GCC(1)
🌈 GCC(1) — GNU — GCC(1) 🌈
okay bestie, welcome to the GCC(1) page from GNU — yes, the header is literally GCC(1) on both sides, we love a symmetrical moment 💅
💖 NAME 💖
gcc — GNU project C and C++ compiler ✨
🦋 SYNOPSIS 🦋
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
🎀 DESCRIPTION 🎀
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The “overall options” allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as kB and KiB for kilobyte and kibibyte, respectively, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes.
🔥 OPTIONS 🔥
✨ Option Summary ✨
Here’s a summary of all the options, grouped by type — explanations are in the following sections, bestie 🫶
Overall Options
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -ffile-prefix-map=old=new -fcanon-prefix-map
-fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=fileC Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -fpermitted-flt-eval-methods=standard
-fplan9-extensions -fsigned-bitfields -funsigned-bitfields
-fsigned-char -funsigned-char -fstrict-flex-arrays[=n]
-fsso-struct=endiannessC++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fno-assume-sane-operators-new-delete -fchar8_t -fcheck-new
-fconcepts -fconstexpr-depth=n -fconstexpr-cache-depth=n
-fconstexpr-loop-limit=n -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-immediate-escalation -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fno-pretty-templates -frange-for-ext-temps
-fno-rtti -fsized-deallocation -fstrong-eval-order[=kind]
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value -Wcatch-value=n
-Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept
-Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder -Wregister
-Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local
-Wno-terminate -Wno-vexing-parse -Wvirtual-inheritance
-Wno-virtual-move-assign -WvolatileObjective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selectorOpenMP and OpenACC Options
-foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom
-fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type]Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file]
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes]
-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]💅 Warning Options 💅
okay bestie, buckle up, because this is the big one. these are all the -W flags (plus a few -f friends hanging out) that tell gcc what to yell at you about. and yes, every single one is here, because we do NOT leave flags behind in this house 💖
the way this works is iconic in its simplicity: -Wfoo turns a warning on, -Wno-foo turns it off again. that's the whole vibe. -Wno- is literally just the off switch, iykyk.
here's the first batch, exactly as gcc wrote them, no cap:
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors
-fpermissive -w -Wextra -Wall -Wabi=n -Waddress
-Wno-address-of-packed-member -Waggregate-return -Walloc-size
-Walloc-size-larger-than=byte-size -Walloc-zero -Walloca
-Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations
-Warith-conversion -Warray-bounds -Warray-bounds=n -Warray-compare
-Warray-parameter -Warray-parameter=n -Wno-attributes
-Wattribute-alias=n -Wno-attribute-alias -Wno-attribute-warning
-Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c23-compat -Wc23-c2y-compat -Wc++-compat -Wc++11-compat
-Wc++14-compat -Wc++17-compat -Wc++20-compat -Wno-c++11-extensions
-Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions
-Wno-c++23-extensions -Wcalloc-transposed-args -Wcast-align
-Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcomment
-Wcompare-distinct-pointer-types -Wno-complain-wrong-lang
-Wconversion -Wno-coverage-mismatch -Wno-cpp -Wdangling-else
-Wdangling-pointer -Wdangling-pointer=n -Wdate-time -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers
-Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wno-endif-labels -Wenum-compare -Wenum-conversion
-Wenum-int-mismatch -Werror -Werror=* -Wexpansion-to-defined
-Wfatal-errors -Wflex-array-member-not-at-end -Wfloat-conversion
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n
-Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wheader-guard -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Whardened -Wimplicit
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wno-implicit-function-declaration -Wno-implicit-int
-Winfinite-recursion -Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8
-Wno-unicode -Wjump-misses-init -Wlarger-than=byte-size
-Wleading-whitespace=kind -Wlogical-not-parentheses -Wlogical-op
-Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentationa few faves in there, if we're doing a vibe check 👑
-w— lowercase w, the "make it stop" button. it's giving "I don't want to know."-Werror— the icon. turns warnings into errors. dramatic? yes. effective? also yes 💀-Werror=*— same energy but you get to pick which warnings level up. main character control.-Wfatal-errors— for when you want the compiler to take its ball and go home after the first error.-Wall,-Wextra,-Wpedantic,-pedantic-errors— the classic set. friends of every student ever.-Wformat=2— the format warnings, dialled up.-Wbidi-chars=[none|unpaired|any|ucn]— yes, the whole bracket of options is part of the flag name. copy it exactly, bestie.
and here's the rest of the warnings list, no skips, no cuts, fr fr:
-Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs
-Wmissing-noreturn -Wmusttail-local-addr
-Wmaybe-musttail-local-addr -Wno-missing-profile -Wno-multichar
-Wmultistatement-macros -Wnonnull -Wnonnull-compare
-Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr
-Wopenacc-parallelism -Wopenmp -Wopenmp-simd -Wno-overflow
-Woverlength-strings -Wno-override-init-side-effects -Wpacked
-Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas
-Wno-pragma-once-outside-header -Wno-prio-ctor-dtor -Wno-psabi
-Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type
-Wno-scalar-storage-order -Wsequence-point -Wshadow
-Wshadow=global -Wshadow=local -Wshadow=compatible-local
-Wno-shadow-ivar -Wno-shift-count-negative
-Wno-shift-count-overflow -Wshift-negative-value
-Wno-shift-overflow -Wshift-overflow=n -Wsign-compare
-Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector
-Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wstring-compare
-Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation -Wstrict-flex-arrays
-Wsuggest-attribute=attribute-name -Wswitch -Wno-switch-bool
-Wswitch-default -Wswitch-enum -Wno-switch-outside-range
-Wno-switch-unreachable -Wsync-nand -Wsystem-headers
-Wtautological-compare -Wtrailing-whitespace
-Wtrailing-whitespace=kind -Wtrampolines -Wtrigraphs
-Wtrivial-auto-var-init -Wno-tsan -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants
-Wunterminated-string-initialization -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wunused-const-variable -Wunused-const-variable=n -Wunused-function
-Wunused-label -Wunused-local-typedefs -Wunused-macros
-Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wuse-after-free -Wuse-after-free=n
-Wuseless-cast -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings
-Wno-xor-used-as-pow -Wzero-as-null-pointer-constant
-Wzero-length-boundsnotable creatures in that batch 🌸
-Wshadow, plus its three flavours-Wshadow=global,-Wshadow=local,-Wshadow=compatible-local— choose your fighter.-Wstrict-overflowand-Wstrict-overflow=n— the numbered version is right there, don't skip it.-Wstack-usage=byte-size,-Wlarger-than=byte-size,-Wframe-larger-than=byte-size,-Wvla-larger-than=byte-size— all take a byte-size, keep the=.-Wtrailing-whitespace=kind— takes a kind. that's the literal placeholder, bestie.-Wuse-after-freeand-Wuse-after-free=n— both exist, both listed.-Wnormalized=[none|id|nfc|nfkc]— brackets included, exactly like that.
🔍 Static Analyzer Options 🔍
okay so this is the part where gcc puts on its detective hat and investigates your code. -fanalyzer is the headline act, and then there's a whole squad of -fdump-analyzer-* flags for dumping out what it found, plus a frankly iconic number of -Wno-analyzer-* switches for silencing each individual detective complaint.
all of them, in order, untouched:
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-show-events-in-system-headers -fno-analyzer-state-merge
-fno-analyzer-state-purge -fno-analyzer-suppress-followups
-fanalyzer-transitivity -fno-analyzer-undo-inlining
-fanalyzer-verbose-edges -fanalyzer-verbose-state-changes
-fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3
-fdump-analyzer-exploded-paths -fdump-analyzer-feasibility
-fdump-analyzer-infinite-loop -fdump-analyzer-json
-fdump-analyzer-state-purge -fdump-analyzer-stderr
-fdump-analyzer-supergraph -fdump-analyzer-untrackedquick callouts, because some of these are begging to be noticed 💅
-fanalyzer-checker=name— takes a name. that's the placeholder, keep it.-fanalyzer-verbosity=level— takes a level.-fdump-analyzer-exploded-nodes-2and-fdump-analyzer-exploded-nodes-3— yes, there's a 2 and a 3. do not merge them. do not "fix" them. they are different flags and they are both slaying.
and now the warning silencers, the -Wno-analyzer-* wall of fame:
-Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file
-Wno-analyzer-exposure-through-uninit-copy
-Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close
-Wno-analyzer-fd-leak -Wno-analyzer-fd-phase-mismatch
-Wno-analyzer-fd-type-mismatch -Wno-analyzer-fd-use-after-close
-Wno-analyzer-fd-use-without-check -Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap -Wno-analyzer-imprecise-fp-arithmetic
-Wno-analyzer-infinite-loop -Wno-analyzer-infinite-recursion
-Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leak
-Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument
-Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds
-Wno-analyzer-overlapping-buffers
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size
-Wno-analyzer-tainted-assertion -Wno-analyzer-tainted-array-index
-Wno-analyzer-tainted-divisor -Wno-analyzer-tainted-offset
-Wno-analyzer-tainted-size -Wanalyzer-symbol-too-complex
-Wanalyzer-too-complex -Wno-analyzer-undefined-behavior-ptrdiff
-Wno-analyzer-undefined-behavior-strtok
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-va-arg-type-mismatch -Wno-analyzer-va-list-exhausted
-Wno-analyzer-va-list-leak -Wno-analyzer-va-list-use-after-va-end
-Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal⚠️ eyes on-Wanalyzer-symbol-too-complexand-Wanalyzer-too-complex— these two do not have theno-in the middle. read them twice, bestie, because your brain wants to add it and that would be a whole different flag fr.
🐣 C and Objective-C-only Warning Options 🐣
these ones only show up for C and Objective-C. very exclusive. very VIP. 🎀
-Wbad-function-cast -Wdeprecated-non-prototype -Wfree-labels
-Wmissing-declarations -Wmissing-parameter-name
-Wmissing-parameter-type -Wdeclaration-missing-parameter-type
-Wmissing-prototypes -Wmissing-variable-declarations
-Wnested-externs -Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-signthat's all of them, all seventeen, sitting pretty. -Wtraditional and -Wtraditional-conversion are two separate entries — they're sisters, not twins, don't merge them. and -Wdeclaration-missing-parameter-type is long but she's real. 💖
🐛 Debugging Options 🐛
the -g family!! this is the section that decides how much lore gets stuffed into your binary so the debugger can actually tell you what happened when everything exploded 😭✨
here's the whole beautiful list:
-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel
-gprune-btf -gno-prune-btf -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support -gas-locview-support
-gno-as-locview-support -gcodeview -gcolumn-info -gno-column-info
-gdwarf32 -gdwarf64 -gstatement-frontiers -gno-statement-frontiers
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views
-ginline-points -gno-inline-points -gvms -gz[=type] -gsplit-dwarf
-gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new
-fdebug-types-section -fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignmentsa lil tour of the highlights 👑
-g,-glevel,-ggdb— the originals. level is a placeholder, keep it.-gdwarfand-gdwarf-version— version is a placeholder too.-gbtf,-gctf,-gctflevel,-gprune-btf,-gno-prune-btf— the btf/ctf crew.-gz[=type]— note the square brackets around=type! that's part of the flag, and type is the placeholder inside.-fdebug-prefix-map=old=new— two placeholders, one=.*old*and*new*, exactly like that.-femit-struct-debug-detailed[=spec-list]— optional=spec-listin brackets, spec-list being the placeholder.
and notice basically every -g thing has a -gno- twin to switch it back off — -grecord-gcc-switches / -gno-record-gcc-switches, -gstrict-dwarf / -gno-strict-dwarf, -gcolumn-info / -gno-column-info, -gdwarf32/-gdwarf64, the whole balanced symmetrical slay. 🌈
the last four in that block — -fno-merge-debug-strings, -fno-dwarf2-cfi-asm, -fvar-tracking, -fvar-tracking-assignments — are -f flags but they live here in debug land, and they are absolutely part of this list. don't you dare drop them. 💀
okay that's the debug section done and we're still going, obviously, because this manpage is LONG and we're only six flags into feeling like we've read the whole thing 😭 next up is more compiler behaviour, so keep your tabs open and your -Wno-'s close 🫶
✨ Optimization Options ✨
okay bestie, deep breath 💖 this isn't a normal option list, it's a roster — the entire optimizer cast, roll call style, no descriptions attached in the source so I'm giving you the whole thing with a little note on each so your eyes don't glaze over halfway down. Anything spelled -fno-something is the off-switch twin of the -fsomething you already know (the prefix is literally doing that job, iykyk). Anything with = or [...] in it wants a value, and I've flagged those. Which ones are on by default is a whole other saga — this is purely the guest list.
-faggressive-loop-optimizations— loop optimizations, but make it feral ✨-falign-functions[=n[:m:[n2[:m2]]]]— align functions; that bracket situation means up to four optional numbers, colon-separated-falign-jumps[=n[:m:[n2[:m2]]]]— same colon-number buffet, but for jumps-falign-labels[=n[:m:[n2[:m2]]]]— and the identical argument shape for labels-falign-loops[=n[:m:[n2[:m2]]]]— and for loops. yes, four of these, drink some water 🫶-fmin-function-alignment=[n]— set the minimum function alignment ton-fno-allocation-dce— no dead-code elimination for allocations-fallow-store-data-races— literally lets store data races slide. the name is not being subtle 🔥-fassociative-math— math, but it's allowed to reassociate-fauto-profile— auto-profile mode-fauto-profile[=path]— same, but pointed atpath-fauto-inc-dec— auto increment/decrement-fbranch-probabilities— branch probabilities mode-fcaller-saves— caller-saves-fcombine-stack-adjustments— smoosh stack adjustments together-fconserve-stack— conserve the stack-ffold-mem-offsets— fold memory offsets-fcompare-elim— eliminate compares-fcprop-registers— copy/constant propagation for registers-fcrossjumping— cross-jumping. iconic name, no notes 💅-fcse-follow-jumps— CSE that follows jumps-fcse-skip-blocks— CSE that skips blocks-fcx-fortran-rules— complex arithmetic, Fortran-rules flavour-fcx-limited-range— complex arithmetic, limited-range flavour-fdata-sections— every bit of data gets its own section-fdce— dead code elimination-fdelayed-branch— delayed branch-fdelete-null-pointer-checks— delete the null pointer checks-fdevirtualize— devirtualize-fdevirtualize-speculatively— devirtualize, speculatively-fdevirtualize-at-ltrans— devirtualize at ltrans-fdse— dead store elimination-fearly-inlining— inlining, but early-fipa-sra— IPA scalar replacement of aggregates-fexpensive-optimizations— the expensive ones. she said budget? never heard of her-ffat-lto-objects— fat LTO objects-ffast-math— fast math 🏎️-ffinite-math-only— finite math only-ffloat-store— floats get stored-fexcess-precision=style— excess precision,styleis yours to pick-ffinite-loops— assume the loops are finite-fforward-propagate— forward propagate-ffp-contract=style— floating-point contraction, takes astyle-ffunction-sections— a section per function-fgcse— global common subexpression elimination-fgcse-after-reload— gcse again, after reload-fgcse-las— gcse load-after-store-fgcse-lm— gcse load-motion-fgraphite-identity— graphite, identity transform-fgcse-sm— gcse store-motion-fhoist-adjacent-loads— hoist the adjacent loads-fif-conversion— if-conversion-fif-conversion2— if-conversion, the sequel 🎬-findirect-inlining— indirect inlining-finline-stringops[=fn]— inline string ops, optionally just forfn-finline-functions— inline functions-finline-functions-called-once— inline the ones called exactly once (main character functions 💅)-finline-limit=n— cap the inlining atn-finline-small-functions— inline the little ones-fipa-modref— IPA modref-fipa-cp— IPA constant propagation-fipa-cp-clone— IPA constant propagation plus cloning-fipa-bit-cp— IPA bit-level constant propagation-fipa-vrp— IPA value range propagation-fipa-pta— IPA points-to analysis-fipa-profile— IPA profile-fipa-pure-const— IPA pure/const-fipa-reference— IPA reference-fipa-reference-addressable— IPA reference, the addressable cut-fipa-reorder-for-locality— reorder for locality-fipa-stack-alignment— IPA stack alignment-fipa-icf— identical code folding-fira-algorithm=algorithm— pick your IRAalgorithm-flate-combine-instructions— late combine of instructions-flifetime-dse— lifetime dead store elimination-flive-patching=level— live patching, takes alevel-fira-region=region— IRAregion-fira-hoist-pressure— IRA hoist pressure-fira-loop-pressure— IRA loop pressure-fno-ira-share-save-slots— don't share IRA save slots-fno-ira-share-spill-slots— don't share IRA spill slots-fisolate-erroneous-paths-dereference— isolate erroneous paths that dereference-fisolate-erroneous-paths-attribute— isolate erroneous paths flagged by attribute-fivopts— induction variable optimizations-fkeep-inline-functions— keep the inline functions around-fkeep-static-functions— keep static functions-fkeep-static-consts— keep static constants-flimit-function-alignment— limit function alignment-flive-range-shrinkage— live range shrinkage-floop-block— loop blocking-floop-interchange— loop interchange-floop-strip-mine— loop strip mining ⛏️-floop-unroll-and-jam— unroll and jam-floop-nest-optimize— optimize the loop nest-floop-parallelize-all— parallelize all the loops-flra-remat— local register allocator rematerialization-flto— link time optimization 🔗-flto-compression-level— LTO compression level-flto-partition=alg— LTO partitioning withalg-flto-incremental=path— incremental LTO, viapath-flto-incremental-cache-size=n— incremental cache sizen-fmalloc-dce— dead code elimination for mallocs-fmerge-all-constants— merge all the constants-fmerge-constants— merge constants (a little more restrained)-fmodulo-sched— modulo scheduling-fmodulo-sched-allow-regmoves— modulo scheduling, register moves permitted-fmove-loop-invariants— move the loop invariants out-fmove-loop-stores— move the loop stores-fno-branch-count-reg— no branch count register-fno-defer-pop— don't defer pop-fno-fp-int-builtin-inexact— no inexact FP builtins for int-fno-function-cse— no function CSE-fno-guess-branch-probability— don't guess branch probabilities-fno-inline— no inlining. periodt 🚫-fno-math-errno— no math errno-fno-peephole— no peephole-fno-peephole2— no peephole2-fno-printf-return-value— no printf return value optimization-fno-sched-interblock— no interblock scheduling-fno-sched-spec— no speculative scheduling-fno-signed-zeros— signed zeros don't get distinguished-fno-toplevel-reorder— don't reorder the top level-fno-trapping-math— no trapping math-fno-zero-initialized-in-bss— zero-initialized stuff stays out of bss-fomit-frame-pointer— drop the frame pointer-foptimize-crc— optimize CRC-foptimize-sibling-calls— optimize sibling calls-fpartial-inlining— partial inlining-fpeel-loops— peel the loops 🍌-fpredictive-commoning— predictive commoning-fprefetch-loop-arrays— prefetch loop arrays-fprofile-correction— profile correction-fprofile-use— use the profile data-fprofile-use=path— use the profile data living atpath-fprofile-partial-training— partial training profile mode-fprofile-values— profile values-fprofile-reorder-functions— reorder functions by profile-freciprocal-math— reciprocal math-free— full redundancy elimination-frename-registers— rename registers-freorder-blocks— reorder blocks-freorder-blocks-algorithm=algorithm— whichalgorithmdoes the block reordering-freorder-blocks-and-partition— reorder blocks and partition-freorder-functions— reorder functions-frerun-cse-after-loop— run the CSE again after the loop-freschedule-modulo-scheduled-loops— reschedule the modulo-scheduled loops-frounding-math— rounding math-fsave-optimization-record— save the optimization record-fsched2-use-superblocks— sched2 with superblocks-fsched-pressure— scheduler pressure-fsched-spec-load— speculative load scheduling-fsched-spec-load-dangerous— same, but the name has dangerous in it, so like… be aware 🚨-fsched-stalled-insns-dep[=n]— stalled insns dependency, optionaln-fsched-stalled-insns[=n]— stalled insns, optionaln-fsched-group-heuristic— group heuristic-fsched-critical-path-heuristic— critical path heuristic-fsched-spec-insn-heuristic— speculative insn heuristic-fsched-rank-heuristic— rank heuristic-fsched-last-insn-heuristic— last insn heuristic-fsched-dep-count-heuristic— dep count heuristic-fschedule-fusion— schedule fusion-fschedule-insns— schedule insns-fschedule-insns2— schedule insns, the second one-fsection-anchors— section anchors-fselective-scheduling— selective scheduling-fselective-scheduling2— selective scheduling 2-fsel-sched-pipelining— selective scheduler pipelining-fsel-sched-pipelining-outer-loops— and the outer loops too-fsemantic-interposition— semantic interposition-fshrink-wrap— shrink wrapping 🎀-fshrink-wrap-separate— shrink wrapping, but separate-fsignaling-nans— signaling NaNs-fsingle-precision-constant— single precision constants-fsplit-ivs-in-unroller— split IVs inside the unroller-fsplit-loops— split loops-fsplit-paths— split paths-fsplit-wide-types— split wide types-fsplit-wide-types-early— split wide types, but early-fssa-backprop— SSA backprop-fssa-phiopt— SSA phi optimization-fstdarg-opt— stdarg optimization-fstore-merging— merge the stores-fstrict-aliasing— strict aliasing-fipa-strict-aliasing— IPA strict aliasing-fthread-jumps— thread jumps-ftracer— the tracer-ftree-bit-ccp— tree bit CCP-ftree-builtin-call-dce— dead code elimination for builtin calls-ftree-ccp— tree CCP-ftree-ch— the tree CH pass-ftree-coalesce-vars— coalesce vars-ftree-copy-prop— copy propagation-ftree-cselim— conditional store elimination-ftree-dce— dead code elimination-ftree-dominator-opts— dominator optimizations-ftree-dse— dead store elimination-ftree-forwprop— forward propagation-ftree-fre— full redundancy elimination-fcode-hoisting— code hoisting-ftree-loop-if-convert— convert the loop ifs-ftree-loop-im— loop invariant motion-ftree-phiprop— phi propagation-ftree-loop-distribution— loop distribution-ftree-loop-distribute-patterns— distribute loop patterns-ftree-loop-ivcanon— canonicalize the induction variables-ftree-loop-linear— loop linear-ftree-loop-optimize— optimize the loops-ftree-loop-vectorize— vectorize the loops-ftree-parallelize-loops=n— parallelize the loops,nstyle-ftree-pre— partial redundancy elimination-ftree-partial-pre— partial PRE-ftree-pta— points-to analysis-ftree-reassoc— reassociation-ftree-scev-cprop— scalar evolution copy propagation-ftree-sink— sinking-ftree-slsr— straight-line strength reduction-ftree-sra— scalar replacement of aggregates-ftree-switch-conversion— switch conversion-ftree-tail-merge— tail merging-ftree-ter— temporary expression replacement-ftree-vectorize— vectorize-ftree-vrp— value range propagation-ftrivial-auto-var-init— init the trivial auto vars-funconstrained-commons— unconstrained commons-funit-at-a-time— unit at a time-funroll-all-loops— unroll all the loops, no survivors-funroll-loops— unroll loops-funsafe-math-optimizations— math optimizations with unsafe right there in the name 🙈-funswitch-loops— unswitch loops-fipa-ra— IPA register allocation-fvariable-expansion-in-unroller— variable expansion in the unroller-fvect-cost-model— vectorizer cost model-fvpt— the-fvptpass-fweb— the web pass-fwhole-program— whole program mode-fwpa— whole program analysis-fuse-linker-plugin— use the linker plugin-fzero-call-used-regs— zero out the call-used registers--param name=value— set internal parameternametovalue-O— the plain optimize ✨-O0— level 0-O1— level one-O2— level two-O3— level three-Os— the size one-Ofast— the fast one-Og— the debug-friendly one-Oz— the extra size one
🎀 Program Instrumentation Options 🎀
this is the "make my program tell me about itself" shelf — profiling, coverage, sanitizers, and stack protection. very ✨observability✨ of them.
-p— the classic profiling hook-pg— profiling hook, gprof flavour-fprofile-arcs— instrument the arcs--coverage— the coverage one, spelled with two dashes 👀-ftest-coverage— test coverage-fcondition-coverage— condition coverage-fpath-coverage— path coverage-fprofile-abs-path— absolute paths in the profile-fprofile-dir=path— profile dir atpath-fprofile-generate— generate profile data-fprofile-generate=path— generate it intopath-fprofile-info-section— profile info in a section-fprofile-info-section=name— …in the section namedname-fprofile-note=path— profile note atpath-fprofile-prefix-path=path— prefix pathpath-fprofile-update=method— updatemethod-fprofile-filter-files=regex— include files matchingregex-fprofile-exclude-files=regex— exclude files matchingregex-fprofile-reproducible=[multithreaded|parallel-runs|serial]— reproducible, one ofmultithreaded,parallel-runs, orserial-fsanitize=style— sanitizer,stylepicks which-fsanitize-recover— sanitizer recovers instead of bailing-fsanitize-recover=style— recover, but only forstyle-fsanitize-trap— sanitizer traps-fsanitize-trap=style— trap, forstyle-fasan-shadow-offset=number— ASan shadow offset ofnumber-fsanitize-sections=s1,s2,...— sanitize these sections-fsanitize-undefined-trap-on-error— trap on undefined behavior rather than reporting it-fbounds-check— bounds checking-fcf-protection— control flow protection-fcf-protection=[full|branch|return|none|check]— pick one of exactly those five-fharden-compares— harden the compares-fharden-conditional-branches— harden the conditional branches-fhardened— the big harden-me vibe-fharden-control-flow-redundancy— harden control flow redundancy-fhardcfr-skip-leaf— skip leaf functions-fhardcfr-check-exceptions— check exceptions-fhardcfr-check-returning-calls— check returning calls-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— one ofalways,no-xthrow,nothrow,never-fstack-protector— stack protector-fstack-protector-all— stack protector for all of them-fstack-protector-strong— the strong one 💪-fstack-protector-explicit— only where you said so explicitly-fstack-check— stack checking-fstack-limit-register=reg— stack limit via registerreg-fstack-limit-symbol=sym— stack limit via symbolsym-fno-stack-limit— no stack limit-fsplit-stack— split the stack-fstrub=disable— strub:disable-fstrub=strict— strub:strict-fstrub=relaxed— strub:relaxed-fstrub=all— strub:all-fstrub=at-calls— strub:at-calls-fstrub=internal— strub:internal-fvtable-verify=[std|preinit|none]— vtable verification, one ofstd,preinit,none-fvtv-counts— vtable verify counts-fvtv-debug— vtable verify debug-finstrument-functions— instrument function entry and exit-finstrument-functions-once— …but only once-finstrument-functions-exclude-function-list=sym,sym,...— don't instrument these functions-finstrument-functions-exclude-file-list=file,file,...— don't instrument these files-fprofile-prefix-map=old=new— mapoldontonew-fpatchable-function-entry=N[,M]— patchable function entry withN, optionallyM
🧚 Preprocessor Options 🧚
the before-times! everything that happens before your code is even really your code. macros, includes, charsets, and the -M dependency family.
-Aquestion=answer— assertquestion=answer-A-question[=answer]— assert with the minus form, optionalanswer-C— comments, single-C-CC— comments, double-C ✨-Dmacro[=defn]— definemacro, optionally asdefn-dD— dump the macro definitions-dI— dump the include directives-dM— dump macros-dN— dump macro names-dU— dump macro uses-fdebug-cpp— debug the preprocessor-fdirectives-only— directives only-fdollars-in-identifiers— dollars allowed in identifiers 💸-fexec-charset=charset— execution charsetcharset-fextended-identifiers— extended identifiers-finput-charset=charset— input charsetcharset-fmacro-prefix-map=old=new— mapoldtonewfor macros-fmax-include-depth=depth— include depth limitdepth-fno-canonical-system-headers— no canonical system headers-fpch-deps— PCH deps-fpch-preprocess— PCH preprocess-fpreprocessed— it's already preprocessed, chill-ftabstop=width— tab stopwidth-ftrack-macro-expansion— track macro expansion-fwide-exec-charset=charset— wide execution charsetcharset-fworking-directory— working directory mode-H— the-Hflag (capital H, one letter, no notes)-imacros file— imacros fromfile-include file— includefile-M— the-Mdependency flag-MD— the-MDdependency flag-MF— the-MFdependency flag-MG— the-MGdependency flag-MM— the-MMdependency flag-MMD— the-MMDdependency flag-MP— the-MPdependency flag-MQ— the-MQdependency flag-MT— the-MTdependency flag-Mno-modules— no modules, in-Mform-no-integrated-cpp— don't use the integrated preprocessor-P— capital P, its own thing-pthread— the pthreads flag 🧵-remap— remap-traditional— traditional mode-traditional-cpp— traditional preprocessor mode-trigraphs— trigraphs are back, unfortunately 💀-Umacro— undefinemacro-undef— undefine (the broad one)-Wp,option— handoptionstraight to the preprocessor-Xpreprocessor option— same idea, different doorway
🌈 Assembler Options 🌈
tiny section, two doors to the same room:
-Wa,option— passoptionto the assembler-Xassembler option— passoptionto the assembler, the other way
🔥 Linker Options 🔥
gluing object files into a program. this is where main finally gets friends.
object-file-name— just name an object file and it gets linked in-fuse-ld=linker— uselinker-llibrary— linklibrary-nostartfiles— no start files-nodefaultlibs— no default libs-nolibc— no libc-nostdlib— no standard library at all-nostdlib++— no standard library, the++cut-e entry— entry pointentry--entry=entry— entry pointentry, equals-sign edition-pie— PIE-pthread— pthreads again 🧵-r— relocatable output-rdynamic— dynamic symbols, the r-rated version-s— strip it-static— static-static-pie— static and PIE, overachiever-static-libgcc— static libgcc-static-libstdc++— static libstdc++-static-libasan— static libasan-static-libtsan— static libtsan-static-liblsan— static liblsan-static-libubsan— static libubsan-shared— shared-shared-libgcc— shared libgcc-symbolic— symbolic-T script— linker scriptscript-Wl,option— handoptionto the linker-Xlinker option— handoptionto the linker, alt entrance-u symbol— undefinesymbol-z keyword— linker keywordkeyword
🦋 Directory Options 🦋
where does gcc look for stuff. the answer is: everywhere you tell it. 📂
-Bprefix— prefixprefix-Idir— adddirto the include search-I-— the-I-form-idirafter dir—dir, but after-imacros file— imacrosfile-imultilib dir— multilib dirdir-iplugindir=dir— plugin dirdir-iprefix file— prefixfile-iquote dir— quote-include dirdir-isysroot dir— sysrootdir-isystem dir— system dirdir-iwithprefix dir— with prefixdir-iwithprefixbefore dir— with prefix, before,dir--embed-dir=dir— embed dirdir-Ldir— adddirto the library search-no-canonical-prefixes— no canonical prefixes--no-sysroot-suffix— no sysroot suffix-nostdinc— no standard include dirs-nostdinc++— no standard include dirs,++version--sysroot=dir— sysrootdir, equals-sign edition
💜 Code Generation Options 💜
the deep-magic shelf. calling conventions, position independence, struct layout, and a couple of flags whose names are basically warnings.
-fcall-saved-reg—regis call-saved-fcall-used-reg—regis call-used-ffixed-reg—regis fixed-fexceptions— exceptions-fnon-call-exceptions— non-call exceptions-fdelete-dead-exceptions— delete dead exceptions-funwind-tables— unwind tables-fasynchronous-unwind-tables— asynchronous unwind tables-fno-gnu-unique— no gnu unique-finhibit-size-directive— inhibit the size directive-fcommon— common-fno-ident— no ident-fpcc-struct-return— pcc struct return-fpic— pic-fPIC— PIC, but louder (capital letters matter here!!) 👑-fpie— pie-fPIE— PIE, capital edition-fno-plt— no PLT-fno-jump-tables— no jump tables-fno-bit-tests— no bit tests-frecord-gcc-switches— record the gcc switches-freg-struct-return— reg struct return-fshort-enums— short enums-fshort-wchar— short wchar-fverbose-asm— verbose asm, blessed output 🫶-fpack-struct[=n]— pack the struct, optionaln-fleading-underscore— leading underscore-ftls-model=model— TLSmodel-fstack-reuse=reuse_level— stack reusereuse_level-ftrampolines— trampolines-ftrampoline-impl=[stack|heap]— trampoline impl,stackorheap-ftrapv— trap on overflow-fwrapv— wrap on overflow-fvisibility=[default|internal|hidden|protected]— visibility, one of those four-fstrict-volatile-bitfields— strict volatile bitfields-fsync-libcalls— sync libcalls-fzero-init-padding-bits=value— zero-init padding bits,value
⭐ Developer Options ⭐
for when you're debugging the compiler itself, or you just want it to tell you everything it knows. this section is unhinged in the best way — there are like six different memory reports 💀
-dletters— the-dletters-dumpspecs— dump the specs-dumpmachine— dump the machine-dumpversion— dump the version-dumpfullversion— dump the full version-fcallgraph-info[=su,da]— callgraph info, optionallysu,da-fchecking— checking on-fchecking=n— checking at leveln-fdbg-cnt-list— list the debug counters-fdbg-cnt=counter-value-list— set debug counters fromcounter-value-list-fdisable-ipa-pass_name— disable the IPA passpass_name-fdisable-rtl-pass_name— disable the RTL passpass_name-fdisable-rtl-pass-name=range-list— disable that RTL pass overrange-list-fdisable-tree-pass_name— disable the tree passpass_name-fdisable-tree-pass-name=range-list— disable that tree pass overrange-list-fdump-debug— dump debug-fdump-earlydebug— dump early debug-fdump-noaddr— dump without addresses-fdump-unnumbered— dump unnumbered-fdump-unnumbered-links— dump unnumbered links-fdump-final-insns[=file]— dump final insns, optionally intofile-fdump-ipa-all— dump all IPA-fdump-ipa-cgraph— dump the IPA callgraph-fdump-ipa-inline— dump IPA inline-fdump-lang-all— dump all the language stuff-fdump-lang-switch— dump langswitch-fdump-lang-switch-options— dump lang switch options-fdump-lang-switch-options=filename— ...intofilename-fdump-passes— dump the passes-fdump-rtl-pass— dump the RTL pass-fdump-rtl-pass=filename— dump the RTL pass intofilename-fdump-statistics— dump statistics-fdump-tree-all— dump all the trees 🌳-fdump-tree-switch— dump treeswitch-fdump-tree-switch-options— dump tree switch options-fdump-tree-switch-options=filename— ...intofilename-fcompare-debug[=opts]— compare debug, optionally withopts-fcompare-debug-second— compare debug, second pass-fenable-kind-pass— enablekindpass-fenable-kind-pass=range-list— enable it overrange-list-fira-verbose=n— IRA verbose leveln-flto-report— LTO report-flto-report-wpa— LTO report, WPA edition-fmem-report-wpa— memory report, WPA edition-fmem-report— memory report 🧠-fpre-ipa-mem-report— memory report, before IPA-fpost-ipa-mem-report— memory report, after IPA-fopt-info— optimization info-fopt-info-options[=file]— optimization info withoptions, optionally tofile-fmultiflags— multiflags-fprofile-report— profile report-frandom-seed=string— random seedstring-fsched-verbose=n— scheduler verbose leveln-fsel-sched-verbose— selective scheduler verbose-fsel-sched-dump-cfg— dump the selective scheduler CFG-fsel-sched-pipelining-verbose— selective scheduler pipelining verbose-fstats— stats-fstack-usage— stack usage 🥞-ftime-report— time report ⏱️-ftime-report-details— time report, with details-fvar-tracking-assignments-toggle— toggle var tracking assignments-gtoggle— toggle the debug info-print-file-name=library— print the file name oflibrary-print-libgcc-file-name— print libgcc's file name-print-multi-directory— print the multilib directory-print-multi-lib— print the multilib listing-print-multi-os-directory— print the multilib OS directory-print-prog-name=program— print the path ofprogram-print-search-dirs— print the search dirs-Q— capital Q, one letter, its own icon 💅-print-sysroot— print the sysroot-print-sysroot-headers-suffix— print the sysroot headers suffix-save-temps— save the temps-save-temps=cwd— save temps into the current working dir-save-temps=obj— save temps next to the object-time[=file]— timing, optionally intofile
and that's the roster!! next chunk incoming, don't go anywhere 🫶
🧭 Machine-Dependent Options 🧭
Okay so this is the part where GCC stops being one program and turns into like... every program? Every chip family gets its own little aisle in the store, and your girl is walking you through all of them. Nothing is skipped, nothing is merged, nothing gets "cleaned up" — if it's a flag, it's here, spelled exactly right. The deep explanations for these live further down the page, so for now this is the menu, bestie. 🫶
🦋 AArch64 Options 🦋
AArch64 girlies, this is your aisle ✨ — including every tiny qualifier and bracket situation:
-mabi=name-mbig-endian-mlittle-endian-mgeneral-regs-only-mcmodel=tiny-mcmodel=small-mcmodel=large-mstrict-align-mno-strict-align-momit-leaf-frame-pointer-mtls-dialect=desc-mtls-dialect=traditional-mtls-size=size-mfix-cortex-a53-835769-mfix-cortex-a53-843419-mlow-precision-recip-sqrt-mlow-precision-sqrt-mlow-precision-div-mpc-relative-literal-loads-msign-return-address=scope-mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti|gcs-mharden-sls=opts-march=name-mcpu=name-mtune=name-moverride=string-mverbose-cost-dump-mstack-protector-guard=guard-mstack-protector-guard-reg=sysreg-mstack-protector-guard-offset=offset-mtrack-speculation-moutline-atomics-mearly-ldp-fusion-mlate-ldp-fusion-Wexperimental-fmv-target
The-mbranch-protection=value is exactly that whole pipe-and-bracket situation,+leaf +b-keyand all. Don't retype it from memory, copy it. 💅
🧚 Adapteva Epiphany Options 🧚
Tiny parallel-computing icon, honestly:
-mhalf-reg-file-mprefer-short-insn-regs-mbranch-cost=num-mcmove-mnops=num-msoft-cmpsf-msplit-lohi-mpost-inc-mpost-modify-mstack-offset=num-mround-nearest-mlong-calls-mshort-calls-msmall16-mfp-mode=mode-mvect-double-max-vect-align=num-msplit-vecmove-early-m1reg-reg
🌊 AMD GCN Options 🌊
Short list, immaculate energy:
-march=gpu-mtune=gpu-mstack-size=bytes
🌈 ARC Options 🌈
This one is long long, so buckle up 🎀 — I'm going straight down the list:
-mbarrel-shifter-mjli-always-mcpu=cpu-mA6-mARC600-mA7-mARC700-mdpfp-mdpfp-compact-mdpfp-fast-mno-dpfp-lrsr-mea-mno-mpy-mmul32x16-mmul64-matomic-mnorm-mspfp-mspfp-compact-mspfp-fast-msimd-msoft-float-mswap-mcrc-mdsp-packa-mdvbf-mlock-mmac-d16-mmac-24-mrtsc-mswape-mtelephony-mxy-misize-mannotate-align-marclinux-marclinux_prof-mlong-calls-mmedium-calls-msdata-mirq-ctrl-saved-mrgf-banked-regs-mlpc-width=width-G num-mvolatile-cache-mtp-regno=regno-malign-call-mauto-modify-reg-mbbit-peephole-mno-brcc-mcase-vector-pcrel-mcompact-casesi-mno-cond-exec-mearly-cbranchsi-mexpand-adddi-mindexed-loads-mlra-mlra-priority-none-mlra-priority-compact-mlra-priority-noncompact-mmillicode-mmixed-code-mq-class-mRcq-mRcw-msize-level=level-mtune=cpu-mmultcost=num-mcode-density-frame-munalign-prob-threshold=probability-mmpy-option=multo-mdiv-rem-mcode-density-mll64-mfpu=fpu-mrf16-mbranch-index
Just so we're clear:-mRcqand-mRcwhave capital R's, and-G numreally is a space, not an=. Case-sensitive icons. 👑
🌸 ARM Options 🌸
The ARM section is giving main character energy — it's huge and every flag matters:
-mapcs-frame-mno-apcs-frame-mabi=name-mapcs-stack-check-mno-apcs-stack-check-mapcs-reentrant-mno-apcs-reentrant-mgeneral-regs-only-msched-prolog-mno-sched-prolog-mlittle-endian-mbig-endian-mbe8-mbe32-mfloat-abi=name-mfp16-format=name-mthumb-interwork-mno-thumb-interwork-mcpu=name-march=name-mfpu=name-mtune=name-mprint-tune-info-mstructure-size-boundary=n-mabort-on-noreturn-mlong-calls-mno-long-calls-msingle-pic-base-mno-single-pic-base-mpic-register=reg-mnop-fun-dllimport-mpoke-function-name-mthumb-marm-mflip-thumb-mtpcs-frame-mtpcs-leaf-frame-mcaller-super-interworking-mcallee-super-interworking-mtp=name-mtls-dialect=dialect-mword-relocations-mfix-cortex-m3-ldrd-mfix-cortex-a57-aes-1742098-mfix-cortex-a72-aes-1655431-munaligned-access-mneon-for-64bits-mslow-flash-data-masm-syntax-unified-mrestrict-it-mverbose-cost-dump-mpure-code-mcmse-mfix-cmse-cve-2021-35465-mstack-protector-guard=guard-mstack-protector-guard-offset=offset-mfdpic-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]
Note that ARM's -mbranch-protection= value is a different pipe-and-bracket situation than AArch64's. Both are printed correctly. Do not "harmonise" them, I'm begging 🫠
🍓 AVR Options 🍓
Microcontroller besties, this one's for you:
-mmcu=mcu-mabsdata-maccumulate-args-mcvt-mbranch-cost=cost-mfuse-add=level-mfuse-move=level-mfuse-move2-mcall-prologues-mgas-isr-prologues-mint8-mflmap-mdouble=bits-mlong-double=bits-mno-call-main-mn_flash=size-mfract-convert-truncate-mno-interrupts-mmain-is-OS_task-mrelax-mrmw-mstrict-X-mtiny-stack-mrodata-in-ram-msplit-bit-shift-msplit-ldst-mshort-calls-mskip-bug-muse-nonzero-bits-nodevicelib-nodevicespecs-Waddr-space-convert-Wmisspelled-isr
Yes,-mn_flash=sizehas an underscore in it, and-mstrict-Xhas a capital X. And the last two start with-W, not-m. The vibes are chaotic but the spelling is law 💅
🖤 Blackfin Options 🖤
-mcpu=cpu[-sirevision]-msim-momit-leaf-frame-pointer-mno-omit-leaf-frame-pointer-mspecld-anomaly-mno-specld-anomaly-mcsync-anomaly-mno-csync-anomaly-mlow-64k-mno-low64k-mstack-check-l1-mid-shared-library-mno-id-shared-library-mshared-library-id=n-mleaf-id-shared-library-mno-leaf-id-shared-library-msep-data-mno-sep-data-mlong-calls-mno-long-calls-mfast-fp-minline-plt-mmulticore-mcorea-mcoreb-msdram-micplb
⭐ C6X Options ⭐
-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-type
🦋 CRIS Options 🦋
-mcpu=cpu-march=cpu-mtune=cpu-mmax-stack-frame=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-melf-maout-sim-sim2-mmul-bug-workaround-mno-mul-bug-workaround
Heads up:-simand-sim2are printed without the-m. That's not me being sloppy, that's literally what's there 👀
🎀 C-SKY Options 🎀
-march=arch-mcpu=cpu-mbig-endian-EB-mlittle-endian-EL-mhard-float-msoft-float-mfpu=fpu-mdouble-float-mfdivdu-mfloat-abi=name-melrw-mistack-mmp-mcp-mcache-msecurity-mtrust-mdsp-medsp-mvdsp-mdiv-msmart-mhigh-registers-manchor-mpushpop-mmultiple-stld-mconstpool-mstack-size-mccrt-mbranch-cost=n-mcse-cc-msched-prolog-msim
-EBand-ELare short and unbothered, no-mprefix. Respect their choices 🫶
💜 Cygwin and MinGW Options 💜
-mconsole-mcrtdll=library-mdll-mnop-fun-dllimport-mthread-municode-mwin32-mwindows-fno-set-stack-executable
That last one is-f-prefixed, not-m. It's a whole different letter and it's correct.
🍎 Darwin Options 🍎
Bestie, this list is long, and some of these literally show up twice in the original — I'm reading it exactly as printed, no editing, no deduping 💀:
-all_load-allowable_client-arch-arch_errors_fatal-arch_only-bind_at_load-bundle-bundle_loader-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-filelist-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-headerpad_max_install_names-iframework-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nodefaultrpaths-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded-F-gused-gfull-mmacosx-version-min=version-mkernel-mone-byte-bool
Yeah, I know —-sectobjectsymbols,-segs_read_only_addrand-segs_read_write_addreach appear twice up there. That's what the page says. Not a typo, not me glitching, we keep it 💅
🌊 DEC Alpha Options 🌊
-mno-fp-regs-msoft-float-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-msafe-bwa-msafe-partial-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time
🧚 eBPF Options 🧚
-mbig-endian-mlittle-endian-mframe-limit=bytes-mxbpf-mco-re-mno-co-re-mjmpext-mjmp32-malu32-mv3-atomics-mbswap-msdiv-msmov-mcpu=version-masm=dialect-minline-memops-threshold=bytes
🌈 FR30 Options 🌈
Short and sweet:
-msmall-model-mno-lsim
🔥 FT32 Options 🔥
-msim-mlra-mnodiv-mft32b-mcompress-mnopm
🦋 FRV Options 🦋
Okay this one goes off, look at all these:
-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-cc-mdword-mno-dword-mdouble-mno-double-mmedia-mno-media-mmuladd-mno-muladd-mfdpic-minline-plt-mgprel-ro-multilib-library-pic-mlinked-fp-mlong-calls-malign-labels-mlibrary-pic-macc-4-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
-mTLSand-mtlsare both there and they are different flags. The capitalisation is the whole personality. 👑
🐧 GNU/Linux Options 🐧
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
The android ones switch from-mto-t. That's on purpose, bestie.
🎀 H8/300 Options 🎀
-mrelax-mh-ms-mn-mexr-mno-exr-mint32-malign-300
💅 HPPA Options 💅
Another big one, and I'm not dropping a single flag:
-march=architecture-type-matomic-libcalls-mbig-switch-mcaller-copies-mdisable-fpregs-mdisable-indexing-mordered-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mcoherent-ldcw-mjump-in-delay-mlinker-opt-mlong-calls-mlong-load-store-mno-atomic-libcalls-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-msoft-mult-msio-mwsio-munix=unix-std-nolibdld-static-threads
-nolibdld,-staticand-threadshave no-mat all. Fully their own thing. Also-mpa-risc-1-0has hyphens, not dots. Do not write-mpa-risc-1.0, the linker will simply not ✨
👑 IA-64 Options 👑
This section is huge — and the scheduler flags are giving "I have a very long name and I'm proud of it":
-mbig-endian-mlittle-endian-mgnu-as-mgnu-ld-mno-pic-mvolatile-asm-stop-mregister-names-msdata-mno-sdata-mconstant-gp-mauto-pic-mfused-madd-minline-float-divide-min-latency-minline-float-divide-max-throughput-mno-inline-float-divide-minline-int-divide-min-latency-minline-int-divide-max-throughput-mno-inline-int-divide-minline-sqrt-min-latency-minline-sqrt-max-throughput-mno-inline-sqrt-mdwarf2-asm-mearly-stop-bits-mfixed-range=register-range-mtls-size=tls-size-mtune=cpu-type-milp32-mlp64-msched-br-data-spec-msched-ar-data-spec-msched-control-spec-msched-br-in-data-spec-msched-ar-in-data-spec-msched-in-control-spec-msched-spec-ldc-msched-spec-control-ldc-msched-prefer-non-data-spec-insns-msched-prefer-non-control-spec-insns-msched-stop-bits-after-every-cycle-msched-count-spec-in-critical-path-msel-sched-dont-check-control-spec-msched-fp-mem-deps-zero-cost-msched-max-memory-insns-hard-limit-msched-max-memory-insns=max-insns
Look at-msel-sched-dont-check-control-spec— it starts with-msel, not-msched. That's not a typo on my end, that's genuinely what's printed. Copy it, don't vibe it 😭
🧚 LM32 Options 🧚
Last aisle in this part, and it's a cute little one:
-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabled
And that's the machine-dependent tour so far, bestie — every single flag exactly where it belongs 💖 There's more coming, so don't go anywhere, we're literally not done 🫶
Okay bestie, we're still deep in the architecture-specific option zone, and it's giving “every family brought a plus-one.” 💅 I'm not inventing what each flag does — the page just lists the guest list — so here's every single name, exactly as printed. No cap.
