traps,
and the domain values not include Infinities, denormals or zero denominator.
-maltivec
-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.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared
libraries with non-exec .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 Pow-
erPC 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.
-mspe
-mno-isel
This switch enables or disables the generation of SPE simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-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 reg-
isters.
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 environ-
ment 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 will allocate at least one TOC entry for each unique non-automatic variable
reference in your program. GCC will also place 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 -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 will produce
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 and -mpowerpc, 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 argu-
ments with fewer arguments than declared. IBM XL compilers access floating
point arguments which 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 com-
pilers 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 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 incom-
patible.
-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. Soft-
ware floating point emulation is provided if you use the -msoft-float option,
and pass the option to GCC when linking.
-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
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 the instructions usage 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 the instruc-
tions usage 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.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumu-
late instructions. These instructions are generated by default if hardware
floating is used.
-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 would be aligned to a 4 byte boundary and have a size of 4
bytes. By using -mno-bit-align, the structure would be aligned to a 1 byte
boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned
memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the pro-
gram to be relocated to a different address at runtime. If you use -mrelocat-
able on any module, all objects linked together must be compiled with -mrelo-
catable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the pro-
gram to be relocated to a different address at runtime. Modules compiled with
-mrelocatable-lib can be linked with either modules compiled without -mrelocat-
able and -mrelocatable-lib or with modules compiled with 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 pro-
gram.
-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.
-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 0/1/2 to assign no/highest/second-highest priority to dispatch slot
restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target dur-
ing instruction scheduling. The argument dependence_type takes one of the fol-
lowing values: no: no dependence is costly, all: all dependences are costly,
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 depen-
dence which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be 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 which 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 pro-
cessor grouping. number: Insert nops to force costly dependent insns into sep-
arate 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 conven-
tions that adheres to the March 1995 draft of the System V Application Binary
Interface, PowerPC processor supplement. This is the default unless you con-
figured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris operat-
ing system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU
system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU
system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operat-
ing 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, ieeelongdou-
ble.
-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 Booke 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.
-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 were
passed in the floating point registers in case the function takes a variable
arguments. With -mprototype, only calls to prototyped variable argument func-
tions will 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.
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to
indicate that eabi extended relocations are used.
-meabi
-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 to 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, do not call an initialization function from
"main", and the -msdata option will only use "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 unini-
tialized 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 and static data in
the .sdata section. Put small uninitialized global and static 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
On embedded PowerPC systems, put all initialized global and static data in the
.data section, and all uninitialized data in the .bss section.
-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
Default to making all function calls indirectly, using a register, so that
functions which reside further than 32 megabytes (33,554,432 bytes) from the
current location can be called. This setting can be overridden by the "short-
call" 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" will generate "jbsr callee, L42",
plus a "branch island" (glue code). The two target addresses represent the
callee and the "branch island". The Darwin/PPC linker will prefer the first
address and generate a "bl callee" if the PPC "bl" instruction will reach the
callee directly; otherwise, the linker will generate "bl L42" to call the
"branch island". The "branch island" is appended to the body of the calling
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, we may cause GCC to ignore all longcall specifications when the
linker is known to generate glue.
-pthread
Adds support for multithreading with the pthreads library. This option sets
flags for both the preprocessor and linker.
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 will be used to perform floating-point operations. When -mhard-float
is specified, the compiler generates IEEE floating-point instructions. This is
the default.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64bit 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 that do not understand DWARF-2 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 com-
piled 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 sup-
ported. 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 pur-
poses. 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 subrou-
tine calls. This only works reliably if the total executable size does not
exceed 64k. The default is to use the "basr" instruction 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 gener-
ating 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.
-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 will run on cpu-type, which is the name of a system repre-
senting a certain processor type. Possible values for cpu-type are g5, g6,
z900, and z990. When generating code using the instructions available on
z/Architecture, the default is -march=z900. Otherwise, the default is
-march=g5.
-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 point multiply and accumu-
late 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 pro-
gram runs. It is intended to identify functions which 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
These arguments always have to be used in conjunction. If they are present the
s390 back end emits additional instructions in the function prologue which
trigger a trap if the stack size is stack-guard bytes above the stack-size
(remember that the stack on s390 grows downward). 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.