LoongArch Options (LoongArch Options) 🐉
LoongArch pulled up with the full set:
-march=arch-type-mtune=tune-type-mabi=base-abi-type-mfpu=fpu-type-msimd=simd-type-msoft-float-msingle-float-mdouble-float-mlsx-mno-lsx-mlasx-mno-lasx-mbranch-cost=n-maddr-reg-reg-cost=n-mcheck-zero-division-mno-check-zero-division-mcond-move-int-mno-cond-move-int-mcond-move-float-mno-cond-move-float-memcpy-mno-memcpy-mstrict-align-mno-strict-align-G num-mmax-inline-memcpy-size=n-mexplicit-relocs=style-mexplicit-relocs-mno-explicit-relocs-mdirect-extern-access-mno-direct-extern-access-mcmodel=code-model-mrelax-mpass-mrelax-to-as-mrecip-mrecip=opt-mfrecipe-mno-frecipe-mdiv32-mno-div32-mlam-bh-mno-lam-bh-mlamcas-mno-lamcas-mld-seq-sa-mno-ld-seq-sa-mtls-dialect=opt-mannotate-tablejump-mno-annotate-tablejump
M32C Options (M32C Options) ✨
M32C keeps it short and sweet:
-mcpu=cpu-msim-memregs=number
M32R/D Options (M32R/D Options) 🦋
M32R/D said “here's my whole bag”:
-m32r2-m32rx-m32r-mdebug-malign-loops-mno-align-loops-missue-rate=number-mbranch-cost=number-mmodel=code-size-model-type-msdata=sdata-type-mno-flush-func-mflush-func=name-mno-flush-trap-mflush-trap=number-G num
M680x0 Options (M680x0 Options) 🎀
M680x0 has a whole era's worth of flags:
-march=arch-mcpu=cpu-mtune=tune-m68000-m68020-m68020-40-m68020-60-m68030-m68040-m68060-mcpu32-m5200-m5206e-m528x-m5307-m5407-mcfv4e-mbitfield-mno-bitfield-mc68000-mc68020-mnobitfield-mrtd-mno-rtd-mdiv-mno-div-mshort-mno-short-mhard-float-m68881-msoft-float-mpcrel-malign-int-mstrict-align-msep-data-mno-sep-data-mshared-library-id=n-mid-shared-library-mno-id-shared-library-mxgot-mno-xgot-mlong-jump-table-offsets
MCore Options (MCore Options) 🌿
MCore's list is right here:
-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
MicroBlaze Options (MicroBlaze Options) ⭐
MicroBlaze brought these:
-msoft-float-mhard-float-msmall-divides-mcpu=cpu-mmemcpy-mxl-soft-mul-mxl-soft-div-mxl-barrel-shift-mxl-pattern-compare-mxl-stack-check-mxl-gp-opt-mno-clearbss-mxl-multiply-high-mxl-float-convert-mxl-float-sqrt-mbig-endian-mlittle-endian-mxl-reorder-mxl-mode-app-model-mpic-data-is-text-relative
MIPS Options (MIPS Options) 🐚
MIPS brought the whole extended family — grab a snack, bestie:
-EL-EB-march=arch-mtune=arch-mips1-mips2-mips3-mips4-mips32-mips32r2-mips32r3-mips32r5-mips32r6-mips64-mips64r2-mips64r3-mips64r5-mips64r6-mips16-mno-mips16-mflip-mips16-minterlink-compressed-mno-interlink-compressed-minterlink-mips16-mno-interlink-mips16-mabi=abi-mabicalls-mno-abicalls-mshared-mno-shared-mplt-mno-plt-mxgot-mno-xgot-mgp32-mgp64-mfp32-mfpxx-mfp64-mhard-float-msoft-float-mno-float-msingle-float-mdouble-float-modd-spreg-mno-odd-spreg-mabs=mode-mnan=encoding-mdsp-mno-dsp-mdspr2-mno-dspr2-mmcu-mmno-mcu-meva-mno-eva-mvirt-mno-virt-mxpa-mno-xpa-mcrc-mno-crc-mginv-mno-ginv-mmicromips-mno-micromips-mmsa-mno-msa-mloongson-mmi-mno-loongson-mmi-mloongson-ext-mno-loongson-ext-mloongson-ext2-mno-loongson-ext2-mfpu=fpu-type-msmartmips-mno-smartmips-mpaired-single-mno-paired-single-mdmx-mno-mdmx-mips3d-mno-mips3d-mmt-mno-mt-mllsc-mno-llsc-mlong64-mlong32-msym32-mno-sym32-Gnum-mlocal-sdata-mno-local-sdata-mextern-sdata-mno-extern-sdata-mgpopt-mno-gopt-membedded-data-mno-embedded-data-muninit-const-in-rodata-mno-uninit-const-in-rodata-mcode-readable=setting-msplit-addresses-mno-split-addresses-mexplicit-relocs-mno-explicit-relocs-mexplicit-relocs=release-mcheck-zero-division-mno-check-zero-division-mdivide-traps-mdivide-breaks-mload-store-pairs-mno-load-store-pairs-mstrict-align-mno-strict-align-mno-unaligned-access-munaligned-access-mmemcpy-mno-memcpy-mlong-calls-mno-long-calls-mmad-mno-mad-mimadd-mno-imadd-mfused-madd-mno-fused-madd-nocpp-mfix-24k-mno-fix-24k-mfix-r4000-mno-fix-r4000-mfix-r4400-mno-fix-r4400-mfix-r5900-mno-fix-r5900-mfix-r10000-mno-fix-r10000-mfix-rm7000-mno-fix-rm7000-mfix-vr4120-mno-fix-vr4120-mfix-vr4130-mno-fix-vr4130-mfix-sb1-mno-fix-sb1-mflush-func=func-mno-flush-func-mbranch-cost=num-mbranch-likely-mno-branch-likely-mcompact-branches=policy-mfp-exceptions-mno-fp-exceptions-mvr4130-align-mno-vr4130-align-msynci-mno-synci-mlxc1-sxc1-mno-lxc1-sxc1-mmadd4-mno-madd4-mrelax-pic-calls-mno-relax-pic-calls-mmcount-ra-address-mframe-header-opt-mno-frame-header-opt
MMIX Options (MMIX Options) 🧚
MMIX's guest list:
-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 (MN10300 Options) 🌈
MN10300 said “don't forget these”:
-mmult-bug-mno-mult-bug-mno-am33-mam33-mam33-2-mam34-mtune=cpu-type-mreturn-pointer-on-d0-mno-crt0-mrelax-mliw-msetlb
Moxie Options (Moxie Options) 🫶
Moxie keeps it tiny:
-meb-mel-mmul.x-mno-crt0
MSP430 Options (MSP430 Options) 🍓
MSP430's lineup:
-msim-masm-hex-mmcu=-mcpu=-mlarge-msmall-mrelax-mwarn-mcu-mcode-region=-mdata-region=-msilicon-errata=-msilicon-errata-warn=-mhwmult=-minrt-mtiny-printf-mmax-inline-shift=
NDS32 Options (NDS32 Options) 💜
NDS32 brought this whole batch:
-mbig-endian-mlittle-endian-mreduced-regs-mfull-regs-mcmov-mno-cmov-mext-perf-mno-ext-perf-mext-perf2-mno-ext-perf2-mext-string-mno-ext-string-mv3push-mno-v3push-m16bit-mno-16bit-misr-vector-size=num-mcache-block-size=num-march=arch-mcmodel=code-model-mctor-dtor-mrelax
Nvidia PTX Options (Nvidia PTX Options) 🦋
Nvidia PTX keeps it chill:
-m64-mmainkernel-moptimize
OpenRISC Options (OpenRISC Options) ✨
OpenRISC's list:
-mboard=name-mnewlib-mhard-mul-mhard-div-msoft-mul-msoft-div-msoft-float-mhard-float-mdouble-float-munordered-float-mcmov-mror-mrori-msext-msfimm-mshftimm-mcmodel=code-model
PDP-11 Options (PDP-11 Options) 🎀
PDP-11 said “here's the archive”:
-mfpu-msoft-float-mac0-mno-ac0-m40-m45-m10-mint32-mno-int16-mint16-mno-int32-msplit-munix-asm-mdec-asm-mgnu-asm-mlra
PowerPC Options — see RS/6000 and PowerPC Options. That's literally the whole entry, bestie. 💜
PRU Options (PRU Options) 🔥
PRU's lineup:
-mmcu=mcu-minrt-mno-relax-mloop-mabi=variant
RISC-V Options (RISC-V Options) 🌸
RISC-V brought a very full bag:
-mbranch-cost=N-instruction-mplt-mno-plt-mabi=ABI-string-mfdiv-mno-fdiv-mfence-tso-mno-fence-tso-mdiv-mno-div-misa-spec=ISA-spec-string-march=ISA-string-mtune=processor-string-mpreferred-stack-boundary=num-msmall-data-limit=N-bytes-msave-restore-mno-save-restore-mshorten-memrefs-mno-shorten-memrefs-mstrict-align-mno-strict-align-mcmodel=medlow-mcmodel=medany-mcmodel=large-mexplicit-relocs-mno-explicit-relocs-mrelax-mno-relax-mriscv-attribute-mno-riscv-attribute-malign-data=type-mbig-endian-mlittle-endian-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset-mcsr-check-mno-csr-check-mmovcc-mno-movcc-minline-atomics-mno-inline-atomics-minline-strlen-mno-inline-strlen-minline-strcmp-mno-inline-strcmp-minline-strncmp-mno-inline-strncmp-mtls-dialect=desc-mtls-dialect=trad
RL78 Options (RL78 Options) 🌿
RL78's list:
-msim-mmul=none-mmul=g13-mmul=g14-mallregs-mcpu=g10-mcpu=g13-mcpu=g14-mg10-mg13-mg14-m64bit-doubles-m32bit-doubles-msave-mduc-in-interrupts
RS/6000 and PowerPC Options (RS/6000 and PowerPC Options) 👑
Okay, this is the big one — the whole PowerPC group chat is here:
-mcpu=cpu-type-mtune=cpu-type-mcmodel=code-model-mpowerpc64-maltivec-mno-altivec-mpowerpc-gpopt-mno-powerpc-gpopt-mpowerpc-gfxopt-mno-powerpc-gfxopt-mmfcrf-mno-mfcrf-mpopcntb-mno-popcntb-mpopcntd-mno-popcntd-mfprnd-mno-fprnd-mcmpb-mno-cmpb-mhard-dfp-mno-hard-dfp-mfull-toc-mminimal-toc-mno-fp-in-toc-mno-sum-in-toc-m64-m32-mxl-compat-mno-xl-compat-mpe-malign-power-malign-natural-msoft-float-mhard-float-mmultiple-mno-multiple-mupdate-mno-update-mavoid-indexed-addresses-mno-avoid-indexed-addresses-mfused-madd-mno-fused-madd-mbit-align-mno-bit-align-mstrict-align-mno-strict-align-mrelocatable-mno-relocatable-mrelocatable-lib-mno-relocatable-lib-mtoc-mno-toc-mlittle-mlittle-endian-mbig-mbig-endian-mdynamic-no-pic-mswdiv-msingle-pic-base-mprioritize-restricted-insns=priority-msched-costly-dep=dependence_type-minsert-sched-nops=scheme-mcall-aixdesc-mcall-eabi-mcall-freebsd-mcall-linux-mcall-netbsd-mcall-openbsd-mcall-sysv-mcall-sysv-eabi-mcall-sysv-noeabi-mtraceback=traceback_type-maix-struct-return-msvr4-struct-return-mabi=abi-type-msecure-plt-mbss-plt-msplit-patch-nops-mlongcall-mno-longcall-mpltseq-mno-pltseq-mblock-move-inline-limit=num-mblock-compare-inline-limit=num-mblock-compare-inline-loop-limit=num-mno-block-ops-unaligned-vsx-mstring-compare-inline-limit=num-misel-mno-isel-mvrsave-mno-vrsave-mmulhw-mno-mulhw-mdlmzb-mno-dlmzb-mprototype-mno-prototype-msim-mmvme-mads-myellowknife-memb-msdata-msdata=opt-mreadonly-in-sdata-mvxworks-G num-mrecip-mrecip=opt-mno-recip-mrecip-precision-mno-recip-precision-mveclibabi=type-mfriz-mno-friz-mpointers-to-nested-functions-mno-pointers-to-nested-functions-msave-toc-indirect-mno-save-toc-indirect-mpower8-fusion-mno-mpower8-fusion-mcrypto-mno-crypto-mhtm-mno-htm-mquad-memory-mno-quad-memory-mquad-memory-atomic-mno-quad-memory-atomic-mcompat-align-parm-mno-compat-align-parm-mfloat128-mno-float128-mfloat128-hardware-mno-float128-hardware-mgnu-attribute-mno-gnu-attribute-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset-mprefixed-mno-prefixed-mpcrel-mno-pcrel-mmma-mno-mmma-mrop-protect-mno-rop-protect-mprivileged-mno-privileged
…and we're still doing the per-CPU flag tour, bestie, breathe 💅 here's the rest of the platform menu — every single flag, no skips, no cap.
RX Options (RX Options) — -m64bit-doubles, -m32bit-doubles, -fpu, -nofpu, -mcpu=, -mbig-endian-data, -mlittle-endian-data, -msmall-data, -msim, -mno-sim, -mas100-syntax, -mno-as100-syntax, -mrelax, -mmax-constant-size=, -mint-register=, -mpid, -mallow-string-insns, -mno-allow-string-insns, -mjsr, -mno-warn-multiple-fast-interrupts, -msave-acc-in-interrupts.
S/390 and zSeries Options (S/390 and zSeries Options) — -mtune=cpu-type, -march=cpu-type, -mhard-float, -msoft-float, -mhard-dfp, -mno-hard-dfp, -mlong-double-64, -mlong-double-128, -mbackchain, -mno-backchain, -mpacked-stack, -mno-packed-stack, -msmall-exec, -mno-small-exec, -mmvcle, -mno-mvcle, -m64, -m31, -mdebug, -mno-debug, -mesa, -mzarch, -mhtm, -mvx, -mzvector, -mtpf-trace, -mno-tpf-trace, -mtpf-trace-skip, -mno-tpf-trace-skip, -mfused-madd, -mno-fused-madd, -mwarn-framesize, -mwarn-dynamicstack, -mstack-size, -mstack-guard, -mhotpatch=halfwords,halfwords.
SH Options (SH Options) — -m1, -m2, -m2e, -m2a-nofpu, -m2a-single-only, -m2a-single, -m2a, -m3, -m3e, -m4-nofpu, -m4-single-only, -m4-single, -m4, -m4a-nofpu, -m4a-single-only, -m4a-single, -m4a, -m4al, -mb, -ml, -mdalign, -mrelax, -mbigtable, -mfmovd, -mrenesas, -mno-renesas, -mnomacsave, -mieee, -mno-ieee, -mbitops, -misize, -minline-ic_invalidate, -mpadstruct, -mprefergot, -musermode, -multcost=number, -mdiv=strategy, -mdivsi3_libfunc=name, -mfixed-range=register-range, -maccumulate-outgoing-args, -matomic-model=atomic-model, -mbranch-cost=num, -mzdcbranch, -mno-zdcbranch, -mcbranch-force-delay-slot, -mfused-madd, -mno-fused-madd, -mfsca, -mno-fsca, -mfsrra, -mno-fsrra, -mpretend-cmove, -mtas.
Solaris 2 Options (Solaris 2 Options) — -mclear-hwcap, -mno-clear-hwcap, -mimpure-text, -mno-impure-text, -pthreads.
SPARC Options (SPARC Options) — -mcpu=cpu-type, -mtune=cpu-type, -mcmodel=code-model, -mmemory-model=mem-model, -m32, -m64, -mapp-regs, -mno-app-regs, -mfaster-structs, -mno-faster-structs, -mflat, -mno-flat, -mfpu, -mno-fpu, -mhard-float, -msoft-float, -mhard-quad-float, -msoft-quad-float, -mstack-bias, -mno-stack-bias, -mstd-struct-return, -mno-std-struct-return, -munaligned-doubles, -mno-unaligned-doubles, -muser-mode, -mno-user-mode, -mv8plus, -mno-v8plus, -mvis, -mno-vis, -mvis2, -mno-vis2, -mvis3, -mno-vis3, -mvis3b, -mno-vis3b, -mvis4, -mno-vis4, -mvis4b, -mno-vis4b, -mcbcond, -mno-cbcond, -mfmaf, -mno-fmaf, -mfsmuld, -mno-fsmuld, -mpopc, -mno-popc, -msubxc, -mno-subxc, -mfix-at697f, -mfix-ut699, -mfix-ut700, -mfix-gr712rc.
System V Options (System V Options) — -Qy, -Qn, -YP,paths, -Ym,dir.
V850 Options (V850 Options) — -mlong-calls, -mno-long-calls, -mep, -mno-ep, -mprolog-function, -mno-prolog-function, -mspace, -mtda=n, -msda=n, -mzda=n, -mapp-regs, -mno-app-regs, -mdisable-callt, -mno-disable-callt, -mv850e2v3, -mv850e2, -mv850e1, -mv850es, -mv850e, -mv850, -mv850e3v5, -mloop, -mrelax, -mlong-jumps, -msoft-float, -mhard-float, -mgcc-abi, -mrh850-abi, -mbig-switch.
VAX Options (VAX Options) — -munix, -mgnu, -md, -md-float, -mg, -mg-float, -mlra.
Visium Options (Visium Options) — -mdebug, -msim, -mfpu, -mno-fpu, -mhard-float, -msoft-float, -mcpu=cpu-type, -mtune=cpu-type, -msv-mode, -muser-mode.
VMS Options (VMS Options) — -mvms-return-codes, -mdebug-main=prefix, -mmalloc64, -mpointer-size=size.
VxWorks Options (VxWorks Options) — -mrtp, -msmp, -non-static, -Bstatic, -Bdynamic, -Xbind-lazy, -Xbind-now.
x86 Options (x86 Options) — okay this one is a whole entire meal, bestie 🍽️👑 grab a snack. Here's the first batch: -mtune=cpu-type, -march=cpu-type, -mtune-ctrl=feature-list, -mdump-tune-features, -mno-default, -mfpmath=unit, -masm=dialect, -mno-fancy-math-387, -mno-fp-ret-in-387, -m80387, -mhard-float, -msoft-float, -mno-wide-multiply, -mrtd, -malign-double, -mpreferred-stack-boundary=num, -mincoming-stack-boundary=num, -mcld, -mcx16, -msahf, -mmovbe, -mcrc32, -mmwait, -mrecip, -mrecip=opt, -mvzeroupper, -mprefer-avx128, -mprefer-vector-width=opt, -mpartial-vector-fp-math, -mmove-max=bits, -mstore-max=bits, -mnoreturn-no-callee-saved-registers, -mmmx, -msse, -msse2, -msse3, -mssse3, -msse4.1, -msse4.2, -msse4, -mavx, -mavx2, -mavx512f, -mavx512cd, -mavx512vl, -mavx512bw, -mavx512dq, -mavx512ifma, -mavx512vbmi, -msha, -maes, -mpclmul, -mfsgsbase, -mrdrnd, -mf16c, -mfma, -mpconfig, -mwbnoinvd, -mptwrite, -mclflushopt, -mclwb, -mxsavec, -mxsaves, -msse4a, -m3dnow, -m3dnowa, -mpopcnt, -mabm, -mbmi, -mtbm, -mfma4, -mxop, -madx, -mlzcnt, -mbmi2, -mfxsr, -mxsave, -mxsaveopt, -mrtm, -mhle, -mlwp, -mmwaitx, -mclzero, -mpku, -mthreads, -mgfni, -mvaes, -mwaitpkg, -mshstk, -mmanual-endbr, -mcet-switch, -mforce-indirect-call, -mavx512vbmi2, -mavx512bf16, -menqcmd, -mvpclmulqdq, -mavx512bitalg, -mmovdiri, -mmovdir64b, -mavx512vpopcntdq, -mavx512vnni, -mprfchw, -mrdpid, -mrdseed, -msgx, -mavx512vp2intersect, -mserialize, -mtsxldtrk, -mamx-tile, -mamx-int8, -mamx-bf16, -muintr, -mhreset, -mavxvnni, -mamx-fp8, -mavx512fp16, -mavxifma, -mavxvnniint8, -mavxneconvert, -mcmpccxadd, -mamx-fp16, -mprefetchi, -mraoint, -mamx-complex, -mavxvnniint16, -msm3, -msha512, -msm4, -mapxf, -musermsr, -mavx10.1, -mavx10.1-256, -mavx10.1-512, -mevex512, -mavx10.2, -mamx-avx512, -mamx-tf32, -mamx-transpose, -mmovrs, -mamx-movrs, -mcldemote.
Second batch, still x86, still going 🏃♀️💨 — -mms-bitfields, -mno-align-stringops, -minline-all-stringops, -minline-stringops-dynamically, -mstringop-strategy=alg, -mkl, -mwidekl, -mmemcpy-strategy=strategy, -mmemset-strategy=strategy, -mpush-args, -maccumulate-outgoing-args, -m128bit-long-double, -m96bit-long-double, -mlong-double-64, -mlong-double-80, -mlong-double-128, -mregparm=num, -msseregparm, -mveclibabi=type, -mvect8-ret-in-mem, -mpc32, -mpc64, -mpc80, -mdaz-ftz, -mstackrealign, -momit-leaf-frame-pointer, -mno-red-zone, -mno-tls-direct-seg-refs, -mcmodel=code-model, -mabi=name, -maddress-mode=mode, -m32, -m64, -mx32, -m16, -miamcu, -mlarge-data-threshold=num, -msse2avx, -mfentry, -mrecord-mcount, -mnop-mcount, -m8bit-idiv, -minstrument-return=type, -mfentry-name=name, -mfentry-section=name, -mavx256-split-unaligned-load, -mavx256-split-unaligned-store, -malign-data=type, -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol, -mgeneral-regs-only, -mcall-ms2sysv-xlogues, -mrelax-cmpxchg-loop, -mindirect-branch=choice, -mfunction-return=choice, -mindirect-branch-register, -mharden-sls=choice, -mindirect-branch-cs-prefix, -mneeded, -mno-direct-extern-access, -munroll-only-small-loops, -mlam=choice.
x86 Windows Options — no flags dropped here, it just redirects you: see Cygwin and MinGW Options. 🫶
Xstormy16 Options (Xstormy16 Options) — -msim.
Xtensa Options (Xtensa Options) — -mconst16, -mno-const16, -mfused-madd, -mno-fused-madd, -mforce-no-pic, -mserialize-volatile, -mno-serialize-volatile, -mtext-section-literals, -mno-text-section-literals, -mauto-litpools, -mno-auto-litpools, -mtarget-align, -mno-target-align, -mlongcalls, -mno-longcalls, -mabi=abi-type, -mextra-l32r-costs=cycles, -mstrict-align, -mno-strict-align.
zSeries Options — also just a redirect, bestie: see S/390 and zSeries Options. 🌀
✨ Options Controlling the Kind of Output ✨
Okay so compile-time is giving four-act play 🎭 — you've got preprocessing, compilation proper, assembly, and linking, and they always go down in that exact order. No shuffling the lineup.
GCC can preprocess and compile a bunch of files either into several assembler input files, or all into one assembler input file. Then each assembler input file makes an object file, and linking pulls all the object files together — the freshly compiled ones and the ones you handed in as input — into one executable file. Slay, that's the whole pipeline 💖
For any given input file, the file name suffix is what decides which kind of compilation happens. Iconic little lookup table, let's go:
file.c— C source code, and it must be preprocessed.file.i— C source code that should not be preprocessed.file.ii— C++ source code that should not be preprocessed.file.m— Objective-C source code. Heads up though: you have to link with thelibobjclibrary for an Objective-C program to actually work.file.mi— Objective-C source code that should not be preprocessed.file.mm,file.M— Objective-C++ source code. And yeah, same deal: you must link with thelibobjclibrary to make an Objective-C++ program work. Also note that.Mmeans a literal capitalM, no lowercase shenanigans.file.mii— Objective-C++ source code that should not be preprocessed.file.h— a C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or a C, C++ header file to be turned into an Ada spec (via the-fdump-ada-specswitch).file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source code that must be preprocessed. Tiny detail that matters: in.cxxthe last two letters have to both be literallyx. Same energy for.C— that's a literal capitalC.file.mm,file.M— Objective-C++ source code that must be preprocessed.file.mii— Objective-C++ source code that should not be preprocessed. (yes, she's back 💀 the page really does list her twice)file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— a C++ header file to be turned into a precompiled header or an Ada spec.file.f,file.for,file.ftn,file.fi— fixed form Fortran source code that should not be preprocessed.file.F,file.FOR,file.fpp,file.FPP,file.FTN— fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).file.f90,file.f95,file.f03,file.f08,file.fii— free form Fortran source code that should not be preprocessed.file.F90,file.F95,file.F03,file.F08— free form Fortran source code that must be preprocessed (with the traditional preprocessor).file.cob,file.COB,file.cbl,file.CBL— COBOL source code.file.go— Go source code.file.d— D source code.file.di— a D interface file.file.dd— D documentation code (Ddoc).file.ads— an Ada source code file holding a library unit declaration (that's a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These files also get called specs, iykyk.file.adb— an Ada source code file containing a library unit body (a subprogram or package body). These ones are also called bodies.file.s— assembler code.file.S,file.sx— assembler code that must be preprocessed.- other — literally anything else is an object file to be fed straight into linking. Any file name with no recognized suffix gets treated this way, no questions asked. 🫡
Fr fr, the suffix is the whole vibe check here — get it wrong and GCC will treat your precious source as a random object file and send it straight to the linker. 🙈
okay so you can also just tell gcc what language your files are in, instead of making it guess from the file name like some kind of detective 🕵️♀️
-xlanguage — bestie, this says explicitly what language the following input files are in (instead of letting the compiler pick a default from the file name suffix). It applies to all the following input files until the next-xshows up. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-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
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
goheads up 💜-xdoes not imply a particular language standard. Like,-x f77may also need-std=legacyfor some older source codes. it's giving "yes i know the language, no i don't know the rules".
-x none— turns off any language specification, so the files after it get handled by their file name suffixes again (as if-xhad never been used at all). a total reset slay ✨
so 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 -c, -S, or -E to say where gcc is to stop. just know that some combos (like -x cpp-output -E) literally instruct gcc to do nothing at all 😭 like okay go off do absolutely nothing i guess.
-c— compile or assemble the source files, but do not link. the linking stage simply is not done, periodt. the ultimate output is 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 that don't need compilation or assembly are just ignored. not their business, they're not invited 🎀
-S— stop after the stage of compilation proper; do not assemble. the output is an assembler code file for each non-assembler input file you 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; don't run the compiler proper. output is preprocessed source code, which gets sent to standard output. she's a pipeline girlie 💅
input files that don't require preprocessing are ignored.
-ofile — put the primary output in file. this applies to whatever sort of output is being produced, whether it's an executable file, an object file, an assembler file or preprocessed C code. one flag, all the vibes.
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.
so like, -o names only the primary output, but bestie it also affects the naming of auxiliary and dump outputs 😳 see the examples below. unless overridden, both auxiliary outputs and dump outputs go in the same directory as the primary output. in auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. in compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. if both share the same base name (suffix disregarded), the result of the combination is that base name, otherwise they're concatenated, separated by a dash. iconic little rule, honestly 👑
gcc -c foo.c ...will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g. aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
if a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
a linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.
the one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
the location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj. okay now we're getting into the naming lore, buckle up 🦋
🧚 the dump-naming extended universe (still OPTIONS)
-dumpbasedumpbase — sets the base name for auxiliary and dump output files. it does not affect the name of the primary output file. intermediate outputs, when preserved, are not regarded as primary outputs but as auxiliary outputs:
gcc -save-temps -S foo.csaves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.cpreprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.
absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. -save-temps, -gsplit-dwarf, -fcallgraph-info) without it. iykyk 💀
similar suffix differentiation of dump and aux outputs can be attained for an explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.
if dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir. she said "i'm the main character now" 💅
when -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
an empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values:
gcc -c foo.c -o dir/foobar.o -dumpbase " ...will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. notice how their basenames are taken from the input name, but the directory still defaults to that of the output 🎀
the empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...the compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.
-dumpbase-extauxdropsuf — when forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. if not specified, this option defaults to the suffix of a default dumpbase, i.e. the suffix of the input file when-dumpbaseis not present in the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. dump outputs retain the suffix: dir/x-foo.c.*.
this option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdirdumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. dump outputs also take the input name suffix: pfx-foo.c.*.
if dumppfx is to be used as a directory name, it must end with a directory separator (yes, this matters, don't come for me 🙈):
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. dump outputs also take the input name suffix: dir/bar.c.*.
it defaults to the location of the output file, unless the output file is a special file like /dev/null. options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. in case multiple such options are given, the last one prevails (last one wins, always, that's the rule 🌈):
gcc -dumpdir pfx- -c foo.c -save-temps=obj ......it's literally throwing foo.o out, with the auxiliary outputs named foo.* — because -save-temps=* overrides the dumppfx that the earlier -dumpdir option set up. And honestly it does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. The dump outputs are named foo.c.*. 🌸
When you're compiling from multiple input files, if -dumpbase is specified, then dumpbase (minus an auxdropsuf suffix) plus a dash get appended to — or override, if it contains any directory components — an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs. 💅
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...That outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. The dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast that with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...which, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.
When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes that same dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name — minus auxdropsuf if specified, or the executable suffix otherwise — plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option. 🧚
When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd. ✨
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overriden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase — as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such — is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them. 😭
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers. 🫶
-v— Prints (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program and of the preprocessor and the compiler proper. It's giving "show me everything you're doing" and we love that for us. 👑-###— Like-vexcept the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or./-_. This is useful for shell scripts to capture the driver-generated command lines. ngl this one's the sneaky icon.--help— Prints (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified then--helpis also passed on to the various processes invoked bygcc, so that they can display the command-line options they accept. And if the-Wextraoption has also been specified (prior to the--helpoption), then command-line options that have no documentation associated with them are also displayed. ✨--target-help— Prints (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.--help={class|[^]qualifier}[,...]— Prints (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. 💖
These are the supported classes:
optimizers— Display all of the optimization options supported by the compiler.warnings— Display all of the options controlling warning messages produced by the compiler.target— Display target-specific options. Unlike the--target-helpoption however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended--help=syntax.params— Display the values recognized by the--paramoption.language— Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to selectcommonclass.common— Display the options that are common to all languages.
These are the supported qualifiers:
undocumented— Display only those options that are undocumented.joined— Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target.separate— Display options taking an argument that appears as a separate word following the original option, such as:-o output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers. 🚫
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. 💅
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizersAlternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled--version— Display the version number and copyrights of the invoked GCC. the way I check this every single time before filing a bug 😭-pass-exit-codes— Normally thegccprogram exits with the code of1if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, thegccprogram instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if an internal compiler error is encountered. 💀-pipe— Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. lowkey free speed, highkey situational 🦋-specs=file— Process file after the compiler reads in the standard specs file, in order to override the defaults which thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. More than one-specs=filecan be specified on the command line, and they are processed in order, from left to right. 🎀-wrapper— Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... not me debugging cc1 at 3am 💀🙈
-ffile-prefix-map=old=new— okay so when you're compiling files that live in directory old, this records any references to them in the compile output as if they lived in directory new instead. Specifying this one option is literally equivalent to specifying all the individual-f*-prefix-mapoptions 🫶. Use it for reproducible builds that are location independent. Heads up: directories referenced by directives are not affected by these options. See also-fmacro-prefix-map,-fdebug-prefix-map,-fprofile-prefix-mapand-fcanon-prefix-map.-fcanon-prefix-map— so for the-f*-prefix-mapoptions, the comparison of old prefix against the filename that would normally be referenced in the compile output is normally done with a plain textual comparison of the prefixes… or, on case insensitive filesystems, ignoring character case, and on DOS based filesystems treating slashes and backslashes as equal 💅. This option makes those comparisons happen on canonicalized paths of old and the referenced filename instead. Fancy.-fplugin=name.so— loads the plugin code in filename.so, assumed to be a shared object that getsdlopen'd by the compiler 🧚. The base name of the shared object file is what identifies the plugin for argument parsing (peep-fplugin-arg-name-key=valuebelow). Every plugin should define the callback functions specified in the Plugins API. no cap.-fplugin-arg-name-key=value— defines an argument called key with a value of value for the plugin called name. that's it, that's the whole vibe 🎀-fdump-ada-spec[-slim]— for C and C++ source and include files, generates corresponding Ada specs ✨.-fada-spec-parent=unit— works together with-fdump-ada-spec[-slim]up there 👆, generating Ada specs as child units of parent unit.-fdump-go-spec=file— for input files in any language, generates corresponding Go declarations in file. It generates Go"const","type","var", and"func"declarations, which honestly may be a super useful way to start writing a Go interface to code written in some other language 🌈.@file— reads command-line options from file. The options read get inserted in place of the original@fileoption. And if file doesn't exist, or can't be read? Then the option is treated literally, and not removed 🙈.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options are processed recursively 🌀.
💖 Compiling C++ Programs 💖
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm. so many suffixes bestie, but we memorized them all 💅
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 — and this is the part everyone gets wrong 😭 — the use of gcc does not add the C++ library.
g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++ 👑
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. pick your fighter 🥊
🌸 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. this is the whole personality section fr fr ✨
-ansi— in C mode, this is equivalent to-std=c90. In C++ mode, it is equivalent to-std=c++98. simple as that 💫-std=— determines the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:"" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard 😳. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. iconic behaviour honestly 👑
A value for this option must be provided; possible values are:
c90c89iso9899:1990— support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code.iso9899:199409— ISO C90 as modified in amendment 1.c99c9xiso9899:1999iso9899:199x— ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). Seehttps://gcc.gnu.org/c99status.htmlfor more information. The namesc9xandiso9899:199xare deprecated 💀c11c1xiso9899:2011— ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated.c17c18iso9899:2017iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with-std=c11) and a new value of"__STDC_VERSION__", and so is supported to the same extent as C11 🫶c23c2xiso9899:2024— ISO C23, the 2023 revision of the ISO C standard (published in 2024). The namec2xis deprecated.c2y— the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete 🙈gnu90gnu89— GNU dialect of ISO C90 (including some C99 features).gnu99gnu9x— GNU dialect of ISO C99. The namegnu9xis deprecated.gnu11gnu1x— GNU dialect of ISO C11. The namegnu1xis deprecated.gnu17gnu18— GNU dialect of ISO C17.gnu23gnu2x— GNU dialect of ISO C23. This is the default for C code. 👑 The namegnu2xis deprecated.gnu2y— the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The namegnu2xis deprecated.c++98c++03— the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code.gnu++98gnu++03— GNU dialect of-std=c++98.c++11c++0x— the 2011 ISO C++ standard plus amendments. The namec++0xis deprecated.gnu++11gnu++0x— GNU dialect of-std=c++11. The namegnu++0xis deprecated.c++14c++1y— the 2014 ISO C++ standard plus amendments. The namec++1yis deprecated.gnu++14gnu++1y— GNU dialect of-std=c++14. The namegnu++1yis deprecated.c++17c++1z— the 2017 ISO C++ standard plus amendments. The namec++1zis deprecated.gnu++17gnu++1z— GNU dialect of-std=c++17. This is the default for C++ code. 👑 The namegnu++1zis deprecated.c++20c++2a— the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The namec++2ais deprecated.gnu++20gnu++2a— GNU dialect of-std=c++20. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2ais deprecated.c++23c++2b— the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The namec++2bis deprecated.gnu++23gnu++2b— GNU dialect of-std=c++23. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2bis deprecated.c++2cc++26— the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases 💀gnu++2cgnu++26— GNU dialect of-std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-aux-info filename— outputs 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 🤫-fno-asm— okay so this one tells GCC not to recognizeasm,inlineortypeofas keywords, so your code can literally use those words as identifiers ✨. Instead you can use the keywords__asm__,__inline__and__typeof__. In C,-ansiimplies-fno-asm.
In C++, inline is a standard keyword and is not affected by this switch. You probably want -fno-gnu-keywords instead — that disables typeof but not asm and inline. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the asm keyword, since typeof is a standard keyword in ISO C23. 💅
-fno-builtin,-fno-builtin-function— this is the "don't recognize built-in functions that do not begin with__builtin_as prefix" one 🫶.
GCC normally generates special code to handle certain built-in functions more efficiently — for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library 😭. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory 🔥.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fcond-mismatch— allow conditional expressions with mismatched types in the second and third arguments 😳. The value of such an expression isvoid. Heads up bestie: this option is not supported for C++.-ffreestanding— assert that compilation targets a freestanding environment. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be atmain🧚. The most obvious example is an OS kernel. This is equivalent to-fno-hosted.-fgimple— enables parsing of function definitions marked with__GIMPLE⭐. This is an experimental feature that allows unit testing of GIMPLE passes.-fgnu-tm— when the option-fgnu-tmis specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 👑. This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature 🙈.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). 💀
-fgnu89-inline— tells GCC to use the traditional GNU semantics forinlinefunctions when in C99 mode 🌸.
Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode 🚫.
The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions 💖.
-fhosted— assert that compilation targets a hosted environment. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in whichmainhas a return type ofint✨. Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-flax-vector-conversions— allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types 😬. Bestie... this option should not be used for new code. Consider this your vibe check warning 🚩.-fms-extensions— accept some non-standard constructs used in Microsoft header files 🪟.
In C++ code, this allows member names in structures to be similar to previous types declarations:
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi. 📝
-fpermitted-flt-eval-methods=style— okay so ISO/IEC TS 18661-3 defines new permissible values forFLT_EVAL_METHODthat indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type 🦋. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values ofFLT_EVAL_METHOD. As such, code conforming to C11 may not have been written expecting the possibility of the new values 👀.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of FLT_EVAL_METHOD specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either c11 or ts-18661-3 as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3 💜.
The -fdeps-* options are used to extract structured dependency information for a source 🎀. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well. Genuinely so useful, no cap.
-fdeps-file=file— where to write structured dependency information ✍️.-fdeps-format=format— the format to use for structured dependency information.p1689r5is the only supported format right now 🙈. Note that when this argument is specified, the output of-MFis stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools 👑-fdeps-target=file— analogous to-MTbut for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources 💅.-fplan9-extensions— accept some non-standard constructs used in Plan 9 code 🌈.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++ 💀.
-fsigned-bitfields,-funsigned-bitfields,-fno-signed-bitfields,-fno-unsigned-bitfields— these options control whether a bit-field is signed or unsigned, when the declaration does not use eithersignedorunsigned🎀. By default, such a bit-field is signed, because this is consistent: the basic integer types such asintare signed types. Slay, consistency.-fsigned-char— let the typecharbe signed, likesigned 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 🫶.
-funsigned-char— let the typecharbe unsigned, likeunsigned 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. She's her own person, okay?? 👑
-fstrict-flex-arrays(C and C++ only),-fstrict-flex-arrays=level(C and C++ only) — control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array 🦋. The value of level controls the level of strictness.
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards 🔥.
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members 🌈.
There are two more levels hanging out between 0 and 3, and they exist purely to be nice to older code that uses the GCC zero-length array extension ([0]) or the one-element array as a flexible array member ([1]) 🫶. When level is 1, the trailing array counts as a flexible array member when it's declared as either [], [0], or [1]. When level is 2, the trailing array counts as a flexible array member when it's declared as either [], or [0].
You can vibe-check this behavior for one specific trailing array field of a structure by using the variable attribute strict_flex_array attribute. iykyk ✨
The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're literally in conversation, bestie 💬
-fsso-struct=endianness— sets the default scalar storage order of structures and unions to whatever endianness you name. The accepted values arebig-endian,little-endianandnativefor the native endianness of the target (that last one is the default). Heads up: this option is not supported for C++ 🙅♀️
⚠️ Warning, and I mean it: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it, if the specified endianness is not the native endianness of the target. So like… know what you're doing 💀
💖 Options Controlling C++ Dialect 💖
This section is all about the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.CIn this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC. She's versatile like that 👑
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here's the list of options that are only for compiling C++ programs — buckle up, it's a long one 🎢:
-fabi-version=n— use version n of the C++ ABI. The default is version0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (Yeah, it's giving "living document" energy 😅)
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. (Oopsie 🙈)
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. Redemption arc ✨
Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.
Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags.
Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.
See also -Wabi. 👀
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. Teamwork, honestly 🤝
-fno-access-control— turn off ALL access checking. This switch is mainly useful for working around bugs in the access control code. It's giving "just turn off the security system, what could go wrong" 🚨😭-faligned-new— enable support for C++17newof types that require more alignment thanvoid* ::operator new(std::size_t)provides. A numeric argument such as-faligned-new=32can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default ofalignof(std::max_align_t).
This flag is enabled by default for -std=c++17. Slay, standards compliance 💅
-fno-assume-sane-operators-new— okay this one's a whole essay, stay with me 📖. The C++ standard allows replacing the globalnew,new[],deleteanddelete[]operators, though a lot of C++ programs don't replace them and just use the implementation provided version. Furthermore, the C++ standard allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed if__builtin_operator_newor__builtin_operator_deletefunctions are used). This option allows control over some optimizations around calls to those operators. With-fassume-sane-operators-new-deleteoption GCC may assume that calls to the replaceable global operators from new or delete expressions or from__builtin_operator_newor__builtin_operator_deletecalls don't read or modify any global variables or variables whose address could escape to the operators (global state; except forerrnofor thenewandnew[]operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unlessmallocimplementation details are observable in the code or unlessmallochooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default. With-fno-assume-sane-operators-new-deleteoption GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and code needs to expect such behavior. Phew. No cap, that was a lot 🫠-fchar8_t-fno-char8_t— enable support forchar8_tas adopted for C++20. This includes the addition of a newchar8_tfundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new__cpp_char8_tand__cpp_lib_char8_tfeature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified. Brace yourself 😬
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requestedIt's giving "your perfectly good C++11 code suddenly has opinions" 😭💀
-fcheck-new— this one's the vibe check for pointers coming back fromoperator new💅 it makes the compiler confirm the pointer is non-null before it starts poking at the storage it allocated. Ngl, this check is normally unnecessary, because the C++ standard saysoperator newonly returns 0 if it's declaredthrow(), and in that case the compiler always checks the return value even without this option. In every other case, whenoperator newhas a non-empty exception specification, memory exhaustion gets signalled by throwingstd::bad_allocinstead. See alsonew (nothrow)✨-fconcepts— turns on support for the C++ Concepts feature, aka constraining your template arguments so they stop accepting literally anything 🎀 with-std=c++20and above, Concepts are part of the language standard, so-fconceptsdefaults to on. Periodt.
Some constructs that the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), allowed but that didn't make it into the standard could additionally be enabled by -fconcepts-ts. Heads up though: -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15, so users are expected to convert their code to C++20 concepts. It's giving "the old thing is gone, bestie." 💀
- *
-fconstexpr-depth=n* — sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need a limit here so the compiler can actually detect endless recursion while it's evaluating constant expressions 😭 the minimum specified by the standard is 512. - *
-fconstexpr-cache-depth=n* — sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic, okay, it's out here trading off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows absolutely massive from highly recursive evaluations) 🦋 the default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. Main character experimentation energy ✨ -fconstexpr-fp-except— Annex F of the C standard says IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, and instead treat floating point division by zero as non-constant even though it has a well defined value 🙈 this flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except- *
-fconstexpr-loop-limit=n* — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. Again, a limit is needed to detect infinite loops during constant expression evaluation 💀 the default is 262144 (1<<18). - *
-fconstexpr-ops-limit=n* — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long 😭 the default is 33554432 (1<<25). -fcontracts— enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820) 🎀 the implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard. It's giving "work in progress, don't ship your whole codebase on it."
On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining
void
handle_contract_violation (const std::experimental::contract_violation&);There are different sets of additional flags that can be used together to specify which contracts will be checked and how, for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together 🚫
-fcontract-mode=[on|off]— control whether any contracts have any semantics at all. Defaults toon.-fcontract-assumption-mode=[on|off]— [N4820] control whether contracts with level axiom should have the assume semantic. Defaults toon.-fcontract-build-level=[off|default|audit]— [N4820] specify which level of contracts to generate checks for. Defaults todefault.-fcontract-continuation-mode=[on|off]— [N4820] control whether to allow the program to continue executing after a contract violation. That is, do checked contracts have the maybe semantic described below rather than the never semantic. Defaults tooff.- *
-fcontract-role=<name>:<default>,<audit>,<axiom>* — [P1332] specify the concrete semantics for each contract level of a particular contract role. - *
-fcontract-semantic=[default|audit|axiom]:<semantic>* — [P1429] specify the concrete semantic for a particular contract level. -fcontract-strict-declarations=[on|off]— control whether to reject adding contracts to a function after its first declaration. Defaults tooff.
The possible concrete semantics that can be specified with -fcontract-role or -fcontract-semantic are:
"ignore"— this contract has no effect. Ghosted. 👻"assume"— this contract is treated like C++23[[assume]]."check_never_continue","never","abort"— this contract is checked. If it fails, the violation handler is called. If the handler returns,std::terminateis called 💀"check_maybe_continue","maybe"— this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally. Slay, we live to run another line 🌸
-fcoroutines— enables support for the C++ coroutines extension (experimental) ✨ yes bestie, it's experimental, act accordingly.-fdiagnostics-all-candidates— permits the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected 🔥 so when your overload resolution flops, you get the full receipts on everyone who was in the running.-fno-elide-constructors— the C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases 😭 this option also causes G++ to call trivial member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. So it's not fully back, it's just doing side quests now.
-fno-enforce-eh-specs— don't generate code to check for violation of exception specifications at run time. Okay so real talk: this option violates the C++ standard, but it may be useful for reducing code size in production builds, much like definingNDEBUG🙈 this does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. Not a cute look, be warned 💀-fextern-tls-init,-fno-extern-tls-init— the C++11 and OpenMP standards allowthread_localandthreadprivatevariables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization 🦋 when the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there's significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the-fno-extern-tls-initoption.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init. Target-dependent defaults, iykyk ✨
-ffold-simple-inlines,-fno-fold-simple-inlines— permit the C++ frontend to fold calls tostd::move,std::forward,std::addressofandstd::as_const🎀 in contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless-fno-inlineis active. Lowkey a free win.-fno-gnu-keywords— do not recognizetypeofas a keyword, so that code can use this word as an identifier 🦋 you can use the keyword__typeof__instead. This option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc.-fno-immediate-escalation— do not enable immediate function escalation whereby certain functions can be promoted toconsteval, as specified in P2564R3 👑 for example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}compiles in C++20: f is an immediate-escalating function (due to the auto it is a function template and is declared constexpr) and id(t) is an immediate-escalating expression, so f is promoted to consteval 💅 consequently, the call to id(t) is in an immediate context, so doesn't have to produce a constant (that is the mechanism allowing consteval function composition). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) is not a constant expression, the compiler rejects the code 😭
This option is turned on by default; it is only effective in C++20 mode or later.
-fimplicit-constexpr— make inline functions implicitlyconstexpr, if they satisfy the requirements for a constexpr function ✨ this option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations. Free upgrade energy, honestly 🫶-fno-implicit-templates— okay so bestie, this one says: never emit code for non-inline templates that are instantiated implicitly (like, just 'cause someone used them). Only emit code for explicit instantiations, periodt. And yeah, if you turn this on you gotta actually structure your code yourself and include all the necessary explicit instantiations, otherwise you'll be getting undefined symbols at link time and crying. 💅-fno-implicit-inline-templates— same vibe but for inline templates: don't emit code for implicit instantiations of those either. The default handles inlines differently so that compiling with and without optimization needs the same set of explicit instantiations. slay, consistency ✨-fno-implement-inlines— to save space, don't emit out-of-line copies of inline functions that are controlled by#pragma implementation. Lowkey important: this causes linker errors if those functions aren't inlined everywhere they're called. 💀-fmodules,-fno-modules— turn on support for C++20 modules 🦋.-fno-modulesis usually not even needed, 'cause that's the default already. And ngl, even though this is a C++20 feature, it is not currently implicitly enabled just by picking that standard version, so you gotta ask for it.-fmodule-header,-fmodule-header=user,-fmodule-header=system— compile a header file to create an importable header unit 🎀-fmodule-implicit-inline— member functions defined inside their class definitions are not implicitly inline for modular code. That's different from traditional C++ behavior, and for good reasons. But it may cause difficulty during code porting 😭. This option makes such function definitions implicitly inline again. Warning though: it generates an ABI incompatibility, so you must use it everywhere or nowhere, fr fr. (Definitions like that outside of a named module stay implicitly inline regardless.)-fno-module-lazy— disable lazy module importing and module mapper creation. bye lazy era 🦋-fmodule-mapper=...— an oracle to query for module name → filename mappings. The forms are:
-fmodule-mapper=[hostname]:port[?ident]
-fmodule-mapper=|program[?ident] args...
-fmodule-mapper==socket[?ident]
-fmodule-mapper=<>[inout][?ident]
-fmodule-mapper=<in>out[?ident]
-fmodule-mapper=file[?ident]If you leave it unspecified, the CXX_MODULE_MAPPER environment variable gets used, and if that's unset too, an in-process default is provided for you. 🫶
-fmodule-only— only emit the Compiled Module Interface, inhibiting any object file. just the CMI, nothing else bestie ✨-fms-extensions— disableWpedanticwarnings about constructs used in MFC, like implicitintand getting a pointer to member function via non-standard syntax. letting the old-school stuff slide 🙈-fnew-inheriting-ctors— enable the P0136 adjustment to the semantics of C++11 constructor inheritance. It's part of C++17 but also considered a Defect Report against C++11 and C++14. This flag is enabled by default unless-fabi-version=10or lower is specified. 👑-fnew-ttp-matching— enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this lets a template with default template arguments be used as an argument for a template template parameter with fewer template parameters. Enabled by default for-std=c++17. 🎀-fno-nonansi-builtins— disable built-in declarations of functions that aren't mandated by ANSI/ISO C. The squad includesffs,alloca,_exit,index,bzero,conjf, and other related functions. ✨-fnothrow-opt— treat athrow()exception specification as if it were anoexceptspecification to reduce or eliminate the text size overhead compared to a function with no exception specification. Cute detail: if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call toterminaterather thanunexpected. noted. 💅-fno-operator-names— do not treat the operator name keywordsand,bitand,bitor,compl,not,orandxoras synonyms as keywords. nope, not synonyms anymore 🙈-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. so chill about it 🧚-fno-pretty-templates— okay so normally, when an error message refers to a specialization of a function template, the compiler prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.void f(T) [with T = int]rather thanvoid f(int)) so it's clear which template is involved. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use-fno-pretty-templatesto disable them. your call bestie 😭-frange-for-ext-temps— enable lifetime extension of C++ range basedfortemporaries. With-std=c++23and above this is part of the language standard, so the lifetime of the temporaries is extended until the end of the loop by default. This option lets you enable that behavior in earlier versions of the standard too. 🌸-fno-rtti— disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_castandtypeid). If you don't use those parts of the language, you can save some space with this flag. Note that exception handling uses the same information, but G++ generates it as needed. Thedynamic_castoperator can still be used for casts that don't require run-time type information, i.e. casts tovoid *or to unambiguous base classes. 😌
🚨 Mixing code compiled with-frttiwith code compiled with-fno-rttimay not work. For example, programs may fail to link if a class compiled with-fno-rttiis used as a base for a class compiled with-frtti. don't do it bestie 💀
-fsized-deallocation— enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. And the flag -Wsized-deallocation warns about places that might want to add a definition. 👑
-fstrict-enums— allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). Lowkey this assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. Heads up: this option has no effect for an enumeration type with a fixed underlying type. 💅-fstrong-eval-order,-fstrong-eval-order=kind— evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17.-fstrong-eval-orderis equivalent to-fstrong-eval-order=all, and is enabled by default with-std=c++17or later. ✨-fstrong-eval-order=someenables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.-fstrong-eval-order=noneis equivalent to-fno-strong-eval-order.
-ftemplate-backtrace-limit=n— set the maximum number of template instantiation notes for a single warning or error to n. The default value is10. so your terminal doesn't get absolutely obliterated 🫶-ftemplate-depth=n— set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than17(changed to1024in C++11). The default value is900, as the compiler can run out of stack space before hitting1024in some situations. iconic that it protects you from yourself 😭-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. only if you're sure bestie 💀-fuse-cxa-atexit— register destructors for objects with static storage duration with the__cxa_atexitfunction rather than theatexitfunction. This option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports__cxa_atexit. no cap 🌈-fno-use-cxa-get-exception-ptr— don't use the__cxa_get_exception_ptrruntime routine. This causesstd::uncaught_exceptionto be incorrect, but it's necessary if the runtime routine isn't available. trade-offs, babes 🙈-fvisibility-inlines-hidden— this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. 👑
The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't appear in the export table of a DSO and don't require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. ate. 🔥
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it doesn't affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. 👀
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. slay, boundary safety 🦋
-fvisibility-ms-compat— okay so this one is giving "let me try to make GCC's C++ linkage model match Microsoft Visual Studio's" and it does that by messing with visibility settings. Basically a compatibility era 💅
Here's what it actually changes in GCC's linkage model:
- It sets the default visibility to
"hidden", like-fvisibility=hidden. - Types, but not their members, are not hidden by default.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
For real though, in new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 😭
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. 🫠
-fno-weak— do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available.
Bestie, this option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. So like… don't. 🙈
-fext-numeric-literals(C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
-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.) ✨-flang-info-include-translate,-flang-info-include-translate-not, *-flang-info-include-translate=header* — inform of include translation events. The first will note accepted include translations, the second will note declined include translations.
The header form will inform of include translations relating to that specific header. If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path. 🦋
-flang-info-module-cmi, *-flang-info-module-cmi=module* — inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames.
The module form will not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in "<>" or ""). 🎀
-stdlib=libstdc++,libc++** — when G++ is configured to support this option, it allows specification of alternate C++ runtime libraries.
Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards).
The option switches G++ to use the headers from the specified library and to emit "-lstdc++" or "-lc++" respectively, when a C++ runtime is required for linking. 🔥
In addition, these warning options have meanings only for C++ programs: 👑
-Wabi-tag(C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.-Wcomma-subscript(C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in"( )"is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated.
This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. 💀
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides.
This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD.
This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;-Wctor-dtor-privacy(C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.
Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🫶
-Wdangling-reference(C++ and Objective-C++ only) — warn when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is danglingIn the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling.
This warning also detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is danglingCertain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic popThe "#pragma" can also surround the class; in that case, the warning will be disabled for all the member functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);where "std::minmax" returns "std::pair<const int&, const int&>", and both references dangle after the end of the full expression that contains the call to "std::minmax".
The warning does not warn for "std::span"-like classes. We consider classes of the form:
template<typename T>
struct Span {
T* data_;
std::size len_;
};as "std::span"-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor.
The warning can be disabled by using the "gnu::no_dangling" attribute.
This warning is enabled by -Wextra. 😭
-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warn when"delete"is used to destroy an instance of a class that has virtual functions and non-virtual destructor.
It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. Like, genuinely unsafe, not just vibe-unsafe. 💀
This warning is enabled by -Wall.
-Wdeprecated-copy(C++ and Objective-C++ only) — warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up.
This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.
-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated. 🌸
-Wdeprecated-literal-operator(C++ and Objective-C++ only) — bestie, if you declare a user-defined literal operator with a space before the suffix, GCC is gonna give you the side-eye. This one's on by default in C++23, or whenever you pass explicit-Wdeprecated. the space is lowkey not the vibe 💅
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred-Wdeprecated-variadic-comma-omission(C++ and Objective-C++ only) — snitching on you when you skip the comma before the varargs...at the end of a function parameter list. omitting it is deprecated, bestie. Enabled by default in C++26, or with explicit-Wdeprecated✨
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated-Wno-elaborated-enum-base— for C++11 and above, this warns if an (invalid) additional enum-base shows up in an elaborated-type-specifier 🫶. Like, if an enum with a given underlying type and no enumerator list gets used in a declaration that isn't just a standalone declaration of the enum. Enabled by default, and it gets upgraded to an error with-pedantic-errors. No cap.-Wno-init-list-lifetime(C++ and Objective-C++ only) — turns OFF the warning about uses ofstd::initializer_listthat are probably gonna leave you with dangling pointers 😭. Why does this warning even exist? Because the underlying array for aninitializer_listis handled like a normal C++ temporary object, so it's so easy to accidentally keep a pointer to that array past the end of its lifetime. It's giving "you had one job." For example:- If a function returns a temporary
initializer_list, or a localinitializer_listvariable, the array's lifetime ends at the end of the return statement — so the value you returned has a dangling pointer 💀 - If a new-expression creates an
initializer_list, the array only lives until the end of the enclosing full-expression, so theinitializer_listin the heap has a dangling pointer. - When an
initializer_listvariable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression — so at the very next statement, yourinitializer_listvariable has a dangling pointer. the betrayal 🙈
- If a function returns a temporary
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior- When a list constructor stores the
beginpointer from theinitializer_listargument, that doesn't extend the lifetime of the array — so if a class variable is constructed from a temporaryinitializer_list, the pointer is left dangling by the end of the variable declaration statement. it's giving cursed 🧚
-Winvalid-constexpr— warns when a function never produces a constant expression, fr fr. In C++20 and earlier, for everyconstexprfunction and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's totally possible to have a function or function template markedconstexprfor which no invocation satisfies the requirements of a core constant expression 😬
This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it'll be an ordinary warning. Like this:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}-Winvalid-imported-macros— verifies all imported macro definitions are valid at the end of compilation 👑. Not enabled by default, because it requires additional processing to figure out. It may be useful when you're preparing sets of header-units to ensure consistent macros. niche but iconic ✨-Wno-literal-suffix(C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix that does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens so it can maintain backwards compatibility with code that uses formatting macros from<inttypes.h>. For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}In this case, PRId64 is treated as a separate preprocessing token 🤓
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization. These warnings are enabled by default, iykyk.
-Wno-narrowing(C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard, periodt. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but-Wno-narrowingsuppresses the diagnostic. Note that this does not affect the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts 🫶
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat.
-Wnoexcept(C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()ornoexcept) but is known by the compiler to never throw an exception 🌸. the compiler knows things 👀-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature makingnoexceptpart of a function type changes the mangled name of a symbol relative to C++14. Enabled by-Wabiand-Wc++17-compat. Example time:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>. glow-up or glow-down? honestly both 💅
-Wclass-memaccess(C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function such asmemsetormemcpyis an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers 💀. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call tomemsetbelow is undefined because it modifies a non-trivial class object, so it gets diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning 🫶
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class — in which case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class 😭. This warning is automatically enabled if-Weffc++is specified. The-Wdelete-non-virtual-dtoroption (enabled by-Wall) should be preferred because it warns about the unsafe cases without false positives. upgrade your warning, bestie ✨-Wregister(C++ and Objective-C++ only) — warns on uses of theregisterstorage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using theregisterkeyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17. rip register, you had a good run 🕊️-Wreorder(C++ and Objective-C++ only) — warns when the order of member initializers given in the code doesn't match the order in which they must be executed 👑. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall. order matters, always has 💅
-Wno-pessimizing-move(C++ and Objective-C++ only) — this warning warns when a call tostd::moveprevents copy elision 😬. A typical scenario where copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, the std::move call prevents copy elision. not me sabotaging my own optimization 💀
This warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — this warning warns about redundant calls tostd::move; that is, when a move operation would have been performed even without thestd::movecall 🫶. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:
Here, the std::move call is redundant. And because G++ implements Core Issue 1579, there's another example:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}In this one, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue 😌
This warning is enabled by -Wextra.
-Wrange-loop-construct(C++ and Objective-C++ only) — bestie, this one calls out the moment a C++ range-based for-loop is making an unnecessary copy 🫶 Like, the range declaration isn't a reference but honestly it probably should be. Example time:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. So tiny stuff gets a pass ✨
It also warns when a loop variable in a range-based for-loop is initialized with a value of a different type, resulting in a copy. Like:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, in every single iteration of the loop a temporary value of type double is created and destroyed, and the reference const double & is bound to it 😭 every. iteration.
This warning is enabled by -Wall.
-Wredundant-tags(C++ and Objective-C++ only) — warn about redundant class-key and enum-key in references to class types and enumerated types, in contexts where the key can be eliminated without causing an ambiguity 💅 e.g.:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedOn the other hand, in this example there's no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessarySee how the keyword is doing a job there? Iconic of it honestly 👑
-Wno-subobject-linkage(C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace, or depends on a type with no linkage. Here's the tea: if a type A depends on a type B with no or internal linkage, then defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit 😬 If A only shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. The compiler doesn't give this warning for types defined in the main.Cfile, because those are unlikely to have multiple definitions. And hey —-Wsubobject-linkageis enabled by default 🎀-Weffc++(C++ and Objective-C++ only) — warn about violations of these style guidelines from Scott Meyers' Effective C++ series of books 📚:- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have
operator=return a reference to*this. - Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload
&&,||, or,.
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations 💅 However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. No cap, you'll want it 🙈
-Wno-exceptions(C++ and Objective-C++ only) — disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers 👀 (so it's lowkey useful, but you asked for it off, so off it goes)-Wstrict-null-sentinel(C++ and Objective-C++ only) — warn about the use of an uncastedNULLas sentinel. When compiling only with GCC this is a valid sentinel, asNULLis defined to__null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers — so like, cute for you, not cute for everyone else 😅-Wno-non-template-friend(C++ and Objective-C++ only) — disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such asfriend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function 🌸 the warning exists to diagnose compatibility problems, and it's enabled by default.-Wold-style-cast(C++ and Objective-C++ only) — warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast,static_cast,reinterpret_cast, andconst_cast) are less vulnerable to unintended effects and much easier to search for 🔍 periodt, that's the whole reason they exist.-Woverloaded-virtual(C++ and Objective-C++ only)-Woverloaded-virtual=_n_ (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class 💀 For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int); // does not override
};the A class version of f is hidden in B, and code like:
B* b;
b->f();fails to compile 😭 not the vibe.
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add using A::f; to B ✨ slay.
The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}This pattern is less likely to be a mistake 🫶 if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall.
-Wno-pmf-conversions(C++ and Objective-C++ only) — disable the diagnostic for converting a bound pointer to member function to a plain pointer. That's it, that's the whole thing 💅-Wsign-promo(C++ and Objective-C++ only) — warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size 👀 Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. The standard said what it said.-Wtemplates(C++ and Objective-C++ only) — warn when a primary template declaration is encountered ⭐ Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL 🎀 One may also instantiate or specialize templates.-Wmismatched-new-delete(C++ and Objective-C++ only) — warn for mismatches between calls tooperator neworoperator deleteand the corresponding call to the allocation or deallocation function 🔥 This includes invocations of C++operator deletewith pointers returned from either mismatched forms ofoperator new, or from other functions that allocate objects for which theoperator deleteisn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned fromoperator newfor which the deallocation function isn't suitable. Basically: the new said one thing, the delete said another, and we're calling it out 😤
For example, the delete expression in the function below is diagnosed because it doesn't match the array form of the new expression the pointer argument was returned from. Similarly, the call to free is also diagnosed:
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete 🧚
-Wmismatched-new-deleteis included in-Wall.
-Wmismatched-tags(C++ and Objective-C++ only) — warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided 🦋
For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition:
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);It is not wrong to declare a class with the class-key struct as the example above shows 🙈 The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations 💅 In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys 👑 The option can be used either on its own or in conjunction with -Wredundant-tags.
-Wmultiple-inheritance(C++ and Objective-C++ only) — this one comes for you when a class is defined with multiple direct base classes 👀. Some coding rules literally disallow multiple inheritance, and this warning is how you enforce that rule. Bestie, the warning is inactive inside a system header file, such as the STL, so you can still use the STL, no cap. One may also define classes that indirectly use multiple inheritance. Slay.-Wvirtual-inheritance— warns when a class is defined with a virtual direct base class 🦋. Same vibe as above: some coding rules disallow multiple inheritance and this is the enforcement tool. The warning is inactive inside a system header file, such as the STL, so the STL stays usable. One may also define classes that indirectly use virtual inheritance.-Wno-virtual-move-assign— suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator 🙈. And this is dangerous fr fr, because if the virtual base is reachable along more than one path, it gets moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. Periodt.-Wnamespaces— warns when a namespace definition is opened ✨. Some coding rules disallow namespaces, and this is how you enforce that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL. One may also useusingdirectives and qualified names.-Wno-template-body(C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template 😌. This flag can also be used to downgrade such errors into warnings withWno-error=or-fpermissive.-Wno-template-id-cdtor(C++ and Objective-C++ only) — disables the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237 💀. Like, for example:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat.
-Wtemplate-names-tu-local— warns when a template body hides an exposure of a translation-unit-local entity 🌸. In most cases, referring to a translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a module's CMI is an error. However, within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error is emitted.
This can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it 😭. This warning attempts to detect cases where this might occur. The presence of an explicit instantiation silences the warning.
This flag is enabled by -Wextra.
-Wno-terminate(C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call toterminate💅.-Wno-vexing-parse(C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity 🫠. It warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead 🔥.
This warning is enabled by default.
-Wno-class-conversion(C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void 🙃; such a conversion function will never be called.-Wvolatile(C++ and Objective-C++ only) — warns about deprecated uses of thevolatilequalifier ⚠️. This includes postfix and prefix++and--expressions ofvolatile-qualified types, using simple assignments where the left operand is avolatile-qualified non-class type for their value, compound assignments where the left operand is avolatile-qualified non-class type,volatile-qualified function return type,volatile-qualified parameter type, and structured bindings of avolatile-qualified type. This usage was deprecated in C++20.
Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.
-Waligned-new— warns about a new-expression of a type that requires greater alignment than thealignof(std::max_align_t)but uses an allocation function without an explicit alignment parameter 🎯. This option is enabled by-Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions.
-Wno-placement-new,-Wplacement-new=n— warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object 💀. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large:
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— this is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code 🫶. For example, the followingnewexpression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array:
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();-Wplacement-new=2— at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed 😤. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2:
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value,-Wcatch-value=n(C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference 🎀. With-Wcatch-value=1(or-Wcatch-valuefor short) warn about polymorphic class types that are caught by value. With-Wcatch-value=2warn about all class types that are caught by value. With-Wcatch-value=3warn about all types that are not caught by reference.-Wcatch-valueis enabled by-Wall.-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs 🦋.-Wno-defaulted-function-deleted(C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler 👀. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.-Wno-delete-incomplete(C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime 😬. This warning is enabled by default.-Wextra-semi(C++, Objective-C++ only) — warns about redundant semicolons 🧐. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
void foo () {};
};Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
};
;And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
int a;
;
};-Wno-global-module(C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives 💅.-Wno-inaccessible-base(C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity 🌈. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the-Wextraoption.
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor 😵; the warning is on by default because the ellipsis is not inherited.-Wno-invalid-offsetof(C++ and Objective-C++ only) — suppresses warnings from applying theoffsetofmacro to a non-POD type 🫣. According to the 2014 ISO C++ standard, applyingoffsetofto a non-standard-layout type is undefined. In existing C++ implementations, however,offsetoftypically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard 👑.
-Wsized-deallocation(C++ and Objective-C++ only) — ok so this one warns you when you've defined an unsized deallocation function:
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;...but you didn't define the matching sized one:
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;or the other way around (sized without unsized, same energy). Enabled by -Wextra along with -fsized-deallocation. 💅
-Wsuggest-final-types— this one's a lil glow-up suggestion ngl ✨ It warns about types with virtual methods where your code quality would literally improve if the type were declared with the C++11finalspecifier, or — if possible — declared in an anonymous namespace. Why? Because that lets GCC devirtualize the polymorphic calls way more aggressively. Hot tip: it's way more effective with link-time optimization, where the info about the class hierarchy graph is more complete. iykyk 🦋-Wsuggest-final-methods— same vibe but for virtual methods 🎀 Warns about virtual methods where code quality would improve if the method were declared with the C++11finalspecifier, or if possible its type were declared in an anonymous namespace or with thefinalspecifier. Also more effective with link-time optimization (again: more complete class hierarchy graph info). The recommended flow: first consider the suggestions from-Wsuggest-final-types, then rebuild with the new annotations. Do it in that order bestie, trust 🙏-Wsuggest-override— warns about overriding virtual functions that aren't marked with theoverridekeyword. slay, label your overrides 💜-Wno-conversion-null(C++ and Objective-C++ only) — do not warn for conversions betweenNULLand non-pointer types. Just so you know:-Wconversion-nullis enabled by default, so this is you turning it off. 🙈
💙 Options Controlling Objective-C and Objective-C++ Dialects 💙
NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section is the ✨ only meaningful for Objective-C and Objective-C++ ✨ zone. You can still use most of the language-independent GNU compiler options, bestie — it's not a whole different world. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mIn this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; the other options you can use with any language GCC supports. 🌈
Also keep in mind: since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Same deal flipped — Objective-C++ compilations may use C++-specific options (e.g., -Wabi). Crossover icon behavior 🎀
Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:
- *
-fconstant-string-class=class-name* — useclass-nameas the name of the class to instantiate for each literal string specified with the syntax@"...". The default class name isNXConstantStringif the GNU runtime is being used, andNSConstantStringif the NeXT runtime is being used (see below). On Darwin / macOS platforms, the-fconstant-cfstringsoption, if also present, overrides the-fconstant-string-classsetting and causes@"..."literals to be laid out as constant CoreFoundation strings. Note that-fconstant-cfstringsis an alias for the target-specific-mconstant-cfstringsequivalent. 🍓 -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 / macOS. The macro__NEXT_RUNTIME__is predefined if (and only if) this option is used. 💀-fno-nil-receivers— assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is notnil. That lets the runtime use more efficient entry points. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. 🔥- *
-fobjc-abi-version=n* — use versionnof the Objective-C ABI for the selected runtime. Currently only supported for the NeXT runtime. In that case:- Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
- Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
- Version 2 is the modern (64-bit) ABI.
If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. pick your version wisely bestie 💅
-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_constructinstance method which runs non-trivial default constructors on any such instance variables, in order, and then returnsself. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special- (void) .cxx_destructmethod which runs all such default destructors, in reverse order. 😭 the symmetry
The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It's the Objective-C runtime's job to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance. ⏱️
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the- (id) .cxx_constructand- (void) .cxx_destructmethods.
-fobjc-direct-dispatch— allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. zoom zoom 🦋-fobjc-exceptions— enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. This option is required to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. Available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). 🔥-fobjc-gc— enable garbage collection (GC) in Objective-C and Objective-C++ programs. Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that doesn't require special compiler flags. 🗑️✨-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using-fno-objc-nilcheck. Class methods and super calls are never checked for nil this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 🙈-fobjc-std=objc1— conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards — that's controlled by the separate C/C++ dialect option flags, okurrr. When used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that GCC 4.0 doesn't recognize is rejected. Super useful if you need to make sure your Objective-C code can be compiled with older versions of GCC. 👑-freplace-objc-classes— emit a special marker instructingld(1)not to statically link in the resulting object file, and allowdyld(1)to load it in at run time instead. 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 needing to restart the program itself. iconic tbh. 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 toobjc_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-fzero-linksuppresses this behavior and causes calls toobjc_getClass("...")to be retained. Useful in Zero-Link debugging mode, since it allows individual class implementations to be modified during program execution. The GNU runtime currently always retains calls toobjc_get_class("...")regardless of command-line options. 💅-fno-local-ivars— by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. That can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. 👀 Specifying-fno-local-ivarsdisables this behavior, thus avoiding variable shadowing issues. periodt.-fivar-visibility=[public|protected|private|package]— set the default instance variable visibility to the specified option, so that instance variables declared outside the scope of any access modifier directives default to that visibility. 🌸-gen-decls— dump interface declarations for all classes seen in the source file to a file namedsourcename.decl. 📝-Wassign-intercept(Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector. caught in 4k 💀-Wno-property-assign-default(Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified. 🙈-Wno-protocol(Objective-C and Objective-C++ only) — okay so picture this: a class says it implements a protocol, and by default GCC is gonna warn you about every method in that protocol the class doesn't implement. And bestie, that's strict — even if the implementation comes from the superclass, the default still yells at you if it's not explicitly implemented in the class itself. Turn on-Wno-protocoland the vibe relaxes: methods inherited from the superclass count as implemented, and no warning gets issued for them. 🫶-Wobjc-root-class(Objective-C and Objective-C++ only) — warns you if a class interface has no superclass at all. Which is lowkey suspicious, since most classes inherit fromNSObject(orObject), for example. But if you're intentionally declaring root classes, you can hush the warning by marking their interfaces with__attribute__((objc_root_class)). Slay, intentional design. 💅-Wselector(Objective-C and Objective-C++ only) — warns you if multiple methods with different types for the same selector show up during compilation. The check happens on the list of methods in the final stage of compilation. On top of that, there's a check for each selector in a@selector(...)expression, and whether a corresponding method for that selector has been found during compilation. Now here's the catch, and it's giving "read the fine print": because these checks scan the method table only at the end of compilation, the warnings don't get produced if the final stage isn't reached — like if an error is found during compilation, or if you're using the-fsyntax-onlyoption. 😭-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns you if multiple methods with differing argument and/or return types are found for a given selector when you're trying to send a message with that selector to a receiver of typeidorClass. When this flag is off (and off is the default behavior, fyi), the compiler just omits those warnings if any differences it found are confined to types that share the same size and alignment. So like, same-size differences get a pass. 🙈-Wundeclared-selector(Objective-C and Objective-C++ only) — warns you if a@selector(...)expression refers to an undeclared selector. A selector counts as undeclared if no method with that name has been declared before the@selector(...)expression — either explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. This option is the eager one: it always performs its checks as soon as a@selector(...)expression is found, while-Wselectoronly performs its checks in the final stage of compilation. It also enforces that coding style convention where methods and selectors must be declared before being used. Declare first, ask questions later. ✨-print-objc-runtime-info— generates a C header describing the largest structure that is passed by value, if any. That's it, that's the whole thing. 🎀
🌈 Options Controlling OpenMP and OpenACC 🌈
GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. Parallel brain rot, but make it productive. 🔥
-foffload=disable,-foffload=default,-foffload=target-list— these tell GCC for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to-foffload=default, is to generate code for all supported offload targets. The-foffload=disableform generates code only for the host fallback, while-foffload=target-listgenerates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. Very "iykyk" of you to check. 💜
-foffload-options=options,-foffload-options=target-triplet-list=options— with-foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the-foffload-options=target-list=optionsform. The target-list is a comma-separated list in the same format as for the-foffload=option.
Typical command lines are:
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906-fopenacc— enables handling of OpenACC directives#pragma accin C/C++ and!$accin free-form Fortran and!$acc,c$accand*$accin fixed-form Fortran. Heads up though: this option implies-pthread, and thus is only supported on targets that have support for-pthread. No pthread, no party. 🦋- *
-fopenacc-dim=geom* — specifies default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of:-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a target-specific default value. Pick your faves and skip the rest. ⭐ -fopenmp— enables handling of OpenMP directives#pragma omp,[[omp::directive(...)]],[[omp::decl(...)]], and[[omp::sequence(...)]]in C/C++. In Fortran, it enables!$ompand the conditional compilation sentinel!$. In fixed source form Fortran, the sentinels can also start withcor*.
This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. The implications are implying, bestie. 😭
-fopenmp-simd— enables handling of OpenMP'ssimd,declare simd,declare reduction,assume,ordered,scanandloopdirective, and of combined or composite directives withsimdas constituent with#pragma omp,[[omp::directive(...)]],[[omp::sequence(...)]]and[[omp::decl(...)]]in C/C++ and!$ompin Fortran. It additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. Other OpenMP directives are ignored. And unless-fopenmpis additionally specified, theloopregion binds to the current task region, independent of the specifiedbindclause. Noted, periodt. 📌-fopenmp-target-simd-clone, *-fopenmp-target-simd-clone=device-type* — in addition to generating SIMD clones for functions marked with thedeclare simddirective, GCC also generates clones for functions marked with the OpenMPdeclare targetdirective that are suitable for vectorization when this option is in effect. The device-type may be one ofnone,host,nohost, andany, which correspond to keywords for thedevice_typeclause of thedeclare targetdirective; clones are generated for the intersection of devices specified.-fopenmp-target-simd-cloneis equivalent to-fopenmp-target-simd-clone=anyand-fno-openmp-target-simd-cloneis equivalent to-fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default. Optimization said "nohost, not today." 💀
🎀 Options to Control Diagnostic Messages Formatting 🎀
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). Which, like, rude to your terminal honestly. You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options — so don't come for GCC if one of them ignores you. 🙈
- *
-fmessage-length=n* — tries to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not however affect the pragma GCC warning and pragma GCC error pragmas. Those two are in their own lane. 💅
-fdiagnostics-plain-output— this option requests that diagnostic output look as plain as possible, which may be useful when runningdejagnuor other utilities that need to parse diagnostics output and prefer that it remain more stable over time.-fdiagnostics-plain-outputis currently equivalent to the following options:
-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers
-fdiagnostics-color=never -fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events
-fdiagnostics-text-art-charset=none
-fno-diagnostics-show-event-linksIn the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. So the list is a grower, not a shower. 🌱
-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. Emit once, then chill. 🧘-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. Every line gets the prefix. Consistency icon behavior, honestly. 👑-fdiagnostics-color[=WHEN],-fno-diagnostics-color— use color in diagnostics.WHENisnever,always, orauto. The default depends on how the compiler has been configured — it can be any of the aboveWHENoptions or alsoneverif theGCC_COLORSenvironment variable isn't present in the environment, andautootherwise.automakes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell. And the forms-fdiagnostics-colorand-fno-diagnostics-colorare aliases for-fdiagnostics-color=alwaysand-fdiagnostics-color=never, respectively. Color-coded drama, but make it readable. 🌈
okay bestie, the colors are literally defined by the environment variable GCC_COLORS 💅 Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (Peep the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include:
1— bold4— underline5— blink7— inverse39— default foreground color30to37— foreground colors90to97— 16-color mode foreground colors38;5;0to38;5;255— 88-color and 256-color modes foreground colors49— default background color40to47— background colors100to107— 16-color mode background colors48;5;0to48;5;255— 88-color and 256-color modes background colors
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. The supported capabilities are as follows.
error=— SGR substring forerror:markers.warning=— SGR substring forwarning:markers.note=— SGR substring fornote:markers.path=— SGR substring for colorizing paths of control-flow events as printed via-fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns.range1=— SGR substring for first additional range.range2=— SGR substring for second additional range.locus=— SGR substring for location information,file:lineorfile:line:columnetc.quote=— SGR substring for information printed within quotes.fnname=— SGR substring for names of C++ functions.targs=— SGR substring for C++ function template parameter bindings.fixit-insert=— SGR substring for fix-it hints suggesting text to be inserted or replaced.fixit-delete=— SGR substring for fix-it hints suggesting text to be deleted.diff-filename=— SGR substring for filename headers within generated patches.diff-hunk=— SGR substring for the starts of hunks within generated patches.diff-delete=— SGR substring for deleted lines within generated patches.diff-insert=— SGR substring for inserted lines within generated patches.type-diff=— SGR substring for highlighting mismatching types within template arguments in the C++ frontend.valid=— SGR substring for highlighting valid elements within text art diagrams.invalid=— SGR substring for highlighting invalid elements within text art diagrams.highlight-a=,highlight-b=— SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See-fdiagnostics-show-highlight-colors.-fdiagnostics-urls[=WHEN]— use escape sequences to embed URLs in diagnostics 🌈 For example, when-fdiagnostics-show-optionemits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option.WHENisnever,always, orauto.automakes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.- The default depends on how the compiler has been configured. It can be any of the above
WHENoptions. - GCC can also be configured (via the
--with-diagnostics-urls=auto-if-envconfigure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to usingautoif eitherGCC_URLSorTERM_URLSenvironment variables are present and non-empty in the environment of the compiler, orneverif neither are. - However, even with
-fdiagnostics-urls=alwaysthe behavior is dependent on those environment variables: IfGCC_URLSis set to empty orno, do not embed URLs in diagnostics. If set tost, URLs use ST escape sequences. If set tobel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. IfGCC_URLSis not set, useTERM_URLSas a fallback. Note: ST is an ANSI escape sequence, string terminatorESC \, BEL is an ASCII character,CTRL-Gthat usually sounds like a beep. - At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently
xfce4-terminal, certain known to be buggygnome-terminalversions, thelinux console, andmingw. This check can be skipped with the-fdiagnostics-urls=always.
-fno-diagnostics-show-option— by default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the-fno-diagnostics-show-optionflag suppresses that behavior, no cap 🫶-fno-diagnostics-show-caret— by default, each diagnostic emitted includes the original source line and a caret^indicating the column. This option suppresses this information. The source line is truncated toncharacters, if the-fmessage-length=noption is given. When the output is done to the terminal, the width is limited to the width given by theCOLUMNSenvironment variable or, if not set, to the terminal width.-fno-diagnostics-show-labels— by default, when printing source code (via-fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intThis option suppresses the printing of these labels (in the example above, the vertical bars and the char * and long int text). It’s giving “no labels, just vibes” ✨
-fno-diagnostics-show-event-links— by default, when printing execution paths (via-fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as the line connecting events 1 and 2 in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'This option suppresses the printing of such connector lines. Lowkey tragic for the diagram girlies, but valid 💀
-fno-diagnostics-show-cwe— diagnostic messages can optionally have an associated CWE identifier (https://cwe.mitre.org/index.html). GCC itself only provides such metadata for some of the-fanalyzerdiagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.-fno-diagnostics-show-rules— diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.-fno-diagnostics-show-highlight-colors— GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user’s source. For example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s- the
%iandintin the message and theintin the quoted source are colored usinghighlight-a(bold green by default), and - the
const char *in the message and in the quoted source are both colored usinghighlight-b(bold blue by default).
The intent is to draw the reader’s eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones. This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors. Obsessed with the color-coding, honestly 🎀
-fno-diagnostics-show-line-numbers— by default, when printing source code (via-fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin.-fdiagnostics-minimum-margin-width=width— this option controls the minimum width of the left margin printed by-fdiagnostics-show-line-numbers. It defaults to6. Tiny margin, big main character energy 🌸-fdiagnostics-parseable-fixits— spits out fix-it hints in a machine-parseable format, literally made for IDEs to snack on ✨
For every fix-it, a line gets printed after the relevant diagnostic, starting with the string fix-it:. Bestie, like this:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"The location is a half-open range, counted in bytes, starting at byte 1 for the initial column 💅 So in that example up there, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_allThe filename and replacement string escape backslash as \\, tab as \t, newline as \n, double quotes as \", non-printable characters as octal (e.g. vertical tab as \013).
An empty replacement string means the given range is to be removed 🗑️ And an empty range (e.g. 45:3-45:3) means the string is to be inserted at the given position. Cute, right?
-fdiagnostics-generate-patch— prints fix-it hints to stderr in unified diff format, after all the diagnostics are printed 🩹
For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}The diff may or may not be colorized, following the same rules as for diagnostics (peep -fdiagnostics-color) 🌈
-fdiagnostics-show-template-tree— C++ frontend glow-up for template mismatch diagnostics 🌳
In the C++ frontend, when printing diagnostics showing mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>The parts that differ are highlighted with color (double and float in this case) 🎨
-fno-elide-type— stops GCC from being all mysterious with[...]🙈
By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as [...] to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag 👑
-fdiagnostics-path-format=KIND— tells GCC how to print paths of control-flow events for diagnostics that have such a path attached to them 🛤️
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths. Simple as that.
separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source 🎀
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped 🫶
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)-fdiagnostics-show-path-depths— extra tea ☕ when printing control-flow paths associated with a diagnostic
If this is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow 👩💻 Ngl, it's giving insider info.
-fno-show-column— do not print column numbers in diagnostics 🙅♀️
This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.
-fdiagnostics-column-unit=UNIT— picks the units for the column number 📏
This affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if requested.
The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes 🙂 (that's literally the slightly smiling face, iconic).
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0 🕰️
-fdiagnostics-column-origin=ORIGIN— sets the origin for column numbers, i.e. the column number assigned to the first column 🔢
The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified. Zero-based girlies, this one's for you 💖
-fdiagnostics-escape-format=FORMAT— controls how unprintable bytes get escaped when GCC shows you source lines 🦋
When GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines.
This option controls how such bytes should be escaped.
The default FORMAT, unicode, displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX> ✨
For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-text-art-charset=CHARSET— picks which characters get used for "text art" diagrams 🎨
Some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font.
This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji 🧚♀️ (yes, emoji is a real option and honestly? main character behavior).
The none value suppresses the printing of such diagrams. The ascii value makes absolutely sure those diagrams are pure ASCII ("ASCII art"). The unicode value lets you conservatively use unicode drawing characters (box-drawing characters, etc.). The emoji value goes even further and adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character).
The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii.
-fdiagnostics-format=FORMAT— picks a different format for printing diagnostics.FORMATistext,sarif-stderr,sarif-file,json,json-stderr, orjson-file.
Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=.
The default is text.
The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or to a file named source.sarif, respectively.
The various json, json-stderr, and json-file values are deprecated and refer to a legacy JSON-based output format. The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to. With json-stderr, the JSON is emitted to stderr, whereas with json-file it is written to source.gcc.json.
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC— adds an additional "output sink" for emitting diagnostics.
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in this form:
<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>etc.
🚨 Schemes, keys, or values with a name prefixed experimental may change or be removed without notice.
SCHEME can be:
text— emit diagnostics to stderr using GCC's classic text output format.
Supported keys are:
color=[yes|no]— override colorization settings from-fdiagnostics-colorfor this text output.experimental-nesting=[yes|no]— enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages, displaying location information on separate lines.experimental-nesting-show-locations=[yes|no]— ifexperimental-nesting=yes, then by default locations are shown; set this key tonoto disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.experimental-nesting-show-levels=[yes|no]— this is a debugging option for use withexperimental-nesting=yes. Set this key toyesto print explicit nesting levels in the output. This exists for use by GCC developers.
sarif— emit diagnostics to a file in SARIF format.
Supported keys are:
file=FILENAME— specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.sarif.version=[2.1|2.2-prerelease]— specify the version of SARIF to use for the output. If not specified, defaults to2.1.2.2-prereleaseuses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarifwould add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively.
In EBNF:
<diagnostics-output-specifier> = <diagnostics-output-name>
| <diagnostics-output-name>, ":", <key-value-pairs>;
<diagnostics-output-name> = "text" | "sarif";
<key-value-pairs> = <key-value-pair>
| <key-value-pair> "," <key-value-pairs>;
<key-value-pair> = <key> "=" <value>;
<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC— works in a similar way to-fdiagnostics-add-output=except that instead of adding an additional "output sink" for diagnostics, it replaces all existing output sinks, such as from-fdiagnostics-format=,-fdiagnostics-add-output=, or a prior call to-fdiagnostics-set-output=.-fno-diagnostics-json-formatting— by default, when JSON is emitted for diagnostics (via-fdiagnostics-format=sarif-stderr,-fdiagnostics-format=sarif-file,-fdiagnostics-format=json,-fdiagnostics-format=json-stderr,-fdiagnostics-format=json-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON.
Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect.
This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases.
✨ Options to Request or Suppress Warnings ✨
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only— check the code for syntax errors, but don't do anything beyond that.-fmax-errors=n— limits the maximum number of error messages ton, at which point GCC bails out rather than attempting to continue processing the source code. Ifnis0(the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option.-w— inhibit all warning messages.-Werror— turn all warnings into errors.-Werror=— turn the specified warning into an error. The specifier for a warning is appended; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. This switch takes a negative form, to be used to negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings not be errors, even when-Werroris in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.
-Wfatal-errors— this option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wunused-variable to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-variable. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives.
-Wpedantic,-pedantic— issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any-stdoption used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors).
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. 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 warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign.
Okay so when the standard you picked with -std is a GNU extended C dialect — like gnu90 or gnu99 — there’s a matching base standard, aka the ISO C version that GNU dialect is built on 🧚. Warnings from -Wpedantic show up wherever the base standard says they’re required. (And no, it doesn’t make sense to give those warnings only for stuff that’s not in the specified GNU C dialect — because by definition the GNU dialects of C include all the features the compiler supports with that option, so there’d literally be nothing to warn about 💀.)
-pedantic-errors— bestie, this one gives an error whenever the base standard (see-Wpedantic) requires a diagnostic, in some cases where there’s undefined behavior at compile-time, and in some other cases that don’t prevent compilation of programs that are valid according to the standard. It is not equivalent to-Werror=pedantic: that latter option is unlikely to be useful, because it only makes errors out of the diagnostics controlled by-Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than-Wpedantic👑.
If you want the required diagnostics that are warnings by default to become errors instead, but you don’t also want to enable the -Wpedantic diagnostics, you can specify -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings) ✨.
Some required diagnostics are errors by default, but can be reduced to warnings using -fpermissive or their specific warning option, e.g. -Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made errors by -pedantic-errors. For instance:
-Wattributes(for standard attributes)-Wchanges-meaning(C++)-Wcomma-subscript(C++23 or later)-Wdeclaration-after-statement(C90 or earlier)-Welaborated-enum-base(C++11 or later)-Wimplicit-int(C99 or later)-Wimplicit-function-declaration(C99 or later)-Wincompatible-pointer-types-Wint-conversion-Wlong-long(C90 or earlier)-Wmain-Wnarrowing(C++11 or later)-Wpointer-arith-Wpointer-sign-Wincompatible-pointer-types-Wregister(C++17 or later)-Wvla(C90 or earlier)-Wwrite-strings(C++11 or later)
-fpermissive— this downgrades some required diagnostics about nonconformant code from errors to warnings 😌. So using-fpermissiveallows some nonconforming code to compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:-Wdeclaration-missing-parameter-type(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Wincompatible-pointer-types(C and Objective-C only)-Wint-conversion(C and Objective-C only)-Wnarrowing(C++ and Objective-C++ only)-Wreturn-mismatch(C and Objective-C only)-Wtemplate-body(C++ and Objective-C++ only)
The -fpermissive option is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90) 🫶.
-Wall— this enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros 🌈. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress-Waligned-new(C++ and Objective-C++ only)-Warray-bounds=1(only with-O2)-Warray-compare-Warray-parameter=2-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wc++17compat-Wc++20compat-Wcatch-value(C++ and Objective-C++ only)-Wchar-subscripts-Wclass-memaccess(C++ and Objective-C++ only)-Wcomment-Wdangling-else-Wdangling-pointer=2-Wdelete-non-virtual-dtor(C++ and Objective-C++ only)-Wduplicate-decl-specifier(C and Objective-C only)-Wenum-compare(in C/ObjC; this is on by default in C++)-Wenum-int-mismatch(C and Objective-C only)-Wformat=1-Wformat-contains-nul-Wformat-diag-Wformat-extra-args-Wformat-overflow=1-Wformat-truncation=1-Wformat-zero-length-Wframe-address-Wimplicit(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Winfinite-recursion-Winit-self(C++ and Objective-C++ only)-Wint-in-bool-context-Wlogical-not-parentheses-Wmain(only for C/ObjC and unless-ffreestanding)-Wmaybe-uninitialized-Wmemset-elt-size-Wmemset-transposed-args-Wmisleading-indentation(only for C/C++)-Wmismatched-dealloc-Wmismatched-new-delete(C++ and Objective-C++ only)-Wmissing-attributes-Wmissing-braces(only for C/ObjC)-Wmultistatement-macros-Wnarrowing(C++ and Objective-C++ only)-Wnonnull-Wnonnull-compare-Wopenmp-simd(C and C++ only)-Woverloaded-virtual=1(C++ and Objective-C++ only)-Wpacked-not-aligned-Wparentheses-Wpessimizing-move(C++ and Objective-C++ only)-Wpointer-sign(only for C/ObjC)-Wrange-loop-construct(C++ and Objective-C++ only)-Wreorder(C++ and Objective-C++ only)-Wrestrict-Wreturn-type-Wself-move(C++ and Objective-C++ only)-Wsequence-point-Wsign-compare(C++ and Objective-C++ only)-Wsizeof-array-div-Wsizeof-pointer-div-Wsizeof-pointer-memaccess-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtautological-compare-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-Wunused-but-set-variable-Wunused-const-variable=1(only for C/ObjC)-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-value-Wunused-variable-Wuse-after-free=2-Wvla-parameter-Wvolatile-register-var-Wzero-length-bounds
Note that some warning flags are not implied by -Wall 🙈. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.
-Wextra— this enables some extra warning flags that are not enabled by-Wall⭐. (This option used to be called-W. The older name is still supported, but the newer name is more descriptive.)-Wabsolute-value(only for C/ObjC)-Walloc-size-Wcalloc-transposed-args-Wcast-function-type-Wclobbered-Wdangling-reference(C++ only)-Wdeprecated-copy(C++ and Objective-C++ only)-Wempty-body-Wenum-conversion(only for C/ObjC)-Wexpansion-to-defined-Wignored-qualifiers(only for C/C++)-Wimplicit-fallthrough=3-Wmaybe-uninitialized-Wmissing-field-initializers-Wmissing-parameter-name(C/ObjC only)-Wmissing-parameter-type(C/ObjC only)-Wold-style-declaration(C/ObjC only)-Woverride-init(C/ObjC only)-Wredundant-move(C++ and Objective-C++ only)-Wshift-negative-value(in C++11 to C++17 and in C99 and newer)-Wsign-compare(C++ and Objective-C++ only)-Wsized-deallocation(C++ and Objective-C++ only)-Wstring-compare-Wtype-limits-Wuninitialized-Wunterminated-string-initialization-Wunused-parameter(only with-Wunusedor-Wall)-Wunused-but-set-parameter(only with-Wunusedor-Wall)
The option -Wextra also prints warning messages for the following cases:
- A pointer is compared against integer zero with
<,<=,>, or>=. - (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
- (C++ only) Ambiguous virtual bases.
- (C++ only) Subscripting an array that has been declared
register. - (C++ only) Taking the address of a variable that has been declared
register. - (C++ only) A base class is not initialized in the copy constructor of a derived class.
-Wabi(C, Objective-C, C++ and Objective-C++ only) — warn about code affected by ABI changes 🦋. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. The latter warnings are also controlled separately by-Wpsabi, which is implied by-Wabi.
Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI compatibility problems if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code 😭. There may also be cases where warnings are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This was fixed in -fabi-version=3.
- SIMD vector types declared using
__attribute ((vector_size))were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
__attribute ((const))andnoreturnwere mangled as type qualifiers, anddecltypeof a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5 💅.
- scoped enumerators passed as args to a variadic function — they get promoted like unscoped enumerators do, which makes
va_argthrow a whole tantrum 😤. On most targets this doesn't actually mess with the parameter-passing ABI, because there's literally no way to pass an argument smaller thanintanyway. Ngl, kind of a nothingburger.
Also the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.
All of this got fixed in -fabi-version=6.
- lambdas in default argument scope — these were mangled wrong, and the ABI also changed the mangling of
nullptr_t.
Fixed in -fabi-version=7. 💅
- mangling a function type with function-cv-qualifiers — the un-qualified function type was being treated as a substitution candidate when it absolutely should not have been.
Fixed in -fabi-version=8, which is the default for GCC 5.1.
decltype(nullptr)had alignment 1 — bestie. that's so wrong. It led to unaligned accesses. Heads up though: this did not affect the ABI of a function with anullptr_tparameter, because parameters get a minimum alignment regardless.
Fixed in -fabi-version=9, the default for GCC 5.2.
- target-specific attributes that affect a type's identity — like ia32 calling conventions on a function type (stdcall, regparm, etc.) — these didn't affect the mangled name, which caused name collisions when function pointers were used as template arguments. Chaos. 💀
Fixed in -fabi-version=10, the default for GCC 6.1.
-Wpsabi (C, Objective-C, C++ and Objective-C++ only)
-Wpsabi turns on warnings about processor-specific ABI changes — stuff like changes in alignment requirements or how function arguments get passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have shifted between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect the binary compatibility of compiled code. With -Wpsabi, GCC can spot potentially incompatible usages and flag them for you. Iconic of it honestly. ✨
-Wpsabi is enabled by default, and it's also implied by -Wabi.
-Wno-changes-meaning (C++ and Objective-C++ only)
C++ requires that unqualified uses of a name inside a class mean the same thing across the whole scope of the class — so declaring the name after you use it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaningBy default, the B1 case is only a warning because both declarations have the same type, while the B2 case is straight-up an error. Both of these diagnostics can be silenced with -Wno-changes-meaning. Or, alternatively, you can downgrade the error case to a warning with -Wno-error=changes-meaning or -fpermissive.
Both diagnostics are also suppressed by -fms-extensions. 🫶
-Wchar-subscripts
Warns you if an array subscript has type char. This is a super common source of bugs, because programmers constantly forget that this type is signed on some machines. 😭 This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warns if feedback profiles don't match when you're using the -fprofile-use option. If a source file gets changed between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file, and then GCC just can't use the profile feedback info at all. By default this warning is enabled and treated as an error. You can use -Wno-coverage-mismatch to disable the warning, or -Wno-error=coverage-mismatch to disable the error.
Turning off the error for this warning can leave you with poorly optimized code, and it's only really useful in the case of very minor changes like bug fixes to an existing codebase. Fully disabling the warning is not recommended, bestie. Don't do it.
-Wno-coverage-too-many-conditions
Warns if -fcondition-coverage is used and an expression has too many terms, so GCC gives up on coverage. Coverage gets abandoned when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default.
-Wno-coverage-too-many-paths
Warns if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. The giving-up part is controlled by -fpath-coverage-limit. This warning is enabled by default.
-Wno-coverage-invalid-line-number
Warns when a function ends earlier than it begins, because of invalid linenum macros. The warning is only emitted with --coverage enabled.
By default this warning is enabled and treated as an error. -Wno-coverage-invalid-line-number disables the warning, or -Wno-error=coverage-invalid-line-number disables the error.
-Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppresses warning messages emitted by #warning directives. That's it. That's the whole thing. 💅
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Gives you a warning when a value of type float gets implicitly promoted to double. Here's the tea: CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but they emulate double in software. On a machine like that, doing computations with double values is so much more expensive because of all the overhead that software emulation drags in. 😬
And it's genuinely easy to accidentally compute with double, because floating-point literals are implicitly of type double. For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler does the entire computation with double, because that floating-point literal is a double. Sneaky.
-Wduplicate-decl-specifier (C and Objective-C only)
Warns if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall.
-Wformat
-Wformat=n
Checks calls to printf and scanf, etc., to make sure the arguments you supplied have types that actually fit the format string you specified, and that the conversions in the format string make sense. This covers standard functions, plus others specified by format attributes, across the printf, scanf, strftime and strfmon families (that last one is an X/Open extension, not in the C standard) — or other target-specific families. Which functions get checked without format attributes having been specified depends on the standard version you selected, and those attribute-less checks are disabled by -ffreestanding or -fno-builtin.
Formats are checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all of these features — GCC does not warn about features that go beyond a particular library's limitations. However, if -Wpedantic is used together with -Wformat, you do get warnings about format features that aren't in the selected standard version (but not for strfmon formats, since those aren't in any version of the C standard at all). 🦋
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by these options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.
-Wformat=2
Enables -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. Slay. 🔥
-Wno-format-contains-nul
If -Wformat is specified, don't warn about format strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, don't warn about excess arguments to a printf or scanf format function. The C standard says such arguments are just ignored, so like… whatever.
Where the unused arguments sit between used arguments that are specified with $ operand number specifications, warnings are normally still given, because the implementation couldn't know what type to pass to va_arg in order to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warns about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes a format directive writes can't be figured out at compile time, it gets estimated based on heuristics that depend on the level argument and on optimization. Enabling optimization will in most cases improve how accurate the warning is — but it can also hand you false positives, so keep that in mind. 🙈
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow, the one enabled by -Wformat, takes a conservative approach: it only warns about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments known to be bounded to a subrange of their type — or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals — are assumed to take on the value within the range that produces the most bytes on output.
For example, the call to sprintf below gets diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') that the function appends to the destination buffer will be written past its end. Bumping the buffer size up by a single byte is enough to dodge the warning — though it may not be enough to actually avoid the overflow. 😅
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— so level 2 is where it gets extra 🍓. It also clocks calls that might overflow the destination buffer if you hand it an argument that's long or big enough. How does it guess? At level 2, any unknown numeric argument is assumed to be the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. And unknown string arguments? If their length can't be assumed bounded by the directive's precision, or by a finite set of string literals they might evaluate to, or by the character array they might point at — they get assumed to be 1 character long. iykyk 💅
At level 2, the call in the example up there gets diagnosed again — but this time it's because with a equal to a 32-bit INT_MIN, that first %i directive is gonna write some of its digits past the end of the destination buffer 😭. If you want the call to be safe no matter what the two variables hold, you gotta bump the destination buffer up to at least 34 bytes. And GCC is nice about it: it drops the minimum size of the buffer into an informational note right after the warning 🫶.
Don't wanna grow the buffer? Then constrain the range of formatted values instead, bestie. The max length of string arguments can be bounded by specifying the precision in the format directive. And when numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier shrinks the required buffer size. For example — if a and b in the example above can be assumed to be within the precision of the short int type, then using either the %hi format directive or casting the argument to short drops the maximum required buffer size down to 24 bytes ✨:
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}-Wno-format-zero-length— if-Wformatis on, this tells it not to warn about zero-length formats. And like, the C standard literally says zero-length formats are allowed, so chill 💀.-Wformat-nonliteral— if-Wformatis specified, also warn when the format string isn't a string literal and therefore can't be checked — unless the format function takes its format arguments as ava_list.-Wformat-security— if-Wformatis specified, also warn about uses of format functions that look like possible security problems 👀. Right now this warns about calls toprintfandscanffunctions where the format string isn't a string literal and there are no format arguments, likeprintf (foo);. That can be a whole security hole if the format string came from untrusted input and contains%n😭. (Fun fact: this is currently a subset of what-Wformat-nonliteralwarns about — but future warnings may get added to-Wformat-securitythat aren't in-Wformat-nonliteral, so don't get too comfy.)-Wformat-signedness— if-Wformatis specified, also warn when the format string wants an unsigned argument but the argument is signed, or vice versa. matching matters, bestie 💅.-Wformat-truncation- *
-Wformat-truncation=level*</italic>* — warn about calls to formatted input/output functions likesnprintfandvsnprintfthat might result in output truncation ✂️. When the exact number of bytes written by a format directive can't be figured out at compile-time, it gets estimated using heuristics that depend on the level argument and on optimization. Turning on optimization will in most cases improve the accuracy of the warning — but it may also give you false positives, so 🙈. Except where noted otherwise, this option uses the same logic as-Wformat-overflow.-Wformat-truncation-Wformat-truncation=1— level 1 of-Wformat-truncation, enabled by-Wformat, takes a conservative approach: it only warns about calls to bounded functions whose return value is unused and that will most likely result in output truncation. safe, lowkey 🎀-Wformat-truncation=2— level 2 also warns about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude. spicy 🔥
-Wformat-y2k— if-Wformatis specified, also warn aboutstrftimeformats that may yield only a two-digit year. it's giving Y2K bug nostalgia fr fr 💀-Wnonnull— warn about passing a null pointer for arguments that are marked as requiring a non-null value by thenonnullfunction attribute. no nulls allowed, periodt 🚫
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Wnonnull-compare— warn when you compare an argument marked with thenonnullfunction attribute against null inside the function. like bestie it literally can't be null, why are you checking 😭
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference— warn if the compiler spots paths that trigger erroneous or undefined behavior by dereferencing a null pointer 💀. Heads up: this option is only active when-fdelete-null-pointer-checksis active, which gets enabled by optimizations on most targets. And the precision of the warnings depends on the optimization options you used — so it's a vibe, not a guarantee.-Wno-musttail-local-addr— don't warn about passing a pointer (or in C++, a reference) to a local variable or label as an argument of amusttailcall. Those variables go out of scope before the tail call instruction, so it's fine 🫶.-Wmaybe-musttail-local-addr— warn when the address of a local variable can escape to amusttailcall — unless it already goes out of scope before thatmusttailcall. look at this whole example, it's iconic:
int foo (int *);
int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C<musttail> attribute this call would not
be tail called, because address of the C<a> variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C<a> variable isn't already in scope, so even when it
escaped, even without C<musttail> attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}This warning is enabled by -Wextra.
-Wnrvo(C++ and Objective-C++ only) — warn if the compiler doesn't elide the copy from a local variable to the return value of a function in a context where[class.copy.elision]allows it. This elision is commonly known as the Named Return Value Optimization 🎀. For instance, in the example below the compiler can't elide copies from bothv1andv2, so it elides neither:
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}-Winfinite-recursion— warn about infinitely recursive calls 🔁. It's effective at all optimization levels, but it needs optimization to detect infinite recursion in calls between two or more functions.-Winfinite-recursionis included in-Wall.
Compare with -Wanalyzer-infinite-recursion, which gives a similar diagnostic but is implemented a totally different way (as part of -fanalyzer).
-Winit-self(C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves 🙈. Note this option can only be used with the-Wuninitializedoption.
For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++.
-Wno-implicit-int(C and Objective-C only) — this option controls warnings when a declaration doesn't specify a type 👀. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone with-Wno-error=implicit-int.
This warning is upgraded to an error by -pedantic-errors.
-Wno-implicit-function-declaration(C and Objective-C only) — this option controls warnings when a function is used before being declared 😬. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone with-Wno-error=implicit-function-declaration.
This warning is upgraded to an error by -pedantic-errors.
-Wimplicit(C and Objective-C only) — same as-Wimplicit-intand-Wimplicit-function-declaration. This warning is enabled by-Wall.-Whardened— warn when-fhardeneddidn't enable an option from its set (for which see-fhardened) 👑. For instance, using-fhardenedand-fstack-protectorat the same time on the command line causes-Whardenedto warn, because-fstack-protector-strongwon't be enabled by-fhardened.
This warning is enabled by default and has effect only when -fhardened is enabled.
-Wimplicit-fallthrough—-Wimplicit-fallthroughis the same as-Wimplicit-fallthrough=3, and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0. easy math 💅- *
-Wimplicit-fallthrough=n*</italic>* — warn when a switch case falls through 🍂. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}Okay so this warning is not gonna yell at you when the last statement of a case literally can't fall through — like when there's a return in there, or a call to a function declared with the noreturn attribute. Bye, false alarm 💅
And plot twist: -Wimplicit-fallthrough= is smart enough to look at control flow statements too — ifs and friends — and it only warns when it actually should. For example:
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}Now, sometimes a falling-through switch case is exactly what you want (iconic, intentional behavior ✨), so GCC gives you an attribute — __attribute__ ((fallthrough)) — which you use together with a null statement to suppress the warning that would normally pop up:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 has a standard way to shut off the -Wimplicit-fallthrough warning, using [[fallthrough]]; instead of the GNU attribute. And if you're on C++11 or C++14, you can use [[gnu::fallthrough]];, which is a GNU extension. 🫶
Instead of these attributes, you can also drop a fallthrough comment in there to silence the warning — the whole body of the C or C++ style comment needs to match the regular expressions listed down below. The option argument n tells GCC what kind of comments get accepted:
-Wimplicit-fallthrough=0— disables the warning entirely. Done, muted, no notes 💀-Wimplicit-fallthrough=1— matches the".*"regular expression, so literally any comment counts as a fallthrough comment.-Wimplicit-fallthrough=2— case insensitively matches the".*falls?[ \t-]*thr(ough|u).*"regular expression.-Wimplicit-fallthrough=3— case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
-Wimplicit-fallthrough=4— case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t]*FALLTHR(OUGH|U)[ \t]*"
-Wimplicit-fallthrough=5— straight up doesn't recognize any comments as fallthrough comments; only attributes can disable the warning. Strict queen behavior 👑
The comment has to be followed — after optional whitespace and other comments — by the case or default keywords, or by a user label that comes right before some case or default label. Like so:
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}Also bestie, -Wimplicit-fallthrough=3 is the one that gets enabled by -Wextra. 🌸
-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)
This one decides whether warnings triggered by the warn_if_not_aligned attribute get issued. And heads up — these warnings are on by default. 🎀
-Wignored-qualifiers (C and C++ only)
Warns you when the return type of a function has a type qualifier like const. For ISO C, that kind of qualifier does absolutely nothing, because the value a function returns isn't an lvalue. For C++, the warning only fires for scalar types or void. And ISO C straight up prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 😭
This warning is also enabled by -Wextra.
-Wno-ignored-attributes (C and C++ only)
This option controls warnings for when an attribute gets ignored. It's different from the -Wattributes option because it warns whenever the compiler decides to drop an attribute — not because the attribute is unknown or used in the wrong place or whatever. This warning is enabled by default. 🙈
-Wmain
Warns if the type of main is suspicious. main should be a function with external linkage, returning int, and taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic. 👑
And this warning gets upgraded to an error by -pedantic-errors.
-Wmisleading-indentation (C and C++ only)
Warns when your code's indentation does not reflect the block structure — i.e. when your vibes are lying to you. Specifically, a warning fires for if, else, while, and for clauses that have a guarded statement with no braces, followed by an unguarded statement sitting at the same indentation. 😭
In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */When it's mixed tabs and spaces, the warning uses the -ftabstop= option to figure out whether the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic, like this example:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);And it's not issued after a #line directive, since that usually means the code is autogenerated and no assumptions can be made about the layout of the file that the directive references. 🤷♀️
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warns when a declaration of a function is missing one or more attributes that a related function is declared with — and whose absence might mess with the correctness or efficiency of the generated code. For example, the warning fires for declarations of aliases that use attributes to specify less restrictive requirements than the ones their targets have. That's typically a potential optimization opportunity, bestie. 💅 By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could actually lead to incorrect code generation. 😱
Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.