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.
-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-preci-
sion mode by default.
-m4 Generate code for the SH4.
-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-preci-
sion 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.
-ml Compile code for the processor in little endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conven-
tions, and thus some functions from the standard C library will 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.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by Renesas.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conven-
tions were available. This option is the default for all targets of the SH
toolchain except for sh-symbianelf.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code. At the moment, this is equiv-
alent to -fno-finite-math-only. When generating 16 bit SH opcodes, getting
IEEE-conforming results for comparisons of NANs / infinities incurs extra over-
head in every floating point comparison, therefore the default is set to -ffi-
nite-math-only.
-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.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls using the Global
Offset Table instead of the Procedure Linkage Table.
-musermode
Generate a library function call to invalidate instruction cache entries, after
fixing up a trampoline. This library function call doesn't assume it can write
to the whole memory address space. This is the default when the target is
"sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of:
call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call, inv:call2, inv:fp .
"fp" performs the operation in floating point. This has a very high latency,
but needs only a few instructions, so it might be a good choice if your code
has enough easily exploitable ILP to allow the compiler to schedule the float-
ing point instructions together with other instructions. Division by zero
causes a floating point exception. "inv" uses integer operations to calculate
the inverse of the divisor, and then multiplies the dividend with the inverse.
This strategy allows cse and hoisting of the inverse calculation. Division by
zero calculates an unspecified result, but does not trap. "inv:minlat" is a
variant of "inv" where if no cse / hoisting opportunities have been found, or
if the entire operation has been hoisted to the same place, the last stages of
the inverse calculation are intertwined with the final multiply to reduce the
overall latency, at the expense of using a few more instructions, and thus
offering fewer scheduling opportunities with other code. "call" calls a
library function that usually implements the inv:minlat strategy. This gives
high code density for m5-*media-nofpu compilations. "call2" uses a different
entry point of the same library function, where it assumes that a pointer to a
lookup table has already been set up, which exposes the pointer load to cse /
code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all use the
"inv" algorithm for initial code generation, but if the code stays unoptimized,
revert to the "call", "call2", or "fp" strategies, respectively. Note that the
potentially-trapping side effect of division by zero is carried by a separate
instruction, so it is possible that all the integer instructions are hoisted
out, but the marker for the side effect stays where it is. A recombination to
fp operations or a call is not possible in that case. "inv20u" and "inv20l"
are variants of the "inv:minlat" strategy. In the case that the inverse calcu-
lation was nor separated from the multiply, they speed up division where the
dividend fits into 20 bits (plus sign where applicable), by inserting a test to
skip a number of operations in this case; this test slows down the case of
larger dividends. inv20u assumes the case of a such a small dividend to be
unlikely, and inv20l assumes it to be likely.
-mdivsi3_libfunc=name
Set the name of the library function used for 32 bit signed division to name.
This only affect the name used in the call and inv:call division strategies,
and the compiler will still expect the same sets of input/output/clobbered reg-
isters as if this option was not present.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This option only has
an effect if the gcc code base supports the TARGET_ADJUST_UNROLL_MAX target
hook.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is
only safe if the hardware and/or OS implement 32 bit wrap-around semantics for
the indexed addressing mode. The architecture allows the implementation of
processors with 64 bit MMU, which the OS could use to get 32 bit addressing,
but since no current hardware implementation supports this or any other way to
make the indexed addressing mode safe to use in the 32 bit ABI, the default is
-mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The default is 2 if
-mpt-fixed is in effect, 100 otherwise.