In C++, the warning fires when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning..
You can use the copy attribute to slap the same set of attributes onto a declaration as another declaration has, without explicitly listing them all out. This attribute can be applied to declarations of functions, variables, or types. ✨
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size, the declaration of the explicit specialization of the template gets diagnosed because it's missing one of the attributes:
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);-Wmissing-braces
Warns if an aggregate or union initializer isn't fully bracketed. In the following example, the initializer for a is not fully bracketed, but the one 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, Objective-C++ and Fortran only)
Warns if a user-supplied include directory doesn't exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it's partially enabled by default — warning for -I and -J, only. 🦋
-Wno-missing-profile
This option controls warnings about missing feedback profiles when you're using the -fprofile-use option. It diagnoses those cases where a new function or a new file got added between compiling with -fprofile-generate and compiling with -fprofile-use, without regenerating the profiles. In those cases, the profile feedback data files don't contain any profile feedback info for the newly added function or file. 😭 Also, when profile count data (.gcda) files get removed, GCC can't use any profile feedback information at all. In all these cases, warnings get issued to let you know that a profile generation step is due. Ignoring the warning can result in poorly optimized code. You can use -Wno-missing-profile to disable the warning, but that's not recommended — only do it when non-existent profile data is genuinely justified. 🙈
-Wmismatched-dealloc
Warns for calls to deallocation functions with pointer arguments returned from allocation functions where the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by using the attribute malloc. Unless it's disabled by the -fno-builtin option, the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete, are implicitly associated as matching allocators and deallocators. In the following example, mydealloc is the deallocator for pointers returned from myalloc. 💅
void mydealloc (void*); __attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete. 💅
Option -Wmismatched-dealloc is included in -Wall. ✨
-Wmultistatement-macros— warns you about those unsafe multiple-statement macros that look like they're guarded by anif,else,for,switch, orwhileclause, when actually only the first statement ends up guarded after the macro expands. 😭 Sneaky!
Like, look at this:
#define DOIT x++; y++
if (c)
DOIT;bestie, that increments y unconditionally, not just when c holds. 🙈 The fix is usually to wrap the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;This warning is enabled by -Wall in C and C++. 💖
-Wparentheses— warns when parentheses are omitted in certain contexts — like when there's an assignment in a spot where a truth value is expected, or when operators are nested in the kind of precedence tangle that people always get confused about. 🌀
It also warns if a comparison like x<=y<=z shows up — because that's equivalent to (x<=y ? 1 : 0) <= z, which is a totally different interpretation than ordinary mathematical notation. Not the math you learned, bestie. 😭
It also warns about dangerous uses of the GNU extension to ?: with an omitted middle operand. When the condition in the ? : operator is a boolean expression, the omitted value is always 1. But programmers often expect it to be a value computed inside the conditional expression instead. 🙈
For C++, it also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}This warning is enabled by -Wall. ⭐
-Wno-self-move(C++ and Objective-C++ only) — this warning warns when a value is moved to itself withstd::move. Such astd::movetypically has no effect. 💀 Like, why are you moving yourself? Main character energy but pointless.
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}This warning is enabled by -Wall. ✨
-Wsequence-point— warns about code that may have undefined semantics because it violates the sequence point rules in the C and C++ standards. 🚨
So the C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. 😵💫 All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. 💅
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable. 🎲
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs. 🔍
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. 🫶
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>. 📚
This warning is enabled by -Wall for C and C++. 💖
-Wno-return-local-addr— do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. 🙈 Yeah, that's a dangling thing, but okay, you asked.-Wreturn-mismatch— warns aboutreturnstatements without an expression in functions which do not returnvoid. Also warns about areturnstatement with an expression in a function whose return type isvoid, unless the expression type is alsovoid. As a GNU extension, the latter case is accepted without a warning unless-Wpedanticis used. 🎀
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. 💀
This warning is specific to C and enabled by default. In C99 and later language dialects, it is treated as an error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch. 🔥
-Wreturn-type— warns whenever a function is defined with a return type that defaults toint(unless-Wimplicit-intis active, which takes precedence). Also warns if execution may reach the end of the function body, or if the function does not contain any return statement at all. 🌸
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. 😭
Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. 😱
This warning is enabled by default in C++ and by -Wall otherwise. ⭐
-Wno-shift-count-negative— controls warnings if a shift count is negative. This warning is enabled by default. 🌀-Wno-shift-count-overflow— controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default. 🌀-Wshift-negative-value— warns if you left shift a negative value. This warning is enabled by-Wextrain C99 (and newer) and C++11 to C++17 modes. 💅-Wno-shift-overflow,-Wshift-overflow=n— these options control warnings about left shift overflows. 🚨-Wshift-overflow=1— this is the warning level of-Wshift-overflowand is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap. 🌀-Wshift-overflow=2— this warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active. 🔥
-Wswitch— warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration. (The presence of adefaultlabel prevents this warning.)caselabels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with theflag_enumattribute. This warning is enabled by-Wall. 🎀-Wswitch-default— warns whenever aswitchstatement does not have adefaultcase. 🙈-Wswitch-enum— warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with theflag_enumattribute. The only difference between-Wswitchand this option is that this option gives a warning about an omitted enumeration code even if there is adefaultlabel. ✨-Wno-switch-bool— do not warn when aswitchstatement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs. 💖
-Wno-switch-outside-range— this option controls warnings when aswitchcase has a value that is outside of its respective type range. This warning is enabled by default for C and C++ programs. 🦋-Wno-switch-unreachable— do not warn when aswitchstatement contains statements between the controlling expression and the first case label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}💀 Like bestie, that i = 15; is never ever running.
and the plot twist 🎀 — -Wswitch-unreachable does not come for you when the thing sitting between the controlling expression and the first case label is literally just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs. 💅
-Wsync-nand (C and C++ only)
Warns you whenever the __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions get used. Why do we care? Because these functions changed semantics in GCC 4.4 😭 so if you're seeing this warning, bestie, your code is giving "pre-4.4 vibes" and you should lowkey check it.
-Wtrivial-auto-var-init
Warns when -ftrivial-auto-var-init can't initialize the automatic variable. The most common situation? An automatic variable that's declared between the controlling expression and the first case label of a switch statement. (Yeah. Same little cursed zone as above. ✨)
-Wunused-but-set-parameter
Warns whenever a function parameter gets assigned to but is otherwise unused (aside from its declaration). Like, you wrote to it and then just… never spoke to it again 🙈.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warns whenever a local variable gets assigned to but is otherwise unused (aside from its declaration). This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warns whenever a static function is declared but not defined, or a non-inline static function is unused. This warning is enabled by -Wall. 💀 (declared and then ghosted, honestly)
-Wunused-label
Warns whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warns when a typedef that's locally defined inside a function is not used. This warning is enabled by -Wall. 🦋
-Wunused-parameter
Warns whenever a function parameter is unused aside from its declaration. Heads up: this option is not enabled by -Wunused unless -Wextra is also specified. 👑
To suppress this warning use the unused attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with the warn_unused_result attribute doesn't use its return value. The default is -Wunused-result. (So like, by default GCC will call you out for ignoring that return value 😭)
-Wunused-variable
Warns whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-const-variable / -Wunused-const-variable=n
Warns whenever a constant static variable is unused aside from its declaration. 🎀
To suppress this warning use the unused attribute.
-Wunused-const-variable=1 — warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this isn't an error since const variables take the place of #defines. iykyk ✨
-Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). It's equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++, because it might be hard to clean up all headers included. 😭 (fr fr, header cleanup is a whole saga)
-Wunused-value
Warns whenever a statement computes a result that is explicitly not used. To suppress this warning, cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not. 💅
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined, except those documented as needing to be specified explicitly. 🌈
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 and/or -Wunused-but-set-parameter.
-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. 🫶
-Wuse-after-free (C, Objective-C, C++ and Objective-C++ only) / -Wuse-after-free=n
Warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels, but may yield different results with optimization than without. 👀
-Wuse-after-free=1 — at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-free calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
-Wuse-after-free=2 — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to free in the following function is diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}-Wuse-after-free=3 — at level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined, but equality tests sometimes appear after calls to realloc as an attempt to determine whether the call resulted in relocating the object to a different address. They're diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. 🧚 (honestly a glow-up for your codebase)
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast (C, Objective-C, C++ and Objective-C++ only)
Warns when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type, as that's a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}-Wuninitialized
Warns if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warns if a non-static reference or non-static const member appears in a class without constructors. 😭
In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized 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 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. (the compiler simply ✨ deletes the evidence ✨)
In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about b being uninitialized in the following snippet:
struct A {
int a;
int b;
A() : a(b) { }
};-Wno-invalid-memory-model
This option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the memory_order enumeration. For example, since the __atomic_store and __atomic_store_n built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default. 💜
-Wmaybe-uninitialized
For an object with automatic or allocated storage duration: if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. 🔥 (the "maybe" is doing so much heavy lifting here, ngl)
Okay so also, bestie: passing a pointer (or, in C++, a reference) to an uninitialized object to a const-qualified function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) If you annotate the function with attribute access (none), that says the argument isn't used to access the object—and avoids the warning. 💅
These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables. Like, no optimization, no variable-state surveillance. 😭
These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— Warn when a#pragmadirective is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the-Wallcommand-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.-Wno-pragma-once-outside-header— Do not warn when#pragma onceis used in a file that is not a header file, such as a main file.-Wno-prio-ctor-dtor— Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution beforemainis called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between0--100for the implementation.-Wstrict-aliasing— This option is only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in-Wall. It is equivalent to-Wstrict-aliasing=3.-Wstrict-aliasing=n— This option is only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way-Oworks.-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-fstrict-aliasingstill breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only. - Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
- Level 3 (default for
-Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end:*(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-Wstrict-overflow,-Wstrict-overflow=n— This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1— Warn about cases that are both questionable and easy to avoid. For example the compiler simplifiesx + 1 > xto 1. This level of-Wstrict-overflowis enabled by-Wall; higher levels are not, and must be explicitly requested.-Wstrict-overflow=2— Also warn about other cases where a comparison is simplified to a constant. For example:abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, becauseabs (INT_MIN)overflows toINT_MIN, which is less than zero.-Wstrict-overflow(with no level) is the same as-Wstrict-overflow=2.-Wstrict-overflow=3— Also warn about other cases where a comparison is simplified. For example:x + 1 > 1is simplified tox > 0.-Wstrict-overflow=4— Also warn about other simplifications not covered by the above cases. For example:(x * 10) / 5is simplified tox * 2.-Wstrict-overflow=5— Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example:x + 2 > yis simplified tox + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.
-Wstring-compare— Warn for calls tostrcmpandstrncmpwhose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case ofstrncmp). Such calls could be mistakes. For example, the call tostrcmpbelow is diagnosed because its result is necessarily non-zero irrespective of the contents of the arraya.
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow,-Wstringop-overflow,-Wstringop-overflow=type— Warn for code that can be statically determined to cause buffer overflows or memory overruns, such as calls tomemcpyandstrcpythat overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for string functions that operate on character arrays; raw memory functions likememcpyalways use type-zero Object Size Checking.
The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes.
The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as __builtin___memcpy_chk, which can perform run-time checking if the access cannot be identified as safe at compile time.
For example, the option issues a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}The effect of this option is not limited to string or memory manipulation functions. In this example, a warning is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow, -Wstringop-overflow=1
okay so the -Wstringop-overflow=1 option uses type-zero Object Size Checking to figure out the sizes of destination objects. At this setting it does not warn for writes past the end of subobjects of larger objects accessed by pointers — unless the size of the largest surrounding object is known. When the destination could be one of several objects, it just assumes it's the biggest one of them. And on Linux systems, when optimization is enabled at this setting, the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. slay.
-Wstringop-overflow=2
-Wstringop-overflow=2 uses type-one Object Size Checking to determine the sizes of destination objects. At this setting it warns about overflows when writing to members of the largest complete objects whose exact size is known. But ngl it does not warn for excessive writes to the same members of unknown objects referenced by pointers — because those pointer pookies may point to arrays containing unknown numbers of elements 💀. This is the default setting of the option, bestie.
-Wstringop-overflow=3
-Wstringop-overflow=3 uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. Heads up: this is the most restrictive setting of the option, and it may result in warnings for safe code 😭. It's giving extra.
-Wstringop-overflow=4
-Wstringop-overflow=4 uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3, this setting of the option may result in warnings for benign code 🙈.
-Wno-stringop-overread
Warns for calls to string manipulation functions like memchr or strcpy that are determined to read past the end of the source sequence. Rude of them honestly.
Fun fact: the option -Wstringop-overread is enabled by default ✨.
-Wno-stringop-truncation
This one says: don't warn for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that may either truncate the copied string or leave the destination unchanged.
So in the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result the copy of the source will be truncated, and so the call is diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound 👑
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}As another example, the following call to strncpy results in copying to d just the characters preceding the terminating NUL — without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a common mistake (like, everyone does it 😭), and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}And in the following example, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the result of the copy will not be NUL-terminated. Therefore the call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL.
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}When a character array is intended to store a sequence of bytes with no terminating NUL, you can annotate such an array with the attribute nonstring to avoid this warning. But — and this is important bestie — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of such arrays, GCC issues warnings unless it can prove that the use is safe 🫶.
-Wstrict-flex-arrays (C and C++ only)
Warns about improper usages of flexible array members — according to the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure if it's available, otherwise according to the level of the option -fstrict-flex-arrays=level. -Wstrict-flex-arrays is only effective when level is greater than 0, iykyk.
- When
level=1, warnings are issued for a trailing array reference of a structure that have 2 or more elements if the trailing array is referenced as a flexible array member. - When
level=2, in addition tolevel=1, additional warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member. - When
level=3, in addition tolevel=2, additional warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.
This option is more effective when -ftree-vrp is active (the default for -O2 and above), but some warnings may be diagnosed even without optimization ⭐.
-Wsuggest-attribute=attribute-name
Warns for cases where adding an attribute may be beneficial. The attribute-names currently supported are listed below 💅:
-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc-Wsuggest-attribute=returns_nonnull
These warn about functions that might be candidates for attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis ✨.
-Wsuggest-attribute=format-Wmissing-format-attribute
These warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning 🎀.
GCC also warns about function definitions that might be candidates for format attributes. Again, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf — but this might not always be the case, and some functions for which format attributes are appropriate may not be detected 🙈.
-Wsuggest-attribute=cold
Warns about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions which always lead to a call to another cold function — like wrappers of C++ throw or fatal error reporting functions leading to abort 💀.
-Walloc-size
Warns about calls to allocation functions decorated with the attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Like bestie, that's not enough room, no cap.
-Walloc-zero
Warns about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated), relying on it may result in subtle portability bugs and should be avoided 😭.
-Wcalloc-transposed-args
Warns about calls to allocation functions decorated with the attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning, periodt. The first argument to calloc is documented to be the number of elements in an array, while the second argument is the size of each element — so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n) 👑.
If having sizeof in the earlier argument and not the latter is intentional, the warning can be suppressed by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4) or using sizeof in the later argument as well.
-Walloc-size-larger-than=byte-size
Warns about calls to functions decorated with the attribute alloc_size that attempt to allocate objects larger than the specified number of bytes — or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default ✨. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disables the -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger 💅.
-Walloca
This option warns on all uses of alloca in the source. Every single one. It's watching 👀.
-Walloca-larger-than=byte-size— okay so this one's the alloca watchdog 🐕. It yells at you whenever somebody callsallocawith an integer argument whose value is either zero, or that isn't bounded by a controlling predicate keeping it at most byte-size. It also warns when the bound value is just… unknown. And lowkey, arguments of non-integer types? Treated as totally unbounded, even if they look like they're constrained to the range you expected. Vibes only, no trust.
Fr fr, here's what a bounded alloca looks like:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}So in that example, passing -Walloca-larger-than=1000 would not warn, because the call to alloca is known to be at most 1000 bytes. But if you passed -Walloca-larger-than=500? Bestie, the compiler would absolutely emit a warning. 💀
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like this lil guy:
void func ()
{
void *p = alloca (n);
f (p);
}If -Walloca-larger-than=500 were passed, the above would trigger a warning — but this time because of the lack of bounds checking. Not me getting called out for having zero guardrails 🙈.
And heads up, even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}Because in that example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. The audacity of signed integers honestly 😭.
This option also warns when alloca is used in a loop. Yes, in a loop. We see you.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default, but it's usually only effective when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size. Sibling energy 👯
-Wno-alloca-larger-than— turns off-Walloca-larger-than=warnings entirely. It's equivalent to-Walloca-larger-than=SIZE_MAXor larger. Basically "I choose peace" ✨-Warith-conversion— do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from-Wconversion,-Wfloat-conversion, and-Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}Like, the second one is technically fine and yet we're still gonna talk about it 🎀
-Warray-bounds/-Warray-bounds=n— warns about out of bounds subscripts or offsets into arrays. Enabled by-Wall💅. It's way more effective when-ftree-vrpis active (the default for-O2and above), but a subset of instances are issued even without optimization. So like, don't think you're slick just because you didn't optimize.
By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or a one-element array ([1]). As a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default. Sneaky but valid.
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats trailing array of a structure as a flexible array member:
- when level
<=1, no change to the default behavior. - when level
=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays; - when level
=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays.
-Warray-bounds=1 — this is the default warning level of -Warray-bounds and is enabled by -Wall. Higher levels are not enabled, and must be explicitly requested. You gotta ask nicely 💁♀️
-Warray-bounds=2 — this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. Warning tho: this level may give a larger number of false positives and is deactivated by default. It's giving paranoid friend energy 🔥
-Wunterminated-string-initialization(C and Objective-C only) — warns about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has thenonstringattribute. Like so:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */This warning is enabled by -Wextra. And if -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the nonstring warning, because in C++ such initializations are an error. C++ really said "no 💀".
-Warray-compare— warns about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;-Warray-compare is enabled by -Wall. Comparing arrays directly is so not the move bestie 😭
-Warray-parameter/-Warray-parameter=n— warns about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds.
So here's the tea ☕: if the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.
For example, the warning triggers for the second declaration of f because the first one with the keyword static specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to f. Inconsistent much? 💅
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments. VLA girlies stay winning 🌸
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0.
-Wattribute-alias=n/-Wno-attribute-alias— warns about declarations using thealiasand similar attributes whose target is incompatible with the type of the alias. It's giving mismatched friendship bracelets 🫶
-Wattribute-alias=1 — the default warning level of the -Wattribute-alias option, diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. Iconic red flag behavior honestly 🚩
-Wattribute-alias=2 — at this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation 😬. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive. Overachiever vs. underachiever, we stan both.
Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. Whole squad, accounted for ✨
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]— warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. Like, the code is lying to your face 😭 literally a jumpscare.
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified. Combo options, we love to see it 🎀
-Wbool-compare— warns about boolean expression compared with an integer value different fromtrue/false. For instance, the following comparison is always false. Bestie it's giving "checking if water is wet but with extra steps":
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall. A bool is not out here living in the 2's world, and it's never gonna be 🦋
-Wbool-operation— warns you when someone does something sus with a boolean expression. like, bitwise-negating a boolean? bestie that is almost certainly a bug, no cap. for C it also yells at you for incrementing or decrementing a boolean, which lowkey never makes sense (in C++, decrementing a boolean is always invalid, and incrementing is invalid in C++17 and deprecated everywhere else 🫠). enabled by-Wall. ⭐-Wduplicated-branches— calls out anif-elsewhere both branches are literally the same thing. it sniffs out stuff like:
if (p != NULL)
return 0;
else
return 0;it does not warn when both branches are just a null statement. it also warns for conditional operators:
int i = x ? *p : *p;like girl why 😭
-Wduplicated-cond— snitches on duplicated conditions in anif-else-ifchain. example of what gets flagged:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }same condition twice?? pick a lane 💅
-Wframe-address— warns when__builtin_frame_addressor__builtin_return_addressis called with an argument greater than 0. those calls may return indeterminate values or crash the program, so it's giving danger. included in-Wall. 💀-Wno-discarded-qualifiers(C and Objective-C only) — do not warn if type qualifiers on pointers are being discarded. typically the compiler warns when a"const char *"variable gets passed to a function that takes a"char *"parameter. this option suppresses that warning. 🎀-Wno-discarded-array-qualifiers(C and Objective-C only) — do not warn if type qualifiers on arrays which are pointer targets are being discarded. typically the compiler warns when a"const int (*)[]"variable is passed to a function that takes a"int (*)[]"parameter. use this to hush that warning. ✨-Wno-incompatible-pointer-types(C and Objective-C only) — do not warn when there's a conversion between pointers that have incompatible types. this covers cases that-Wno-pointer-signdoesn't (that one handles pointer argument passing or assignment with different signedness). heads up though: by default, in C99 and later dialects of C, GCC treats this as an error. you can downgrade the error to a warning with-fpermissive(along with certain other errors), or for this error alone with-Wno-error=incompatible-pointer-types. also, this warning gets upgraded to an error by-pedantic-errors. 🙈-Wno-int-conversion(C and Objective-C only) — do not warn about incompatible integer-to-pointer and pointer-to-integer conversions. this is about implicit conversions; for explicit ones use-Wno-int-to-pointer-castand-Wno-pointer-to-int-cast. same deal as above: by default, in C99 and later dialects of C, GCC treats this as an error. downgrade with-fpermissive(along with certain other errors), or for this error alone with-Wno-error=int-conversion. and yeah,-pedantic-errorsupgrades it to an error too. 😬-Wzero-as-null-pointer-constant— warns when a literal0is used as a null pointer constant. like babe just use the actual null constant 🧚-Wzero-length-bounds— warns about accesses to elements of zero-length array members that might overlap other members of the same object. declaring interior zero-length arrays is discouraged because accessing them is undefined behaviour. fr fr.
for example, the first two stores in function "bad" are diagnosed because the array elements overlap the subsequent members "b" and "c". the third store is diagnosed by -Warray-bounds because it's beyond the bounds of the enclosing object:
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}option -Wzero-length-bounds is enabled by -Warray-bounds. 💜
-Wno-div-by-zero— do not warn about compile-time integer division by zero. floating-point division by zero is not warned about, because it can be a legitimate way of obtaining infinities and NaNs. so like, don't come for float division, it's valid 👑-Wsystem-headers— prints warning messages for constructs found in system header files. warnings from system headers are normally suppressed, on the assumption that they usually don't 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. but bestie, note: using-Walltogether with this option does not warn about unknown pragmas in system headers — for that,-Wunknown-pragmasmust also be used. iykyk 🌈-Wtautological-compare— warns if a self-comparison always evaluates to true or false. catches various mistakes like:
int i = 1;
...
if (i > i) { ... }it also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }which will always be false. enabled by -Wall. 🔥
-Wtrailing-whitespace-Wtrailing-whitespace=kind— warns about trailing whitespace at the end of lines, including inside comments, but excluding trailing whitespace in raw string literals."-Wtrailing-whitespace"is equivalent to"-Wtrailing-whitespace=blanks"and warns just about trailing space and horizontal tab characters."-Wtrailing-whitespace=any"warns about those or trailing form feed or vertical tab characters."-Wno-trailing-whitespace"or"-Wtrailing-whitespace=none"disables the warning, which is the default. this is a coding style warning 🎀-Wleading-whitespace=kind— warns about style issues in leading whitespace, but not about the amount of indentation. some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of"-ftabstop=n") followed by zero or fewer than n spaces. no warning is emitted on lines which contain solely whitespace (although"-Wtrailing-whitespace="warning might be emitted), and no warnings are emitted inside raw string literals. warnings are also emitted for leading whitespace inside multi-line comments."-Wleading-whitespace=spaces"warns about leading whitespace other than spaces for projects which want to indent just by spaces."-Wleading-whitespace=tabs"warns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs."-Wleading-whitespace=blanks"warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is the argument of"-ftabstop=n", 8 by default)."-Wleading-whitespace=none"disables the warning, which is the default. also a coding style warning ✨-Wtrampolines— warns about trampolines generated for pointers to nested functions. a trampoline is a small piece of data or code that's created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. for some targets it's made up of data only and thus requires no special treatment. but for most targets it's made up of code, and thus requires the stack to be made executable for the program to work properly. it's giving "we're gonna need an executable stack" 😭-Wfloat-equal— warns if floating-point values are used in equality comparisons. the idea behind this is that sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. if you're doing that, you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). in particular, instead of testing for equality, you should check whether the two values have ranges that overlap — and that's done with the relational operators, so equality comparisons are probably mistaken. no cap, floats are not the equality-comparison girlies 🦋-Wtraditional(C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided:- Macro parameters that appear within string literals in the macro body. in traditional C, macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. traditional preprocessors only considered a line to be a directive if the
#appeared in column 1 on the line. therefore-Wtraditionalwarns about directives that traditional C understands but ignores because the#does not appear as the first character on the line. it also suggests you hide directives like"#pragma"not understood by traditional C by indenting them. some traditional implementations do not recognize"#elif", so this option suggests avoiding it altogether. - A function-like macro that appears without arguments.
- The unary plus operator.
- The
Uinteger constant suffix, or theForLfloating-point constant suffixes. (traditional C does support theLsuffix on integer constants.) note: these suffixes appear in macros defined in the system headers of most modern systems, e.g. the_MIN/_MAXmacros 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 than its traditional type 😱 but bestie, this warning only fires if the base of the constant is ten. Hexadecimal or octal values, which are usually just bit patterns anyway, get a free pass 🎀
- Usage of ISO string concatenation is detected 👍
- Initialization of automatic aggregates 🫶
- Identifier conflicts with labels — traditional C straight up lacks a separate namespace for labels, which is giving "no room in the group chat" energy 💀
- Initialization of unions. If the initializer is zero, the warning is omitted — the assumption is that the zero initializer in user code is conditioned on something like
"__STDC__"to dodge missing-initializer warnings, and it relies on default initialization to zero in the traditional C case ✨ - Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. Heads up: this is a subset of the possible conversion warnings — for the full set use
-Wtraditional-conversion🔥 - Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions, because these ISO C features show up in your code when you're using libiberty's traditional C compatibility macros,
"PARAMS"and"VPARAMS". It's also bypassed for nested functions, because that feature is already a GCC extension and thus not relevant to traditional C compatibility 🧚 -Wtraditional-conversion(C and Objective-C only) — warns if a prototype causes a type conversion that's different from what would happen to the same argument without a prototype. This includes conversions of fixed point to floating and vice versa, plus conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. No cap 💅-Wdeclaration-after-statement(C and Objective-C only) — warns when a declaration pops up after a statement in a block. This construct, which C++ girlies know and love, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 🫣
This warning is upgraded to an error by -pedantic-errors.
-Wshadow— warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed 👀 Note that in C++ the compiler warns if a local variable shadows an explicit typedef, but not if it shadows astruct/class/enum. If this warning is enabled, it includes all instances of local shadowing too — which means-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis used. Same as-Wshadow=global.-Wno-shadow-ivar(Objective-C only) — do not warn whenever a local variable shadows an instance variable in an Objective-C method 🙈-Wshadow=global— warn for any shadowing. Same as-Wshadow🌈-Wshadow=local— warn when a local variable shadows another local variable or parameter 🍓-Wshadow=compatible-local— warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, "type compatibility" here means the type of the shadowing variable can be converted to that of the shadowed variable. The whole idea behind this flag (on top of-Wshadow=local) is that when a local variable shadows one of an incompatible type, it's most likely intentional, not a bug or typo — like this iconic little situation:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since the two variables "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning anyway 💅 Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means shadowing "const char *i" by "char *i" does not emit a warning.
This warning is also enabled by -Wshadow=local ✨
-Wlarger-than=byte-size — warns whenever an object is defined whose size exceeds byte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by this option can be disabled either by specifying byte-size ofSIZE_MAXor more, or by-Wno-larger-than💖
It also warns for calls to bounded functions like "memchr" or "strnlen" that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These particular warnings can only be disabled by -Wno-larger-than 🫣
-Wno-larger-than— disables-Wlarger-than=warnings. The option is equivalent to-Wlarger-than=SIZE_MAXor larger 🎀-Wframe-larger-than=byte-size — warns if the size of a function frame exceeds byte-size. Heads up: the computation done to determine the stack frame size is approximate and not conservative, so the actual requirements may be somewhat greater than byte-size even if you get no warning 😭 On top of that, any space allocated via"alloca", variable-length arrays, or related constructs is not included by the compiler when deciding whether or not to issue a warning.-Wframe-larger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size ofSIZE_MAXor more, or by-Wno-frame-larger-than.-Wno-frame-larger-than— disables-Wframe-larger-than=warnings. The option is equivalent to-Wframe-larger-than=SIZE_MAXor larger 🌸-Wfree-nonheap-object— warns when you try to deallocate an object that either wasn't allocated on the heap, or that you're deallocating via a pointer that wasn't returned from a prior call to the corresponding allocation function 💀 For example, because the call to"stpcpy"returns a pointer to the terminating nul character and not to the beginning of the object, the call to"free"below gets diagnosed:
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall 🔥
-Wstack-usage=byte-size — warns if the stack usage of a function might exceed byte-size. Good news: the computation done to determine stack usage is conservative 🫶 Any space allocated via"alloca", variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage:
- If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-stack-usage 💜
-Wno-stack-usage— disables-Wstack-usage=warnings. The option is equivalent to-Wstack-usage=SIZE_MAXor larger ✨-Wunsafe-loop-optimizations— warns if the loop cannot be optimized because the compiler can't assume anything about the bounds of the loop indices 😤 And with-funsafe-loop-optimizations, it warns if the compiler makes such assumptions. Iconic mutual beef.-Wno-pedantic-ms-format(MinGW targets only) — when used together with-Wformatand-pedanticwithout GNU extensions, this option disables the warnings about non-ISO"printf"/"scanf"format width specifiers"I32","I64", and"I"used on Windows targets, which depend on the MS runtime 🎀-Wpointer-arith— warns about anything that depends on the "size of" a function type or of"void". GNU C assigns these types a size of 1, for convenience in calculations with"void *"pointers and pointers to functions 👑 In C++, it also warns when an arithmetic operation involves"NULL". This warning is also enabled by-Wpedantic.
This warning is upgraded to an error by -pedantic-errors 💅
-Wno-pointer-compare— do not warn if a pointer is compared with a zero character constant. That usually means the pointer was meant to be dereferenced, so like… pay attention bestie 🙈 For example:
const char *p = foo ();
if (p == '\0')
return 42;Note that the code above is invalid in C++11 🫣
This warning is enabled by default.
-Wno-tsan— disables warnings about unsupported features in ThreadSanitizer 🧵 ThreadSanitizer does not support"std::atomic_thread_fence"and can report false positives. Not the vibes.-Wtype-limits— warns if a comparison is always true or always false due to the limited range of the data type, but does not warn for constant expressions 💀 For example, it warns if an unsigned variable is compared against zero with<or>=. This warning is also enabled by-Wextra.-Wabsolute-value(C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available 🌈 For example, callingabs(3.14)triggers the warning, because the appropriate function to call to compute the absolute value of adoubleargument is"fabs". The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast, and it's also enabled by-Wextra✨-Wcomment-Wcomments— warns whenever a comment-start sequence/*appears in a/*comment, or whenever a backslash-newline appears in a//comment 😭 This warning is enabled by-Wall.-Wtrigraphs— warns if any trigraphs are encountered that might change the meaning of the program 👀 Trigraphs within comments are not warned about, except those that would form escaped newlines.
This one's implied by -Wall — and even if you don't pass -Wall, it's still on unless trigraphs are turned on. So if you want trigraph conversion but zero warnings about it, while still getting the rest of the -Wall goodies, you go -trigraphs -Wall -Wno-trigraphs. 💅
-Wundef— yells at you if an undefined identifier shows up inside an#ifdirective. Those identifiers get replaced with zero, which is exactly the kind of silent chaos this warning exists to catch. 🫠-Wexpansion-to-defined— warns wheneverdefinedappears during the expansion of a macro (yes, including when the macro gets expanded by an#ifdirective). That usage is not portable, bestie. Also switched on by-Wpedanticand-Wextra.-Wunused-macros— snitches on macros defined in the main file that never get used. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hadn't been used by the time it got redefined or undefined. 💀
Built-in macros, macros defined on the command line, and macros defined in include files? Not warned about. They're safe.
Note: if a macro is actually used but only inside skipped conditional blocks, the preprocessor will still report it as unused. To dodge that warning, you could tighten the scope of the macro's definition — like moving it into the first skipped block. Or you can just hand it a dummy use, something like: #if defined the_macro_causing_the_warning #endif
-Wno-endif-labels— turns off the warning for when an#elseor an#endifis followed by text. Older programs do this constantly, in code shaped like:
#if FOO
...
#else FOO
...
#endif FOOThat second and third FOO should honestly be in comments 🙈. This warning is on by default.
-Wbad-function-cast(C and Objective-C only) — warns when a function call gets cast to a non-matching type. Like, warn if you call a function returning an integer type and then cast it to a pointer type. Bestie. Why. 😭-Wc90-c99-compat(C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. For instance: variable length arrays, thelong longtype, thebooltype, compound literals, designated initializers, and so on. This one's independent of standards mode. Warnings are disabled inside the expression that follows__extension__. ✨-Wc99-c11-compat(C and Objective-C only) — warns about features missing from ISO C99 but present in ISO C11. For instance: anonymous structures and unions, the_Atomictype qualifier, the_Thread_localstorage-class specifier, the_Alignasspecifier, theAlignofoperator, the_Generickeyword, and so on. Also independent of standards mode, and warnings are disabled in the expression following__extension__.-Wc11-c23-compat,-Wc11-c2x-compat(C and Objective-C only) — warns about features that aren't in ISO C11 but are in ISO C23. For instance: omitting the string in_Static_assert, using[[]]syntax for attributes, using decimal floating-point types, and so on. Independent of standards mode again; warnings are disabled in the expression after__extension__. Heads up: the name-Wc11-c2x-compatis deprecated. 🫡
When you're not compiling in C23 mode, -pedantic-errors upgrades these warnings to errors.
-Wc23-c2y-compat,-Wc23-c2y-compat(C and Objective-C only) — warns about features not present in ISO C23 but present in ISO C2Y. For instance:_Genericselecting with a type name instead of an expression. Independent of standards mode, warnings disabled in the expression following__extension__.
When you're not compiling in C2Y mode, -pedantic-errors upgrades these to errors. 🔥
-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. asking for an implicit conversion fromvoid *to a pointer to a non-voidtype.-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers that were fine in ISO C++ 1998 but became keywords in ISO C++ 2011. This one turns on-Wnarrowingand is enabled by-Wall. 👑-Wc++14-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by-Wall.-Wc++17-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by-Wall.-Wc++20-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by-Wall.-Wno-c++11-extensions(C++ and Objective-C++ only) — don't warn about C++11 constructs when the code is being compiled with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if you're using-Wpedantic.-Wno-c++14-extensions(C++ and Objective-C++ only) — same vibe, but for C++14 constructs compiled under an older standard. And again, without this option some C++14 constructs are only diagnosed with-Wpedantic.-Wno-c++17-extensions(C++ and Objective-C++ only) — ditto for C++17 constructs in older-standard compilation. Some of them only get diagnosed under-Wpedanticanyway.-Wno-c++20-extensions(C++ and Objective-C++ only) — ditto for C++20 constructs. Yep,-Wpedanticis still the gatekeeper for some of them.-Wno-c++23-extensions(C++ and Objective-C++ only) — ditto for C++23 constructs. Some only diagnosed with-Wpedantic.-Wno-c++26-extensions(C++ and Objective-C++ only) — ditto for C++26 constructs. Some only diagnosed with-Wpedantic. 🌈
-Wcast-qual— warns whenever a pointer gets cast in a way that strips a type qualifier off the target type. Example: warning if aconst char *is cast to a plain oldchar *. 🚩
It also warns when a cast introduces a type qualifier in an unsafe way. Like casting char ** to const char ** — that's unsafe, bestie, and here's why:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';Yeah. Pain. 💀
-Wcast-align— warns whenever a pointer cast increases the required alignment of the target. For example: warning if achar *is cast to anint *on machines where integers can only be accessed at two- or four-byte boundaries.-Wcast-align=strict— same deal, but warns whenever a pointer cast increases the target's required alignment regardless of the target machine. Like achar *cast to anint *, periodt.-Wcast-function-type— warns when a function pointer gets cast to an incompatible function pointer. Details, because there are a lot: in a cast involving function types with a variable argument list, only the types of the initial arguments that are actually provided get considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored — likeintvs.longon ILP32 targets. Type qualifiers are ignored too. The function typevoid (*) (void)is special and matches everything, which you can use to suppress this warning. And in a cast involving pointer to member types, this warning fires whenever the type cast changes the pointer to member type. Enabled by-Wextra. ✨-Wcast-user-defined— warns when a cast to reference type doesn't involve a user-defined conversion that the programmer might expect to be called.
struct A { operator const int&(); } a;
auto r = (int&)a; // warningThis warning is enabled by default.
-Wwrite-strings— in C, this gives string constants the typeconst char[length], so copying the address of one into a non-constchar *pointer produces a warning. These warnings help you find, at compile time, code that might try to write into a string constant — but only if you've been super careful withconstin your declarations and prototypes. Otherwise it's just a nuisance, which is exactly why-Walldoesn't request these warnings. 🙃
In C++, it warns about the deprecated conversion from string literals to char *. That warning is enabled by default for C++ programs.
-pedantic-errors upgrades this warning to an error in C++11 mode or later.
-Wclobbered— warns about variables that might get changed bylongjmporvfork. Also enabled by-Wextra. 😬-Wno-complain-wrong-lang— by default, language front ends complain when a command-line option is valid but not applicable to that front end.-Wno-complain-wrong-langturns that off, which is mostly useful when you're invoking a single compiler driver for multiple source files written in different languages. For example:
$ g++ -fno-rtti a.cc b.f90Here the driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which you can silence with -Wno-complain-wrong-lang. 🎀
-Wcompare-distinct-pointer-types(C and Objective-C only) — warns if pointers of distinct types get compared without a cast. This warning is enabled by default. 🦋-Wconversion— okay so this one is your lil' bestie who taps you on the shoulder every time an implicit conversion might quietly change a value 💅. It's giving: conversions between real and integer (likeabs (x)whenxisdouble), conversions between signed and unsigned (likeunsigned ui = -1), and conversions to smaller types (likesqrtf (M_PI)). But it is not gonna come for you if you wrote an explicit cast —abs ((int) x)andui = (unsigned) -1are totally fine — nor if the value isn't actually changed, like inabs (2.0). If the signed ↔ unsigned warnings are too loud for you, you can mute just those with-Wno-sign-conversion.
For C++ it also warns about confusing overload resolution for user-defined conversions, and about conversions that never use a type conversion operator: conversions to void, to the same type, to a base class, or to a reference to them. Heads up tho — in C++ the signed ↔ unsigned conversion warnings are off by default unless you explicitly turn on -Wsign-conversion. 🙈
Oh and!! Warnings about conversion from arithmetic on a small type back to that same type are only given with -Warith-conversion. iykyk.
-Wdangling-else— this is the "whichifdoes theelsebelong to??" warning, and honestly it's saved so many of us 😭. Here's the classic situation:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). That is probably not what the programmer expected — as the indentation above literally shows 🙈. When there's potential for this confusion, GCC throws a warning if this flag is specified. To make it go away, just add explicit braces around the innermost if so there's no way the else can belong to the enclosing if. Like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses. ✨
-Wdangling-pointer-Wdangling-pointer=n— warn about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended 💀. That includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. It also warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels, but ngl it may give you different results with optimization than without.-Wdangling-pointer=1— at level 1, the warning diagnoses only unconditional uses of dangling pointers.-Wdangling-pointer=2— at level 2, in addition to unconditional uses, it also diagnoses conditional uses of dangling pointers.
The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 both do the same "nope, off" thing. -Wdangling-pointer=2 is included in -Wall.
This example triggers the warning at level 1 — the address of the unnamed temporary is unconditionally referenced outside of its scope:
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}In the next function, the store of the address of the local variable x in the escaped pointer *p triggers the warning at level 1:
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}And in this one, the array a is out of scope when the pointer s is used. Since the code that sets s is conditional, the warning triggers at level 2:
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}-Wdate-time— warns when the macros__TIME__,__DATE__or__TIMESTAMP__show up, because they might ruin bit-wise-identical reproducible compilations 🕰️. Reproducibility girlies stay winning.-Wempty-body— warns if you've got an empty body in anif,elseordo whilestatement. (Bestie. Where's the body.) This warning is also enabled by-Wextra.-Wno-endif-labels— do not warn about stray tokens after#elseand#endif. Cute and chill.-Wenum-compare— warns about a comparison between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions are also diagnosed, and the warning is enabled by default. In C this warning is enabled by-Wall.-Wenum-conversion— warns when a value of an enumerated type gets implicitly converted to a different enumerated type. In C this warning is enabled by-Wextra.-Wenum-int-mismatch(C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. Example time:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. But since the choice of the underlying type of an enumerated type is implementation-defined, mismatches like this may cause portability issues 🫠. In C++, such mismatches are straight-up an error. In C, this warning is enabled by -Wall and -Wc++-compat.
-Wjump-misses-init(C, Objective-C only) — warns if agotostatement or aswitchstatement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. It only warns about variables that are initialized when they're declared, okurrr. And this warning is only supported for C and Objective-C — in C++ that kind of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. You can turn it off with the -Wno-jump-misses-init option.
-Wsign-compare— warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned 😬. In C++ this warning is also enabled by-Wall. In C, it's also enabled by-Wextra.-Wsign-conversion— warns about implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning ✨. In C, this option is also enabled by-Wconversion.-Wflex-array-member-not-at-end(C and C++ only) — warns when a structure containing a C99 flexible array member as the last field is not at the end of another structure. Like, why is it in the middle, bestie 💀. It warns about stuff like:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };-Wfloat-conversion— warns about implicit conversions that reduce the precision of a real value. That includes real → integer conversions, and higher-precision real → lower-precision real conversions. This option is also enabled by-Wconversion.-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order. Say less.-Wsizeof-array-div— warns about divisions of twosizeofoperators when the first one is applied to an array and the divisor doesn't equal the size of the array element. In that case the computation won't give you the number of elements in the array, which is almost certainly what you wanted 😭. It warns about stuff like:
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}This warning is enabled by -Wall.
-Wsizeof-pointer-div— warns about suspicious divisions of twosizeofexpressions that divide the pointer size by the element size — which is the usual way to compute the array size, but it does not work correctly with pointers 🫠. It warns about stuff likesizeof (ptr) / sizeof (ptr[0])ifptris not an array but a pointer. This warning is enabled by-Wall.-Wsizeof-pointer-memaccess— warns about suspicious length parameters to certain string and memory built-in functions when the argument usessizeof. It triggers for example onmemset (ptr, 0, sizeof (ptr));ifptris not an array but a pointer — and it suggests a possible fix ✨ — or onmemcpy (&foo, ptr, sizeof (&foo));.-Wsizeof-pointer-memaccessalso warns about calls to bounded string copy functions likestrncatorstrncpythat specify as the bound asizeofexpression of the source array. For example, in this function the call tostrncatspecifies the size of the source string as the bound — that is almost certainly a mistake, so the call gets diagnosed:
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument— do not warn when thesizeofoperator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs.-Wmemset-elt-size— warns about suspicious calls to thememsetbuilt-in function: if the first argument references an array, and the third argument is a number equal to the number of elements but not equal to the size of the array in memory, that's a red flag 🚩. It means the user forgot to multiply by the element size. This warning is enabled by-Wall.-Wmemset-transposed-args— this one's a lil detective 🕵️♀️ for suspicious calls to thememsetbuilt-in function where the second argument is not zero and the third argument is zero. For example, the callmemset (buf, sizeof buf, 0)gets diagnosed, because bestie, you clearly meantmemset (buf, 0, sizeof buf). The diagnostic is only emitted if the third argument is a literal zero — otherwise, if it's an expression that gets folded to zero, or a cast of zero to some type, it's far less likely that the arguments were mistakenly transposed, so no warning comes out. This warning is enabled by-Wall.-Waddress— warns about suspicious uses of address expressions, and honestly the vibe check here is strong. These include comparing the address of a function or a declared object to the null pointer constant, like this:
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, like this:
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}and tests of the results of pointer addition or subtraction for equality to null, like this:
void f (const int *p, int i)
{
return p + i == NULL;
}Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so using them in a conditional might mean you're missing parentheses in a function call, or missing a dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. Comparisons against string literals result in unspecified behavior and aren't portable, and they suggest the intent was to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.
-Wno-address-of-packed-member— do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. 🙈-Wlogical-op— warns about suspicious uses of logical operators in expressions ✨. This includes using logical operators in contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }-Wlogical-not-parentheses— warns about logical not used on the left-hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its whole purpose is detecting suspicious code like the following:
int a;
...
if (!a > 1) { ... }It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }This warning is enabled by -Wall.
-Waggregate-return— warns 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.) Like, okay, diva 💅-Wno-aggressive-loop-optimizations— do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations.-Waggressive-loop-optimizationsis enabled by default.-Wno-attributes— do not warn if an unexpected__attribute__is used, such as unrecognized attributes, function attributes applied to variables, etc. Important caveat bestie: this does not stop errors for incorrect use of supported attributes.
Warnings about ill-formed uses of standard attributes are upgraded to errors by -pedantic-errors.
Additionally, using -Wno-attributes=, it's possible to suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:
[[vendor::attr]] void f();It's also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor::, which prevents warning about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();Note that -Wno-attributes= does not imply -Wno-attributes. 🫶
-Wno-builtin-declaration-mismatch— warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match those expected by the function. When-Wextrais specified, it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. To avoid the warning, include the appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of__TIMESTAMP__,__TIME__,__DATE__,__FILE__, and__BASE_FILE__.-Wfree-labels(C and Objective-C only) — warns if a label is applied to a non-statement, or occurs at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before C23 by -pedantic-errors.
-Wheader-guard— warns if a valid preprocessor header multiple inclusion guard has a#definedirective right after#ifndefor#if !defineddirective for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding#ifndefdirective at the end of the header, and the#definedirective defines an object-like macro with empty definition. In such a case, it often is just a misspelled guard name, either in the#ifndefor#if !defineddirective or in the subsequent#definedirective 😭. This warning is enabled by-Wall.-Wstrict-prototypes(C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types.) Classic vibe of "just tell me the types, bestie."-Wold-style-declaration(C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers likestaticaren't the first things in a declaration. This warning is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there's a previous prototype. A definition using()is not considered an old-style definition in C23 mode, because it's equivalent to(void)in that case, but it is considered an old-style definition for older standards. 🫠-Wdeprecated-non-prototype(C and Objective-C only) — warns if a function declared with an empty parameter list()is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration. Both cases are errors in C23 and later dialects of C.
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name(C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. This can be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before C23 by -pedantic-errors.
-Wmissing-parameter-type(C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }This warning is also enabled by -Wextra.
-Wno-declaration-missing-parameter-type(C and Objective-C only) — do not warn if a function declaration contains a parameter name without a type. Such function declarations do not provide a function prototype and prevent most type checking in function calls. 💀
This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes(C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. Heads up: this option is not valid for C++ because all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. Use-Wmissing-declarationsto detect missing declarations in C++.-Wmissing-variable-declarations(C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matchingexterndeclaration in a header file. ✨-Wmissing-declarations— okay so this one yells at you if a global function is defined without a previous declaration. It does this even if the definition itself provides a prototype. Use it to catch global functions that aren’t declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use-Wmissing-prototypesto detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. no cap, it’s picky in exactly those ways ✨-Wmissing-field-initializers— warns if a structure’s initializer has some fields missing. For example, the following code causes such a warning, because"x.h"is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };In C this option does not warn about designated initializers, so the following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Likewise, in C++ this option does not warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. the loopholes are real and very specific 🎀
-Wno-missing-requires— by default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write
bool satisfied = requires { requires C<T> };so satisfied is only true if concept C is satisfied for type T.
This warning can be disabled with -Wno-missing-requires. the double requires is the whole plot twist 🦋
-Wno-missing-template-keyword— the member access tokens.,->and::must be followed by the"template"keyword if the parent object is dependent and the member being named is a template.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}This warning can be disabled with -Wno-missing-template-keyword. parentheses are literally little vibe shields here 💅
-Wno-multichar— do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user’s code, as they have implementation-defined values, and should not be used in portable code. fr fr, they’re giving typo energy 🔥-Wnormalized=[none|id|nfc|nfkc]— in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there’s no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. bestie, “literally impossible to see” is not a flex, it’s a warning label 😭
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. homoglyph chaos, honestly 🧚
-Wno-attribute-warning— do not warn about usage of functions declared with"warning"attribute. By default, this warning is enabled.-Wno-attribute-warningcan be used to disable the warning or-Wno-error=attribute-warningcan be used to disable the error when compiled with-Werrorflag. two different switches for two different moods 🌸-Wno-deprecated— do not warn about usage of deprecated features.
In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards, specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission. that’s a whole receipt of deprecated stuff 👑
-Wno-deprecated-declarations— do not warn about uses of functions, variables, and types marked as deprecated by using the"deprecated"attribute. basically “I know it’s old, I’m doing it anyway” 🙈-Wno-overflow— do not warn about compile-time overflow in constant expressions. math goblin mode, silenced 💀-Wno-odr— warn about One Definition Rule violations during link-time optimization. Enabled by default. yes, the-Wno-name is a little gremlin, but that’s what the page says 🫶-Wopenacc-parallelism— warn about potentially suboptimal choices related to OpenACC parallelism. OpenACC gets a vibe check 🌈-Wno-openmp— warn about suspicious OpenMP code. suspicious means suspicious, no further notes 🔍-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model. cost model said “actually, no” and GCC is telling on it ⭐-Woverride-init(C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init. the -Wextra combo meal, but hold the pickles 🍓
-Wno-override-init-side-effects(C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default. side effects get a pass here ✨-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 variablef.xinstruct baris misaligned even thoughstruct bardoes not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};misaligned for little benefit is such a tragedy 😭
-Wnopacked-bitfield-compat— the 4.1, 4.2 and 4.3 series of GCC ignore the"packed"attribute on bit-fields of type"char". This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between fieldaandbin this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning. compiler archaeology, but make it a warning 🏺
-Wpacked-not-aligned(C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued onstruct S, like,warning: alignment 1 of 'struct S' is less than 8, in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall. -Wall said “I gotchu” 👑
-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. field rearrangement is self-care 🎀-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. redundant but valid? still getting called out 💅-Wrestrict— warn when an object referenced by arestrict-qualified parameter (or, in C++, a__restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to thestrcpyfunction below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL intoa[4], the copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall. overlap detected, friendship with strcpy ended 💀
-Wnested-externs(C and Objective-C only) — warn if anexterndeclaration is encountered within a function. nested externs? in this economy? 🦋-Winline— warns you when a function that's declaredinlinejust... doesn't get inlined 😭. One caveat tho: even with this flag, the compiler stays quiet about failures to inline stuff declared in system headers. No snitching on the system 💅.
Also, real talk: the compiler uses a whole bunch of heuristics to decide whether to inline a function — like how big the function is, and how much inlining it's already done in the current function. So a seemingly tiny change in your source can make these warnings appear or vanish. It's giving "why is my build different today" energy. Spooky but true 🦋.
-Winterference-size— warns you if you're using C++17'sstd::hardware_destructive_interference_sizewithout telling it what value to use via--param destructive-interference-size. It also side-eyes questionable values for that option 👀.
So what's this variable even for? It's meant to control class layout, specifically to dodge false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};Here one and two are meant to sit far enough apart that storing to one doesn't force the other's cache line to reload. Iconic cache-line separation behavior ✨.
By default, --param destructive-interference-size and --param constructive-interference-size get set based on your current -mtune option — usually the L1 cache line size for that particular target CPU, sometimes a whole range if you're tuning for a generic target. So like, every translation unit that depends on ABI compatibility for these variables must be compiled with the same -mtune (or -mcpu). No mixing, bestie 🚫.
If ABI stability matters to you (say, you're using this in a library header), you should probably just not use the hardware interference size variables at all. Or alternatively, force a particular value with --param.
And if you're confident your use of the variable doesn't affect ABI outside a single build of your project, you can shut the warning off with -Wno-interference-size 🙈.
-Wint-in-bool-context— flags suspicious use of integer values where booleans belong. Like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g.if (a <= b ? 2 : 3). Or left shifting signed integers in boolean context, likefor (a = 0; 1 << a; a++);. Same deal for all kinds of multiplications, no matter the data type. Enabled by-Wall🎀.-Wno-int-to-pointer-cast— suppresses warnings from casts to pointer type of an integer of a different size. Note that in C++, casting to a pointer type of smaller size is straight-up an error.Wint-to-pointer-castis enabled by default.-Wno-pointer-to-int-cast(C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size.-Winvalid-pch— warns if a precompiled header is chilling in the search path but can't actually be used. Sad precompiled header hours 😔.-Winvalid-utf8— warns if an invalid UTF-8 character is found. This one's on by default for C++23 if-finput-charset=UTF-8is used, and gets turned into an error with-pedantic-errors.-Wno-unicode— don't diagnose invalid forms of delimited or named escape sequences that are treated as separate tokens.Wunicodeis enabled by default.-Wlong-long— warns if thelong longtype is used. Enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. Wanna silence it? Use-Wno-long-long🤫.
Heads up: this warning gets upgraded to an error by -pedantic-errors. Fr fr.
-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either-Wpedanticor-Wtraditional. To inhibit the warning messages, use-Wno-variadic-macros.-Wno-varargs— don't warn upon questionable usage of the macros used to handle variable arguments likeva_start. These warnings are enabled by default. So this is your "please stop yelling at my varargs" button 💀.-Wvector-operation-performance— warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning 🔥. A vector operation can be implemented:- "piecewise" — the scalar operation is performed on every vector element;
- "in parallel" — the vector operation uses scalars of wider type, which normally is more performance efficient;
- "as a single scalar" — the vector fits into a scalar type.
-Wvla— warns if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array.
Upgraded to an error by -pedantic-errors. You've been warned 💅.
-Wvla-larger-than=byte-size— if you use this option, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceed byte-size bytes. Works similar to how-Walloca-larger-than=byte-sizeworks, but with variable-length arrays 🌸.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not even trigger for those. Sneaky.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size 🫶.
-Wno-vla-larger-than— disables-Wvla-larger-than=warnings. This option is equivalent to-Wvla-larger-than=SIZE_MAXor larger.-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds👑.
If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function, and the maximum number of elements accessed by it. Fail to provide arguments of sufficient size, or access more than the maximum number of elements, and you might get diagnosed 😭.
For example, the warning triggers for these redeclarations because the first one allows an array of any size to be passed to f, while the second one specifies that the array argument must have at least n elements. Also, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning too:
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}-Wvla-parameter is included in -Wall. And the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments ⭐.
-Wvolatile-register-var— warns if a register variable is declaredvolatile. The thing is, thevolatilemodifier doesn't actually inhibit all the optimizations that may eliminate reads and/or writes to register variables. So like... it's not doing what you think it's doing, bestie 🫠. Enabled by-Wall.-Wno-xor-used-as-pow(C, C++, Objective-C and Objective-C++ only) — disables warnings about uses of^, the exclusive or operator, where it looks like the code meant exponentiation. Specifically, the warning fires when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant.
In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. Different language, different vibe 😅.
You can silence this warning by converting one of the operands to hexadecimal, as well as by compiling with -Wno-xor-used-as-pow.
-Wdisabled-optimization— warns if a requested optimization pass is disabled. Ngl, this doesn't generally mean anything's wrong with your code; it just means GCC's optimizers couldn't handle the code effectively. Often the problem is your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. It's not you, it's the optimizer's schedule 💀.-Wpointer-sign(C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. It's implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign.
Upgraded to an error by -pedantic-errors 🔥.
-Wstack-protector— this option is only active when-fstack-protectoris active. It warns about functions that aren't protected against stack smashing. Not them being vulnerable and everyone knowing 😭.-Woverlength-strings— warns about string constants longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants way longer than the standard's minimum limit, but very portable programs should avoid using longer strings 🎀.
The limit applies after string constant concatenation, and doesn't count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 doesn't specify a normative minimum maximum, so we don't diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants(C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix. When used together with-Wsystem-headers, it warns about such constants in system header files too 👀. This can be useful when preparing code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99.-Wno-lto-type-mismatch— during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires-fltoto be enabled. Enabled by default.-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that's been marked with thedesignated_initattribute. Positively iconically permissive of you 🌈.
🌸 Options That Control Static Analysis 🌸
Okay bestie, this next batch is the analysis girlies — the flags that make GCC actually sit down and think about your code instead of just compiling it and running away. 💅
-fanalyzer
This turns on a static analysis of program flow — it goes hunting for "interesting" interprocedural paths through your code and yells at you about problems it finds on them. 🔥
Heads up: this analysis is much more expensive than other GCC warnings. Like, it will take a minute. Pay for the good stuff, bestie. 💸
In technical terms (iykyk), it performs coverage-guided symbolic execution of the code being compiled. It is neither sound nor complete: it can have false positives and false negatives. It's a bug-finding tool — not a tool for proving program correctness. Periodt.
The analyzer is only suitable for use on C code in this release. 🫶
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalThis option is only available if GCC was configured with analyzer support enabled. No support, no analysis, no slay. 💀
-Wanalyzer-symbol-too-complex
So if -fanalyzer is enabled, the analyzer uses various heuristics to try to track the state of memory — but sufficiently complicated code can absolutely defeat them. 😭
By default, the analysis silently stops tracking values of expressions if they exceed the threshold defined by --param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for those expressions. The -Wanalyzer-symbol-too-complex option warns you if this occurs. ✨
-Wanalyzer-too-complex
Same energy, different heuristics: with -fanalyzer enabled, the analyzer tries to explore the control flow and data flow in the program — but again, complicated code can defeat it. 🙈
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns you if this occurs. ⭐
-Wno-analyzer-allocation-size
This warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size. 🎀
This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer).
See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).
-Wno-analyzer-deref-before-check
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it. 🎀
This diagnostic warns for paths through the code in which a pointer is checked for NULL after it has already been dereferenced — which, um, suggests the pointer could have been NULL the whole time. 😬 Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, and thus will complain if such values are checked for NULL after returning from such a function call.
Real talk: this diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically consider such checks for NULL as redundant and optimize them away before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. Iconic but picky, honestly.
-Wno-analyzer-double-fclose
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it. 🎀
This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-double-free
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to disable it. 🎀
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once — either free, or a deallocator referenced by attribute malloc.
See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).
-Wno-analyzer-exposure-through-output-file
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it. 🎀
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Bestie. Do not log the password. 😭
See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).
-Wanalyzer-exposure-through-uninit-copy
This warning requires both -fanalyzer and the use of a plugin to specify a function that copies across a "trust boundary". Use -Wno-analyzer-exposure-through-uninit-copy to disable it. 🫶
This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 🙈
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).
-Wno-analyzer-fd-access-mode-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it. 🎀
This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa.
This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.
-Wno-analyzer-fd-double-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to disable it. 🎀
This diagnostic warns for paths through code in which a file descriptor can be closed more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-fd-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable it. 🎀
This diagnostic warns for paths through code in which an open file descriptor is leaked. 🚰
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-fd-phase-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it. 🎀
This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it. It's giving "skipping steps in the tutorial". 💀
See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).
-Wno-analyzer-fd-type-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-type-mismatch to disable it. 🎀
This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket. 🧦 Wrong shoe, wrong foot.
-Wno-analyzer-fd-use-after-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-after-close to disable it. 🎀
This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor. 🙈
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.
-Wno-analyzer-fd-use-without-check
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-without-check to disable it. 🎀
This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. 👑
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.
-Wno-analyzer-file-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it. 🎀
This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked. 🫧
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-free-of-non-heap— okay so this one needs-fanalyzerto even exist (that's what turns it on), and you use-Wno-analyzer-free-of-non-heapto turn it back off. What it does is warn you about code paths wherefreegets called on a pointer that was, like… never on the heap? Think an on-stack buffer, or a global. Not it. 😭
See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).
-Wno-analyzer-imprecise-fp-arithmetic— requires-fanalyzer, which enables it; disable with-Wno-analyzer-imprecise-fp-arithmetic. This one flags paths where floating-point arithmetic gets used in places that genuinely need precise computation. And ngl, at the moment it only warns on floating-point operands showing up inside the calculation of an allocation size. That's the whole scope, bestie. 🎯-Wno-analyzer-infinite-loop— requires-fanalyzer, which enables it; disable with-Wno-analyzer-infinite-loop. It warns about paths through the code that look like they lead to an infinite loop.
Specifically, the analyzer drops this warning when it "sees" a loop where:
- no externally-visible work could be being done within the loop
- there is no way to escape from the loop
- the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true
One way this warning gets emitted: there's an execution path through a loop where taking the path on one iteration implies that the same path will be taken on all subsequent iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}The analyzer will complain about the above case because if opcode ever matches none of the cases, the switch follows the implicit default case, which makes the body of the loop a "no-op" with cpu_state.pc unchanged — so you get the same value of opcode on all subsequent iterations, and boom, infinite loop. 💀
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).
-Wno-analyzer-infinite-recursion— requires-fanalyzer, which enables it; disable with-Wno-analyzer-infinite-recursion. Warns about paths through the code that appear to lead to infinite recursion.
Specifically, when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with that at the entry to the previous frame of that function on the stack. The warning fires if nothing in memory appears to be changing; any changes observed to parameters or globals are assumed to lead to termination of the recursion and therefore suppress the warning.
Heads up though: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're trying to trigger this one, fr fr.
Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented a totally different way.
See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).
-Wno-analyzer-jump-through-null— requires-fanalyzer, which enables it; disable with-Wno-analyzer-jump-through-null. Warns about paths through the code where aNULLfunction pointer is called. Like, you called nothing, bestie. 🙈-Wno-analyzer-malloc-leak— requires-fanalyzer, which enables it; disable with-Wno-analyzer-malloc-leak. This one warns about paths where a pointer allocated via an allocator gets leaked: eithermalloc, or a function marked with attributemalloc. You forgot to free it, didn't you. 😭
See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html).
-Wno-analyzer-mismatching-deallocation— requires-fanalyzer, which enables it; disable with-Wno-analyzer-mismatching-deallocation. Warns about paths where the wrong deallocation function is called on a pointer value, based on which function was used to allocate that pointer value. The diagnostic warns about mismatches betweenfree, scalardeleteand vectordelete[], plus those marked as allocator/deallocator pairs using attributemalloc. Matching sets only, babes ✨
See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).
-Wno-analyzer-out-of-bounds— requires-fanalyzer, which enables it; disable with-Wno-analyzer-out-of-bounds. Warns about paths through the code where a buffer is definitely read or written out-of-bounds. The diagnostic applies for cases where the analyzer can determine a constant offset, and for accesses past the end of a buffer (also a constant capacity). Further, it does limited checking for accesses past the end when the offset as well as the capacity is symbolic.
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).
And when the analyzer is able, it will emit a whole text-art diagram visualizing the spatial relationship between the memory region the analyzer predicts would be accessed, versus the range of memory that's valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. Iconic, honestly. 🎨 That diagram can be suppressed using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers— requires-fanalyzer, which enables it; disable with-Wno-analyzer-overlapping-buffers. Warns about paths where overlapping buffers get passed to an API for which the behavior on such buffers is undefined. (Undefined behavior = no promises, bestie, none.)
Specifically, the diagnostic occurs on calls to these functions:
memcpystrcatstrcpy
for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument— requires-fanalyzer, which enables it; disable with-Wno-analyzer-possible-null-argument. Warns about paths where a possibly-NULL value gets passed to a function argument marked with__attribute__((nonnull))as requiring a non-NULL value. The attribute said no, you said maybe. 🙃
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-possible-null-dereference— requires-fanalyzer, which enables it; disable with-Wno-analyzer-possible-null-dereference. Warns about paths where a possibly-NULL value gets dereferenced. Possibly. Which is enough to ruin your whole day.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-null-argument— requires-fanalyzer, which enables it; disable with-Wno-analyzer-null-argument. Warns about paths where a value known to be NULL gets passed to a function argument marked with__attribute__((nonnull))as requiring a non-NULL value. Not "possibly" this time — it knows. 💀
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-null-dereference— requires-fanalyzer, which enables it; disable with-Wno-analyzer-null-dereference. Warns about paths where a value known to be NULL gets dereferenced. Same energy, dereference edition.
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-putenv-of-auto-var— requires-fanalyzer, which enables it; disable with-Wno-analyzer-putenv-of-auto-var. Warns about paths where a call toputenvgets a pointer to an automatic variable or an on-stack buffer. That stack frame is gonna be gone, bestie, and the env is gonna be pointing at ghosts. 👻
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).
-Wno-analyzer-shift-count-negative— requires-fanalyzer, which enables it; disable with-Wno-analyzer-shift-count-negative. Warns about paths where a shift is attempted with a negative count. It's analogous to the-Wshift-count-negativediagnostic implemented in the C/C++ front ends, but this one is implemented based on analyzing interprocedural paths instead of just parsing the syntax tree. Fair warning though: the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. 🌸-Wno-analyzer-shift-count-overflow— requires-fanalyzer, which enables it; disable with-Wno-analyzer-shift-count-overflow. Warns about paths where a shift is attempted with a count greater than or equal to the precision of the operand's type. Analogous to the-Wshift-count-overflowdiagnostic implemented in the C/C++ front ends, but again it's built on analyzing interprocedural paths rather than merely parsing the syntax tree. Same caveat: the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.-Wno-analyzer-stale-setjmp-buffer— requires-fanalyzer, which enables it; disable with-Wno-analyzer-stale-setjmp-buffer. Warns about paths wherelongjmpis called to rewind to ajmp_bufrelating to asetjmpcall in a function that has already returned. That buffer is expired, bestie — rewinding into a function that's already left the chat. 🫠
so yeah — when setjmp is called on a jmp_buf to bookmark a rewind point, it saves the stack frame. But the second that function containing the setjmp call returns, that stack frame is toast. Trying to rewind to it with longjmp? You're referencing a stack frame that literally doesn't exist anymore, and that's giving crash (or worse) energy. 💀
-Wno-analyzer-tainted-allocation-size— bestie, this warning requires-fanalyzer(that's what turns it on); use-Wno-analyzer-tainted-allocation-sizeto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an allocation without being sanitized, so that an attacker could inject an excessively large allocation and potentially cause a denial of service attack.
See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).
-Wno-analyzer-tainted-assertion— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-assertionto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as part of a condition without being first sanitized, and that condition guards a call to a function marked with attribute noreturn (such as the function __builtin_unreachable). Functions like that typically indicate abnormal termination of the program, like for assertion failure handlers. For example:
assert (some_tainted_value < SOME_LIMIT);In such cases:
- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
- when assertion-checking is disabled, such as by defining
NDEBUG, an attacker could inject data that subverts the process, since it presumably violates a precondition that is being assumed by the code.
Real talk: when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer ever has a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled. 🫶
For the purpose of this warning, any function marked with attribute noreturn is considered as a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.
See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}despite the above not being an assertion failure, strictly speaking. 🙈
-Wno-analyzer-tainted-array-index— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-array-indexto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the index of an array access without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-tainted-divisor— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-divisorto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the divisor in a division or modulus operation without being sanitized, so that an attacker could inject a division-by-zero.
See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).
-Wno-analyzer-tainted-offset— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-offsetto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as a pointer offset without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).
-Wno-analyzer-tainted-size— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-sizeto disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-undefined-behavior-ptrdiff— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-ptrdiffto disable it.
This diagnostic warns for paths through the code in which a pointer subtraction occurs where the pointers refer to different chunks of memory. Code like that is relying on undefined behavior, because pointer subtraction is only defined for cases where both pointers point to within (or just after) the same array.
See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).
-Wno-analyzer-undefined-behavior-strtok— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-strtokto disable it.
This diagnostic warns for paths through the code in which a call is made to strtok with undefined behavior.
Specifically, passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior.
-Wno-analyzer-unsafe-call-within-signal-handler— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-unsafe-call-within-signal-handlerto disable it.
This diagnostic warns for paths through the code in which a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. 😭
See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).
-Wno-analyzer-use-after-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-use-after-freeto disable it.
This diagnostic warns for paths through the code in which a pointer is used after a deallocator is called on it: either free, or a deallocator referenced by attribute malloc.
See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).
-Wno-analyzer-use-of-pointer-in-stale-stack-frame— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-pointer-in-stale-stack-frameto disable it.
This diagnostic warns for paths through the code in which a pointer is dereferenced that points to a variable in a stale stack frame.
-Wno-analyzer-va-arg-type-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-va-arg-type-mismatchto disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use va_arg to extract a value passed to a variadic call, but uses a type that does not match that of the expression passed to the call.
See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).
-Wno-analyzer-va-list-exhausted— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-exhaustedto disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use va_arg to access the next value passed to a variadic call, but all of the values in the va_list have already been consumed. 💀
See CWE-685: Function Call With Incorrect Number of Arguments (https://cwe.mitre.org/data/definitions/685.html).
-Wno-analyzer-va-list-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-leakto disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects that va_start or va_copy has been called on a va_list without a corresponding call to va_end.
-Wno-analyzer-va-list-use-after-va-end— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-use-after-va-endto disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list.
-Wno-analyzer-write-to-const— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable it.
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a const object. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-write-to-string-literal— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-string-literalto disable it.
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a string literal. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-use-of-uninitialized-value— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-uninitialized-valueto disable it.
This diagnostic warns for paths through the code in which an uninitialized value is used.
See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).
The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:
alloca- the built-in functions
__builtin_alloc,__builtin_alloc_with_align,__builtin_calloc,__builtin_free,__builtin_malloc,__builtin_memcpy,__builtin_memcpy_chk,__builtin_memset,__builtin_memset_chk,__builtin_realloc,__builtin_stack_restore, and__builtin_stack_save callocfreemallocmemsetoperator deleteoperator delete []operator newoperator new []reallocstrdupstrndup
...and of these iconic functions for wrangling file descriptors:
openclosecreatdup,dup2anddup3isattypipe, andpipe2readwritesocket,bind,listen,accept, andconnect
...and of these ones for working with <stdio.h> streams, bestie:
- the built-in functions
__builtin_fprintf,__builtin_fprintf_unlocked,__builtin_fputc,__builtin_fputc_unlocked,__builtin_fputs,__builtin_fputs_unlocked,__builtin_fwrite,__builtin_fwrite_unlocked,__builtin_printf,__builtin_printf_unlocked,__builtin_putc,__builtin_putchar,__builtin_putchar_unlocked,__builtin_putc_unlocked,__builtin_puts,__builtin_puts_unlocked,__builtin_vfprintf, and__builtin_vprintf fopenfcloseferrorfgetsfgets_unlockedfilenofreadgetcgetcharfprintfprintffwrite
and also these absolute legends of functions:
- the built-in functions
__builtin_expect,__builtin_expect_with_probability,__builtin_strchr,__builtin_strcpy,__builtin_strcpy_chk,__builtin_strlen,__builtin_va_copy, and__builtin_va_start - the GNU extensions
erroranderror_at_line getpasslongjmpputenvsetjmpsiglongjmpsignalsigsetjmpstrcatstrchrstrlen
Plus — plot twist — a whole bunch of functions with an __analyzer_ prefix have special meaning to the analyzer, and those are documented in the GCC Internals manual. iykyk 💅
The parameters that actually matter for controlling the exploration are these cuties:
--param analyzer-bb-explosion-factor=value--param analyzer-max-enodes-per-program-point=value--param analyzer-max-recursion-depth=value--param analyzer-min-snodes-for-call-summary=value
And now, the options that control the analyzer. Buckle up, there's like twenty of them 🎀
-fanalyzer-call-summaries— makes interprocedural analysis wayyy simpler by computing the effect of certain calls instead of exploring every single path through the function from callsite to each possible return. Fr fr, so much walking saved. If it's enabled, call summaries only get used for functions with more than one call site and that are sufficiently complicated (as per--param analyzer-min-snodes-for-call-summary=value).-fanalyzer-checker=name — restricts the analyzer so it only runs the named checker, and enables it. Just the one. Narrow the vibes. ✨-fanalyzer-debug-text-art-headings— this one's for analyzer developers, bestie. If you turn it on, the analyzer adds extra annotations to any diagrams it generates. Cute little labels. 🎨-fno-analyzer-feasibility— this option is intended for analyzer developers.
By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits — like, that the conditions it's holding onto aren't mutually exclusive. No cap. Diagnostics where no feasible path can be found get rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.
-fanalyzer-fine-grained— this option is intended for analyzer developers.
Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information. 💥
By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. One statement, one edge. Main character energy for every single line.
-fanalyzer-show-duplicate-count— this option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. Like "hey, there were more of these, we just hid them 🫣".-fanalyzer-show-events-in-system-headers— by default the analyzer emits simplified diagnostics paths by hiding events that are fully located within a system header. With-fanalyzer-show-events-in-system-headersthose events are no longer suppressed. Nothing stays hidden. 🙈-fno-analyzer-state-merge— this option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge— this option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues. Keep everything. Hoarder mode. 🗃️
-fno-analyzer-suppress-followups— this option is intended for analyzer developers.
By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics. Honestly, respectful.
The diagnostics that terminate analysis along a path are:
-Wanalyzer-null-argument-Wanalyzer-null-dereference-Wanalyzer-use-after-free-Wanalyzer-use-of-pointer-in-stale-stack-frame-Wanalyzer-use-of-uninitialized-value
With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. ⚠️ Go off, I guess.
-fanalyzer-transitivity— this option enables transitivity of constraints within the analyzer. Math girlies rise. 🔥-fno-analyzer-undo-inlining— this option is intended for analyzer developers.
-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. 😭
By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer.
-fanalyzer-verbose-edges— this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. 🗣️-fanalyzer-verbose-state-changes— this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.-fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.
The level can be one of:
- 0 — at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-
freediagnostic, both calls tofreewill be shown. - 1 — as per the previous level, but also show events for the entry to each function.
- 2 — as per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional).
This level is the default. ⭐
- 3 — as per the previous level, but show all control flow events, not just significant ones. Maximal tea. 🫖
- 4 — this level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.
-fdump-analyzer— dumps internal details about what the analyzer is doing to file.analyzer.txt. Heads up:-fdump-analyzer-stderroverrides this option.-fdump-analyzer-stderr— dumps internal details about what the analyzer is doing to stderr. This option overrides-fdump-analyzer. 💅-fdump-analyzer-callgraph— dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. Pretty graph time 🌸-fdump-analyzer-exploded-graph— dumps a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. It's giving aesthetic and informative.-fdump-analyzer-exploded-nodes— emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source.-fdump-analyzer-exploded-nodes-2— dumps a textual representation of the "exploded graph" to file.eg.txt.-fdump-analyzer-exploded-nodes-3— dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt*. Word of warning: this is typically a large number of dump files. 💀 Like, a lot a lot.-fdump-analyzer-exploded-paths— dumps a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt.-fdump-analyzer-feasibility— dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form file..fg.dot, file..tg.dot, and file..fpath.txt.-fdump-analyzer-infinite-loop— dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the form file..infinite-loop.dot. 🔁 Round and round and round~-fdump-analyzer-json— dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. Heads up though: the precise format is subject to change. ⚠️ Don't get too attached, bestie.-fdump-analyzer-state-purge— okay so this is like-fdump-analyzer-supergraph(see above, bestie 👆), it dumps a representation of the "supergraph" that's suitable for viewing with GraphViz, but plot twist: it annotates the graph with info on what state will be purged at each node. The graph gets written tofile.state-purge.dot. Slay.-fdump-analyzer-supergraph— dumps representations of the "supergraph" suitable for viewing with GraphViz, intofile.supergraph.dotandfile.supergraph-eg.dot. These show all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the "exploded graph" and diagnostics associated with them. Two files, one flag, main character energy 💅-fdump-analyzer-untracked— emits custom warnings with internal details that are intended for analyzer developers. iykyk, this one's not for you unless you're building the analyzer itself 🧚
✨ Options for Debugging Your Program ✨
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options. So cute, they can be friends 💖
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all (😭 she's gone); flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. Fr fr, optimized debugging is real and it works.
If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. Lowkey the move 🫶
-g— produces debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. ⚠️ So like, exclusive bestie behaviour. If you want to control for certain whether to generate the extra information, use -gvms (see below).
-ggdb— produces debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. She's doing the absolute most and we love her for it 💅-gdwarf-gdwarf-version— produces debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4).
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm (yes the paren really is in there 😭) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. Don't let the name fool you, bestie.
-gbtf— request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. Two formats, one object file, iconic 🌈-gprune-btf-gno-prune-btf— prune BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations. In addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced withBTF_KIND_FWD, as though those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF information, but at significant loss in the amount of detailed type information. ⚠️ It is primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.
This option is enabled by default for the BPF target when generating BTF information.
-gctf-gctflevel— request CTF debug information and use level to specify how much CTF debug information should be produced. If-gctfis specified without a value for level, the default level of CTF debug information is 2.
CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. Besties coexisting 🫶
Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf. 💀
Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gvms— produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. Vintage, ngl 🎀-gcodeview— produces debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows.-glevel-ggdblevel-gvmslevel— request debugging information and also use level to specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables. Just the vibes, no details.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. She's giving ✨everything✨.
If you use multiple -g options, with or without level numbers, the last such option is the one that is effective. Last one wins, periodt.
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF. So like -gdwarf plus -g3, not -gdwarf3, got it? 🙈
-fno-eliminate-unused-debug-symbols— by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols. She collects everyone, no one left behind 💜-femit-class-debug-always— instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. ⚠️ This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two. Big trade-off, bestie, be careful.-fno-merge-debug-strings— directs the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.- *
-fdebug-prefix-map=old*=new** — when compiling files residing in directory *old*, record debugging information describing them as if the files resided in directory *new* instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using.for *new*. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-mapand-fcanon-prefix-map`. Sneaky path remapping, we stan 🦋 -fvar-tracking— run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments— annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of-gdwarf-4is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. 😭 So much effort for nothing. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
-gsplit-dwarf— if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension.dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading.dwofiles. Split it, diva 💅-gdwarf32-gdwarf64— if DWARF debugging information is enabled, the-gdwarf32selects the 32-bit DWARF format and the-gdwarf64selects the 64-bit DWARF format. The default is target specific, on most targets it is-gdwarf32though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. ⚠️ 64-bit is big and brave but not everyone can handle her.-gdescribe-dies— add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. Giving the unnamed ones a little name tag 🎀-gpubnames— emits those DWARF.debug_pubnamesand.debug_pubtypessections for you 💅-ggnu-pubnames— same.debug_pubnamesand.debug_pubtypessections, but formatted so they can be converted into a GDB index. Heads up though: this one's only useful if your linker can actually produce GDB index version 7, bestie 🫶-fdebug-types-section— with DWARF Version 4 or higher, type DIEs can live in their own.debug_typessection instead of being crammed into.debug_info. Putting them in a separate comdat section is more efficient because the linker can then yeet the duplicates ✨ but ngl, not every DWARF consumer supports.debug_typessections yet, and on some objects.debug_typesactually makes your debugging info bigger instead of smaller 😭-grecord-gcc-switches,-gno-record-gcc-switches— this switch appends the command-line options used to invoke the compiler (the ones that may affect code generation) to theDW_AT_producerattribute in DWARF debugging info. The options get concatenated with spaces separating them from each other and from the compiler version. It's enabled by default 👑 and psst, there's also-frecord-gcc-switchesif you want another way of storing compiler options into the object file.-gstrict-dwarf— disallows using extensions from a later DWARF standard version than whatever you selected with-gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed, but not with this one 🚫-gno-strict-dwarf— allows using extensions of later DWARF standard versions than the one selected with-gdwarf-version.-gas-loc-support— tells the compiler the assembler supports.locdirectives, so it may then use them for the assembler to generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can generate itself 🌸
It'll be enabled by default if, at GCC configure time, the assembler was found to support such directives.
-gno-as-loc-support— forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated at all.-gas-locview-support— tells the compiler the assembler supports "view" assignment and reset assertion checking in.locdirectives.
Enabled by default if, at GCC configure time, the assembler was found to support them 💖
-gno-as-locview-support— forces GCC to assign view numbers internally, if-gvariable-location-viewsis explicitly requested.-gcolumn-info,-gno-column-info— emits location column information into DWARF debugging info, not just file and line. Enabled by default ✨-gstatement-frontiers,-gno-statement-frontiers— makes GCC create markers in the internal representation at the beginning of statements, and keep them roughly in place throughout compilation, using them to guide the output ofis_stmtmarkers in the line number table. Enabled by default when compiling with optimization (-Os,-O1,-O2, ...) and outputting DWARF 2 debug information at the normal level.-gvariable-location-views,-gvariable-location-views=incompat5,-gno-variable-location-views— augments variable location lists with progressive view numbers implied from the line number table, so debug info consumers can inspect state at certain points of the program even if no instructions associated with the corresponding source locations are present there. If the assembler lacks support for view numbers in line number tables, this makes the compiler emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so debug info consumers that don't know about these augmentations can still consume them — they just won't get any benefit from them 🙈
Enabled by default when outputting DWARF 2 debug information at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled, and -gstrict-dwarf is not. When assembler support isn't available, this may still be enabled, but it'll force GCC to output internal line number tables, and if -ginternal-reset-location-views isn't enabled, that will most certainly lead to silently mismatching location views 😬
There's also a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, enabled with -gvariable-location-views=incompat5. This option may be removed in the future, and it's only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists that use it 💀
-ginternal-reset-location-views,-gno-internal-reset-location-views— attempts to determine location views that can be omitted from location view lists. This needs the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when you're using an assembler that doesn't support location view lists. The GNU assembler will flag any such error as a "view number mismatch". Only enabled on ports that define a reliable estimation function 🎀-ginline-points,-gno-inline-points— generates extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points so that address and view numbers can be computed and output in debug info. This can be enabled independently of location views, in which case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it's only enabled by default if location views are enabled.-gz[=type]— produces compressed debug sections in DWARF format, if that's supported. If type isn't given, the default depends on the capabilities of the assembler and linker you're using. type can be one ofnone(don't compress debug sections) orzlib(use zlib compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected 🚫 Otherwise, if the assembler doesn't support them,-gzis silently ignored when producing object files.-femit-struct-debug-baseonly— emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined.
This substantially reduces the size of your debugging information, but at a significant potential loss in type information to the debugger 😭 Check -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed for more detailed control.
Works only with DWARF debug output.
-femit-struct-debug-reduced— emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the type is defined, unless the struct is a template or defined in a system header.
Significantly reduces the size of debugging information, with some potential loss in type info to the debugger. See -femit-struct-debug-baseonly for a more aggressive option, and -femit-struct-debug-detailed for more detailed control.
Works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]— specifies the struct-like types for which the compiler generates debug info. The whole intent here is reducing duplicate struct debug info between different object files within the same program 🫶
This is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and it serves most needs.
A specification has the syntax
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the spec to structs used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed doesn't yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug info. The values none and any have the normal meaning. The value base means the base of the name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice that means when compiling foo.c, debug info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.
You may need to experiment to figure out the best settings for your application 🔬
The default is -femit-struct-debug-detailed=all.
Works only with DWARF debug output.
-fno-dwarf2-cfi-asm— emits DWARF unwind info as a compiler-generated.eh_framesection instead of using GAS.cfi_*directives.-fno-eliminate-unused-debug-types— normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. But sometimes it's genuinely useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often than not though, this just results in a significant amount of wasted space 🙈
🌈 Options That Control Optimization 🌈
These options control various sorts of optimizations, bestie. Buckle up 💅
Okay so bestie, when there’s no optimization option at all, the compiler is in “reduce the cost of compilation and make debugging produce the expected results” mode. Statements are independent: if you stop the program with a breakpoint between statements, you can 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 expect from the source code. ✨
Turning on optimization flags makes the compiler attempt to improve performance and/or code size at the expense of compilation time and possibly the ability to debug the program. Tradeoffs, bestie. The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. Important vibe check!!
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level.
-O,-O1— Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O is the recommended optimization level for large machine-generated code as a sensible balance between time taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic complexity than at -O.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time-O2— Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to-O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap⚠️ Please note the warning under-fgcseabout invoking-O2on programs that use computed gotos.
-O3— Optimize yet more.-O3turns on all optimizations specified by-O2and also turns on the following optimization flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— Reduce compilation time and make debugging produce the expected results. This is the default.
At -O0, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on the command line, or are listed by -Q --help=optimizers as being enabled by default. Many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it would not be useful to run individual optimization passes in isolation.
-Os— Optimize for size.-Osenables all-O2optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcIt also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.
-Ofast— Disregard strict standards compliance.-Ofastenables all-O3optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on-ffast-math,-fallow-store-data-racesand the Fortran-specific-fstack-arrays, unless-fmax-stack-var-sizeis specified, and-fno-protect-parens. It turns off-fsemantic-interposition.-Og— Optimize debugging experience.-Ogshould be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than-O0for producing debuggable code because some compiler passes that collect debug information are disabled at-O0.
Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra-Oz— Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode.-Ozbehaves similarly to-Osincluding enabling most-O2optimizations.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Last one wins, bestie!!
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the list below, only one of the forms is listed—the one you typically use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.
-fno-defer-pop— For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels-O1and higher,-fdefer-popis the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once.-fforward-propagate— Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.
-favoid-store-forwarding,-fno-avoid-store-forwarding— Many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value. Disabled by default. 🫶-ffp-contract=style—-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them.-ffp-contract=onenables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.
The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.
-fomit-frame-pointer— Omit the frame pointer in functions that don’t need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn’t guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc— Detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation. Detect 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC.
Enabled by default at -O2 and higher. 💅
-foptimize-sibling-calls— optimizes sibling and tail recursive calls, bestie 🔁. Enabled at levels-O2,-O3,-Os.-foptimize-strlen— turns various standard C string functions (likestrlen,strchrorstrcpy) and their_FORTIFY_SOURCEcounterparts into faster alternatives ✨. Enabled at levels-O2,-O3.-finline-stringops[=fn]— expands memory and string operations (for now, onlymemset) inline, even when the length is variable or big enough that it would require looping 💅. This is most useful along with-ffreestandingand-fno-builtin.
In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches 😭. This option is disabled by default.
-fno-inline— do not expand any functions inline apart from those marked with thealways_inlineattribute 🚫. This is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the noinline attribute.
-finline-small-functions— integrates functions into their callers when their body is smaller than expected function call code (so the overall size of the program gets smaller) 🫶. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining— inlines also indirect calls that are discovered to be known at compile time thanks to previous inlining ✨. This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-finline-functions— considers all functions for inlining, even if they are not declared inline 👑. The compiler heuristically decides which functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once— considers allstaticfunctions called once for inlining into their caller even if they are not markedinline💫. If a call to a given function is integrated, then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining— inlines functions marked byalways_inlineand functions whose body seems smaller than the function call overhead early before doing-fprofile-generateinstrumentation and the 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.
-fipa-sra— performs interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value 🧚. Enabled at levels-O2,-O3and-Os.-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit.nis the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-singleis set ton/2.max-inline-insns-autois set ton/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.
-fno-keep-inline-dllexport— this is a more fine-grained version of-fkeep-inline-functions, which applies only to functions that are declared using thedllexportattribute or declspec 🎀.-fkeep-inline-functions— in C, emitstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers 💖. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, emit any and all inline functions into the object file.-fkeep-static-functions— emitstaticfunctions into the object file, even if the function is never used 🫶.-fkeep-static-consts— emit variables declaredstatic constwhen optimization isn't turned on, even if the variables aren't referenced ✨.
GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants— attempts to merge identical constants (string constants and floating-point constants) across compilation units 🌈.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants— attempts to merge identical constants and identical variables 💀.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior.
-fmodulo-sched— performs swing modulo scheduling immediately before the first scheduling pass 🔁. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.-fmodulo-sched-allow-regmoves— performs more aggressive SMS-based modulo scheduling with register moves allowed 🔥. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with-fmodulo-schedenabled.-fno-branch-count-reg— disables the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result 🙈. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the-fno-branch-count-regoption doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-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 except in Ada.
-fthread-jumps— performs optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found 🦋. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types— when using a type that occupies multiple registers, such aslong longon a 32-bit system, split the registers apart and allocate them independently 💅. This normally generates better code for those types, but may make debugging more difficult.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early— fully split wide types early, instead of very late ⭐. This option has no effect unless-fsplit-wide-typesis turned on.
This is the default on some targets.
-fcse-follow-jumps— in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path 🧚. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— this is similar to-fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks 💖. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blockscauses CSE to follow the jump around the body of theif.
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-runs common subexpression elimination after loop optimizations are performed 🔁.
Enabled at levels -O2, -O3, -Os.
-fgcse— performs a global common subexpression elimination pass 🌈. This pass also performs global constant and copy propagation.
Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
okay bestie, next batch of optimization flags — these are the deep-lore ones, so grab a snack 🍓 and let's go, because the compiler is about to do some unhinged amounts of cleverness on your code 💅
-fgcse-lm— with this one on, global common subexpression elimination tries to move loads that are only killed by stores into themselves 👀 which means a loop holding a load/store sequence can get rewritten into a load outside the loop plus a copy/store inside the loop. Iconic. Enabled by default whenever-fgcseis enabled.-fgcse-sm— turn this on and a store motion pass runs after global common subexpression elimination, and it tries to move stores out of loops 🌀 when you pair it with-fgcse-lm, loops with a load/store sequence can become a load before the loop and a store after the loop. Not enabled at any optimization level, so it's strictly an opt-in vibe ✨-fgcse-las— with this enabled, the global common subexpression elimination pass yeets redundant loads that come after stores to the same memory location (both partial and full redundancies, no cap 💀). Not enabled at any optimization level.-fgcse-after-reload— enables a redundant load elimination pass after reload 🫶 the whole point of this pass is cleaning up redundant spilling. Enabled by-O3,-fprofile-useand-fauto-profile.-faggressive-loop-optimizations— tells the loop optimizer to use language constraints to derive bounds for how many iterations a loop runs 🔥 and heads up, this assumes your loop code doesn't invoke undefined behavior — like, for example, causing signed integer overflows or out-of-bound array accesses 😭 those iteration bounds get used to guide loop unrolling and peeling and loop exit test optimizations. Enabled by default, she's always on.-funconstrained-commons— tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds 🙈-fcrossjumping— performs the cross-jumping transformation, which unifies equivalent code and saves code size 🎀 plot twist though: the resulting code may or may not perform better than without cross-jumping. Enabled at levels-O2,-O3,-Os.-fauto-inc-dec— combines increments or decrements of addresses with memory accesses ⚡ this pass is always skipped on architectures that don't have instructions to support it. Enabled by default at-O1and higher on architectures that support this.-fdce— performs dead code elimination (DCE) on RTL 💀 enabled by default at-O1and higher.-fdse— performs dead store elimination (DSE) on RTL 🗑️ enabled by default at-O1and higher.-fif-conversion— attempts to transform conditional jumps into branch-less equivalents, which includes using conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics 🧚 the use of conditional execution on chips where it's available is controlled by-fif-conversion2. Enabled at levels-O1,-O2,-O3,-Os, but not with-Og.-fif-conversion2— uses conditional execution (where available) to transform conditional jumps into branch-less equivalents ✂️ enabled at levels-O1,-O2,-O3,-Os, but not with-Og.-fdeclone-ctor-dtor— okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards 🏗️ for a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function, and we simply cannot. With this option, the base and complete variants are changed to be thunks that call a common implementation. Enabled by-Os.-fdelete-null-pointer-checks— assumes that programs cannot safely dereference null pointers, and that no code or data element resides at address zero 🚫 this option enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — these assume that a memory access to address zero always results in a trap, so if a pointer is checked after it's already been dereferenced, it cannot be null. Lowkey clever.
⚠️ but note: in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior, bestie please 😭
This option is enabled by default on most targets. On AVR and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize— attempts to convert calls to virtual functions into direct calls 👑 done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels-O2,-O3,-Os.-fdevirtualize-speculatively— attempts to convert calls to virtual functions into speculative direct calls 🔮 based on the analysis of the type inheritance graph, it determines for a given call the set of likely targets, and if that set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, such as inlining; when they seem useless after further optimization, they get converted back into their original form. Recycling era ♻️-fdevirtualize-at-ltrans— streams the extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 📡 this enables more devirtualization but significantly increases the size of streamed data. For this reason it's disabled by default.-fexpensive-optimizations— performs a number of minor optimizations that are relatively expensive 💸 enabled at levels-O2,-O3,-Os.-free— attempts to remove redundant extension instructions 🌸 especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels-O2,-O3,-Os.-fno-lifetime-dse— in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed 🪦 normally dead store elimination takes advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because youroperator newclears the object storage) but still treat the object as dead after the destructor, you can use-flifetime-dse=1. The default behavior can be explicitly selected with-flifetime-dse=2, and-flifetime-dse=0is equivalent to-fno-lifetime-dse. iykyk 💅-flive-range-shrinkage— attempts to decrease register pressure through register live range shrinkage 🎈 helpful for fast processors with small or moderate size register sets.-fira-algorithm=algorithm— uses the specified coloring algorithm for the integrated register allocator 🎨 the algorithm argument can bepriority, which specifies Chow's priority coloring, orCB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it's the default because it generates better code. Slay.-fira-region=region— uses the specified regions for the integrated register allocator 🗺️ the region argument should be one of the following:all— use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.mixed— use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O,-O2, ...).one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-O0.
-fira-hoist-pressure— uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions 🏋️ this usually results in smaller code, but it can slow the compiler down. Enabled at level-Osfor all targets.-fira-loop-pressure— uses IRA to evaluate register pressure in loops for decisions to move loop invariants 🌀 this usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down. Enabled at level-O3for some targets.-fno-ira-share-save-slots— disables sharing of stack slots used for saving call-used hard registers living through a call 📚 each hard register gets a separate stack slot, and as a result function stack frames are larger. Bigger frames, zero sharing, that's the tradeoff bestie.-fno-ira-share-spill-slots— disables sharing of stack slots allocated for pseudo-registers 🧺 each pseudo-register that doesn't get a hard register gets a separate stack slot, and as a result function stack frames are larger.-flra-remat— enables CFG-sensitive rematerialization in LRA ✨ instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Enabled at levels-O2,-O3,-Os.-fdelayed-branch— if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions 🎯 enabled at levels-O1,-O2,-O3,-Os, but not at-Og.-fschedule-insns— if supported for the target machine, attempts to reorder instructions to eliminate execution stalls due to required data being unavailable ⏳ this helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required. Obsessed with a scheduler that actually gets it 💖
Conventionally enabled at optimization levels -O2 and -O3 — but like, many targets override this behavior, so don't come for me. For example, on x86 it's disabled at all levels, while on AArch64 it's enabled only at -O3. It's giving "it depends" energy fr fr.
-fschedule-insns2— pretty much the same as-fschedule-insns, but it asks for an additional pass of instruction scheduling after register allocation has already happened. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. Lowkey a lifesaver there.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock— turns off instruction scheduling across basic blocks, which is normally enabled when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fno-sched-spec— turns off speculative motion of non-load instructions, which is normally enabled when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-pressure— enables register pressure sensitive insn scheduling before register allocation. Heads up bestie: this only makes sense when scheduling before register allocation is already on, i.e. with-fschedule-insnsor at-O2or higher. Using this can improve the generated code and shrink its size by preventing register pressure from rising above the number of available hard registers (and the subsequent spills in register allocation that follow 😭).-fsched-spec-load— allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-spec-load-dangerous— allows speculative motion of more load instructions. Yes the name is dramatic, no I'm not joking. Only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-stalled-insns-fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely out of the queue of stalled insns and into the ready list during the second scheduling pass.-fno-sched-stalled-insnsmeans no insns are moved prematurely, and-fsched-stalled-insns=0means there's, like, no limit on how many queued insns can be moved prematurely.-fsched-stalled-insnswith no value is equivalent to-fsched-stalled-insns=1. Cute default ngl.-fsched-stalled-insns-dep-fsched-stalled-insns-dep=n — defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only does anything during the second scheduling pass, and only if-fsched-stalled-insnsis used.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0, and-fsched-stalled-insns-depwith no value is equivalent to-fsched-stalled-insns-dep=1.-fsched2-use-superblocks— when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules ✨. It's experimental though — not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. So, you know, proceed with caution.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic— enables the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. It's enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-critical-path-heuristic— enables the critical-path heuristic in the scheduler. This one favors instructions on the critical path (main character instructions, obviously 👑). Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-spec-insn-heuristic— enables the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-rank-heuristic— enables the rank heuristic in the scheduler. It favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-last-insn-heuristic— enables the last-instruction heuristic in the scheduler. This one favors the instruction that's less dependent on the last instruction scheduled. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-dep-count-heuristic— enables the dependent-count heuristic in the scheduler. It favors the instruction that has more instructions depending on it. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-freschedule-modulo-scheduled-loops— okay so modulo scheduling happens before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.-fselective-scheduling— schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. Swapping out the whole vibe, basically.-fselective-scheduling2— schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.-fsel-sched-pipelining— enables software pipelining of innermost loops during selective scheduling. No effect unless one of-fselective-schedulingor-fselective-scheduling2is turned on. Bestie the dependency is real.-fsel-sched-pipelining-outer-loops— when pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless-fsel-sched-pipeliningis turned on.-fsemantic-interposition— some object formats, like ELF, allow interposing of symbols by the dynamic linker. What that means: for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question might change. This feature is useful — say, to rewrite memory allocation functions with a debugging implementation — but it's expensive in terms of code quality 💀.
With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak.
-fshrink-wrap— emits function prologues only before the parts of the function that actually need it, instead of at the very top of the function. This flag is enabled by default at-Oand higher.-fshrink-wrap-separate— shrink-wraps separate parts of the prologue and epilogue separately, so those parts are only executed when needed. This option is on by default, but has no effect unless-fshrink-wrapis also turned on and the target supports this.-fcaller-saves— enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation only happens when it seems to result in better code.
This option is always enabled by default on certain machines — usually those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments— tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Matchmaker behavior, honestly 🫶
Enabled by default at -O1 and higher.
-fipa-ra— uses caller save registers for allocation if those registers aren't used by any called function. In that case it's not necessary to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it.
Enabled at levels -O2, -O3, -Os — however, the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL).
-fconserve-stack— attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. It's giving "priorities" 😌 This option implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400.-ftree-reassoc— performs reassociation on trees. This flag is enabled by default at-O1and higher.-fcode-hoisting— performs code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. It's especially useful as a code size optimization, but it often helps for code speed as well. This flag is enabled by default at-O2and higher.-ftree-pre— performs partial redundancy elimination (PRE) on trees. This flag is enabled by default at-O2and-O3.-ftree-partial-pre— makes partial redundancy elimination (PRE) more aggressive. Go off, PRE 💅 This flag is enabled by default at-O3.-ftree-forwprop— performs forward propagation on trees. This flag is enabled by default at-O1and higher.-ftree-fre— performs full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at-O1and higher.-ftree-phiprop— performs hoisting of loads from conditional pointers on trees. This pass is enabled by default at-O1and higher.-fhoist-adjacent-loads— bestie, imagine an if-then-else where both branches grab stuff from adjacent spots in the same struct, and your target arch has a conditional move instruction — this speculatively hoists those loads up so they both happen and can be merged. iconic. it's on by default at-O2and higher ✨-ftree-copy-prop— does copy propagation on trees, which nukes pointless copy operations so you're not wasting cycles shuffling values around. on by default at-O1and higher 🎀-fipa-pure-const— figures out which functions are pure or constant (iykyk, no sneaky side effects). enabled by default at-O1and higher 🌸-fipa-reference— sniffs out which static variables do not escape the compilation unit, so it knows what it can keep local. enabled by default at-O1and higher 🦋-fipa-reference-addressable— the upgrade era: discovers read-only, write-only and non-addressable static variables. enabled by default at-O1and higher 💫-fipa-reorder-for-locality— groups call chains close together in the binary layout so the code has better locality and jumps between frequently called functions are shorter. literally keeping the squad together 🫶. unlike-freorder-functionsit looks at the call chains between functions and groups those together, rather than dumping all hot/normal/cold/never-executed functions into separate sections. and unlike-fprofile-reorder-functionsit's going for locality throughout the whole runtime of the program, not just program startup. heads up: this option is incompatible with an explicit-flto-partition=option, since it enforces a custom partitioning scheme, so pick one bestie. if you do use it, also using profile feedback is recommended — but otherwise the option is not enabled by default 💅-fipa-stack-alignment— reduces stack alignment on call sites when possible. on by default ✅-fipa-pta— interprocedural pointer analysis plus interprocedural modification and reference analysis. so powerful but it can cause excessive memory and compile-time usage on large compilation units 😭. not enabled by default at any optimization level — you have to opt in and pay the toll 💀-fipa-profile— interprocedural profile propagation, the vibe-checker: functions called only from cold functions get marked cold, and functions executed once (like"cold","noreturn", static constructors or destructors) get identified. cold functions and the loop-less parts of functions-executed-once then get optimized for size. enabled by default at-O1and higher 🔥-fipa-modref— interprocedural mod/ref analysis: it reads the side effects of functions (the memory locations that get modified or referenced) and uses that to optimize better across the function-call boundary. enabled by default at-O1and higher 👑-fipa-cp— interprocedural constant propagation, the main character of cross-function optimization 💖. it analyzes the program to figure out when values passed to functions are constants, then optimizes accordingly. if your app passes constants to functions, this can substantially increase performance. enabled by default at-O2,-Osand-O3, and also enabled by-fprofile-useand-fauto-profile🌈-fipa-cp-clone— function cloning to make interprocedural constant propagation even stronger. when enabled, it clones a function when an externally visible function can be called with constant arguments. because it can create multiple copies of functions, it may significantly increase code size (see--param ipa-cp-unit-growth=value). enabled by default at-O3, and also enabled by-fprofile-useand-fauto-profile⭐-fipa-bit-cp— when enabled, does interprocedural bitwise constant propagation. enabled by default at-O2, and by-fprofile-useand-fauto-profile. it requires that-fipa-cpis enabled 🧚-fipa-vrp— when enabled, does interprocedural propagation of value ranges. enabled by default at-O2. it requires that-fipa-cpis enabled 💜-fipa-icf— performs Identical Code Folding for functions and read-only variables 🎀. it reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name (yes, that's a real caveat, don't @ me). works more effectively with link-time optimization enabled. the behavior is similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels, so the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold. enabled by default at-O2and-Os💅-flate-combine-instructions— enables two instruction combination passes that run pretty late in the compilation process: one before register allocation and one after. the main aim of the passes is to substitute definitions into all uses. most targets enable this flag by default at-O2and-Os✨-flive-patching=level— controls GCC's optimizations so the output is suitable for live-patching 🩹.
so here's the lore: if the compiler's optimization uses a function's body, or information extracted from its body, to optimize/change another function, that other function is called an impacted function of the first one. if a function is patched, its impacted functions should be patched too.
the impacted functions are determined by the compiler's interprocedural optimizations. for example, a caller is impacted when a function is inlined into its caller, when a function is cloned and its caller is changed to call the new clone, or when a function's pureness/constness info is extracted to optimize its direct or indirect callers, etc.
usually the more IPA optimizations you enable, the larger the number of impacted functions for each function. to control how many impacted functions there are (and to make computing the list of impacted functions easier), IPA optimizations can be partially enabled at two different levels.
the level argument should be one of these:
inline-clone— only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. so when you patch a function, all its callers and its clones' callers are impacted, meaning they need to be patched too.
-flive-patching=inline-clone disables these optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enable inlining of static functions. so when you patch a static function, all its callers are impacted and need to be patched too.
on top of all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static also disables these additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
when -flive-patching is specified without any value, the default value is inline-clone.
this flag is disabled by default 🙈.
heads up:-flive-patchingis not supported with link-time optimization (-flto) 💀.
-fisolate-erroneous-paths-dereference— detects paths that trigger erroneous or undefined behavior because of dereferencing a null pointer 😭. it isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. enabled by default at-O2and higher, and it depends on-fdelete-null-pointer-checksalso being enabled 🔥-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior because a null value is used in a way that's forbidden by a"returns_nonnull"or"nonnull"attribute. isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. this is not currently enabled, but it may be enabled by-O2in the future 👀-ftree-sink— performs forward store motion on trees. enabled by default at-O1and higher 🎀-ftree-bit-ccp— does sparse conditional bit constant propagation on trees and propagates pointer alignment info. this pass only operates on local scalar variables, and it's enabled by default at-O1and higher — except for-Og. it requires that-ftree-ccpis enabled 🌸-ftree-ccp— performs sparse conditional constant propagation (CCP) on trees. this pass only operates on local scalar variables, and it's enabled by default at-O1and higher 🦋-fssa-backprop— propagates information about a value's uses up the definition chain to simplify the definitions. for example, it strips sign operations if the sign of a value never actually matters 🙌. enabled by default at-O1and higher 💫-fssa-phiopt— does pattern matching on SSA PHI nodes to optimize conditional code. enabled by default at-O1and higher — except for-Og🫶-ftree-switch-conversion— converts simple initializations in aswitchinto initializations from a scalar array. enabled by default at-O2and higher ✨-ftree-tail-merge— hunts for identical code sequences, and when it finds them, replaces one with a jump to the other. this optimization is known as tail merging or cross jumping 💅. enabled by default at-O2and higher. the compilation time in this pass can be limited using themax-tail-merge-comparisonsparameter and themax-tail-merge-iterationsparameter ⏱️-ftree-cselim— performs conditional store elimination on trees. enabled by default at-O1and higher on targets that have conditional move instructions 🌈-ftree-dce— performs dead code elimination (DCE) on trees. enabled by default at-O1and higher 🗑️-ftree-builtin-call-dce— performs conditional dead code elimination (DCE) for calls to built-in functions that may set"errno"but are otherwise free of side effects. enabled by default at-O2and higher — as long as-Osis not also specified 🍓-ffinite-loops— okay so this one's like, "bestie, trust me, if a loop has an exit it WILL eventually take it" — no infinite looping era. Which means the compiler gets to yeet loops that have no side-effects, because it's not out here worrying about them spinning forever. 💅
This one's on by default at -O2 for C++ when you're on -std=c++11 or higher. Slay.