-mpt-fixed
Assume pt* instructions won't trap. This will generally generate better sched-
uled code, but is unsafe on current hardware. The current architecture defini-
tion says that ptabs and ptrel trap when the target anded with 3 is 3. This
has the unintentional effect of making it unsafe to schedule ptabs / ptrel
before a branch, or hoist it out of a loop. For example, __do_global_ctors, a
part of libgcc that runs constructors at program startup, calls functions in a
list which is delimited by -1. With the -mpt-fixed option, the ptabs will be
done before testing against -1. That means that all the constructors will be
run a bit quicker, but when the loop comes to the end of the list, the program
crashes because ptabs loads -1 into a target register. Since this option is
unsafe for any hardware implementing the current architecture specification,
the default is -mno-pt-fixed. Unless the user specifies a specific cost with
-mgettrcost, -mno-pt-fixed also implies -mgettrcost=100; this deters register
allocation using target registers for storing ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the
compiler will always be valid to load with movi/shori/ptabs or
movi/shori/ptrel, but with assembler and/or linker tricks it is possible to
generate symbols that will cause ptabs / ptrel to trap. This option is only
meaningful when -mno-pt-fixed is in effect. It will then prevent cross-basic-
block cse, hoisting and most scheduling of symbol loads. The default is
-mno-invalid-symbols.
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. 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.
-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 req-
uisite libraries are not available for all SPARC targets. Normally the facili-
ties 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.
-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 particu-
lar, 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 sup-
port 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.
-mno-faster-structs
-mfaster-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 changed alignment directly violates the SPARC ABI.
Thus, it's intended only for use on targets where the developer acknowledges
that their resulting code will not be directly in line with the rules of the
ABI.
-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
will 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.
This option is only available on SunOS and Solaris.
-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, sparclite, f930, f934, hypersparc, sparclite86x, sparclet, tsc701,
v9, ultrasparc, ultrasparc3, and niagara.
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 implementa-
tions.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3, niagara
By default (unless configured otherwise), GCC generates code for the V7 variant
of the SPARC architecture. With -mcpu=cypress, the compiler additionally opti-
mizes 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 multi-
ply 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 com-
piler 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.
-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.
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, f930, f934, sparclite86x, tsc701,
ultrasparc, ultrasparc3, and niagara.
-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-bit wide.
This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 proces-
sors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual
Instruction Set extensions. The default is -mno-vis.
These -m options are supported in addition to the above on SPARC-V9 processors in
64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only avail-
able for a few configurations and most notably not on Solaris and Linux.
-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=medlow
Generate code for 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.
-mcmodel=medmid
Generate code for 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.
-mcmodel=medany
Generate code for 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 text seg-
ment.
-mcmodel=embmedany
Generate code for 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.
-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.
These switches are supported in addition to the above on Solaris:
-threads
Add support for multithreading using the Solaris threads library. This option
sets flags for both the preprocessor and linker. This option does not affect
the thread safety of object code produced by the compiler or that of libraries
supplied with it.
-pthreads
Add support for multithreading using the POSIX threads library. This option
sets flags for both the preprocessor and linker. This option does not affect
the thread safety of object code produced by the compiler or that of libraries
supplied with it.
-pthread
This is a synonym for -pthreads.
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" assem-
bler 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.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are c30, c31, c32,
c40, and c44. The default is c40 to generate code for the TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small memory model
assumed that all data fits into one 64K word page. At run-time the data page
(DP) register must be set to point to the 64K page containing the .bss and
.data program sections. The big memory model is the default and requires
reloading of the DP register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the block count
register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch, DBcond(D),
instructions. This is enabled by default for the C4x. To be on the safe side,
this is disabled for the C3x, since the maximum iteration count on the C3x is
2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note
that GCC will try to reverse a loop so that it can utilize the decrement and
branch instruction, but will give up if there is more than one memory reference
in the loop. Thus a loop where the loop counter is decremented can generate
slightly more efficient code, in cases where the RPTB instruction cannot be
utilized.
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an interrupt service routine
(ISR), reloaded to point to the data section, and restored on exit from the
ISR. This should not be required unless someone has violated the small memory
model by modifying the DP register, say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a
library call to guarantee 32-bit results. Note that if one of the operands is
a constant, then the multiplication will be performed using shifts and adds.