-ftree-dominator-opts— the lil' glow-up pass ✨ It does a whole bunch of simple scalar cleanups based on a dominator tree traversal: constant/copy propagation, redundancy elimination, range propagation, and expression simplification. Plus it does jump threading (to reduce jumps to jumps — jumps to jumps to jumps are so not the vibe). Enabled by default at-O1and higher.-ftree-dse— dead store elimination (DSE) on trees, icon behavior tbh. A "dead store" is when you write to a memory location and then later something else overwrites it without any loads in between 🫠 so like... that first store literally did nothing? Embarrassing. It gets deleted. Enabled by default at-O1and higher.-ftree-ch— loop header copying on trees, and it's giving main character energy because it makes code motion optimizations way more effective. Also saves one jump, we love a bonus. Enabled by default at-O1and higher. Not enabled for-Osthough, because it usually makes your code bigger (and-Osis on a strict "stay small" diet).-ftree-loop-optimize— loop optimizations on trees. Enabled by default at-O1and higher. That's it, that's the post 💖-ftree-loop-linear,-floop-strip-mine,-floop-block— these three are the same thing, loop nest optimizations, identical to-floop-nest-optimize. To actually use this code transformation, GCC has to be configured with--with-islto enable the Graphite loop transformation infrastructure. Like, no config, no transformation, iykyk.-fgraphite-identity— turns on the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. So with-fgraphite-identityyou can literally vibe-check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations also get done by the code generator isl, like index splitting and dead code elimination in loops. 🌀-floop-nest-optimize— enables the isl based loop nest optimizer, a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. Heads up though: this option is experimental 🙈 (so like, it's cute but proceed with awareness).-floop-parallelize-all— uses the Graphite data dependence analysis to find loops that can be parallelized, then parallelizes all the loops it can analyze as not containing loop carried dependences. And it does not check whether parallelizing is actually profitable. Not me making zero financial decisions 💀 — it just goes for it.-ftree-coalesce-vars— while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables instead of just compiler temporaries. Real talk: this may severely limit your ability to debug an optimized program compiled with-fno-var-tracking-assignments😭 In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise (lowkey does nothing, respectfully).-ftree-loop-if-convert— attempts to transform conditional jumps in the innermost loops into branch-less equivalents 🦋 The whole point is removing control-flow from the innermost loops so the vectorization pass can actually handle them. Enabled by default if vectorization is enabled.-ftree-loop-distribution— performs loop distribution, which can improve cache performance on big loop bodies and unlock further loop optimizations like parallelization or vectorization. Example time, bestie — this loop:
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOgets transformed to:
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOEnabled by default at -O3. Also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns— performs loop distribution on patterns that can be code generated with calls to a library. Enabled by default at-O2and higher, and by-fprofile-useand-fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero ⭐ For example, this loop:
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOis transformed to:
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand then the initialization loop becomes a call to memset zero. Chef's kiss 🎀
-floop-interchange— performs loop interchange outside of graphite. Can improve cache performance on loop nests and lets further loop optimizations like vectorization happen. Example, this:
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];becomes this:
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];see how j and k swapped? iconic 💅 Enabled by default at -O3. Also enabled by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam— applies unroll and jam transformations on feasible loops. In a loop nest, this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at-O3. Also enabled by-fprofile-useand-fauto-profile.-ftree-loop-im— performs loop invariant motion on trees. It only moves invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With-funswitch-loopsit also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion, periodt.-ftree-loop-ivcanon— creates a canonical counter for the number of iterations in loops where figuring out the iteration count needs complicated analysis. Then later optimizations can determine the number easily 😌 Super useful especially in connection with unrolling.-ftree-scev-cprop— performs final value replacement. If a variable gets modified in a loop in a way where its value when exiting the loop can be figured out using only its initial value and the number of loop iterations, it replaces uses of the final value with that computation — as long as it's sufficiently cheap. Reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher. 🔥-fivopts— performs induction variable optimizations on trees: strength reduction, induction variable merging, and induction variable elimination. Enabled by default at-O1and higher.-ftree-parallelize-loops=n— parallelize loops, i.e., split their iteration space to run in n threads 🧵 This only works for loops whose iterations are independent and can be arbitrarily reordered. Also the optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive rather than constrained e.g. by memory bandwidth. This option implies-pthread, and thus is only supported on targets that have support for-pthread.-ftree-pta— performs function-local points-to analysis on trees. Enabled by default at-O1and higher, except for-Og(sorry-Og, not your era apparently).-ftree-sra— performs scalar replacement of aggregates. This pass replaces structure references with scalars so we don't commit structures to memory too early (way too eager, honestly). Enabled by default at-O1and higher, except for-Og.-fstore-merging— performs merging of narrow stores to consecutive memory addresses. It merges contiguous stores of immediate values narrower than a word into fewer wider stores so there are fewer instructions overall. Enabled by default at-O2and higher as well as-Os🌸-ftree-ter— performs temporary expression replacement during the SSA->normal phase. Single use/single def temporaries get replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, which means better RTL generation 💖 Enabled by default at-O1and higher.-ftree-slsr— performs straight-line strength reduction on trees. It recognizes related expressions involving multiplications and replaces them with less expensive calculations when possible. Enabled by default at-O1and higher.-ftree-vectorize— performs vectorization on trees. This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif they're not explicitly specified. ✨-ftree-loop-vectorize— performs loop vectorization on trees. Enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftree-slp-vectorize— performs basic block vectorization on trees. Enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftrivial-auto-var-init=choice — initializes automatic variables with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use 🛡️ Important though: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized,-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the firstcaseof a"switch"statement. Use-Wtrivial-auto-var-initto report all such cases.
The three values of choice are:
uninitialized— doesn't initialize any automatic variables. This is C and C++'s default, no cap 🫶pattern— initializes automatic variables with values that will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and are not values programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte0xFE. The values used for pattern initialization might be changed in the future. 💀zero— initializes automatic variables with zeroes. ✨
The default is uninitialized. Periodt.
Okay but real talk, important vibe check: the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), not to their interpretation as numerical values. This distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the range 160..400 will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range — and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say 0X5A and 0xA5 for false and true, respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for false, even if it would for a static variable of the same type. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with false (zero), even when pattern is requested.
You can control this behavior for a specific variable by using the variable attribute uninitialized.
-fvect-cost-model=model— alter the cost model used for vectorization. The model argument should be one ofunlimited,dynamic,cheaporvery-cheap. With theunlimitedmodel the vectorized code-path is assumed to be profitable, while with thedynamicmodel a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. Thecheapmodel disables vectorization of loops where doing so would be cost prohibitive, for example due to required runtime checks for data dependence or alignment, but otherwise is equal to thedynamicmodel. Thevery-cheapmodel disables vectorization of loops when any runtime check for data dependence or alignment is required; it also disables vectorization of epilogue loops but otherwise is equal to thecheapmodel. 🔥
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model— alter the cost model used for vectorization of loops marked with the OpenMPsimddirective. The model argument should be one ofunlimited,dynamic,cheap. All values of model have the same meaning as described in-fvect-cost-modeland by default a cost model defined with-fvect-cost-modelis used. ✨-ftree-vrp— perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at-O2and higher. Null pointer check elimination is only done if-fdelete-null-pointer-checksis enabled. 🦋-fsplit-paths— split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at-O3and above. 🌸-fsplit-ivs-in-unroller— enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes. 💅
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller— with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. 👑
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining— inline parts of functions. This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions. 🎀
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning— perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops. 🔁
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays— if supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays. 🌈
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.
Disabled at level -Os.
-fno-printf-return-value— do not substitute constants for known return value of formatted output functions such assprintf,snprintf,vsprintf, andvsnprintf(but notprintfoffprintf). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when-fprintf-return-valueis in effect, both the branch and the body of theifstatement (but not the call tosnprint) can be optimized away wheniis a 32-bit or smaller integer because the return value is guaranteed to be at most 8. 😭
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default. ⭐
-fno-peephole-fno-peephole2— disable any machine-specific peephole optimizations. The difference between-fno-peepholeand-fno-peephole2is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. 🙈
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability— do not guess branch probabilities using heuristics. 🧚
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.
It is also possible to specify expected probability of the expression with __builtin_expect_with_probability built-in function.
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 -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm— use the specified algorithm for basic block reordering. The algorithm argument can besimple, which does not increase code size (except sometimes due to secondary effects like alignment), orstc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code. 🔥
The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition— in addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and.ofiles, to improve paging and cache locality performance. 🫶
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions— reorder functions in the object file in order to improve code locality. Unlike-fipa-reorder-for-localitythis option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. This is implemented by using special subsections.text.hotfor most frequently executed functions and.text.unlikelyfor unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way. 🌟
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing— okay bestie, this one lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++), it switches on optimizations based on the type of expressions. Specifically: an object of one type is assumed to never live at the same address as an object of a different type, unless the types are almost the same. For example, anunsigned intcan alias anint, but not avoid*or adouble. A character type may alias any other type. 💅
Pay special attention to code like this, fr fr:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So the code above works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}🫶 The-fstrict-aliasingoption is enabled at levels-O2,-O3,-Os.
-fipa-strict-aliasing— this one controls whether the rules of-fstrict-aliasingget applied across function boundaries. Note that if multiple functions gets inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary. ✨
-fipa-strict-aliasing is enabled by default and is effective only in combination with -fstrict-aliasing.
-falign-functions-falign-functions=n-falign-functions=n:m-falign-functions=n:m:n2-falign-functions=n:m:n2:m2— align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's an optimization of code performance, and alignment is ignored for functions considered cold. If alignment is required for all functions, use-fmin-function-alignment. 🌸
If m is not specified, it defaults to n.
Examples, bestie: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment— if this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. 🎀
-falign-labels-falign-labels=n-falign-labels=n:m-falign-labels=n:m:n2-falign-labels=n:m:n2:m2— align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops-falign-loops=n-falign-loops=n:m-falign-loops=n:m:n2-falign-loops=n:m:n2:m2— align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. It's an optimization of code performance, and alignment is ignored for loops considered cold. 🦋
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps-falign-jumps=n-falign-jumps=n:m-falign-jumps=n:m:n2-falign-jumps=n:m:n2:m2— align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. This is an optimization of code performance, and alignment is ignored for jumps considered cold. 💜
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment— specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike-falign-functions, this alignment is applied also to all functions (even those considered cold). The alignment is also not affected by-flimit-function-alignment. 👑
-fno-allocation-dce— do not remove unused C++ allocations (using operatornewand operatordelete) in dead code elimination. 🧚
See also -fmalloc-dce.
-fallow-store-data-races— allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads. ⚠️
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast.
-funit-at-a-time— this option is left for compatibility reasons.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors. 🍓
Enabled by default.
-fno-toplevel-reorder— do not reorder top-level functions, variables, andasmstatements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. 💀
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps— with this option, the compiler turns calls to__builtin_unreachableinto traps, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler. 🙈
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.
This option is enabled by default at -O0 and -Og.
-fweb— constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register". 😭
Enabled by default with -funroll-loops.
-fwhole-program— assume that the current compilation unit represents the whole program being compiled. All public functions and variables — with the exception ofmainand those merged by attributeexternally_visible— become static functions and in effect are optimized more aggressively by interprocedural optimizers. 🌈
With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).
-flto[=n]— this option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. 🔥
Okay so the link-time optimizer — LTO — is a whole vibe, and you literally use it by putting -flto and your optimization options at compile time and at the final link. 💖 The manual lowkey recommends compiling all the files participating in the same link with the same options and also specifying those options at link time. Example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oThe first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. Iconic cross-file behavior, no cap. ✨
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cThe above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.
The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions. Bestie, this is the “trust me, nothing outside is touching these symbols” arc. 🫶
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. Mix-and-match chaos, but make it compiler-certified. 🌈
Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpieCertain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. Do not fight the ABI, bestie, it will win. 💀
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time. Warning-vibes can shift mid-optimization, iykyk. ⭐
When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.
To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line and mixing different settings in different translation units is discouraged. 🎀
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. Undefined behavior stays undefined, bestie — the diagnostic is just a heads-up, not a rescue. 😭
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Multilingual main character energy, fr fr. 👑
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. Aggregated GIMPLE image sounds like a group chat merge, honestly. 🦋
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. Strict version check era — no mixing and matching bytecode across GCC versions, periodt. 💅
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. Parallel jobs: because waiting is not the move. ⚡
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. 🌸
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. Auto-detect and slay. 🎀
-flto-partition=alg— this is the “how do we chop up the LTO work” knob, bestie. The value is either1to1for partitioning that mirrors the original source files,balancedfor partitioning into equally sized chunks whenever possible,maxto create a new partition for every symbol where possible, orcacheto balance chunk sizes while keeping related symbols together for better caching in incremental LTO. Specifyingnoneas an algorithm disables partitioning and streaming completely. The default value isbalanced.
1to1 can be used as a workaround for various code ordering issues, but the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.
-flto-incremental=path— enable incremental LTO, with its cache in the given existing directory. This can significantly shorten edit-compile cycles with LTO.
When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are likely to be encountered again when recompiling with small code changes, leading to recompile time reduction.
Multiple GCC instances can use the same cache in parallel. cute, right? 💅
-flto-incremental-cache-size=n— specifies the number of cache entries in incremental LTO after which to prune old entries. This is a soft limit, so temporarily there may be more entries. soft limit! not a hard cap, bestie.-flto-compression-level=n— this option specifies the level of compression used for intermediate language written to LTO object files, and it’s only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms: forzstd, valid values are0(no compression) to19(maximum compression), whilezlibsupports values from0to9. Values outside this range are clamped to either the minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used.-fuse-linker-plugin— enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available ingoldor in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects— fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with-fltoand is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim— after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O1, -O2, -O3, -Os.
-ffold-mem-offsets,-fno-fold-mem-offsets— try to eliminate add instructions by folding them in memory loads/stores.
Enabled at levels -O2, -O3.
-fcprop-registers— after register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O1, -O2, -O3, -Os.
-fprofile-correction— profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
This option is enabled by -fauto-profile.
-fprofile-partial-training— with-fprofile-use, all portions of programs not executed during the train run are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, a program may contain functions specific for a given hardware and training may not cover all hardware configurations the program is run on.) With-fprofile-partial-training, profile feedback is ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when the train run is not representative, but also leads to significantly bigger code.-fprofile-use,-fprofile-use=path— enable profile feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functionsBefore you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
-fauto-profile,-fauto-profile=path— enable sampling-based feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correctionpath is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/.
e.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo.
e.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdoThe following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
-fexcess-precision=style— this option allows control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. An example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs, the excess precision is harmless, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values.
By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision.
So -fexcess-precision=standard is lowkey not implemented for any language that isn't C or C++, bestie 💀 and on the x86 it has no effect at all if you've got -mfpmath=sse or -mfpmath=sse+387 on the go — in the former case, IEEE semantics apply and there's simply no excess precision to worry about, and in the latter, the rounding is unpredictable. 🙈
-ffloat-store— don't store floating-point variables in registers, and inhibit all the other options that might change whether a floating-point value gets taken from a register or from memory. Ngl this one has generally been subsumed by-fexcess-precision=standard, which is the way more general icon 👑. But if you do go with-ffloat-store, you may need to tweak your program to explicitly store intermediate computations in temporary variables — because-ffloat-storehandles rounding to IEEE format only on assignments, and not on casts like-fexcess-precision=standarddoes. 🎀-ffast-math— sets the options-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fast. 💅
This option causes the preprocessor macro __FAST_MATH__ to be defined. ✨
It is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications. Speed vs. correctness, choose your fighter.
-fno-math-errno— do not seterrnoafter 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 is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications.
The default is -fmath-errno. 📌
On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default there. 🍓
-funsafe-math-optimizations— allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. 😭
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math— allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. ⚠️ NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — so it cannot be used on code that relies on rounding behavior like(x + 2**52) - 2**52. It may also reorder floating-point comparisons, so it may not be used when ordered comparisons are required. This option requires that both-fno-signed-zerosand-fno-trapping-mathbe in effect. Moreover, it doesn't make much sense with-frounding-math. For Fortran the option is automatically enabled when both-fno-signed-zerosand-fno-trapping-mathare in effect. 🧚
The default is -fno-associative-math.
-freciprocal-math— allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For examplex / ycan be replaced withx * (1/y), which is useful if(1/y)is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 🎀
The default is -fno-reciprocal-math.
-ffinite-math-only— allow optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs. 👀
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros— allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct+0.0and-0.0values, which then prohibits simplification of expressions such asx+0.0or0.0*x(even with-ffinite-math-only). This option implies that the sign of a zero result isn't significant. 💅
The default is -fsigned-zeros.
-fno-trapping-math— compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that-fno-signaling-nansbe in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example. 🔥
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS. 🔮
-frounding-math— disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest). This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. It 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 along with -ftrapping-math to specify the default state for FENV_ACCESS. 🫶
-fsignaling-nans— compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies-ftrapping-math. 😤
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. ✨
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact— do not allow the built-in functionsceil,floor,roundandtrunc, and theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so. 🧐
The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661. 💀
-fsingle-precision-constant— treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. 🎀-fcx-limited-range— when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there's no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case. The default is-fno-cx-limited-range, but it's enabled by-ffast-math. 💖
This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.
-fcx-fortran-rules— complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there's no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case. 🦋
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options. Heads up bestie: this section includes experimental options that may produce broken code. 😭⚠️
-fbranch-probabilities— after running a program compiled with-fprofile-arcs, you can compile it a second time using-fbranch-probabilitiesto improve optimizations based on the number of times each branch was taken. When a program compiled with-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor 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. No cap, they've gotta match exactly. 👑 See details about the file naming in -fprofile-arcs.
...and here's the tea on that whole thing: with -fbranch-probabilities, GCC drops a REG_BR_PROB note on every single JUMP_INSN and CALL_INSN 🫶. These bad boys can be used to make optimization chef's kiss. Currently though, they're only used in one place: in reorg.cc, instead of straight-up guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. No cap, actual math 💅.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— if you pair this with-fprofile-arcs, it adds code so that some data about values of expressions in the program gets gathered 📊. Then with-fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations. Enabled by-fprofile-generate,-fprofile-use, and-fauto-profile✨.-fprofile-reorder-functions— function reordering based on profile instrumentation, bestie. It collects the first time of execution of a function and orders these functions in ascending order, aiming to optimize program startup through more efficient loading of text segments 🚀. Enabled with-fprofile-use.-fvpt— if combined with-fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions 👀. With-fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator — iykyk, that's the good stuff 🔥. Enabled with-fprofile-useand-fauto-profile.-frename-registers— attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation ✨. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register" 😭. Enabled by default with-funroll-loops.-fschedule-fusion— performs a target dependent pass over the instruction stream to schedule instructions of same type together, because the target machine can execute them more efficiently if they are adjacent to each other in the instruction flow 👯♀️. Enabled at levels-O2,-O3,-Os.-ftracer— perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, allowing other optimizations to do a better job 💖. Enabled by-fprofile-useand-fauto-profile.-funroll-loops— unroll loops whose number of iterations can be determined at compile time or upon entry to the loop 🌀.-funroll-loopsimplies-frerun-cse-after-loop,-fweband-frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster — it's giving "tradeoffs," ngl. Enabled by-fprofile-useand-fauto-profile.-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-loopsimplies the same options as-funroll-loops.-fpeel-loops— peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis) 🍓. It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). Enabled by-O3,-fprofile-use, and-fauto-profile.-fmalloc-dce— controls whether"malloc"(and its variants such as"calloc"or"strdup") can be optimized away, provided its return value is only used as a parameter of a"free"call or compared with"NULL"🧚. If-fmalloc-dce=1is used, only calls to"free"are allowed, while with-fmalloc-dce=2also comparisons with"NULL"pointer are considered safe to remove. The default is-fmalloc-dce=2. See also-fallocation-dce👑-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer 🌈. Enabled at level-O1and higher, except for-Og.-fmove-loop-stores— enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration 🔄. Note: for this option to have an effect,-ftree-loop-imhas to be enabled as well ⚠️. Enabled at level-O1and higher, except for-Og.-fsplit-loops— split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other ✂️. Enabled by-fprofile-useand-fauto-profile.-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) 🌸. Enabled by-fprofile-useand-fauto-profile.-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one 🦋. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses 💫. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-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 have linkers with such optimizations 💅. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping) 🗑️.
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats though 👀.
Only use these options when there are significant benefits from doing so, bestie. When you specify these options, the assembler and linker create larger object and executable files and are also slower 😭. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.
-fstdarg-opt— optimize the prologue of variadic argument functions with respect to usage of those arguments ✨.-fsection-anchors— try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects ⚓. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead 🎯. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option 🙈.
-fzero-call-used-regs=choice— zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers 🔒. The possible values of choice are the same as for the"zero_call_used_regs"attribute. The default isskip. You can control this behavior for a specific function by using the function attribute"zero_call_used_regs"💜.--param name=value— in some places, GCC uses various constants to control the amount of optimization that is done 🎛️. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the--paramoption.
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 order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options 🩺.
In each case, the value is an integer. The following choices of name are recognized for all targets:
phiopt-factor-max-stmts-live— when factoring statements out of if/then/else, this is the max # of statements after the defining statement to be allowed to extend the lifetime of a name 🏷️.predictable-branch-outcome— when a branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable 🔮.max-rtl-if-conversion-insns— RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable 🧠.file-cache-files— max number of files in the file cache. The file cache is used to print source lines in diagnostics and do some source checks like-Wmisleading-indentation📁.file-cache-lines— max number of lines to index into file cache. When0this is automatically sized. The file cache is used to print source lines in diagnostics and do some source checks like-Wmisleading-indentation📄.
max-rtl-if-conversion-predictable-cost— okay so RTL if-conversion is literally trying to yeet conditional branches around a block and replace them with conditionally executed instructions 💅. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internalseq_costmetric. The compiler will try to provide a reasonable default for this parameter using theBRANCH_COSTtarget macro ✨.max-crossjump-edges— the maximum number of incoming edges to consider for cross-jumping. The algorithm used by-fcrossjumpingisO(N^2)in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size 🌀.min-crossjump-insns— the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched 👀.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 📈.max-goto-duplication-insns— the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoidO(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 block with no more thanmax-goto-duplication-insnsare unfactored 🧩.max-delay-slot-insn-search— the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time ⏳.max-delay-slot-live-search— when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph 🛠️.max-gcse-memory— the approximate maximum amount of memory inkBthat can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done 💾.max-gcse-insertion-ratio— if the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream 🔁.max-pending-list-length— the maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources 🧠.max-modulo-backtrack-attempts— the maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time 🔥.max-inline-functions-called-once-loop-depth— maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once 🎯.max-inline-functions-called-once-insns— maximal estimated size of functions produced while inlining functions called once 📏.max-inline-insns-single— several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++) 🌳.max-inline-insns-auto— when you use-finline-functions(included in-O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto) 🔍.max-inline-insns-small— this is the bound applied to calls that are considered relevant with-finline-small-functions🐣.max-inline-insns-size— this is the bound applied to calls that are optimized for size. Small growth may be desirable to anticipate optimization opportunities exposed by inlining 🎀.uninlined-function-insns— number of instructions accounted by inliner for function overhead such as function prologue and epilogue 🧾.uninlined-function-time— extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue ⏱️.inline-heuristics-hint-percent— the scale (in percents) applied toinline-insns-single,inline-insns-single-O2,inline-insns-autowhen inline heuristics hints that inlining is very profitable (will enable later optimizations) 💡.uninlined-thunk-insns,uninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-timebut applied to function thunks 🫶.inline-min-speedup— when estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on--param max-inline-insns-singleand--param max-inline-insns-auto🚀.large-function-insns— the limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by--param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end 🏋️.large-function-growth— specifies maximal growth of large functions caused by inlining in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size 📊.large-unit-insns— the limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by--param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to--param large-unit-insnsbefore applying--param inline-unit-growth🧱.lazy-modules— maximum number of concurrently open C++ module files when lazy loading 📂.inline-unit-growth— specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size ❄️.ipa-cp-unit-growth— specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size 🧬.ipa-cp-large-unit-insns— the size of translation unit that IPA-CP pass considers large 🐘.large-stack-frame— the limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much 🥞.large-stack-frame-growth— specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size 🌱.max-inline-insns-recursive,max-inline-insns-recursive-auto— specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.--param max-inline-insns-recursiveapplies to functions declared inline. For functions not declared inline, recursive inlining happens only when-finline-functions(included in-O3) is enabled;--param max-inline-insns-recursive-autoapplies instead 🔄.max-inline-recursive-depth,max-inline-recursive-depth-auto— specifies the maximum recursion depth used for recursive inlining.--param max-inline-recursive-depthapplies to functions declared inline. For functions not declared inline, recursive inlining happens only when-finline-functions(included in-O3) is enabled;--param max-inline-recursive-depth-autoapplies instead 🪜.min-inline-recursive-probability— recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers. When profile feedback is available (see-fprofile-generate) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents) 🎲.
okay bestie, we’re still deep in the --param zone and these dials are lowkey iconic — every single one is here, no skipping 💅
early-inlining-insns— sets how much growth the early inliner can make. In effect, it increases the amount of inlining for code with a big abstraction penalty.max-early-inliner-iterations— limits how many iterations the early inliner does. It basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.comdat-sharing-probability— the probability, in percent, that a C++ inline function withcomdatvisibility gets shared across multiple compilation units.modref-max-bases,modref-max-refs,modref-max-accesses— these set the maximal number of base pointers, references and accesses stored for a single function bymod/refanalysis.modref-max-tests— the maximal number of tests the alias oracle can perform to disambiguate memory locations usingmod/refinformation. This parameter ought to be bigger than--param modref-max-basesand--param modref-max-refs.modref-max-depth— the maximum depth of the DFS walk used bymodrefescape analysis. Setting it to0disables the analysis completely.modref-max-escape-points— the maximum number of escape points tracked bymodrefperSSA-name.modref-max-adjustments— the maximum number of times the access range is enlarged duringmodrefdataflow analysis.profile-func-internal-id— controls whether to use the function internal id in profile database lookup. If the value is0, the compiler uses an id based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc.min-vect-loop-bound— the minimum number of iterations under which loops are not vectorized when-ftree-vectorizeis used. After vectorization, the number of iterations needs to be greater than the value specified by this option to allow vectorization.gcse-cost-distance-ratio— scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., expressions that have cost less thangcse-unrestricted-cost. Specifying0disables hoisting of simple expressions.gcse-unrestricted-cost— cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying0allows all expressions to travel unrestricted distances.max-hoist-depth— the depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in the hoisting algorithm. A value of0does not limit the search, but may slow down compilation of huge functions.max-tail-merge-comparisons— the maximum amount of similarbbs to compare abbwith. This is used to avoid quadratic behavior in tree tail merging.max-tail-merge-iterations— the maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging.store-merging-allow-unaligned— lets the store merging pass introduce unaligned stores if it is legal to do so.max-stores-to-merge— the maximum number of stores to attempt to merge into wider stores in the store merging pass.max-store-chains-to-track— the maximum number of store chains to track at the same time in the attempt to merge them into wider stores in the store merging pass.max-stores-to-track— the maximum number of stores to track at the same time in the attempt to merge them into wider stores in the store merging pass.max-unrolled-insns— the maximum number of instructions a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.max-average-unrolled-insns— the maximum number of instructions, biased by probabilities of their execution, that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.max-unroll-times— the maximum number of unrollings of a single loop.max-peeled-insns— the maximum number of instructions a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.max-peel-times— the maximum number of peelings of a single loop.max-peel-branches— the maximum number of branches on the hot path through the peeled sequence.max-completely-peeled-insns— the maximum number of insns of a completely peeled loop.max-completely-peel-times— the maximum number of iterations of a loop to be suitable for complete peeling.max-completely-peel-loop-nest-depth— the maximum depth of a loop nest suitable for complete peeling.max-unswitch-insns— the maximum number of insns of an unswitched loop.max-unswitch-depth— the maximum depth of a loop nest to be unswitched.lim-expensive— the minimum cost of an expensive expression in loop invariant motion.min-loop-cond-split-prob— when FDO profile information is available, this specifies the minimum threshold for the probability of a semi-invariant condition statement to trigger loop split.iv-consider-all-candidates-bound— bound on the number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.iv-max-considered-uses— induction variable optimizations give up on loops that contain more induction variable uses.iv-always-prune-cand-set-bound— if the number of candidates in the set is smaller than this value, always try to remove unnecessaryivs from the set when adding a new one.avg-loop-niter— average number of iterations of a loop.dse-max-object-size— maximum size, in bytes, of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times.dse-max-alias-queries-per-store— maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores.scev-max-expr-size— bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.scev-max-expr-complexity— bound on the complexity of expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.max-tree-if-conversion-phi-args— maximum number of arguments in aPHIsupported by TREE if conversion unless the loop is marked withsimdpragma.vect-max-layout-candidates— the maximum number of possible vector layouts, such as permutations, to consider when optimizing to-be-vectorized code.vect-max-version-for-alignment-checks— the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.vect-max-version-for-alias-checks— the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.vect-max-peeling-for-alignment— the maximum number of loop peels to enhance access alignment for the vectorizer. Value-1means no limit.max-iterations-to-track— the maximum number of iterations of a loop that the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.hot-bb-count-fraction— the denominatornof fraction1/nof the maximal execution count of a basic block in the entire program that a basic block needs to at least have in order to be considered hot. The default is10000, which means a basic block is considered hot if its execution count is greater than1/10000of the maximal execution count.0means it is never considered hot. Used in non-LTO mode.hot-bb-count-ws-permille— the number of most executed permilles, ranging from0to1000, of the profiled execution of the entire program to which the execution count of a basic block must be part of in order to be considered hot. The default is990, which means a basic block is considered hot if its execution count contributes to the upper990permilles, or99.0%, of the profiled execution of the entire program.0means it is never considered hot. Used in LTO mode.hot-bb-frequency-fraction— the denominatornof fraction1/nof the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is1000, which means a basic block is considered hot in a function if it is executed more frequently than1/1000of the frequency of the entry block of the function.0means it is never considered hot.unlikely-bb-count-fraction— okay so this is the denominator n of the fraction1/nof the total profiled runs of the whole program. A basic block has to be executed below that fraction for it to get branded "unlikely executed" 💅 default is20, which literally means a block counts as unlikely if it runs in fewer than1/20, aka5%, of the program's runs. Set it to0and everything is always considered unlikely executed. chaos, but ok 🫶max-predicted-iterations— the max number of loop iterations we predict statically ✨ genuinely useful when a function has one loop with a known bound and another loop with an unknown bound: the known iteration count gets predicted correctly, while the unknown one averages out to roughly10. which means the boundless loop looks artificially cold next to its sibling 🥶 lowkey unfair to it ngl.builtin-expect-probability— controls the probability of the expression having the specified value. bestie it takes a percentage, like0 ... 100, as input 🎀builtin-string-cmp-inline-length— the maximum length of a constant string for a builtin string cmp call that's eligible for inlining 🔥align-threshold— picks the fraction of the maximal frequency of executions of a basic block in a function, and that's what gets used to align the basic block 💫align-loop-iterations— a loop that's expected to iterate at least the selected number of iterations gets aligned. simple as that 🦋tracer-dynamic-coverage,tracer-dynamic-coverage-feedback— this value limits superblock formation once the given percentage of executed instructions is covered, which caps unnecessary code size expansion (we love a size limit 😌). thetracer-dynamic-coverage-feedbackone is used only when profile feedback is available — because real profiles (as opposed to the statically estimated ones) are way less balanced, which lets the threshold be a larger value 📈tracer-max-code-growth— stops tail duplication once code growth hits the given percentage. it's kind of an artificial limit tbh, since most of the duplicates get eliminated later in cross jumping — so you can set it way higher than the code growth you actually want 👑tracer-min-branch-ratio— stops reverse growth when the reverse probability of the best edge drops below this threshold (in percent) ✂️tracer-min-branch-probability,tracer-min-branch-probability-feedback— stops forward growth if the best edge has a probability lower than this threshold. and yeah, just liketracer-dynamic-coverage, there's two of them:tracer-min-branch-probability-feedbackis for compiling with profile feedback, andtracer-min-branch-probabilityis for compiling without it. the profile-feedback value needs to be more conservative (higher) to actually make tracer effective, no cap 💯stack-clash-protection-guard-size— specifies the size of the operating system provided stack guard as2raised to num bytes. higher values may reduce the number of explicit probes, but a value larger than the OS provided guard leaves your code vulnerable to stack clash style attacks 😭 so like, don't.stack-clash-protection-probe-interval— stack clash protection means probing stack space as it gets allocated. this param controls the maximum distance between probes into the stack as2raised to num bytes. higher values may reduce the number of explicit probes, but again — a value larger than the operating system provided guard leaves code vulnerable to stack clash style attacks 💀 same warning, same energy.max-cse-path-length— the maximum number of basic blocks on a path that CSE considers 🛣️max-cse-insns— the maximum number of instructions CSE processes before flushing 🚽 (no literally, flushing)ggc-min-expand— so GCC uses a garbage collector to manage its own memory allocation, and this parameter specifies the minimum percentage by which the GC's heap is allowed to expand between collections. tuning it may improve compilation speed — it has no effect on code generation, bestie 🎀
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 can't calculate RAM on a particular platform, the lower bound of 30% is used. setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity — which is extremely slow, but can be useful for debugging 🐛
ggc-min-heapsize— minimum size of the garbage collector's heap before it even begins bothering to collect garbage. the first collection happens after the heap expands byggc-min-expand%beyondggc-min-heapsize. again: tuning this may improve compilation speed, and it has no effect on code generation ✨
the default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS aren't exceeded — but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). if GCC can't calculate RAM on a particular platform, the lower bound is used. setting this parameter very large effectively disables garbage collection 🚫🗑️ and 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 instructions reload should look backward for an equivalent register. increasing values mean more aggressive optimization — compilation time goes up, probably with slightly better performance 📈max-cselib-memory-locations— the maximum number of memory locations cselib should take into account. increasing values mean more aggressive optimization, making compilation time increase with probably slightly better performance 💜max-sched-ready-insns— the maximum number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. increasing values mean more thorough searches, which makes compilation time increase with probably little benefit 😬 so like, mostly just slower.max-sched-region-blocks— the maximum number of blocks in a region considered for interblock scheduling 🧩max-pipeline-region-blocks— the maximum number of blocks in a region considered for pipelining in the selective scheduler 🎯max-sched-region-insns— the maximum number of insns in a region considered for interblock scheduling 🧩max-pipeline-region-insns— the maximum number of insns in a region considered for pipelining in the selective scheduler 🎯min-spec-prob— the minimum probability (in percents) of reaching a source block for interblock speculative scheduling 🔮max-sched-extend-regions-iters— the maximum number of iterations through CFG to extend regions. a value of0disables region extensions 🚫max-sched-insn-conflict-delay— the maximum conflict delay for an insn to be considered for speculative motion ⏳sched-spec-prob-cutoff— the minimal probability of speculation success (in percents), so that speculative insns get scheduled ✨sched-state-edge-prob-cutoff— the minimum probability an edge must have for the scheduler to save its state across it 💾sched-mem-true-dep-cost— minimal distance (in CPU cycles) between a store and a load targeting the same memory locations 🕐selsched-max-lookahead— the maximum size of the lookahead window of selective scheduling — basically how deep it searches for available instructions 👀selsched-max-sched-times— the maximum number of times an instruction is scheduled during selective scheduling. this is the limit on the number of iterations through which the instruction may be pipelined 🔁selsched-insns-to-rename— the maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler 🏷️sms-min-sc— the minimum value of stage count that the swing modulo scheduler generates 🎡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 the last known value of that register 📝max-combine-insns— the maximum number of instructions the RTL combiner tries to combine 🧬max-combine-search-insns— the maximum number of instructions the RTL combiner searches in order to find the next use of a given register definition. if this limit is reached without finding such a use, the combiner will stop trying to optimize the definition 🛑
currently this limit only applies after certain successful combination attempts, but it could be extended to other cases in future — so like, watch this space 👀
integer-share-limit— small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed ⚡ this sets the maximum value of a shared integer constant.ssp-buffer-size— the minimum size of buffers (i.e. arrays) that receive stack smashing protection when-fstack-protectoris used 🛡️min-size-for-stack-sharing— the minimum size of variables taking part in stack slot sharing when not optimizing 🧺max-jump-thread-duplication-stmts— maximum number of statements allowed in a block that needs to be duplicated when threading jumps 🧵max-jump-thread-paths— the maximum number of paths to consider when searching for jump threading opportunities. when arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider 🌿 do the math bestie, it's giving budget.max-fields-for-field-sensitive— the max number of fields in a struct that still gets treated all field-sensitive-ly during pointer analysis. ✨prefetch-latency— your estimate of how many instructions, on average, get executed before a prefetch actually finishes. The distance we prefetch ahead is proportional to this constant 💅. Heads up tho: cranking this up can also mean fewer streams get prefetched (peepsimultaneous-prefetches).simultaneous-prefetches— the max number of prefetches allowed to run at the same time. no cap on vibes, cap on prefetches 🫶l1-cache-line-size— size of a cache line in the L1 data cache, in bytes.l1-cache-size— size of the L1 data cache, in kilobytes.l2-cache-size— size of the L2 data cache, in kilobytes.prefetch-dynamic-strides— should the loop array prefetch pass go ahead and drop software prefetch hints for strides that are non-constant? In some cases this can genuinely help, though the whole "the stride isn't constant" thing may make it hard to predict when there's a clear benefit to issuing those hints 🙈.
Set to 1 if prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should only be issued for strides that are known to be constant and below prefetch-minimum-stride.
prefetch-minimum-stride— the minimum constant stride, in bytes, before we start using prefetch hints. If the stride is less than this threshold, no prefetch hints for you, bestie.
This one's lowkey for processors that have hardware prefetchers, where there can be conflicts between the hardware prefetchers and the software prefetchers. If your hardware prefetchers have a maximum stride they can handle, that's the number you should put here — it improves how the software prefetchers get used 🎀.
A value of -1 means we don't have a threshold, so prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and constant.
destructive-interference-size,constructive-interference-size— the values for the C++17 variablesstd::hardware_destructive_interference_sizeandstd::hardware_constructive_interference_size. The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both are the size of an L1 cache line for the target, in bytes 🦋. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size is the small end of the range and the destructive size is the large end.
The destructive interference size is intended to be used for layout, and thus it has ABI impact. The default value is not expected to be stable, and on some targets it varies with -mtune — so using this variable in a context where ABI stability matters, like the public interface of a library, is strongly discouraged. If you do use it there, users can stabilize the value with this option. ⚠️
The constructive interference size is less sensitive, since it's typically only used in a static_assert to make sure a type fits within a cache line.
See also -Winterference-size.
loop-interchange-max-num-stmts— the max number of stmts in a loop that's allowed to be interchanged.loop-interchange-stride-ratio— the minimum ratio between the strides of two loops for interchange to actually be profitable 💸.min-insn-to-prefetch-ratio— the minimum ratio between the number of instructions and the number of prefetches needed to enable prefetching in a loop.prefetch-min-insn-to-mem-ratio— the minimum ratio between the number of instructions and the number of memory references needed to enable prefetching in a loop.use-canonical-types— should the compiler use the "canonical" type system? This should always be1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++ 👑. BUT — if bugs in the canonical type system are causing compilation failures, set this value to0to disable canonical types.switch-conversion-max-branch-ratio— switch initialization conversion refuses to create arrays bigger thanswitch-conversion-max-branch-ratiotimes the number of branches in the switch. periodt.switch-lower-slow-alg-max-cases— max number of cases before the slow switch lowering algorithms get used.max-partial-antic-length— max length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at-O3and above. For some sorts of source code, the enhanced partial redundancy elimination optimization can straight-up run away and consume all the memory available on the host machine 😭. This parameter caps the length of the sets that get computed, which stops the runaway behavior. Setting it to0allows an unlimited set length.rpo-vn-max-loop-depth— the max loop depth that gets value-numbered optimistically. When the limit hits, the innermostrpo-vn-max-loop-depthloops and the outermost loop in the loop nest get value-numbered optimistically, and the rest don't 🙅sccvn-max-alias-queries-per-access— max number of alias-oracle queries we perform when hunting for redundancies in loads and stores. If this limit is hit, the search is aborted and the load or store isn't considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.ira-max-loops-num— IRA uses regional register allocation by default. If a function has more loops than the number this parameter gives, only at most the given number of the most frequently-executed loops form regions for regional register allocation.ira-max-conflict-table-size— even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still eat excessive amounts of memory for huge functions 💀. If a function's conflict table could be bigger than the size in MB given here, the register allocator instead uses a faster, simpler, and lower-quality algorithm that doesn't need to build a pseudo-register conflict table.ira-loop-reserved-regs— IRA can be used to evaluate more accurate register pressure in loops when deciding to move loop invariants (see-O3). The number of available registers reserved for some other purposes is given by this parameter. Its default is the best one found from numerous experiments 🧪.ira-consider-dup-in-all-alts— makes IRA consider the matching constraint (duplicated operand number) heavily across all available alternatives for the preferred register class. If it's set to zero, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. Otherwise, IRA checks all available alternatives for the preferred register class even if it already found a choice with an appropriate register class, and respects the qualified matching constraint it found 🔍.ira-simple-lra-insn-threshold— approximate function insn number, in 1K units, that triggers simple local RA.lra-inheritance-ebb-probability-cutoff— LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance 💖. EBB is used as the region for doing it. This parameter defines a minimal fall-through edge probability, in percentage, used to add a BB to the inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64.loop-invariant-max-bbs-in-loop— loop invariant motion can get very expensive, both in compile time and in the amount of compile-time memory needed, with very large loops 😳. Loops with more basic blocks than this parameter won't get loop invariant motion optimization performed on them.loop-max-datarefs-for-datadeps— building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. Those large loops aren't handled by the optimizations that use loop data dependencies.max-vartrack-size— sets a max number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after all debug insns are removed from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for that function. Setting the parameter to zero makes it unlimited 🌈.max-vartrack-expr-depth— sets a max number of recursion levels when trying to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. Set it too low and value expressions that are available and could be represented in debug info might end up unused 🙈; set it higher and the compiler may find more complex debug expressions, but compile time and memory use may grow.max-debug-marker-count— sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid a complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, those stmts get dropped from the inlined copy of the function, and from its RTL expansion 🎀.
min-nondebug-insn-uid— set the starting uid for nondebug insns to this number. Everything below it is locked away exclusively for debug insns made by-fvar-tracking-assignments, though debug insns can still grab (non-overlapping) uids above it if that reserved range runs out 👑ipa-sra-deref-prob-threshold— IPA-SRA only swaps a pointer that's known not to be NULL for one or more new parameters when the probability (as a percent, relative to function entry) of it actually getting dereferenced is higher than this parameter 💅ipa-sra-ptr-growth-factor— IPA-SRA only replaces a pointer to an aggregate with one or more new parameters when their cumulative size is less than or equal toipa-sra-ptr-growth-factortimes the size of the original pointer parameter 🎀ipa-sra-ptrwrap-growth-factor— the extra maximum allowed growth of the total size of new parameters that IPA-SRA replaces a pointer to an aggregate with, in the case where it points to a local variable that the caller only writes to and passes along as an argument to other functions 🦋ipa-sra-max-replacements— the max number of pieces of an aggregate that IPA-SRA tracks. Which, lowkey, also makes it the maximum number of replacements of a formal parameter ✨sra-max-scalarization-size-Ospeed,sra-max-scalarization-size-Osize— the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are all about replacing scalar parts of aggregates with uses of independent scalar variables. These parameters control the max size, in storage units, of an aggregate that gets considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize) respectively 🌸sra-max-propagations— the max number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, so that copy propagation is easier 💖tm-max-aggregate-size— when copying thread-local variables inside a transaction, this parameter sets the size in bytes past which variables get saved with the logging functions instead of save/restore code sequence pairs. Only applies when you're using-fgnu-tm🧚graphite-max-nb-scop-params— to dodge exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero lifts the bound entirely. A variable whose value is unknown at compilation time and defined outside a SCoP counts as a parameter of the SCoP ⭐hardcfr-max-blocks— turns off-fharden-control-flow-redundancyfor functions that have more blocks than the value you give. Zero removes any limit 🫶hardcfr-max-inline-blocks— forces-fharden-control-flow-redundancyto use out-of-line checking for functions with more basic blocks than the specified value 💜loop-block-tile-size— the loop blocking / strip mining transforms (turned on with-floop-blockor-floop-strip-mine) strip mine each loop in the loop nest by a given number of iterations. You change that strip length with theloop-block-tile-sizeparameter 🔥ipa-jump-function-lookups— specifies how many statements get visited during jump function offset discovery 👑ipa-cp-value-list-size— IPA-CP tries to track all the possible values and types passed to a function's parameter so it can propagate them and do devirtualization.ipa-cp-value-list-sizeis the max number of values and types it stores per one formal parameter of a function 💀ipa-cp-eval-threshold— IPA-CP computes its own score for cloning profitability heuristics and only does the cloning opportunities whose scores beatipa-cp-eval-threshold🎀ipa-cp-max-recursive-depth— maximum depth of recursive cloning for a self-recursive function 🌈ipa-cp-min-recursive-probability— recursive cloning happens only when the probability of the call actually being executed exceeds this parameter ✨ipa-cp-profile-count-base— when you're using-fprofile-use, IPA-CP takes the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation 🦋ipa-cp-recursive-freq-factor— the number of times interprocedural copy propagation expects recursive functions to call themselves 💅ipa-cp-recursion-penalty— the percentage penalty recursive functions get when they're evaluated for cloning 🌸ipa-cp-single-call-penalty— the percentage penalty that functions containing a single call to another function get when they're evaluated for cloning 💖ipa-max-agg-items— IPA-CP can also propagate a number of scalar values passed inside an aggregate, andipa-max-agg-itemscontrols the max number of such values per one parameter 👑ipa-cp-loop-hint-bonus— when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it adds a bonus ofipa-cp-loop-hint-bonusto that candidate's profitability score 🔥ipa-max-loop-predicates— the max number of different predicates IPA will use to describe when loops in a function have known properties ⭐ipa-max-aa-steps— while analyzing function bodies, IPA-CP uses alias analysis to track values pointed to by function parameters. So it doesn't spend forever on huge functions, it gives up and considers all memory clobbered after examiningipa-max-aa-stepsstatements that modify memory 🫶ipa-max-switch-predicate-bounds— the maximal number of boundary endpoints of case ranges of a switch statement. For a switch that goes over this limit, IPA-CP won't construct the cloning cost predicate (the thing used to estimate cloning benefit) for the default case of that switch 💜ipa-max-param-expr-ops— IPA-CP will analyze a conditional statement that references some function parameter to estimate the benefit of cloning on a certain constant value. But if the number of operations in a parameter expression exceedsipa-max-param-expr-ops, that expression gets treated as a complicated one and IPA analysis won't handle it 💀lto-partitions— specify the desired number of partitions produced during WHOPR compilation. The number of partitions should be more than the number of CPUs used for compilation 🧚lto-min-partition— the size of the minimal partition for WHOPR (in estimated instructions). Stops tiny programs from getting split into a million little partitions and wasting everyone's time ✨lto-max-partition— the size of the max partition for WHOPR (in estimated instructions), providing an upper bound on the individual size of a partition. Meant to be used only with balanced partitioning 🎀lto-partition-locality-frequency-cutoff— the denominator n of the fraction 1/n of the callee's execution frequency needed for it to be cloned for a particular caller. The special value of0means always clone, no cut-off 🌸lto-partition-locality-size-cutoff— the size cut-off for a callee (including inlined calls) to be cloned for a particular caller 🦋lto-max-locality-partition— the maximal size of a locality partition for LTO (in estimated instructions). A value of0means the default value gets used 💖lto-max-streaming-parallelism— the maximal number of parallel processes used for LTO streaming 🔥cxx-max-namespaces-for-diagnostic-help— the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier 👑sink-frequency-threshold— the max relative execution frequency (in percents) of the target block compared to a statement's original block that still allows that statement to be sunk. Bigger numbers = more aggressive statement sinking. A small positive adjustment gets applied for statements with memory operands, since those are even more profitable to sink ⭐max-stores-to-sink— the max number of conditional store pairs that can be sunk. Set it to0if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled 💜case-values-threshold— the smallest number of different values for which a jump-table beats a tree of conditional branches. If the value is0, the default for the machine gets used 🌈jump-table-max-growth-ratio-for-size— the max code size growth ratio when expanding into a jump table (in percent). Used when optimizing for size 🧚jump-table-max-growth-ratio-for-speed— the max code size growth ratio when expanding into a jump table (in percent). Used when optimizing for speed ✨tree-reassoc-width— sets the max number of instructions executed in parallel in a reassociated tree. This parameter overrides the target dependent heuristics used by default, as long as it has a nonzero value 🎀sched-pressure-algorithm— choose between the two available implementations of-fsched-pressure. Algorithm 1 is the original implementation and is the one more likely to stop instructions from being reordered. Algorithm 2 was designed as a compromise between algorithm 1's relatively conservative approach and the rather aggressive approach the default scheduler takes. It leans way more heavily on having a regular register file and accurate register pressure classes. Seehaifa-sched.ccin the GCC sources for more details.
The default choice depends on the target 💅
max-slsr-cand-scan— sets the max number of existing candidates considered when searching for a basis for a new straight-line strength reduction candidate 🫶
okay bestie, more --param lore — sanitizer switches, threading knobs, vectorizer settings, all the good stuff. no skips, no cap 💅
asan-globals— turns on buffer overflow detection for global objects. this protection is enabled by default if you're using the-fsanitize=addressoption. to disable global objects protection, use--param asan-globals=0. 🫶asan-stack— enables buffer overflow detection for stack objects. this kind of protection is on by default when using-fsanitize=address. to disable stack protection, use the--param asan-stack=0option. ✨asan-instrument-reads— enables buffer overflow detection for memory reads. it's enabled by default when using-fsanitize=address. to disable memory reads protection, use--param asan-instrument-reads=0. 💖asan-instrument-writes— enables buffer overflow detection for memory writes. also on by default when using-fsanitize=address. to disable memory writes protection, use the--param asan-instrument-writes=0option. 🎀asan-memintrin— enables detection for built-in functions. this protection is enabled by default when using-fsanitize=address. to disable built-in functions protection, use--param asan-memintrin=0. 🦋asan-use-after-return— enables detection of use-after-return. this kind of protection is enabled by default when using the-fsanitize=addressoption. to disable it, use--param asan-use-after-return=0. 🌸
tiny catch, bestie: by default the check is disabled at run time. to enable it, add"detect_stack_use_after_return=1"to the environment variableASAN_OPTIONS. 😭
asan-instrumentation-with-call-threshold— if the number of memory accesses in a function being instrumented is greater or equal to this number, use callbacks instead of inline checks. E.g. to disable inline code, use--param asan-instrumentation-with-call-threshold=0. 💅asan-kernel-mem-intrinsic-prefix— if nonzero, prefix calls to"memcpy","memset"and"memmove"with__asan_or__hwasan_for-fsanitize=kernel-addressor-fsanitize=kernel-hwaddress, respectively. 🔥hwasan-instrument-stack— enables hwasan instrumentation of statically sized stack-allocated variables. this instrumentation is enabled by default when using-fsanitize=hwaddressand disabled by default when using-fsanitize=kernel-hwaddress. to disable stack instrumentation, use--param hwasan-instrument-stack=0, and to enable it, use--param hwasan-instrument-stack=1. 👑hwasan-random-frame-tag— when using stack instrumentation, decides tags for stack variables using a deterministic sequence beginning at a random tag for each frame. with this parameter unset, tags are chosen using the same sequence but beginning from 1. this is enabled by default for-fsanitize=hwaddressand unavailable for-fsanitize=kernel-hwaddress. to disable it, use--param hwasan-random-frame-tag=0. 🌈hwasan-instrument-allocas— enables hwasan instrumentation of dynamically sized stack-allocated variables. it's enabled by default when using-fsanitize=hwaddressand disabled by default when using-fsanitize=kernel-hwaddress. to disable instrumentation of such variables, use--param hwasan-instrument-allocas=0, and to enable it, use--param hwasan-instrument-allocas=1. ⭐hwasan-instrument-reads— enables hwasan checks on memory reads. instrumentation of reads is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. to disable checking memory reads, use--param hwasan-instrument-reads=0. 💀hwasan-instrument-writes— enables hwasan checks on memory writes. instrumentation of writes is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. to disable checking memory writes, use--param hwasan-instrument-writes=0. 🙈hwasan-instrument-mem-intrinsics— enables hwasan instrumentation of builtin functions. instrumentation of these builtin functions is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. to disable instrumentation of builtin functions, use--param hwasan-instrument-mem-intrinsics=0. 🍓use-after-scope-direct-emission-threshold— if the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. 🧚tsan-distinguish-volatile— emit special instrumentation for accesses to volatiles. 💜tsan-instrument-func-entry-exit— emit instrumentation calls to__tsan_func_entry()and__tsan_func_exit(). ✨max-fsm-thread-path-insns— maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path. 🫶threader-debug—threader-debug=[none|all]enables verbose dumping of the threader solver. 💅parloops-chunk-size— chunk size of omp schedule for loops parallelized by parloops. 🎀parloops-schedule— schedule type of omp schedule for loops parallelized by parloops (static,dynamic,guided,auto,runtime). 🌸parloops-min-per-thread— the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. 🦋max-ssa-name-query-depth— maximum depth of recursion when querying properties of SSA names in things like fold routines. one level of recursion corresponds to following a use-def chain. 🔥max-speculative-devirt-maydefs— the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. 👑ranger-debug— specifies the type of debug output to be issued for ranges. 🌈unroll-jam-min-percent— the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. ⭐unroll-jam-max-unroll— the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. 💀max-rtl-if-conversion-unpredictable-cost— maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. 🙈max-variable-expansions-in-unroller— if-fvariable-expansion-in-unrolleris used, the maximum number of times that an individual variable will be expanded during loop unrolling. 🍓partial-inlining-entry-probability— maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen. 🧚max-tracked-strlens— maximum number of strings for which the strlen optimization pass will track string lengths. 💜gcse-after-reload-partial-fraction— the threshold ratio for performing partial redundancy elimination after reload. ✨gcse-after-reload-critical-fraction— the threshold ratio of critical edges execution count that permit performing redundancy elimination after reload. 🫶max-loop-header-insns— the maximum number of insns in loop header duplicated by the copy loop headers pass. 💅vect-epilogues-nomask— enable loop epilogue vectorization using smaller vector size. 🎀vect-partial-vector-usage— controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code.0stops the vectorizer from ever using partial vector loads and stores.1allows partial vector loads and stores if vectorization removes the need for the code to iterate.2allows partial vector loads and stores in all loops. the parameter only has an effect on targets that support partial vector loads and stores. 🌸vect-inner-loop-cost-factor— the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized. the factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. the default value of this parameter is50. 🦋vect-induction-float— enable loop vectorization of floating point inductions. 🔥vect-force-slp— force the use of SLP when vectorizing, fail if not possible. 👑vrp-block-limit— maximum number of basic blocks before VRP switches to a lower memory algorithm. 🌈vrp-sparse-threshold— maximum number of basic blocks before VRP uses a sparse bitmap cache. ⭐vrp-switch-limit— maximum number of outgoing edges in a switch before VRP will not process it. 💀vrp-vector-threshold— maximum number of basic blocks for VRP to use a basic cache vector. 🙈avoid-fma-max-bits— maximum number of bits for which we avoid creating FMAs. 🍓fully-pipelined-fma— whether the target fully pipelines FMA instructions. if non-zero, reassociation considers the benefit of parallelizing FMA's multiplication part and addition part, assuming FMUL and FMA use the same units that can also do FADD. 🧚sms-loop-average-count-threshold— a threshold on the average loop count considered by the swing modulo scheduler. 💜sms-dfa-history— the number of cycles the swing modulo scheduler considers when checking conflicts using DFA. ✨graphite-allow-codegen-errors— whether codegen errors should be ICEs when-fchecking. 🫶sms-max-ii-factor— a factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop. 💅lra-max-considered-reload-pseudos— the max number of reload pseudos which are considered during spilling a non-reload pseudo. 🎀max-pow-sqrt-depth— Caps the maximum depth ofsqrtchains GCC will use when it’s synthesizing exponentiation by a real constant. That’s the whole vibe: no infinite sqrt spiral 😭✨max-dse-active-local-stores— Maximum number of active local stores in RTL dead store elimination. RTL dead store elimination stays on a leash 💅asan-instrument-allocas— Enable asan allocas/VLAs protection. Safety era 🫶max-iterations-computation-cost— Bound on the cost of an expression to compute the number of iterations. Basically a budget cap for that iteration-count math.max-isl-operations— Maximum number of isl operations;0means unlimited. Zero = no cap, iconic 🌈graphite-max-arrays-per-scop— Maximum number of arrays per scop. Graphite’s little limit 💜max-vartrack-reverse-op-size— Max. size ofloclist for which reverse ops should be added. That’sloclist, exactly.fsm-scale-path-stmts— Scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing to--param=max-jump-thread-duplication-stmts. Yes, say that three times fast 😭uninit-control-dep-attempts— Maximum number of nested calls to search for control dependencies during uninitialized variable analysis. Uninit analysis on its little detective arc 🔍uninit-max-chain-len— Maximum number of predicatesandedfor each predicateoredin the normalized predicate chain. We preserve the lore 💀uninit-max-num-chains— Maximum number of predicatesoredin the normalized predicate chain.uninit-max-prune-work— Maximum amount of work done to prune paths where the variable is always initialized. Pruning budget, bestie.sched-autopref-queue-depth— Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at' 'only enable instruction sorting heuristic. Note the weird' 'is literally there — do not touch it 🙈loop-versioning-max-inner-insns— The maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy. Inner-loop size limit, periodt.loop-versioning-max-outer-insns— The maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning. Outer-loop size limit with the inner-loop discount noted 🎀ssa-name-def-chain-limit— The maximum number ofSSA_NAMEassignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics. Big chain energy, but capped 💅store-merging-max-size— Maximum size of a single store merging region in bytes. Merge responsibly 🫶store-forwarding-max-distance— Maximum number of instruction distance that a small store forwarded to a larger load may stall. Value'0'disables the cost checks for the avoid-store-forwarding pass. Zero = off for those checks, noted.hash-table-verification-limit— The number of elements for which hash table verification is done for each searched element. Verification budget, iykyk.max-find-base-term-values— Maximum number ofVALUEs handled during a singlefind_base_termcall. CapitalVALUEs stay capitalized ✨analyzer-max-enodes-per-program-point— The maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point. Exploded nodes, not drama 💥analyzer-max-constraints— The maximum number of constraints per state. State stays constrained, bestie.analyzer-min-snodes-for-call-summary— The minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites. Summary-time threshold 🧚analyzer-max-enodes-for-full-dump— The maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose format. Dot dump gets shy after this depth 🎀analyzer-max-recursion-depth— The maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper. Recursion depth says no 😭analyzer-max-svalue-depth— The maximum depth of a symbolic value, before approximating the value as unknown. Deep symbolics become unknown after this.analyzer-max-infeasible-edges— The maximum number of infeasible edges to reject before declaring a diagnostic as infeasible. Reject budget, then diagnostic says infeasible.gimple-fe-computed-hot-bb-threshold— The number of executions of a basic block which is considered hot. The parameter is used only in GIMPLE FE. GIMPLE FE only — read the label 👑analyzer-bb-explosion-factor— The maximum number of'after supernode'exploded nodes within the analyzer per supernode, before terminating analysis. Explosion factor, but make it per supernode 💥analyzer-text-art-string-ellipsis-threshold— The number of bytes at which to ellipsize string literals in analyzer text art diagrams. Text art gets the ellipsis treatment at this many bytes 🎨analyzer-text-art-ideal-canvas-width— The ideal width in characters of text art diagrams generated by the analyzer. Canvas width goal, ideal only.analyzer-text-art-string-ellipsis-head-len— The number of literal bytes to show at the head of a string literal in text art when ellipsizing it. Head bytes, specified.analyzer-text-art-string-ellipsis-tail-len— The number of literal bytes to show at the tail of a string literal in text art when ellipsizing it. Tail bytes, specified.ranger-logical-depth— Maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges. Ranger looks through logicals only this deep 🔍ranger-recompute-depth— Maximum depth of instruction chains to consider for recomputation in the outgoing range calculator. Recompute depth limit.relation-block-limit— Maximum number of relations the oracle will register in a basic block. Oracle’s per-block relation cap 🌸transitive-relations-work-bound— Work bound when discovering transitive relations from existing relations. Transitive discovery has a work budget.min-pagesize— Minimum page size for warning and early break vectorization purposes. Minimum page size, for those purposes exactly.openacc-kernels— Specify mode of OpenACCkernelsconstructs handling. With--param=openacc-kernels=decompose, OpenACCkernelsconstructs are decomposed into parts, a sequence of compute constructs, each then handled individually. This is work in progress. With--param=openacc-kernels=parloops, OpenACCkernelsconstructs are handled by the parloops pass, en bloc. This is the current default. Decompose = work in progress; parloops = current default 🫶openacc-privatization— Control whether the-fopt-info-omp-noteand applicable-fdump-tree-*-detailsoptions emit OpenACC privatization diagnostics. With--param=openacc-privatization=quiet, don't diagnose. This is the current default. With--param=openacc-privatization=noisy, do diagnose. Quiet is default; noisy diagnoses.cycle-accurate-model— Specifies whether GCC should assume that the scheduling description is mostly a cycle-accurate model of the target processor the code is intended to run on, in the absence of cache misses. Nonzero means that the selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should aggressively spill to try and fill any pipeline bubbles. This is the current default. Zero means the scheduling description might not be available/accurate or perhaps not applicable at all, such as for modern out-of-order processors. Nonzero = current default; zero = maybe not available/accurate/applicable, e.g. modern out-of-order processors.
The following choices of name are available on AArch64 targets: 🦋
aarch64-vect-compare-costs— When vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. This includes:- Trying both SVE and Advanced SIMD, when SVE is available.
- Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
- Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.
aarch64-float-recp-precision— The number of Newton iterations for calculating the reciprocal forfloattype. The precision of division is proportional to this param when division approximation is enabled. The default value is1. Newton iteration count for float reciprocal — default1🍓aarch64-double-recp-precision— The number of Newton iterations for calculating the reciprocal fordoubletype. The precision of division is proportional to this param when division approximation is enabled. The default value is2. Default2, noted.aarch64-autovec-preference— Force an ISA selection strategy for auto-vectorization.default— Use the default heuristics.asimd-only— Use only Advanced SIMD for auto-vectorization.sve-only— Use only SVE for auto-vectorization.prefer-asimd— Use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.prefer-sve— Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
aarch64-ldp-policy— Fine-grained policy for load pairs. With--param=aarch64-ldp-policy=default, use the policy of the tuning structure. This is the current default. With--param=aarch64-ldp-policy=always, emitldpregardless of alignment. With--param=aarch64-ldp-policy=never, do not emitldp. With--param=aarch64-ldp-policy=aligned, emitldponly if the source pointer is aligned to at least double the alignment of the type. Load-pair policy, fully specified 💅aarch64-stp-policy— Fine-grained policy for store pairs. With--param=aarch64-stp-policy=default, use the policy of the tuning structure. This is the current default. With--param=aarch64-stp-policy=always, emitstpregardless of alignment. With--param=aarch64-stp-policy=never, do not emitstp. With--param=aarch64-stp-policy=aligned, emitstponly if the source pointer is aligned to at least double the alignment of the type. Store-pair policy, same deal 🎀aarch64-ldp-alias-check-limit— Limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to form anldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, at the expense of increased compile time. More checks = more aggressive load reordering over stores, but compile time goes up 😭aarch64-ldp-writeback— this one's the vibe check for writeback opportunities in the AArch64 load/store pair fusion pass.0= writeback handling is off, bye.1= we try to form pairs involving one or more existing individual writeback accesses where we can.2= we also opportunistically go looking for writeback opportunities by folding in trailing destructive updates of the base register that a pair is using 👑aarch64-loop-vect-issue-rate-niters— so some AArch64 CPUs' tuning tries to factor in both latencies and issue rates when deciding whether a loop should get vectorized with SVE, vectorized with Advanced SIMD, or just... not vectorized at all, bestie. If you set this parameter to n, GCC won't use this heuristic for loops that are known to run in fewer than n Advanced SIMD iterations 🌀aarch64-vect-unroll-limit— the vectorizer uses the tuning info it's got to figure out whether unrolling the main vectorized loop would be a glow-up and by how much. This parameter sets the upper bound on how much the vectorizer will unroll the main loop. Default isfour🍓
The following choices of name are available on GCN targets:
gcn-preferred-vectorization-factor— preferred vectorization factor:default,32,64.
The following choices of name are available on i386 and x86_64 targets:
x86-stlf-window-ninsns— the instruction count above which the STFL stall penalty can be compensated for.x86-stv-max-visits— the max number of use and def visits when discovering an STV chain before the discovery just gives up and aborts, no cap.
🎀 Program Instrumentation Options 🎀
Okay so GCC has a whole little wardrobe of command-line options that control adding run-time instrumentation to the code it normally generates. Iconic of it honestly. Like — one purpose of instrumentation is to collect profiling statistics for finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses 👀 — and also deliberately hostile attacks such as stack smashing or C++ vtable hijacking, not me being dramatic, it's literally in there. There's also a general hook you can use to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. Versatile queen 💅
-p,-pg— generate extra code to write profile information suitable for the analysis programprof(for-p) orgprof(for-pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. Both times, bestie, no skipping.
You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options 🙈
-fprofile-arcs— adds code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable ✨
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o) 🎀
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. Sneaky but valid 💖
-fcondition-coverage— adds code so that program conditions are instrumented. During execution the program records what terms in a conditional contribute to a decision — which you can use to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read withgcov --conditions🦋-fpath-coverage— adds code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by-fpath-coverage-limit. The result can be read withgcov --prime-paths --prime-paths-lines --prime-paths-source, 💫-fpath-coverage-limit=limit— the threshold at which point-fpath-coveragegives up on instrumenting a function. This limit is approximate and conservative, because GCC uses a pessimistic heuristic that slightly overcounts the running number of paths, and it gives up if the threshold is reached before finding all the paths. Heads up: this option is not for fine grained control over which functions to instrument — rather it's intended to limit the effect of path explosion and keep compile times reasonable. Default is250000⭐--coverage— this option is used to compile and link code instrumented for coverage analysis. It's a synonym for-fprofile-arcs -ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details 👑- Compile the source files with
-fprofile-arcsplus optimization and code generation options. For test coverage analysis, use the additional-ftest-coverageoption. You do not need to profile every source file in a program. - Compile the source files additionally with
-fprofile-abs-pathto create absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. - Link your object files with
-lgcovor-fprofile-arcs(the latter implies the former). - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect,
forkcalls are detected and correctly handled without double counting 🫶
- Compile the source files with
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged. Loyalty 💖
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
-fbranch-probabilities. - For test coverage analysis, use
gcovto produce human readable information from the.gcnoand.gcdafiles. Refer to thegcovdocumentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the 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 🌸
With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression 💅
With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented — as if -fpath-coverage was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. Literally the main character of paths 👑
-ftest-coverage— produce a notes file that thegcovcode-coverage utility can use to show program coverage. Each source file's note file is calledauxname.gcno. Refer to the-fprofile-arcsoption above for a description ofauxnameand instructions on how to generate test coverage data. Coverage data matches the source files more closely if you don't optimize 🧚-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories 🗂️-fprofile-dir=path— set the directory to search for the profile data files in to path. This option affects only the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-arcsand used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, so the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of thesourcename.gcdafile and use it as the file name of a.gcdafile. See details about the file naming in-fprofile-arcs. See similar option-fprofile-note🎀
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p— process ID.%q{VAR}— value of environment variableVAR.
-fprofile-generate,-fprofile-generate=path— enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use-fprofile-generateboth when compiling and when linking your program 🔥
Okay so, the following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp ✨
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
-fprofile-info-section,-fprofile-info-section=name— registers the profile info in the specified section instead of using a constructor/destructor 💅 The section name isnameif it's specified, otherwise it defaults to.gcov_info. A pointer to the profile information generated by-fprofile-arcsis placed in the specified section for each translation unit. This option disables profile information registration through a constructor and disables profile information processing through a destructor. It's not meant for hosted environments like GNU/Linux — it targets freestanding environments (think embedded systems) with limited resources that don't support constructors/destructors or the C library file I/O.
The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}The program could dump the profiling information registered in this linker set for example like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the __gcov_filename_to_gcfn and __gcov_info_to_gcda functions and merge the profile information into .gcda files on the host filesystem 🫶
-fprofile-note=path— ifpathis specified, GCC saves the.gcnofile into thepathlocation. Lowkey heads up: if you combine this option with multiple source files, the.gcnofile will be overwritten 😭-fprofile-prefix-path=path— this one can be used withprofile-generate=profile_dirandprofile-use=profile_dirto tell GCC where the base directory of the built source tree is. By defaultprofile_dirwill contain files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because the profile data will not be found during the optimized build. In such setups,-fprofile-prefix-path=pathwithpathpointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory 💖-fprofile-prefix-map=old=new— when compiling files residing in directoryold, record profiling information (with--coverage) describing them as if the files resided in directorynewinstead. See also-ffile-prefix-mapand-fcanon-prefix-map.-fprofile-update=method— alters the update method for an application instrumented for profile feedback based optimization. Themethodargument should be one ofsingle,atomicorprefer-atomic.singleis useful for single-threaded applications, whileatomicprevents profile corruption by emitting thread-safe code 🔥
Warning: when an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted. 💀
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.
If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, and libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.
For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state 😵💫
-fprofile-filter-files=regex— instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example,-fprofile-filter-files=main\.c;module.*\.cwill instrument onlymain.cand all C files starting with'module'✨-fprofile-exclude-files=regex— instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons). For example,-fprofile-exclude-files=/usr/.*will prevent instrumentation of all files that are located in the/usr/folder.-fprofile-reproducible=[multithreaded|parallel-runs|serial]— control the level of reproducibility of profile gathered by-fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful, for example, for distribution packages.
With -fprofile-reproducible=serial, the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before the fork function is invoked.
Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by the no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible.
With -fprofile-reproducible=parallel-runs, collected profile stays reproducible regardless of the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (such as with make -j). This reduces quality of gathered data, in particular of indirect call profiling 🌸
-fsanitize=address— enables AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. See<https://github.com/google/sanitizers/wiki/AddressSanitizer>for more details. The run-time behavior can be influenced using theASAN_OPTIONSenvironment variable. When set tohelp=1, the available options are shown at startup of the instrumented program. See<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>for a list of supported options. The option cannot be combined with-fsanitize=threador-fsanitize=hwaddress. Note that the only targets-fsanitize=hwaddressis currently supported on are x86-64 (only with-mlam=u48or-mlam=u57options) and AArch64, in both cases only in ABIs with 64-bit pointers 👑
When compiling with -fsanitize=address, you should also use -g to produce more meaningful output. To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). Using -fno-omit-frame-pointer also improves stack traces. Since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On Linux, this can be achieved with setarch `uname -m` -R ./prog 🔥
-fsanitize=kernel-address— enables AddressSanitizer for the Linux kernel. See<https://github.com/google/kernel-sanitizers>for more details 💜-fsanitize=hwaddress— ok so this one turns on Hardware-assisted AddressSanitizer, which is giving genius-level efficiency 💅 it uses a hardware ability to ignore the top byte of a pointer so it can catch memory errors with a really low memory overhead. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. Turning this on also enables-fsanitize-address-use-after-scope. Full lore is at<https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>if you're curious 🦋. You can influence the run-time behavior with theHWASAN_OPTIONSenvironment variable — and when you set it to"help=1", the available options get shown at startup of the instrumented program (so helpful of it ✨). Heads up bestie: this option cannot be combined with-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64 😭.-fsanitize=kernel-hwaddress— same hwaddress energy but specifically for compiling the Linux kernel 👑. It's similar to-fsanitize=kernel-addressbut uses an alternate instrumentation method, and similar to-fsanitize=hwaddressbut with instrumentation differences that are necessary for compiling the Linux kernel. Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.
Note: this option has different defaults to-fsanitize=hwaddress🫶. Instrumenting the stack and alloca calls are not on by default, but they're still possible — just specify the command-line options--param hwasan-instrument-stack=1and--param hwasan-instrument-allocas=1respectively. Also, using a random frame tag is not implemented for kernel instrumentation.
-fsanitize=pointer-compare— instruments comparison operations (<,<=,>,>=) that have pointer operands 🎀. The option must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it cannot be combined with-fsanitize=thread.
Note: by default the check is disabled at run time 🙈. To enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects an invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract— same deal but for subtraction with pointer operands 💜. It must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it cannot be combined with-fsanitize=thread.
Note: by default the check is disabled at run time 🙈. To enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects an invalid operation only when both pointers are non-null.
-fsanitize=shadow-call-stack— enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (like stack buffer overflows, yikes 💀). The way it works: it saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that need to save the return address to the stack — efficient queen ✨.
Currently it only supports the aarch64 platform. It's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option 🌸. For user space programs, runtime support is not currently provided in libc and libgcc — so if you wanna use this feature in user space, you need to provide your own support for the runtime 😭. And it should be noted that this may cause the ABI rules to be broken.
On aarch64, the instrumentation makes use of the platform register "x18". That generally means any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18 — otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer 💀.
Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions 🫶.
See <https://clang.llvm.org/docs/ShadowCallStack.html> for more details.
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector 🔥. Memory access instructions get instrumented to detect data race bugs. See<https://github.com/google/sanitizers/wiki#threadsanitizer>for more details. You can influence the run-time behavior with theTSAN_OPTIONSenvironment variable; see<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>for a list of supported options. This option cannot be combined with-fsanitize=addressor-fsanitize=leak.
When compiling with -fsanitize=thread, you should also use -g so you get more meaningful output 💅 (trust, you want this).
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak— enables LeakSanitizer, a memory leak detector 🦋. This option only matters for linking of executables. The executable is linked against a library that overrides"malloc"and other allocator functions. See<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>for more details. Run-time behavior can be influenced using theLSAN_OPTIONSenvironment variable. This option cannot be combined with-fsanitize=thread.-fsanitize=undefined— enables UndefinedBehaviorSanitizer, a fast undefined behavior detector ⭐. Various computations get instrumented to detect undefined behavior at runtime. See<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html>for more details. Run-time behavior can be influenced using theUBSAN_OPTIONSenvironment variable. Current suboptions are, and buckle up because there's a bunch 🎀:-fsanitize=shift— this one enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions:-fsanitize=shift-baseand-fsanitize=shift-exponent.-fsanitize=shift-exponent— enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument 👑-fsanitize=shift-base— if the second argument of a shift operation is within range, checks that the result of the shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.-fsanitize=integer-divide-by-zero— detects integer division by zero 💀-fsanitize=unreachable— with this option, the compiler turns the"__builtin_unreachable"call into a diagnostics message call instead. When reaching the"__builtin_unreachable"call, the behavior is undefined.-fsanitize=vla-bound— this option instructs the compiler to check that the size of a variable length array is positive ✨-fsanitize=null— enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed to by a NULL pointer 🙈-fsanitize=return— enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value 😭. This option works in C++ only.-fsanitize=signed-integer-overflow— enables signed integer overflow checking 💅. We check that the result of"+","*", and both unary and binary"-"does not overflow in the signed arithmetics. This also detects"INT_MIN / -1"signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— enables instrumentation of array bounds 🦋. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with the exception of flexible array member-like arrays for which"-fstrict-flex-arrays"or"-fstrict-flex-arrays="options or"strict_flex_array"attributes say they shouldn't be treated like flexible array member-like arrays.-fsanitize=bounds-strict— enables strict instrumentation of array bounds 🔥. Most out of bounds accesses are detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented.-fsanitize=alignment— enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object 🎀-fsanitize=object-size— enables instrumentation of memory references using the"__builtin_dynamic_object_size"function. Various out of bounds pointer accesses are detected ⭐-fsanitize=float-divide-by-zero— detects floating-point division by zero. Unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs 💜-fsanitize=float-cast-overflow— enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. Also this option does not work well with"FE_INVALID"exceptions enabled, so like, be aware 😬-fsanitize=nonnull-attribute— enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the"nonnull"function attribute 🫶
-fsanitize=returns-nonnull-attribute
okay so this one instruments every return statement inside functions that are marked with the returns_nonnull function attribute 💅 the whole point is to catch it when somebody returns a null value from a function that swore on its life it would never do that. rude. caught. iconic.
-fsanitize=bool
instrumentation of loads from bool 🎀 if a value that isn't 0 or 1 gets loaded, boom, a run-time error is issued. bestie said binary and it MEANT binary.
-fsanitize=enum
same energy but for loads from an enum type ✨ if a value outside the range of values for that enum type gets loaded, a run-time error is issued. your enum is not a free-for-all, it's a curated list.
-fsanitize=vptr
C++ besties rise 🦋 this instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes — all so it can verify the referenced object has the correct dynamic type. it's giving vibe check but for object types. no cap.
-fsanitize=pointer-overflow
instrumentation of pointer arithmetics 🔥 if the pointer arithmetics overflows, a run-time error is issued. math said no.
-fsanitize=builtin
instrumentation of arguments passed to selected builtin functions 🌸 if an invalid value gets passed to those arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. so like... don't do that 😭
⚠️ heads up bestie: note that sanitizers tend to increase the rate of false positive warnings, most notably those around-Wmaybe-uninitialized. We recommend against combining-Werrorand [the use of] sanitizers. yes this is a real sentence from the page and yes it is a warning. do not come for me.
okay so while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead 💜 this currently works only for the C family of languages.
-fno-sanitize=all
this option disables all previously enabled sanitizers 💀 and no, -fsanitize=all is not allowed, because some sanitizers cannot be used together. it's not a vibe, it's a conflict.
-fasan-shadow-offset=number
this forces GCC to use a custom shadow offset in AddressSanitizer checks ✨ it's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. science <3
-fsanitize-sections=s1,s2,...
sanitize global variables in selected user-defined sections 🎀 and si may contain wildcards. slay.
-fsanitize-recover[=opts]
okay this one controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts 👑
enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened 😳 which means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. yes, success. even with errors. that's the whole deal.
the -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code 🫶
currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address ⭐
for these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental 🔥 and -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
and like, even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal 😭 the runtime library defaults to "halt_on_error=0" for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is "halt_on_error=1". this can be overridden through setting the "halt_on_error" flag in the corresponding environment variable 💖
🙈 also: syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope
enable sanitization of local variables to detect use-after-scope bugs 💅 the option sets -fstack-reuse to none. it's giving "you used it after its moment passed" and honestly? relatable but illegal.
-fsanitize-trap[=opts]
this option instructs the compiler to report, for the sanitizers mentioned in the comma-separated list of opts, undefined behavior using "__builtin_trap" rather than a "libubsan" library routine 🌈
if this option is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, and "__builtin_trap" will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover= 💀 (yes, that's -fsanitizer-recover=, exactly as printed. copying character for character, that's the job.)
the advantage of this is that the "libubsan" library is not needed and is not linked in, so this is usable even in freestanding environments 👑 lowkey perfect for when you have no library and no chill.
currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict ✨ "-fsanitize-trap=all" can also be specified, which enables it for "undefined" suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict.
if "-fsanitize-trap=undefined" or "-fsanitize-trap=all" is used and "-fsanitize=vptr" is enabled on the command line, the instrumentation is silently ignored, because the instrumentation always needs "libubsan" support 🦋 and -fsanitize-trap=vptr is not allowed.
-fsanitize-undefined-trap-on-error
this option is the deprecated equivalent of -fsanitize-trap=all 🥲 still here, still working, still deprecated.
-fsanitize-coverage=trace-pc
enable coverage-guided fuzzing code instrumentation 🎀 inserts a call to "__sanitizer_cov_trace_pc" into every basic block. every. single. one.
-fsanitize-coverage=trace-cmp
enable dataflow guided fuzzing code instrumentation 🔥 this one inserts a call to "__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for integral comparison with both operands variable, or "__sanitizer_cov_trace_const_cmp1", "__sanitizer_cov_trace_const_cmp2", "__sanitizer_cov_trace_const_cmp4" or "__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand constant, and "__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for float or double comparisons, and "__sanitizer_cov_trace_switch" for switch statements. the whole squad fr fr.
-fcf-protection=[full|branch|return|none|check] and -fcf-protection
enable code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid 🛡️ this prevents diverting the flow of control to an unexpected target. it's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). not today, attacker.
the -fcf-protection= keywords are interpreted as follows 💅
branch— tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.return— implements checking of validity at the point of returning from a function.full— an alias for specifying bothbranchandreturn. two for one, we love a bundle.check— used for the final link with link-time optimization (LTO). an error is issued if LTO object files are compiled with different-fcf-protectionvalues. the valuecheckis ignored at the compile time.none— turns off instrumentation. bye.
-fcf-protection is an alias for -fcf-protection=full ✨ to override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind 🫶 (yes, literally kind. that's what it says.)
the macro "__CET__" is defined when -fcf-protection is used 💜 the first bit of "__CET__" is set to 1 for the value branch and the second bit of "__CET__" is set to 1 for the return.
you can also use the "nocf_check" attribute to identify which functions and calls should be skipped from instrumentation 🙈
currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer 🌸
-fharden-compares
for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call "__builtin_trap" if the results do not match 🔥 use with -fharden-conditional-branches to cover all conditionals. double checker energy.
-fharden-conditional-branches
for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call "__builtin_trap" if the result is unexpected 🌈 use with -fharden-compares to cover all conditionals. they're a matching set, don't split them up.
-fharden-control-flow-redundancy
emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph 👑
verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls 💅 tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are available.
tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls would also have verification issued before the call — but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins 🦋 so like, it's a thing, it's just a thing that basically never happens unless you're built different.
-fhardcfr-skip-leaf
disable -fharden-control-flow-redundancy in leaf functions 🍓 short and sweet. leaves get a pass.
-fhardcfr-check-exceptions— so when-fharden-control-flow-redundancyis doing its thing, this makes the compiler check the recorded execution path against the control flow graph at exception escape points 💅 like, it's as if the whole function body got wrapped in a cleanup handler that ran the check and then reraised. It's on by default, bestie — if you're not about it, hit it with-fno-hardcfr-check-exceptionsto turn it off.-fhardcfr-check-returning-calls— when-fharden-control-flow-redundancyis active, this checks the recorded execution path against the control flow graph before any function call that's immediately followed by a return of its result (if there is one), so that tail-call optimization doesn't get blocked 😤 whether or not it actually ends up optimized into a tail call.
It's enabled by default whenever sibling call optimizations are on (peep -foptimize-sibling-calls), but you can flip it on or off explicitly with the negated form regardless of what the optimizations are doing. Slay, options ✨
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— when-fharden-control-flow-redundancyis active, this checks the recorded execution path against the control flow graph beforenoreturncalls. Which ones? Pick your fighter:always— literally all of themno-xthrow— the ones that aren't expected to return control to the caller through an exception (this is the default)nothrow— the ones that may not return control to the caller through an exception eithernever— none of them, we pretend we don't see them 🙈
⚠️ checking before anoreturnfunction that may return control to the caller through an exception can make the check happen more than once — like if the exception gets caught in the caller, whether by a handler or a cleanup. When-fhardcfr-check-exceptionsis also enabled, the compiler avoids associating anoreturncall with the implicitly-added cleanup handler, since it'd be redundant with the check done before the call. But other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it again whenever another checkpoint is hit. And the checkpoint may even be anothernoreturncall, so the checking can end up happening multiple times 😭
Various optimizers may mark calls as noreturn and/or nothrow even when the corresponding attributes aren't there, which can mess with where checks land before calls, plus the implicit cleanup handlers added for them. That unpredictability, and the fact that raising and reraising exceptions lowkey amounts to implicitly calling noreturn functions, is exactly why no-xthrow is the default here: it only excludes from noreturn treatment the internal functions used to (re)raise exceptions, which aren't affected by those optimizations. iykyk 🧚
-fhardened— turns on a whole set of flags for C and C++ that level up the security of the generated code without affecting its ABI 🔥 The exact flags may change between major GCC releases, but right now it's giving:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)You can get the whole list of what -fhardened enables with the --help=hardened option. Cute, right? 🎀
If the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 gets used instead.
This one's meant for production builds, not just debug builds — no cap. Currently -fhardened is only supported on GNU/Linux targets.
Also important: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. So for example -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. Respect the user's choices, periodt 💅
-fstack-protector— emits extra code to check for buffer overflows, like stack smashing attacks 🛡️ How? By adding a guard variable to functions that have vulnerable objects. That includes functions that callalloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and checked again when it exits. If a guard check fails, an error message is printed and the program exits 💀 Only variables that are actually allocated on the stack count — optimized-away variables or variables allocated in registers don't.-fstack-protector-all— same vibe as-fstack-protectorexcept every single function is protected. All of them. No exceptions 👑-fstack-protector-strong— like-fstack-protectorbut it includes extra functions to protect — the ones that have local array definitions, or that have references to local frame addresses. And again: only variables actually allocated on the stack are considered, optimized away variables or register-allocated variables don't count 💜-fstack-protector-explicit— like-fstack-protectorbut it only protects the functions that have thestack_protectattribute. Picky, but make it fashion ✨-fstack-check— generates code to verify you don't go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.
🫶 heads up: this switch does not actually cause checking to be done — the operating system or the language runtime has to do that. The switch just generates code so they can see the stack being extended.
You can also pass a string parameter: no means no checking, generic forces the use of old-style checking, and specific uses the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that needs no specific target support in the compiler, but it comes with these drawbacks:
- Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code 😬
- Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking isn't reliable and the compiler issues a warning.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance gets hampered. rip performance 😭
Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. But it's not generally sufficient to protect against stack-clash attacks — for those you want -fstack-clash-protection. Different tool, different job 🦋
-fstack-clash-protection— generates code to prevent stack clash style attacks. When it's enabled, the compiler only allocates one page of stack space at a time and each page is accessed immediately after allocation. So it stops allocations from jumping over any stack guard page the operating system provides 🔥
Most targets don't fully support stack clash protection. But on those targets, -fstack-clash-protection will protect dynamic stack allocations. It may also give limited protection for static stack allocations if the target supports -fstack-check=specific.
-fstack-limit-register=reg,-fstack-limit-symbol=sym,-fno-stack-limit— generate code to make sure the stack doesn't grow beyond a certain value, either the value of a register or the address of a symbol. If a bigger stack is required, a signal is raised at run time. For most targets the signal gets raised before the stack overruns the boundary, so you can 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 this may only work with the GNU linker.
You can locally override stack limit checking with the no_stack_limit function attribute.
-fsplit-stack— generates code to automatically split the stack before it overflows. The resulting program has a discontiguous stack that can only overflow if the program can't allocate any more memory. Super useful when running threaded programs, because you no longer have to calculate a good stack size for each thread 😌 This is currently only implemented for x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter to run. If compiling all the code — including library code — with -fsplit-stack isn't an option, then the linker can fix up those calls so the code compiled without -fsplit-stack always gets a large stack. Support for that is implemented in the gold linker in GNU binutils release 2.21 and later ⭐
-fstrub=disable— disables stack scrubbing entirely, ignoring anystrubattributes. See-fstrub=strict— functions default tostrubmodedisabled, and strictly apply the restriction that only functions associated withstrub-callablemodes (at-calls,callableandalways_inlineinternal) arecallableby functions withstrub-enabled modes (at-callsandinternal). Strict queen 👑-fstrub=relaxed— restores the default stack scrub (strub) setting, namely,strubis only enabled as required bystrubattributes associated with function and data types. "Relaxed" means strub contexts are only prevented from calling functions explicitly associated withstrubmodedisabled. This option is only useful to override other-fstrub=*options that come before it on the command line 💅-fstrub=at-calls— Turns on theat-callsstrubmode where it’s viable ✨. The main reason this option exists is testing — it exercises thestrubmachinery in scenarios that are strictly local to a translation unit. Thisstrubmode changes function interfaces, so any function that’s visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options can also affect viability. Check thestrubattribute docs for details on viability and eligibility requirements, bestie 🫶.-fstrub=internal— Turns on theinternalstrubmode where it’s viable 💅. Again, the primary use is testing. This option is meant to thoroughly exercise the parts of thestrubmachinery that implement the less efficient but interface-preservingstrubmode. Functions that wouldn’t be affected by this option are pretty uncommon, ngl.-fstrub=all— Turns on somestrubmode wherever it’s viable 👑. When bothstrubmodes are viable,at-callswins.-fdump-ipa-strubmadds function attributes that tell you which mode got picked for each function. The primary use of this option is testing, so you can thoroughly exercise thestrubmachinery 🔥.-fvtable-verify=[std|preinit|none]— This one is only available when compiling C++ code 🎀. It turns on the security feature — or off, if you use-fvtable-verify=none— that verifies at run time, for every virtual call, that the vtable pointer used for the call is valid for the type of the object, and that it hasn’t been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and the program’s execution is immediately halted 💀.
This option causes run-time data structures to be built at program startup, and those structures get used for verifying vtable pointers. The options std and preinit control the timing of when those data structures get built ⏰. In both cases, the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.
If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit; preinit takes priority over std 🫡.
-fvtv-debug— When you use this with-fvtable-verify=stdor-fvtable-verify=preinit, it causes debug versions of the runtime functions for the vtable verification feature to be called 🐛. This flag also makes the compiler log info about which vtable pointers it finds for each class. That info gets written to a file namedvtv_set_ptr_data.login the directory named by the environment variableVTV_LOGS_DIRif that’s defined, or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one 🙈.
-fvtv-counts— This is a debugging flag 🔍. When used with-fvtable-verify=stdor-fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs that info for each compilation unit. The compiler writes this info to a file namedvtv_count_data.login the directory named by the environment variableVTV_LOGS_DIRif that’s defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes that info tovtv_class_set_sizes.login the same directory 📝.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones 🧹.
-finstrument-functions— Generates instrumentation calls for entry and exit to functions 🌸. Just after function entry and just before function exit, these profiling functions get called with the address of the current function and its call site. (On some platforms,__builtin_return_addressdoes not work beyond the current function, so the call site info may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table 🔎.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. That means addressable versions of such functions must be available. If all your uses of a function get expanded inline, this may mean an additional expansion of code size 😭. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute no_instrument_function, in which case this instrumentation is not done 🙅. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-once— Similar to-finstrument-functions, but the profiling functions are called only once per instrumented function 🎯. That means the first profiling function gets called after the first entry into the instrumented function, and the second profiling function gets called before the exit corresponding to this first entry.
The definition of “once” for this option is a little vague because the implementation isn’t protected against data races 😬. As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread, but the calls are always paired — that is, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.
- *
-finstrument-functions-exclude-file-list=file*,file,...— Sets the list of functions that are excluded from instrumentation (see the description of-finstrument-functions). If the file that contains a function definition matches one of file, then that function is not instrumented 🚫. The match is done on substrings: if the file parameter is a substring of the file name, it’s considered a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysexcludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include the letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
- *
-finstrument-functions-exclude-function-list=sym*,sym,...— Similar to-finstrument-functions-exclude-file-list, but this option sets the list of function names to exclude from instrumentation 🧚. The function name to be matched is its user-visible name, such asvector<int> blah(const vector<int> &), not the internal mangled name (e.g.,_Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it’s considered a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. -fpatchable-function-entry=N[,M]— GeneratesNNOPs right at the beginning of each function, with the function entry point before theMth NOP 🩹. IfMis omitted, it defaults to0, so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space that can be used to patch in any instrumentation you want at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interfacegen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.
For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary 🏷️.
Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded.
The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change this.
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops.
✨ Options Controlling the Preprocessor ✨
These options control the C preprocessor, which gets run on each C source file before actual compilation 🧪.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation 💅.
In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options 🌈.
-Dname — Predefine name as a macro, with definition1⭐.-D name=definition— okay so bestie, the contents of definition get tokenized and processed like they literally showed up during translation phase three in a#definedirective ✨. Big tiny detail: the definition gets truncated by embedded newline characters. If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters like spaces that have meaning in shell syntax 🫶. Wanna 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 quote the option, okay? Withshandcsh,-D'name(args...)=definition'works 💅. Also:-Dand-Uoptions are processed in the order they're given on the command line. All-imacros fileand-include fileoptions are processed after all-Dand-Uoptions. Periodt.-U name— cancel any previous definition of name, either built in or provided with a-Doption. Bye-bye macro 👋.-include file— process file as if"#include "file""appeared as the first line of the primary source file 🌸. However! The first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it gets searched for in the remainder of the"#include "...""search chain as normal. If multiple-includeoptions 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 lets you acquire all the macros from a header without also processing its declarations. All files specified by-imacrosare processed before all files specified by-include. Iconic ordering behavior, no cap.-undef— do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined ⭐.-pthread— define additional macros required for using the POSIX threads library 🧵. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Slay across platforms 💅.-M— instead of outputting the result of preprocessing, output a rule suitable formakedescribing the dependencies of the main source file 🔥. The preprocessor outputs onemakerule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from-includeor-imacroscommand-line options. Unless specified explicitly (with-MTor-MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using\-newline. The rule has no commands. This option does not suppress the preprocessor's debug output, such as-dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with-MF, or use an environment variable likeDEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal. Passing-Mto the driver implies-E, and suppresses warnings with an implicit-w. So dramatic, so useful 😌.-MM— like-Mbut do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header 🙈. This implies that the choice of angle brackets or double quotes in an#includedirective does not in itself determine whether that header appears in-MMdependency output. Subtle but important, bestie.-MF file— when used with-Mor-MM, specifies a file to write the dependencies to 🎀. If no-MFswitch is given the preprocessor sends the rules to the same place it would send preprocessed output. When used with the driver options-MDor-MMD,-MFoverrides the default dependency output file. If file is-, then the dependencies are written tostdout. Love astdoutmoment 💖.-MG— in conjunction with an option such as-Mrequesting dependency generation,-MGassumes missing header files are generated files and adds them to the dependency list without raising an error 🦋. The dependency filename is taken directly from the"#include"directive without prepending any path.-MGalso suppresses preprocessed output, as a missing header file renders this useless. This feature is used in automatic updating of makefiles. Very "I got you" energy.-Mno-modules— disable dependency generation for compiled module interfaces. Simple, clean, bye 💀.-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 errorsmakegives if you remove header files without updating theMakefileto match. This is typical output:
test.o: test.c test.h
test.h:Chef's kiss for build systems 🍓.
-MT target— change the target of the rule emitted by dependency generation ✨. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target. An-MToption sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to-MT, or use multiple-MToptions. For example,-MT '$(objpfx)foo.o'might give:
$(objpfx)foo.o: foo.cPrecisely what you asked for, no surprises 💅.
-MQ target— same as-MT, but it quotes any characters which are special to Make 🎀. So-MQ '$(objpfx)foo.o'gives:
$$(objpfx)foo.o: foo.cThe default target is automatically quoted, as if it were given with -MQ. Make-safe and cute.
-MD—-MDis equivalent to-M -MF file, except that-Eis not implied 🔥. The driver determines file based on whether an-ooption is given. If it is, the driver uses its argument but with a suffix of.d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a.dsuffix. If-MDis used in conjunction with-E, any-oswitch is understood to specify the dependency output file, but if used without-E, each-ois understood to specify a target object file. Since-Eis not implied,-MDcan be used to generate a dependency output file as a side effect of the compilation process. Side-effect slay 💖.-MMD— like-MDexcept mention only user header files, not system header files 🌸. Keep it user-only, bestie.-fpreprocessed— indicate to the preprocessor that the input file has already been preprocessed 🧚. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with-Cto the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.-fpreprocessedis implicit if the input file has one of the extensions.i,.iior.mi. These are the extensions that GCC uses for preprocessed files created by-save-temps. Very "already did the work" energy ✨.-fdirectives-only— when preprocessing, handle directives, but do not expand macros 🙅♀️. The option's behavior depends on the-Eand-fpreprocessedoptions. With-E, preprocessing is limited to the handling of directives such as#define,#ifdef, and#error. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the-dDoption is implicitly enabled. With-fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as__LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with-E -fdirectives-only. With both-Eand-fpreprocessed, the rules for-fpreprocessedtake precedence. This enables full preprocessing of files previously preprocessed with-E -fdirectives-only. Layered behavior, iconic 💅.-fdollars-in-identifiers— accept$in identifiers 💸. That's it. That's the vibe.-fextended-identifiers— accept universal character names and extended characters in identifiers 🌈. This option is enabled by default for C99 (and later C standard versions) and C++. We love inclusive identifiers.-fno-canonical-system-headers— when preprocessing, do not shorten system header paths with canonicalization. Keep those paths exactly as they are 🫶.-fmax-include-depth=depth— set the maximum depth of the nested#include🌀. The default is200. Don't go too deep, bestie.-fsearch-include-path[=kind]— look for input files on the#includepath, not just the current directory 🔍. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly. Example time:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cckind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the "#include <...>" search path. Modules found, vibes immaculate ✨.
-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 than1or greater than100, the option is ignored. The default is8. Tab drama handled, periodt.-ftrack-macro-expansion[=level]— track locations of tokens across macro expansions 🧵. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary.- Value
0of level de-activates this option. - Value
1tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. - Value
2tracks tokens locations completely. This value is the most memory hungry.
- Value
When this option is given no argument, the default parameter value is 2. Note that -ftrack-macro-expansion=2 is activated by default 😭. Memory says "bestie please", but diagnostics say "I need this" 💖.
-fmacro-prefix-map=old=new— okay so when the preprocessor is chewing through files that live in directory old, it'll expand the__FILE__and__BASE_FILE__macros like those files were chilling in directory new instead. The classic glow-up move? Set new to.— that turns an absolute path into a relative one, which lowkey gives you more reproducible builds that don't care where they're built. This one also affects__builtin_FILE()during compilation, bestie. See also-ffile-prefix-mapand-fcanon-prefix-map🫶-fexec-charset=charset— sets the execution character set, which is what string and character constants get encoded in. The default isUTF-8. ✨ charset can be literally any encoding your system'siconvlibrary routine supports — the world is your oyster.-fwide-exec-charset=charset— same vibe but for the wide execution character set, used by wide string and character constants. The default is one ofUTF-32BE,UTF-32LE,UTF-16BE, orUTF-16LE— whichever one matches the width ofwchar_tplus the big-endian or little-endian byte order your codegen is doing. Same deal as-fexec-charset: charset can be anythingiconvsupports. ⚠️ But heads up — you will have problems with encodings that don't fit exactly inwchar_t. It's giving "trying to stuff a queen-size duvet into a twin bed" 😭-finput-charset=charset— sets the input character set, i.e. the one used to translate the input file's charset into the source character set GCC actually works with. If the locale doesn't say, or GCC can't get that info from the locale, the default isUTF-8. Both the locale and this command-line option can override it — and right now, if there's a conflict, the command-line option wins. charset can be any encodingiconvsupports. 👑-fpch-deps— when you're using precompiled headers, this flag makes the dependency-output flags also list the files that the precompiled header itself depended on. Without it, only the precompiled header shows up in the list, not the files used to create it — because those files aren't consulted at all when a PCH gets used. ✨-fpch-preprocess— this one lets you use a precompiled header together with-E. It slips a special#pragma, specifically#pragma GCC pch_preprocess "filename", into the output to mark where the precompiled header was found, plus its filename. And when-fpreprocessedis in use, GCC recognizes this#pragmaand loads the PCH. 🎀
This option is off by default, because the preprocessed output it produces is really only suitable as input to GCC. -save-temps switches it on.
Do not write this #pragma in your own code, bestie. That said, it is safe to edit the filename if the PCH file lives somewhere else. The filename can be absolute, or it can be relative to GCC's current directory. 🙈
-fworking-directory— turns on generation of linemarkers in the preprocessor output that let the compiler know what the current working directory was at the time of preprocessing. With this on, the preprocessor emits a second linemarker right after the initial one, holding the current working directory followed by two slashes. When that directory shows up in the preprocessed input, GCC uses it as the directory it emits as the current working directory in certain debugging information formats. It's implicitly enabled whenever debug info is enabled, but you can inhibit it with the negated form-fno-working-directory. And if-Pis on the command line, this option does absolutely nothing — because no#linedirectives get emitted at all. 💅-A predicate=answer— makes an assertion with predicate predicate and answer answer. This form is preferred over the older-A predicate(answer), which is still supported — the newer one wins because it doesn't use shell special characters. 🐚-A -predicate=answer— cancels an assertion with predicate predicate and answer answer.-C— do NOT discard comments. Every comment gets passed through to the output file, except comments in processed directives, which get deleted along with the directive itself.
Be prepared for side effects with-C— it makes the preprocessor treat comments as tokens in their own right. For example, a comment at the start of what would be a directive line turns that line into an ordinary source line, because the first token on the line is no longer a#. 💀
-CC— do not discard comments, including during macro expansion. It's like-Cbut the comments inside macros also get passed through to the output file where the macro is expanded. ✨
On top of all the -C side effects, -CC also converts every C++-style comment inside a macro into a C-style comment — so that later use of that macro can't accidentally comment out the rest of the source line. -CC is generally used to support lint comments. 🔥
-P— inhibits generation of linemarkers in the preprocessor output. Super useful when you're running the preprocessor over something that isn't C code and feeding it to a program that would get confused by linemarkers. 🦋-traditional-traditional-cpp— try to imitate what pre-standard C preprocessors did, as opposed to ISO C preprocessors. The GNU CPP manual has the deets.
Note that GCC doesn't otherwise try to emulate a pre-standard C compiler at all — these options are only supported with the -E switch, or when you invoke CPP explicitly. 🙅♀️
-trigraphs— supports ISO C trigraphs. These are three-character sequences, all starting with??, that ISO C defines to stand for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline. 🤯
The nine trigraphs and their replacements are:
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. 🌈
-remap— enables special code to work around file systems that only permit very short file names, like MS-DOS. Retro problem, retro solution 💾-H— prints the name of every header file used, on top of all the other normal stuff. Each name gets indented to show how deep in the#includestack it is. Precompiled header files get printed too, even when they turn out to be invalid — an invalid one prints with...xand a valid one prints with...!. 🎀-dletters— tells GCC to dump debugging info during compilation as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper or reserved for future GCC versions, so they're silently ignored. And if you pick letters whose behavior conflicts — that's undefined, bestie. 💀-dM— instead of the normal output, generate a list of#definedirectives for every macro defined during the preprocessor run, predefined macros included. Which is how you find out what your particular preprocessor predefines. Assuming you have no filefoo.h, this command:
touch foo.h; cpp -dM foo.hshows you all the predefined macros.
If you use -dM without -E, then -dM is interpreted as a synonym for -fdump-rtl-mach. 👀
-dD— like-dMexcept it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.-dN— like-dD, but it only emits the macro names, not their expansions.-dI— outputs#includedirectives in addition to the result of preprocessing.-dU— like-dD, except only macros that get expanded, or whose definedness is tested in preprocessor directives, are output. The output is delayed until the use or test of the macro, and#undefdirectives also get output for macros tested but undefined at the time. ⏳
-fdebug-cpp— only useful for debugging GCC itself. When used from CPP or with-E, it dumps debugging information about location maps, and every token in the output gets preceded by a dump of the map its location belongs to. 🗺️
When used from GCC without -E, this option does nothing at all.
-Wp,option— you can use-Wp,optionto bypass the compiler driver and shove option straight through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. ⚠️ But lots of options get modified, translated or interpreted by the compiler driver before reaching the preprocessor, and-Wpforcibly skips that whole phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible, avoid-Wpand let the driver handle the options instead. Please. For me. 🙏-Xpreprocessor option— passes option as an option to the preprocessor. Use this for system-specific preprocessor options that GCC doesn't recognize. ✨
If you want to pass an option that takes an argument, you gotta use -Xpreprocessor twice — once for the option and once for the argument. 💅
-no-integrated-cpp— performs preprocessing as a separate pass before compilation. By default GCC does preprocessing as an integrated part of input tokenization and parsing. With this option, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) gets invoked twice instead: once for preprocessing only, and once for actual compilation of the preprocessed input. Handy alongside the-Bor-wrapperoptions if you want to specify an alternate preprocessor or do extra processing of the source between normal preprocessing and compilation. 🧚
🛠️ Passing Options to the Assembler 🛠️
Yep, you can pass options to the assembler too. Iconic of it.