If the -mmpyi option is not specified for the C3x, then squaring operations are
performed inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to an integer
value chooses the nearest integer less than or equal to the floating point
value rather than to the nearest integer. Thus if the floating point number is
negative, the result will be incorrectly truncated an additional code is neces-
sary to detect and correct this case. This option can be used to disable gen-
eration of the additional code required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the RPTB instruc-
tion for zero overhead looping. The RPTB construct is only used for innermost
loops that do not call functions or jump across the loop boundaries. There is
no advantage having nested RPTB loops due to the overhead required to save and
restore the RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction repeat instruction RPTS. If
a repeat block contains a single instruction, and the loop count can be guaran-
teed to be less than the value count, GCC will emit a RPTS instruction instead
of a RPTB. If no value is specified, then a RPTS will be emitted even if the
loop count cannot be determined at compile time. Note that the repeated
instruction following RPTS does not have to be reloaded from memory each itera-
tion, thus freeing up the CPU buses for operands. However, since interrupts
are blocked by this instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB (and DB on the C40) is
2^{31 + 1} since these instructions test if the iteration count is negative to
terminate the loop. If the iteration count is unsigned there is a possibility
than the 2^{31 + 1} maximum iteration count may be exceeded. This switch
allows an unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is happy with.
This also enforces compatibility with the API employed by the TI C3x C com-
piler. For example, long doubles are passed as structures rather than in
floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to functions.
By default, arguments are passed in registers where possible rather than by
pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by default with
-O2.
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided
-mparallel-insns is also specified. These instructions have tight register
constraints which can pessimize the code generation of large functions.
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 will always load the functions address up into a register, and
call 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 regis-
ters. 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).
-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.
-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 will cause r2 and r5 to be used in the code generated by the com-
piler. This setting is the default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants
__v850e1__ and __v850e__ will be defined if this option is used.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant
__v850e__ will be defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target
processor will be chosen and the relevant __v850*__ preprocessor constant will
be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless
of which processor variant is the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for the v850e and
v850e1 flavors of the v850 architecture. The default is -mno-disable-callt
which allows the CALLT instruction to be used.
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 you will assemble
with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-format.
x86-64 Options
These are listed under
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.
-mtext-section-literals
-mno-text-section-literals
Control the treatment of literal pools. The default is -mno-text-section-lit-
erals, 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.
-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 pre-
ceding safe density instructions to align a target, no widening will be per-
formed. The default is -mtarget-align. These options do not affect the treat-
ment of auto-aligned instructions like "LOOP", which the assembler will always
align, either by widening density instructions or by inserting no-op instruc-
tions.
-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 typi-
cally occurs for calls to functions in other source 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 will still show direct call instructions---look at the disas-
sembled object code to see the actual instructions. Note that the assembler
will use an indirect call for every cross-file call, not just those that really
will be out of range.
zSeries Options
These are listed under
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
would be -fno-foo. In the table below, only one of the forms is listed---the one
which is not the default. You can figure out the other form by either removing no-
or adding it.
-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 Java and Fortran front-ends, where this option defaults to
true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, mul-
tiplication operations.
-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.
This option is enabled by default for the Java front-end, as required by the
Java language specification.
-fexceptions
Enable exception handling. Generates extra code needed to propagate excep-
tions. For some targets, this implies GCC will generate frame unwind informa-
tion for all functions, which can produce significant data size overhead,
although it does not affect execution. If you do not specify this option, GCC
will enable it by default for languages like C++ which normally require excep-
tion handling, and disable 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".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static
data, but will not affect the generated code in any other way. You will nor-
mally not enable this option; instead, a language processor that needs this
handling would enable it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 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).
-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 com-
patible with code compiled with the -freg-struct-return switch. Use it to con-
form 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 stan-
dard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary com-
patible with code compiled with the -fpcc-struct-return switch. Use it to con-
form 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. Specifically, the "enum" type will be equivalent to
the smallest integer type which 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-double
Use the same size for "double" as for "float".
Warning: the -fshort-double 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.
-fshared-data
Requests that the data and non-"const" variables of this compilation be shared
data rather than private data. The distinction makes sense only on certain
operating systems, where shared data is shared between processes running the
same program, while private data exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in the data section of the
object file, rather than generating them as common blocks. This has the effect
that if the same variable is declared (without "extern") in two different com-
pilations, you will get an error when you link them. The only reason this
might be useful is if you wish to verify that the program will work on other
systems which always work this way.
-fno-ident
Ignore the #ident directive.
-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 crt-
stuff.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.
-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 and 32k on the m68k and RS/6000.
The 386 has no such limit.)
Position-independent code requires special support, and therefore works only on
certain machines. For the 386, GCC supports PIC for System V but not for the
Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
-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 ta-
ble. This option makes a difference on the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on
certain machines.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position indepen-
dent code can be only linked into executables. Usually these options are used
when -pie GCC option will be used during linking.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more effi-
cient than other code generation strategies. This option is of use in conjunc-
tion with -fpic or -fPIC for building code which 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 descrip-
tion 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 will not save and restore the
register reg.
It is an error to used 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 will produce disastrous results.
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 will save and restore the register reg if they use
it.
It is an error to used 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 will produce disastrous results.
A different sort of disaster will result from the use of this flag for a regis-
ter 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 will be 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.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after
function entry and just before function exit, the following profiling functions
will be 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 func-
tions. The profiling calls will indicate where, conceptually, the inline func-
tion is entered and exited. This means that addressable versions of such func-
tions 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 anyways, but if you get lucky and the optimizer
always expands the functions inline, you might have gotten away without provid-
ing static copies.)
A function may be given the attribute "no_instrument_function", in which case
this instrumentation will not be done. This can be used, for example, for the
profiling functions listed above, 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).
-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 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 operat-
ing system must do that. The switch causes generation of code to ensure that
the operating system sees the stack being extended.
-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 the stack would
grow beyond the value, a signal is raised. 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.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between parameters and
global data.
-fargument-alias specifies that arguments (parameters) may alias each other and
may alias global storage.-fargument-noalias specifies that arguments do not
alias each other, but may alias global storage.-fargument-noalias-global speci-
fies that arguments do not alias each other and do not alias global storage.
Each language will automatically use whatever option is required by the lan-
guage standard. You should not need to use these options yourself.
-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 con-
form 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-dynamic", "initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is
"global-dynamic".
-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified option---all sym-
bols will be 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 ie; 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 will
be "hidden". The default if -fvisibility isn't specified is "default", i.e.,
make every symbol public---this causes the same behavior as previous versions
of GCC.
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 Drep-
per (which can be found at <http://people.redhat.com/~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 DLL's 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 ie; 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 read-
ability and self-documentation of the code. Note that due to ISO C++ specifi-
cation requirements, operator new and operator delete must always be of default
visibility.
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 will use the PLT, so
it is more effective to use __attribute ((visibility)) and/or #pragma GCC visi-
bility to tell the compiler which extern declarations should be treated as hid-
den.
Note that -fvisibility does affect C++ vague linkage entities. This means that,
for instance, an exception class that will be thrown between DSOs must be
explicitly marked with default visibility so that the type_info nodes will be
unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
<http://gcc.gnu.org/wiki/Visibility>.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in For-
tran. When -fopenmp is specified, the compiler generates parallel code accord-
ing to the OpenMP Application Program Interface v2.5 <http://www.openmp.org/>.
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 informa-
tion that allow 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 instal-
lation. 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 would other-
wise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnos-
tic messages.
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 preproces-
sor, which is the input to the compiler proper.
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 will attempt to figure out an appropriate
prefix to use based on the pathname it was 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
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 nor-
mally 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 will search
foo/bar where it would normally search /usr/local/lib/bar. These alternate
directories are searched first; the standard directories come next.
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 can't 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 can't 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 will
use mblen and mbtowc as defined by the default locale to recognize and trans-
late multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special charac-
ter, 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 environ-
ment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particu-
lar 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 sys-
tem header files are not ignored, so it implies -M rather than -MM. However,
the dependence on the main input file is omitted.
BUGS
For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for con-
structors to work. On multi-libbed systems, gcc -shared must select the cor-
rect support libraries to link against. Failing to supply the correct flags
may lead to subtle defects. Supplying them in cases where they are not neces-
sary 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/Contribu-
tors.html>, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
2001, 2002, 2003, 2004, 2005 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 1.2 or any later version pub-
lished by the Free Software Foundation; with the Invariant Sections being "GNU Gen-
eral 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:
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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