-Wa,option— passes option as an option to the assembler. If option contains commas, it gets split into multiple options at the commas. 🎀-Xassembler option— passes option as an option to the assembler. Use this for system-specific assembler options that GCC doesn't recognize.
Same rule as before: 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 💅
okay so these options literally only matter when the compiler is linking object files into an executable output file. if it's not doing a link step? they're meaningless. no cap. 🫶
object-file-name— a file name that doesn't end in a special recognized suffix is considered to name an object file or a library. (the linker tells object files and libraries apart by looking at the file contents, bestie.) and if linking is done, these object files get used as input to the linker. ✨-c,-S,-E— if any of these is used, the linker is not run, and object file names should not be used as arguments. periodt.-flinker-output=type— this one controls code generation of the link-time optimizer. by default the linker output is automatically determined by the linker plugin. for debugging the compiler, and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually. here's the tea on every type:- if type is
exec, code generation produces a static binary. in this case-fpicand-fpieare both disabled. - if type is
dyn, code generation produces a shared library. in this case-fpicor-fPICis preserved, but not enabled automatically. this allows building shared libraries without position-independent code on architectures where that's possible, i.e. on x86. - if type is
pie, code generation produces an-fpieexecutable. this results in similar optimizations asexecexcept that-fpieis not disabled if specified at compilation time. - if type is
rel, the compiler assumes that incremental linking is done. the sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. in addition, if-ffat-lto-objectsis specified, binary code is produced for future non-LTO linking. the object file produced by incremental linking is smaller than a static library produced from the same object files. at link time the result of incremental linking also loads faster than a static library, assuming that the majority of objects in the library are used. 💫 - finally,
nolto-relconfigures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. when multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole program optimizations are lost. 😭
- if type is
during the incremental link (by -r) the linker plugin defaults to rel. with current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. if any of the object files in the incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel.
to maintain whole program optimization, it's recommended to link such objects into a static library instead. alternatively it's possible to use H.J. Lu's binutils with support for mixed objects. 🫡
-fuse-ld=bfd— use the bfd linker instead of the default linker.-fuse-ld=gold— use the gold linker instead of the default linker.-fuse-ld=lld— use the LLVM lld linker instead of the default linker.-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker. ✨-llibrary,-llibrary — search the library named library when linking. (the second alternative, with the library as a separate argument, is only for POSIX compliance and is not recommended. 🙈)
the -l option is passed directly to the linker by GCC. refer to your linker documentation for exact details — the general description below applies to the GNU linker.
the linker searches a standard list of directories for the library. the directories searched include several standard system directories plus any that you specify with -L.
static libraries are archives of object files, and have file names like liblibrary.a. some targets also support shared libraries, which typically have names like liblibrary.so. if both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.
it makes a difference where in the command you write this option: the linker searches and processes libraries and object files in the order they are specified. thus foo.o -lz bar.o searches library z after file foo.o but before bar.o. if bar.o refers to functions in z, those functions may not be loaded. 💀
-lobjc— you need this special case of the-loption 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,-nolibc, or-nodefaultlibsis used. 🎀-nodefaultlibs— do not use the standard system libraries when linking. only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored. the standard startup files are used normally, unless-nostartfilesis used.
heads up, 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. 🫡
-nolibc— do not use the C library or system libraries tightly coupled with it when linking. you still link with the startup files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++, unless options preventing their inclusion are used as well. this typically removes-lcfrom the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example-lpthreador-lmin some configurations. this is intended for bare-board targets when there is indeed no C library available. 🏝️-nostdlib— do not use the standard system startup files or libraries when linking. no startup files, and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored.
the compiler may generate calls to memcmp, memset, memcpy and memmove. these entries are usually resolved by entries in libc. these entry points should be supplied through some other mechanism when this option is specified.
one of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. 👑
in most cases, you need libgcc.a even when you want to avoid other standard libraries. in other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. this ensures that you have no unresolved references to internal GCC library subroutines. (an example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 💖
-nostdlib++— do not implicitly link with standard C++ libraries. simple as that. ✨-eentry,--entry=entry — specify that the program entry point is entry. the argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. 🎯-pie— produce a dynamically linked position independent executable on targets that support it. for predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option.-no-pie— don't produce a dynamically linked position independent executable. that's it. that's the vibe. 🙅-static-pie— produce a static position independent executable on targets that support it. a static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. for predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option. 🌸-pthread— link with the POSIX threads library. this option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. on some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. 🧵-r— produce a relocatable object as output. also known as partial linking.-rdynamic— pass the flag-export-dynamicto 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 ofdlopenor 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 overrides-pieand prevents linking with the shared libraries. on other systems, this option has no effect.-shared— produce a shared object which can then be linked with other objects to form an executable. not all systems support this option. for predictable results, you must also specify the same set of options used for compilation (-fpic,-fPIC, or model suboptions) when you specify this linker option.[1] 💅-shared-libgcc,-static-libgcc— on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. if no shared version of libgcc was built when the compiler was configured, these options have no effect.
there are several situations in which an application should use the shared libgcc instead of the static version. the most common of these is when the application wishes to throw and catch exceptions across different shared libraries. in that case, each of the libraries as well as the application itself should use the shared libgcc.
therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. 👑
Okay so bestie, if, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. vibe check: if GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc. no cap, that part matters.
-static-libasan— when the-fsanitize=addressoption is used to link a program, the GCC driver automatically links againstlibasan. Iflibasanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibasan. The-static-libasanoption directs the GCC driver to linklibasanstatically, without necessarily linking other libraries statically. ✨-static-libtsan— when the-fsanitize=threadoption is used to link a program, the GCC driver automatically links againstlibtsan. Iflibtsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibtsan. The-static-libtsanoption directs the GCC driver to linklibtsanstatically, without necessarily linking other libraries statically. 💅-static-liblsan— when the-fsanitize=leakoption is used to link a program, the GCC driver automatically links againstliblsan. Ifliblsanis available as a shared library, and the-staticoption is not used, then this links against the shared version ofliblsan. The-static-liblsanoption directs the GCC driver to linkliblsanstatically, without necessarily linking other libraries statically. 🎀-static-libubsan— when the-fsanitize=undefinedoption is used to link a program, the GCC driver automatically links againstlibubsan. Iflibubsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibubsan. The-static-libubsanoption directs the GCC driver to linklibubsanstatically, without necessarily linking other libraries statically. 🦋-static-libstdc++— when the g++ program is used to link a C++ program, it normally automatically links againstlibstdc++. Iflibstdc++is available as a shared library, and the-staticoption is not used, then this links against the shared version oflibstdc++. That is normally fine. However, it is sometimes useful to freeze the version oflibstdc++used by the program without going all the way to a fully static link. The-static-libstdc++option directs the g++ driver to linklibstdc++statically, without necessarily linking other libraries statically. slay, but read the whole thing.-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. 🙈-T script— use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the-Toption may be required when linking to avoid references to undefined symbols. ⭐-Xlinker option— pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. fr fr, that one bites.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options. 💜
-Wl,option— pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker. When using the GNU linker, you can also get the same effect with-Wl,-Map=output.map. 🌸-u symbol— pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use-umultiple times with different symbols to force loading of additional library modules. 🔥-z keyword—-zis passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings. 💀
✨ Options for Directory Search ✨
These options specify directories to search for header files, for libraries and for parts of the compiler:
-I dir— add dir to the list of directories to be searched for header files during preprocessing; this one applies to both"#include "file""and"#include <file>". 🎀-iquote dir— add dir to the header search list, but only for the quote form of the directive,"#include "file"". 🌸-isystem dir— add dir to the header search list for both forms; it also marks the directory as a system directory. 🦋-idirafter dir— add dir to the header search list for both forms; it also marks the directory as a system directory. 💜
For all four: if dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of the directive, "#include "file"". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the "#include "file"" and "#include <file>" directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
- For the quote form of the include directive, the directory of the current file is searched first.
- For the quote form of the include directive, the directories specified by
-iquoteoptions are searched in left-to-right order, as they appear on the command line. - Directories specified with
-Ioptions are scanned in left-to-right order. - Directories specified with
-isystemoptions are scanned in left-to-right order. - Standard system directories are scanned.
- Directories specified with
-idirafteroptions are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. iykyk.
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the "#include_next" directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.
-I-— split the include path. This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-and remove the-I-option. 🚨
Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". There is no way to override this effect of -I-.
-iprefix prefix— specify prefix as the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include the final/. 🌈-iwithprefix dir— append dir to the prefix specified previously with-iprefix, and add the resulting directory to the include search path; puts it where-idirafterwould. ⭐-iwithprefixbefore dir— append dir to the prefix specified previously with-iprefix, and add the resulting directory to the include search path; puts it in the same place-Iwould. 💫-isysroot dir— this option is like the--sysrootoption, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the--sysrootoption for more information. 🍓-imultilib dir— use dir as a subdirectory of the directory containing target-specific C++ headers. 🧚-nostdinc— do not search the standard system directories for header files. Only the directories explicitly specified with-I,-iquote,-isystem, and/or-idirafteroptions (and the directory of the current file, if appropriate) are searched. 💖-nostdinc++— do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 🎀--embed-dir=dir— append dir directory to the list of searched directories for"#embed"preprocessing directive or"__has_embed"macro. There are no default directories for"#embed". 🦋
If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
-iplugindir=dir— set the directory to search for plugins that are passed by-fplugin=nameinstead of-fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🙈-Ldir— add directory dir to the list of directories to be searched for-l. 💅-Bprefix— this option specifies where to find the executables, libraries, include files, and data files of the compiler itself. ✨
okay so the compiler driver program runs one or more of the subprograms cpp, cc1, as and ld 🛠️ It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes— Do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix.--sysroot=dir— Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it instead searchesdir/usr/includeanddir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not.
--no-sysroot-suffix— For some targets, a suffix is added to the root directory specified with--sysroot, depending on the other options used, so that headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include. This option disables the addition of such a suffix.
✨ Options for Code Generation Conventions ✨
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one that is not the default. You can figure out the other form by either removing no- or adding it.
-fstack-reuse=reuse-level— This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.reuse_levelcan beall,named_vars, ornone.allenables stack reuse for all local variables and temporaries,named_varsenables the reuse only for user defined local variables with names, andnonedisables stack reuse completely. The default value isall. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization.
-ftrapv— This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective.-fwrapv— This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective.-fwrapv-pointer— This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid.-fstrict-overflow— This option implies-fno-wrapv -fno-wrapv-pointerand when negated implies-fwrapv -fwrapv-pointer.-fexceptions— Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.-fnon-call-exceptions— Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as"SIGALRM". This enables-fexceptions.-fdelete-dead-exceptions— Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with the"pure"or"const"attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.-funwind-tables— Similar to-fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.-fasynchronous-unwind-tables— Generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).-fno-gnu-unique— On systems with recent GNU assembler and C library, the C++ compiler uses the"STB_GNU_UNIQUE"binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of"RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different"RTLD_LOCAL"plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes"dlclose"to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via"dlclose"and"dlopen", you can use-fno-gnu-unique.-fpcc-struct-return— okay so this one makesstructandunionvalues that are "short" get returned in memory (like the long ones do) instead of in registers. 🫶 It's lowkey less efficient, but the whole point is intercallability — like, GCC-compiled files and files from other compilers can actually talk to each other, especially files from the Portable C Compiler (pcc). Iconic compromise behavior.
The precise convention for returning structures in memory depends on the target configuration macros. (Yes, the exact vibe of the return depends on your target. Very "it depends" of it.)
"Short" structures and unions are those whose size and alignment match that of some integer type. That's the whole definition, no cap.
⚠️ Warning: code compiled with the-fpcc-struct-returnswitch is not binary compatible with code compiled with the-freg-struct-returnswitch. Use it to conform to a non-default application binary interface. 💀
-freg-struct-return— the upgrade ✨ returnsstructandunionvalues in registers when possible. For small structures this is straight-up more efficient than-fpcc-struct-return. Slay.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. And if there is no standard convention? GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. Main character energy: we picked the fast one. 👑
⚠️ Warning: code compiled with the-freg-struct-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface. Same warning, opposite direction, both very real. 🙈
-fshort-enums— allocates to anenumtype only as many bytes as it needs for the declared range of possible values. Specifically, theenumtype is equivalent to the smallest integer type that has enough room. Tiny enum, tiny storage, we love to see it. 🎀
Heads up though: this option has no effect for an enumeration type with a fixed underlying type. Not its business, it's already settled.
⚠️ Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar— overrides the underlying type forwchar_tto beshort unsigned intinstead of the default for the target. This option is useful for building programs to run under WINE. 🍷 Yes, that WINE. Very niche, very useful.
⚠️ 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.
-fcommon— in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with theexternkeyword, which do not allocate storage. Two different things, bestie, don't conflate them. 💅
The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. (Read: you get yelled at, which is honestly helpful.)
The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors. Legacy code stays winning, at a cost. 😭
-fno-ident— ignore the#identdirective. That's it. That's the whole option. Short queen. 🦋-finhibit-size-directive— don't output a.sizeassembler 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 compilingcrtstuff.c; you should not need to use it for anything else. Seriously, it's not for you, it's for that one file. 🫶-fverbose-asm— puts 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). Ngl if you're reading this you're basically a compiler girlie now. 🔥
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.
The added comments include:
- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE, - hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbitsThe comments are intended for humans rather than machines and hence the precise format of the comments is subject to change. So don't, like, parse them. 🙈
-frecord-gcc-switches— this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. Receipts! 🧾 This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the-fverbose-asmswitch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also-grecord-gcc-switchesfor another way of storing compiler options into the object file. ✨-fpic— generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system — just so we're clear 💅). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that-fpicdoes not work; in that case, recompile with-fPICinstead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit. Lucky x86, honestly.)
Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros __pic__ and __PIC__ are defined to 1.
-fPIC— if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. The big sister of-fpic, no size cap, no stress. 🦋 This option makes a difference on AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros __pic__ and __PIC__ are defined to 2.
-fpie,-fPIE— these options are similar to-fpicand-fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the-pieGCC option. Executables only, know the assignment. 💅
-fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt— do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. More freedom = better code, we <3 that. 🔥
Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time. Trade-off, but a good one.
Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions. Per-function lewks, iykyk. ✨
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
-fno-jump-tables— do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. Efficiency stays losing sometimes, and that's fine. 💀 This option is of use in conjunction with-fpicor-fPICfor building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. So check before you bother, bestie. 🎀-fno-bit-tests— okay so sometimes GCC is like "ooh a switch statement, lemme use bit tests, it's faster" and you're here like... no bestie, don't. This flag tells it to chill and NOT use bit tests for switch statements, even when that would totally be more efficient than the other code generation strategies. You said what you said 💅-ffixed-reg— treat the register named reg as a fixed register, meaning the generated code should literally never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role, iykyk).
reg has to be the name of a register, and the accepted names are machine-specific — they're defined in the "REGISTER_NAMES" macro in the machine description macro file. Very specific, very demure.
Heads up: this flag does not have a negative form, because it specifies a three-way choice. Not me expecting a -fno- version... there isn't one 🫶
-fcall-used-reg— treat the register named reg as an allocable register that gets clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. She's reusable, she's not sticking around 💅
It is an error to use this flag with the frame pointer or stack pointer. And using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Like, catastrophic. Do not.
Again: no negative form, because it's a three-way choice. Periodt.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. So polite, honestly 🎀
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Yeah, again.
And a different sort of disaster results from using this flag for a register in which function values may be returned. Two flavors of disaster, iconic.
No negative form here either — three-way choice, you know the drill.
-fpack-struct[=n]— with no value, pack all structure members together without holes. When you do give a value (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location). The wording is a whole vibe, I know.
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. So... only when you actually need to, bestie 😭
-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. Old-school era, we respect it 🙈
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch — so like, vibe check your target first.
-ftls-model=model— alters the thread-local storage model to be used. The model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if-fpicis not given on the command line. She's got opinions of her own ✨
The default without -fpic is initial-exec; with -fpic the default is global-dynamic. Bookmark that.
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them — like, for example, HP-PA or IA-64 — do nothing. Literally nothing. Main character energy: shows up only where needed 🧚
A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. Yeah, executable stack. Big deal, security people are screaming.
-fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. Like, the whole program. No mixing bestie 💀
For languages other than Ada, the "-ftrampolines" and "-fno-trampolines" options currently have no effect, and trampolines are always generated on platforms that need them for nested functions. So yeah, noted.
-ftrampoline-impl=[stack|heap]— by default, trampolines are generated onstack. However, certain platforms (such as the Apple M1) do not permit an executable stack. Compiling with-ftrampoline-impl=heapgenerates calls to"__gcc_nested_func_ptr_created"and"__gcc_nested_func_ptr_deleted"in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented inlibgcc, and will only be provided on specific targets:x86_64 Darwin,x86_64andaarch64 Linux.
PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you"setjmp", instantiate nested functions, and then"longjmp"back to a state prior to having allocated those nested functions. So like... don't do that combo? 😭
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Obsessed, honestly 🌈
Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. Chefs kiss 👑
For those adding visibility support to existing code, you may find "#pragma GCC visibility" of use. This works by you enclosing the declarations you wish to set visibility for with (for example) "#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, "operator new" and "operator delete" must always be of default visibility. No exceptions, that's the rule 💅
Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say "#pragma GCC visibility push(default)" before including any such headers. Protect your peace (and your headers) 🫶
"extern" declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to "extern" functions with no explicit visibility use the PLT, so it is more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs. Otherwise, chaos 💀
An overview of these techniques, their benefits and how to use them is at https://gcc.gnu.org/wiki/Visibility.
-fstrict-volatile-bitfields— this option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as"unsigned short"(assumingshortis 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. Hardware says 16, you give it 16. Respect the peripheral ✨
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. Rude, honestly.
In some cases, such as when the "packed" attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. Safety first, bestie 🎀
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So yeah, highkey recommended: define all bits of the field's type as bit-field members. 🫶
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls— This one controls whether any out-of-line instance of the__syncfamily of functions may be used to implement the C++11__atomicfamily of functions. The default is enabled, so the only useful form of the option is-fno-sync-libcalls. It's used in the implementation of thelibatomicruntime library. 🧚-fzero-init-padding-bits=value— Guarantees zero initialization of padding bits in automatic variable initializers. Certain languages guarantee zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ({}), or C++ when using zero-initialization, or C guarantees that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized. The default is-fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. Example time, bestie:
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.-fzero-init-padding-bits=unions— Guarantees zero initialization of padding bits in unions on top of what the standards guarantee, if the initializer of a union is empty (then all bits of the union are zero initialized) or if the initialized member of the union is smaller than the size of the union (in that case guarantees padding bits outside of the initialized member to be zero initialized). This was the GCC behavior before GCC 15, and in the above example guarantees zero initialization of lastsizeof (unsigned long long) - 1bytes in the union. 🌸-fzero-init-padding-bits=all— Guarantees additionally zero initialization of padding bits of other aggregates, so the padding in betweenb.aandb.b(if any) and tail padding in the structure (if any). ✨
🌈 GCC Developer Options 🌈
This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. It's giving compiler-nerd main character energy, no cap. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks. 🙈
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 💅
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. 🎀
-fcallgraph-info,-fcallgraph-info=MARKERS— Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When thesumarker is specified, the callgraph is decorated with stack usage information; it is equivalent to-fstack-usage. When thedamarker is specified, the callgraph is decorated with information about dynamically allocated objects. 🦋
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.
-dletters,-fdump-rtl-pass,-fdump-rtl-pass=filename— Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. 👑
Some -dletters switches have different meaning when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings — iykyk:
-fdump-rtl-alignments— Dump after branch alignments have been computed. ⭐-fdump-rtl-asmcons— Dump after fixing rtl statements that have unsatisfied in/out constraints.-fdump-rtl-auto_inc_dec— Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.-fdump-rtl-barriers— Dump after cleaning up the barrier instructions.-fdump-rtl-bbpart— Dump after partitioning hot and cold basic blocks.-fdump-rtl-bbro— Dump after block reordering.-fdump-rtl-btl1,-fdump-rtl-btl2—-fdump-rtl-btl1and-fdump-rtl-btl2enable dumping after the two branch target load optimization passes.-fdump-rtl-bypass— Dump after jump bypassing and control flow optimizations.-fdump-rtl-combine— Dump after the RTL instruction combination pass.-fdump-rtl-compgotos— Dump after duplicating the computed gotos.-fdump-rtl-ce1,-fdump-rtl-ce2,-fdump-rtl-ce3—-fdump-rtl-ce1,-fdump-rtl-ce2, and-fdump-rtl-ce3enable dumping after the three if conversion passes.-fdump-rtl-cprop_hardreg— Dump after hard register copy propagation.-fdump-rtl-csa— Dump after combining stack adjustments.-fdump-rtl-cse1,-fdump-rtl-cse2—-fdump-rtl-cse1and-fdump-rtl-cse2enable dumping after the two common subexpression elimination passes.-fdump-rtl-dce— Dump after the standalone dead code elimination passes.-fdump-rtl-dbr— Dump after delayed branch scheduling.-fdump-rtl-dce1,-fdump-rtl-dce2—-fdump-rtl-dce1and-fdump-rtl-dce2enable dumping after the two dead store elimination passes.-fdump-rtl-eh— Dump after finalization of EH handling code.-fdump-rtl-eh_ranges— Dump after conversion of EH handling range regions.-fdump-rtl-expand— Dump after RTL generation.-fdump-rtl-fwprop1,-fdump-rtl-fwprop2—-fdump-rtl-fwprop1and-fdump-rtl-fwprop2enable dumping after the two forward propagation passes.-fdump-rtl-gcse1,-fdump-rtl-gcse2—-fdump-rtl-gcse1and-fdump-rtl-gcse2enable dumping after global common subexpression elimination.-fdump-rtl-init-regs— Dump after the initialization of the registers.-fdump-rtl-initvals— Dump after the computation of the initial value sets.-fdump-rtl-into_cfglayout— Dump after converting to cfglayout mode.-fdump-rtl-ira— Dump after iterated register allocation.-fdump-rtl-jump— Dump after the second jump optimization.-fdump-rtl-loop2—-fdump-rtl-loop2enables dumping after the rtl loop optimization passes.-fdump-rtl-mach— Dump after performing the machine dependent reorganization pass, if that pass exists.-fdump-rtl-mode_sw— Dump after removing redundant mode switches.-fdump-rtl-rnreg— Dump after register renumbering.-fdump-rtl-outof_cfglayout— Dump after converting from cfglayout mode.-fdump-rtl-peephole2— Dump after the peephole pass.-fdump-rtl-postreload— Dump after post-reload optimizations.-fdump-rtl-pro_and_epilogue— Dump after generating the function prologues and epilogues.-fdump-rtl-sched1,-fdump-rtl-sched2—-fdump-rtl-sched1and-fdump-rtl-sched2enable dumping after the basic block scheduling passes.-fdump-rtl-ree— Dump after sign/zero extension elimination.-fdump-rtl-seqabstr— Dump after common sequence discovery.-fdump-rtl-shorten— Dump after shortening branches.-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— These options enable dumping after five rounds of instruction splitting.-fdump-rtl-sms— Dump after modulo scheduling. This pass is only run on some architectures.-fdump-rtl-stack— Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.-fdump-rtl-subreg1,-fdump-rtl-subreg2—-fdump-rtl-subreg1and-fdump-rtl-subreg2enable dumping after the two subreg expansion passes.-fdump-rtl-vartrack— Dump after variable tracking.-fdump-rtl-vregs— Dump after converting virtual registers to hard registers.-fdump-rtl-web— Dump after live range splitting.-fdump-rtl-regclass,-fdump-rtl-subregs_of_mode_init,-fdump-rtl-subregs_of_mode_finish,-fdump-rtl-dfinit,-fdump-rtl-dfinish— These dumps are defined but always produce empty files. 💀-da,-fdump-rtl-all— Produce all the dumps listed above. 🔥-dA— annotates the assembler output with miscellaneous debugging info 🫶 literally just sprinkling lil notes everywhere.-dD— dumps all macro definitions, at the end of preprocessing, on top of the normal output 💅.-dH— produces a core dump whenever an error occurs 💀 yes bestie, a whole core dump. On error. Iconic.-dp— annotates the assembler output with a comment saying which pattern and alternative is used ✨ the length and cost of each instruction get printed too 👑-dP— dumps the RTL in the assembler output as a comment before each instruction 🎀 also turns on-dpannotation, so you get both, no cap.-dx— just generates RTL for a function instead of compiling it 🙈 usually used with-fdump-rtl-expand.
-fdump-debug— dumps the debugging information generated during the debug generation phase 🐛✨-fdump-earlydebug— dumps the debugging information generated during the early debug generation phase 🌅 same vibes, earlier timing.-fdump-noaddr— when doing debugging dumps, suppresses address output 🚫📍 this makes it way more feasible to usediffon debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. Fr fr it's the diff-enabler.-freport-bug— collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs 🧊😭 yes it's literally named ICE. Very main character of it.-fdump-unnumbered— when doing debugging dumps, suppresses instruction numbers and address output 🎀 makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g.-fdump-unnumbered-links— when doing debugging dumps (see the-doption above), suppresses instruction numbers for the links to the previous and next instructions in a sequence 🔗✨
🦋 -fdump-ipa-switch, -fdump-ipa-switch-options 🦋
Control the dumping at various stages of inter-procedural analysis language tree to a file 📁 The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. Here's every dump you can get:
all— enables all inter-procedural analysis dumps 👑cgraph— dumps information about call-graph optimization, unused function removal, and inlining decisions 🌸inline— dump after function inlining 🫶strubm— dump after selecting "strub" modes, and recording the selections as function attributes 🎀strub— dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking ✨
Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized 💅
For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 👀
By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis 🧚
🌈 -fdump-ipa-clones 🌈
Creates a dump file containing information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations 🫶 Its main intended use is that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (see -flive-patching). The file name is generated by appending suffix "ipa-clones" to the source file name, and the file is created in the same directory as the output file 📁 Each entry in the file is on a separate line containing semicolon separated fields.
In the case of call graph clone creation, the individual fields are:
- String
"Callgraph clone". - Name of the function being cloned as it is presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
- Name of the new function clone as it is presented to the assembler.
- A number that uniquely represents the new function clone in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the source code location of the new clone points to.
- The line to which the source code location of the new clone points to.
- The column to which the source code location of the new clone points to.
- A string that determines the reason for cloning.
In the case of call graph clone removal, the individual fields are:
- String
"Callgraph removal". - Name of the function being removed as it would be presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
-fdump-lang— dumps language-specific information 🌸 the file name is made by appending.langto the source file name.
💜 -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename 💜
Control the dumping of language-specific information ✨ the options and filename portions behave as described in the -fdump-tree option. These are the switch values accepted:
all— enable all language-specific dumps 👑class— dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. This option is applicable to C++ only 🎀module— dump module information. Optionslineno(locations),graph(reachability),blocks(clusters),uid(serialization),alias(mergeable),asmname(Elrond),eh(mapper) &vops(macros) may provide additional information. This option is applicable to C++ only 🧚 (yes, Elrond. we love a lore drop)raw— dump the raw internal tree data. This option is applicable to C++ only 💅
-fdump-passes— prints onstderrthe list of optimization passes that are turned on and off by the current command-line options 👀 super useful for a vibe check on your flags.-fdump-statistics-option— enables and controls dumping of pass statistics in a separate file 📊 The file name is generated by appending a suffix ending in.statisticsto the source file name, and the file is created in the same directory as the output file. If the-optionform is used,-statscauses counters to be summed over the whole compilation unit while-detailsdumps every event as the passes generate them 🔥 The default with no option is to sum counters for each function compiled.
⭐ -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename ⭐
Control the dumping at various stages of processing the intermediate language tree to a file 🫶 If the -options form is used, options is a list of - separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored 🙈 The following options are available:
address— print the address of each node 📍 Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.asmname— if"DECL_ASSEMBLER_NAME"has been set for a given decl, use that in the dump instead of"DECL_NAME"✨ Its primary use is ease of use working backward from mangled names in the assembly file.slim— when dumping front-end intermediate representations, inhibits 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.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
raw— print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation 🎀details— enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes 🔍stats— enable dumping various statistics about the pass (not honored by every dump option) 📊blocks— enable showing basic block boundaries (disabled in raw dumps) 🧱graph— for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz tofile.passid.pass.dot🌈 Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form ✨
vops— enable showing virtual operands for every statement 👑lineno— enable showing line numbers for statements 🔢uid— enable showing the unique ID ("DECL_UID") for each variable 🏷️verbose— enable showing the tree dump for each statement 💬eh— enable showing the EH region number holding each statement 🛟scev— enable showing scalar evolution analysis details 🌀optimized— enable showing optimization information (only available in certain passes) 🔥missed— enable showing missed optimization information (only available in certain passes) 😭note— enable other detailed optimization information (only available in certain passes) 📝
For the dump switches, here's what those all and optall values mean:
all— turn on all options, exceptraw,slim,verboseandlineno.optall— turn on all optimization options, i.e.optimized,missed, andnote.
🔎 finding the tree dump for a pass (the detective era)
To figure out what tree dumps are available, or to find the dump for a pass you're interested in, follow these steps:
- Invoke GCC with
-fdump-passesand look in thestderroutput for a code that corresponds to the pass you are interested in. For example, the codestree-evrp,tree-vrp1, andtree-vrp2correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. - To enable the creation of the dump file, append the pass code to the
-fdump-option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the-fdump-tree-evrpoption. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below. - Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter
tfor tree passes (and the letterrfor RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file namedmyfile.c.038t.evrpin the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another.
✨ -fopt-info and its many little outfits ✨
-fopt-info— controls optimization dumps from various optimization passes.-fopt-info-options— same control, andoptionsis a list of option keywords separated by-to select the dump details and optimizations.-fopt-info-options=filename— same control, but with afilename.
The options can be divided into three groups:
- options describing what kinds of messages should be emitted,
- options describing the verbosity of the dump, and
- options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.
The following options control which kinds of messages should be emitted:
optimized— print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.missed— print information about missed optimizations. Individual passes control which information to include in the output.note— print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.all— print detailed optimization information. This includesoptimized,missed, andnote.
The following option controls the dump verbosity:
internals— by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to describe a group of optimizations:
ipa— enable dumps from all interprocedural optimizations.loop— enable dumps from all loop optimizations.inline— enable dumps from all inlining optimizations.omp— enable dumps from all OMP (Offloading and Multi Processing) optimizations.vec— enable dumps from all vectorization optimizations.optall— enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
bestie, important: the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.
💾 -fsave-optimization-record (receipts, but make them JSON)
-fsave-optimization-record— write aSRCFILE.opt-record.json.gzfile detailing what optimizations were performed, for those optimizations that support-fopt-info.
heads up: this option is experimental and the format of the data within the compressed JSON file is subject to change.
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted.
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes.
🗓️ -fsched-verbose=n
-fsched-verbose=n— on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs 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.
🧚 enable/disable passes: debug-mode behavior
-fenable-kind-pass,-fdisable-kind-pass=range-list— this is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.-fdisable-ipa-pass— disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fdisable-rtl-pass,-fdisable-rtl-pass=range-list— disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.range-listis a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option-fdump-passes.-fdisable-tree-pass,-fdisable-tree-pass=range-list— disable tree pass pass. See-fdisable-rtlfor the description of option arguments.-fenable-ipa-pass— enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fenable-rtl-pass,-fenable-rtl-pass=range-list— enable RTL pass pass. See-fdisable-rtlfor option argument description and examples.-fenable-tree-pass,-fenable-tree-pass=range-list— enable tree pass pass. See-fdisable-rtlfor the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking— turns on internal consistency checking, bestie. Whether it's on by default depends on how your compiler was configured, so like… it varies. 🙈-fchecking=n— the=nversion. Setting-fchecking=2unlocks even more internal consistency checking, but heads up: that extra checking might actually affect code generation. So it's giving "paranoid mode" ✨-frandom-seed=string— okay so GCC needs random-ish numbers to make certain symbol names that have to be different in every compiled file, and this option gives it a seed to use instead of actual randomness. It's also used to drop unique stamps into coverage data files and the object files that produce them, iykyk. The whole point? You can use-frandom-seedto produce reproducibly identical object files. 🎯
The string can be a number (decimal, octal, or hex) or literally any arbitrary string — and if it's an arbitrary string, it gets converted to a number by computing CRC32. No cap.
The string should be different for every file you compile. Don't be lazy about this one, bestie. 💅
-save-temps— keeps the usual "temporary" intermediate files around permanently instead of yeeting them, and names them as auxiliary output files, as described under-dumpbaseand-dumpdir. 🎀
When you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. If you want that corresponding intermediate file anyway, just rename the source file before using -save-temps. Easy.
-save-temps=cwd— literally equivalent to-save-temps -dumpdir ./. That's it, that's the whole vibe.-save-temps=obj— equivalent to-save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you specified after-o, including any directory separators. And if you never used-o? Then-save-temps=objbehaves like-save-temps=cwd. Slay, it adapts. 🦋-time[=file]— reports the CPU time each subprocess in the compilation sequence took. For C source files, that's the compiler proper and the assembler (plus the linker if linking is happening). ⏱️
Without an output file specified, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01First number on each line = the "user time," aka time spent executing the program itself. Second number = "system time," aka time spent executing operating system routines on behalf of the program. Both are in seconds. 🫶
With an output file specified, the output gets appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>So the "user time" and "system time" get moved before the program name, and the options passed to the program are displayed too — which means later you can tell exactly what file was being compiled and with which options. Iconic detective work fr fr 🔍
-fdump-final-insns[=file]— dumps the final internal representation (RTL) to file. If you omit the optional argument (or if file is.), the dump file's name is determined by appending.gkdto the dump base name — see-dumpbase. 📁-fcompare-debug[=opts]— assuming no error happens during compilation, it runs the compiler a second time, adding opts and-fcompare-debug-secondto the arguments passed to that second compilation. It dumps the final internal representation in both compilations, and prints an error if they differ. Double-checking queen behavior 👑
If you leave out the equal sign, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. And if GCC_COMPARE_DEBUG is defined to a string starting with a dash, then that is used for opts — otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but no opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect. So it's like, the ultimate "no thanks" 💀
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden — which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly, or linking). And if you just want a warning instead? Setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do the trick.
-fcompare-debug-second— this option is implicitly passed to the compiler for the second compilation requested by-fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed so they contain the.gkadditional extension during the second compilation, all so they don't overwrite the ones from the first. 🙈
And when this option is passed to the compiler driver, it causes the first compilation to be skipped — which makes it useful for little other than debugging the compiler proper. So like, niche but valid. 🌸
-gtoggle— turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument in the command line doesn't matter at all — it takes effect after all other options are processed, and it does so only once, no matter how many times you give it. Mainly intended to be used with-fcompare-debug. 🎀-fvar-tracking-assignments-toggle— toggles-fvar-tracking-assignments, in exactly the same way that-gtoggletoggles-g. Symmetry, we love to see it ✨-Q— when used on the command line prior to--help=,-Qacts as a modifier to the help output. Cute.
Otherwise? This option makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. Very "watch me work" energy 👀
-ftime-report— makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes. ⏰
Oh and if SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr, then the -ftime-report information is instead emitted in JSON form as part of the SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr — because they're written out at a slightly different place within the compiler. So like, close enough, don't come for me 😭
-ftime-report-details— records the time consumed by infrastructure parts separately for each pass. Detail-oriented icon 💅-fira-verbose=n— controls how verbose the dump file is for the integrated register allocator. Default value is5. And if n is greater than or equal to10, the dump output gets sent to stderr using the same format as n minus 10. Math but make it debug 🧮-flto-report— prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version, and it's meant to be useful to GCC developers when processing object files in LTO mode (via-flto). Disabled by default. 🧚-flto-report-wpa— like-flto-report, but it only prints for the WPA phase of link-time optimization. Selective reporting, we respect it.-fmem-report— makes the compiler print some statistics about permanent memory allocation when it finishes. Memory tea ☕-fmem-report-wpa— makes the compiler print some statistics about permanent memory allocation for the WPA phase only. Same vibe, narrower window.-fpre-ipa-mem-report,-fpost-ipa-mem-report— makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. Bookends bestie 🫶-fmultiflags— okay this one's a whole thing. This option enables multilib-awareTFLAGSto be used to build target libraries with options different from the ones the compiler is configured to use by default — through the use of specs set up by compiler internals, by the target, or by builders at configure time. 🌈
Like TFLAGS, this allows the target libraries to be built for portable baseline environments, while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. Trade-offs, babes.
Unlike TFLAGS, the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.
Heads up: this option is discarded by the driver once it's done processing driver self spec.
It's also useful to check that TFLAGS are being used to build all target libraries, by configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries. Chef's kiss verification ✨
-fprofile-report— makes the compiler print some statistics about consistency of the (estimated) profile and the effect of individual passes. Receipts, basically 🧾-fstack-usage— makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending.suto the auxname. And auxname is generated from the name of the output file, if explicitly specified and it's not an executable — otherwise it's the basename of the source file. 📊
An entry is made up of three fields:
- The name of the function.
- A number of bytes.
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit, and no stack adjustments are otherwise made in the function. In that case, the second field is this fixed number of bytes. Fr fr precise 💜
The qualifier dynamic means the function messes with the stack dynamically — so on top of that static allocation we just talked about, there are stack adjustments happening in the body of the function, like pushing/popping arguments around function calls 💅. And if the qualifier bounded is also hanging out there, then the amount of those adjustments is bounded at compile time, and the second field is an upper bound on the total stack the function uses. If bounded is not there? Then those adjustments are not bounded at compile time, and the second field only represents the bounded part. Periodt.
-fstats— Emits stats about front-end processing when compilation finishes ✨. Ngl this one is only supported by the C++ front end, and the info is generally only useful to the G++ development team — so like, not really for us, bestie 🫶.-fdbg-cnt-list— Prints the name and the counter upper bound for all the debug counters. Iconic lil inventory moment.- *
-fdbg-cnt=counter-value-list* — Sets the internal debug counter lower and upper bound. The counter-value-list is a comma-separated list ofname:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples, which sets the counter name and a list of closed intervals 🎀. The lower_bound is optional and gets zero-initialized if you don't set it. Example time: with-fdbg-cnt=dce:2-4:10-11,tail_call:10,dbg_cnt(dce)returns true only for the second, third, fourth, tenth and eleventh invocation 🧚. And fordbg_cnt(tail_call), true is returned for the first 10 invocations. - *
-print-file-name=library* — Prints the full absolute name of the library file library that would be used when linking — and does literally nothing else 💅. With this option GCC doesn't compile or link anything; it just prints the file name. -print-multi-directory— Prints the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist inGCC_EXEC_PREFIX.-print-multi-lib— Prints the mapping from multilib directory names to the compiler switches that enable them 🔥. The directory name is separated from the switches by;, and each switch starts with an@instead of the-, with no spaces between multiple switches. This is supposed to ease shell processing, iykyk.-print-multi-os-directory— Prints the path to OS libraries for the selected multilib, relative to somelibsubdirectory. Okay so: if OS libraries are present in thelibsubdirectory and no multilibs are used, this is usually just.; if OS libraries are present inlibsuffixsibling directories this prints e.g.../lib64,../libor../lib32; or if OS libraries are present inlib/subdir subdirectories it prints e.g.amd64,sparcv9orev6🦋.-print-multiarch— Prints the path to OS libraries for the selected multiarch, relative to somelibsubdirectory.- *
-print-prog-name=program* — Like-print-file-name, but it searches for a program such ascpp. -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— Prints the name of the configured installation directory and a list of program and library directoriesgccsearches — and doesn't do anything else 🌸.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing / 🎀.
-print-sysroot— Prints the target sysroot directory that's used during compilation. This is the target sysroot specified either at configure time or using the--sysrootoption, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing 🙈.-print-sysroot-headers-suffix— Prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler isn't configured with such a suffix — and doesn't do anything else 💅.-dumpmachine— Prints the compiler's target machine (for example,i686-pc-linux-gnu) — and doesn't do anything else 👑.-dumpversion— Prints the compiler version (for example,3.0,6.3.0or7) — and doesn't do anything else. This is the compiler version used in filesystem paths and specs 💀. Depending on how the compiler's been configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version).-dumpfullversion— Prints the full compiler version — and doesn't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version ⭐.-dumpspecs— Prints the compiler's built-in specs — and doesn't do anything else. (This is used when GCC itself is being built 🧚.)
🌈 Machine-Dependent Options 🌈
Okay so every target machine supported by GCC can have its own options — like, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine 🫶. By convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform 💖.
🦋 AArch64 Options 🦋
These options are defined for AArch64 implementations:
- *
-mabi=name* — Generates code for the specified data model. Permissible values areilp32for the SysV-like data model whereint,long intand pointers are 32 bits, andlp64for the SysV-like data model whereintis 32 bits, butlong intand pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible 😭 — you must compile your entire program with the same ABI, and link with a compatible set of libraries.
The ilp32 model is deprecated. So like, know that going in, bestie.
-mbig-endian— Generates big-endian code. This is the default when GCC is configured for anaarch64_be-*-*target.-mgeneral-regs-only— Generates code that uses only the general-purpose registers ✨. This prevents the compiler from using floating-point and Advanced SIMD registers — but it won't impose any restrictions on the assembler, ngl.-mlittle-endian— Generates little-endian code. This is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target.-mcmodel=tiny— Generates code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked 💅.-mcmodel=small— Generates code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model 👑.-mcmodel=large— Generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only 💀, and the-mcmodel=largeoption is incompatible with-mabi=ilp32,-fpicand-fPIC.- *
-mtp=name* — Specifies the system register to use as a thread pointer. The valid values aretpidr_el0,tpidrro_el0,tpidr_el1,tpidr_el2,tpidr_el3🎀. For backwards compatibility the aliasesel0,el1,el2,el3are also accepted. The default setting istpidr_el0. It's recommended to compile all code intended to interoperate with the same value of this option — that way you avoid accessing a different thread pointer from the wrong exception level 🙈. -mstrict-align,-mno-strict-align— Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification ⭐.-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— Omit or keep the frame pointer in leaf functions. The former behavior is the default 🌸.- *
-mstack-protector-guard=guard*</italic>, *-mstack-protector-guard-reg=reg*</italic>, *-mstack-protector-guard-offset=offset*</italic> — Generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary orsysregfor a canary in an appropriate system register 🔥.
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as the base register for reading the canary, and from what offset from that base register. There is no default register or offset, as this is entirely for use within the Linux kernel 💜.
-mtls-dialect=desc— Uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default ✨.-mtls-dialect=traditional— Uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.- *
-mtls-size=size* — Specifies bit size of immediate TLS offsets. Valid values are12,24,32,48. This option requires binutils 2.26 or newer 🫶. -mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be ignored if an architecture or CPU is specified on the command line that doesn't need the workaround 💅.-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker 😭. This flag will be ignored if an architecture or CPU is specified on the command line that doesn't need the workaround.-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— Enable or disable the reciprocal square root approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well 🧚. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision — so like, precision girlies be warned 💀.
💅 -mlow-precision-sqrt, -mno-low-precision-sqrt
okay so this one flips the square root approximation on or off. Real talk though — it literally only does anything if you're also using -ffast-math or -funsafe-math-optimizations. Enabling it drops your sqrt precision down to about 16 bits for single precision and 32 bits for double precision. And if you turn it on, it implies -mlow-precision-recip-sqrt too. Package deal, bestie 🎀
🍓 -mlow-precision-div, -mno-low-precision-div
Same energy, but for division this time. Turns the division approximation on or off. Also only vibes if -ffast-math or -funsafe-math-optimizations is in the mix. Turning it on reduces division precision to about 16 bits for single precision and 32 bits for double precision. Less precision, more speed, that's the trade ✨
🦋 -mtrack-speculation, -mno-track-speculation
Enables or disables generating extra code that tracks speculative execution through conditional branches. Then the compiler can use that tracking state when it expands calls to __builtin_speculation_safe_copy so it can emit a more efficient code sequence. Fancy compiler internals, iykyk 💜
👑 -moutline-atomics, -mno-outline-atomics
Enables or disables calls to out-of-line helpers to implement atomic operations. At runtime those helpers figure out whether the LSE instructions from ARMv8.1-A can be used — and if they can't, they fall back to the load/store-exclusive instructions that live in the base ARMv8.0 ISA. Runtime vibe check, essentially.
This one is only applicable when you're compiling for the base ARMv8.0 instruction set. If you're on a later revision — like -march=armv8.1-a or -march=armv8-a+lse — the ARMv8.1-Atomics instructions get used directly. Same story with -mcpu= when the cpu you picked supports the lse feature. And heads up: this option is on by default. 🫶
🌸 -march=name
Tells GCC the name of the target architecture, plus optionally one or more feature modifiers. The form is:
-march=arch{+[no]feature}*Here's the whole menu of permissible arch values and what they turn on by default, babes:
armv8-a: Armv8-A : includes+fp,+simdby defaultarmv8.1-a: Armv8.1-A : includesarmv8-a,+crc,+lse,+rdmaarmv8.2-a: Armv8.2-A : includesarmv8.1-aarmv8.3-a: Armv8.3-A : includesarmv8.2-a,+pauth,+fcma,+jscvtarmv8.4-a: Armv8.4-A : includesarmv8.3-a,+flagm,+fp16fml,+dotprod,+rcpc2armv8.5-a: Armv8.5-A : includesarmv8.4-a,+sb,+ssbs,+predres,+frintts,+flagm2armv8.6-a: Armv8.6-A : includesarmv8.5-a,+bf16,+i8mmarmv8.7-a: Armv8.7-A : includesarmv8.6-a,+wfxt,+xsarmv8.8-a: Armv8.8-a : includesarmv8.7-a,+mopsarmv8.9-a: Armv8.9-a : includesarmv8.8-aarmv9-a: Armv9-A : includesarmv8.5-a,+sve,+sve2armv9.1-a: Armv9.1-A : includesarmv9-a,+bf16,+i8mmarmv9.2-a: Armv9.2-A : includesarmv9.1-a,+wfxt,+xsarmv9.3-a: Armv9.3-A : includesarmv9.2-a,+mopsarmv9.4-a: Armv9.4-A : includesarmv9.3-a,+sve2p1armv9.5-a: Armv9.4-A : includesarmv9.4-a,cpa,+faminmax,+lutarmv8-r: Armv8-R : includesarmv8-r
Now the fun one: the value native is available on native AArch64 GNU/Linux and makes the compiler pick the architecture of the host system. If the compiler can't recognize your host's architecture, this option just does nothing. Also — when -march=native is given and no other -mcpu or -mtune is given, GCC picks the host CPU as the CPU to tune for and selects the architecture features from it. So basically -march=native gets treated as -mcpu=native. chaotic but iconic 🌈
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. If you specify conflicting feature modifiers, the right-most one wins. Periodt.
GCC uses name to decide what kind of instructions it can emit when it generates assembly code. And if you specify -march without also specifying -mtune or -mcpu, the code gets tuned to perform well across a range of target processors implementing that target architecture. 🦋
💜 -mtune=name
Names the target processor GCC should tune the performance of the code for. The permissible values are:
generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78,
cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1,
neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,
neoverse-v2, grace, neoverse-v3, neoverse-v3ae, neoverse-n3,
olympus, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1,
vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98,
octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n,
octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88,
thunderxt88p1, thunderxt81, tsv110, thunderxt83, thunderx2t99,
thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55,
cortex-a76.cortex-a55, cortex-r82, cortex-r82ae, cortex-x1,
cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510,
cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720,
cortex-a720ae, ampere1, ampere1a, ampere1b, cobalt-100, apple-m1,
apple-m2, apple-m3 and native.big.LITTLE besties, this is for you — the values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 all specify that GCC should tune for a big.LITTLE system. 💅
The value neoverse-512tvb means GCC tunes for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, it's telling GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). It's more general than tuning for one specific core like Neoverse V1, but more specific than the default tuning described below. Goldilocks tuning, honestly 🎀
Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. If the compiler can't recognize the processor of the host system, the option has no effect.
And where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
⚠️ Heads up: this option cannot be suffixed by feature modifiers. Not today, satan 💀
⭐ -mcpu=name
Names the target processor, optionally with one or more feature modifiers tacked on. The form is:
-mcpu=cpu{+[no]feature}*The permissible values for cpu are the same ones available for -mtune (scroll up, bestie 👆), and the permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Conflicting feature modifiers? Right-most wins. Same rule as always. ✨
GCC uses name to figure out what kind of instructions it can emit when generating assembly code (as if you'd used -march) and to determine the target processor to tune for performance (as if by -mtune). If you use this alongside -march or -mtune, those options take precedence over the appropriate part of this one. Teamwork, but with a hierarchy.
-mcpu=neoverse-512tvb is special — it doesn't point at one specific core, it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with those properties. And unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code the same way as -mtune=neoverse-512tvb. 🦋
🙈 -moverride=string
Overrides tuning decisions the back-end made in response to a -mtune= switch. Warning bestie: the syntax, semantics and accepted values for string here are not guaranteed to be consistent across releases.
This option is only intended to be useful when developing GCC. So like... not for you unless you're literally building the compiler 🫶
🌈 -mverbose-cost-dump
Turns on verbose cost model dumping inside the debug dump files. Provided for debugging the compiler, so it's giving compiler-dev main character energy 👑
🧚 -mpc-relative-literal-loads, -mno-pc-relative-literal-loads
Enables or disables PC-relative literal loads. With this on, literal pools get accessed using a single instruction and are emitted after each function — which limits the maximum size of functions to 1MB. It's enabled by default for -mcmodel=tiny. 💖
🎀 -msign-return-address=scope
Picks the function scope where return address signing gets applied. The permissible values:
none— disables return address signing entirelynon-leaf— enables pointer signing for functions which are not leaf functionsall— enables pointer signing for all functions
The default value is none.
💀 Heads up: this option has been deprecated by -mbranch-protection.
🔥 -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs
Picks which branch protection features to use. Let's break it down:
none— the default, turns off all types of branch protectionstandard— turns on all types of branch protection features. If a feature has additional tuning options,standardsets it to its standard level.pac-ret[+leaf]— turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argumentleafextends the signing to include leaf functions. The optional argumentb-keysigns functions with the B-key instead of the A-key.bti— turns on the branch target identification mechanismgcs— turns on guarded control stack compatible code generation
Obsessed with how many knobs there are, honestly 🌸
💜 -mharden-sls=opts
Enables compiler hardening against straight line speculation (SLS). opts is a comma-separated list of these:
retbrblr
On top of that, -mharden-sls=all enables all SLS hardening, while -mharden-sls=none disables all SLS hardening. All or nothing, no in-between 🫶
⭐ -mearly-ra=scope
Decides when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, like SME2's strided LD1 and ST1. Fancy vector allocator hours ✨
The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass. 💅
-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. iykyk ✨
-mearly-ldp-fusion— turns on the copy of the AArch64 load/store pair fusion pass that runs before register allocation. It's on by default at-Oand above. 🎀-mlate-ldp-fusion— turns on the copy of that same AArch64 load/store pair fusion pass, but the one that runs after register allocation. Also on by default at-Oand above. two passes, one vibe fr fr 🦋-msve-vector-bits=bits — bestie, this sets how many bits live in an SVE vector register. Heads up: it only does anything when SVE is actually enabled. 🌸
GCC supports two flavors of SVE codegen: "vector-length agnostic" output that works with any size of vector register, and "vector-length specific" output where GCC gets to make assumptions about the vector length when that's useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Saying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. Every other value generates vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths. 😭
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 👑
-Wexperimental-fmv-target— warns you about use of experimental Function Multi Versioning. The Arm C Language Extension spec for Function Multi Versioning is beta and subject to change, so any usage of FMV comes with the caveat that future behavior change and incompatibility is likely. consider yourself warned 💀
🔥 -march and -mcpu Feature Modifiers 🔥
Feature modifiers used with -march and -mcpu can be any of the following, plus their inverses nofeature — so like nocrc, nosimd, etc. slay 🫶
crc— enable CRC extension. On by default for-march=armv8.1-a. ✨crypto— enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.fp— enable floating-point instructions. On by default for all possible values for options-marchand-mcpu. 💖simd— enable Advanced SIMD instructions. This also enables floating-point instructions. On by default for all possible values for options-marchand-mcpu.sve— enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.lse— enable Large System Extension instructions. On by default for-march=armv8.1-a.rdma— enable Round Double Multiply Accumulate instructions. On by default for-march=armv8.1-a.fp16— enable FP16 extension. This also enables floating-point instructions.fp16fml— enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. Enabled by default for-march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported. 🙈rcpc— enable the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension. iconic ✨dotprod— enable the Dot Product extension. This also enables Advanced SIMD instructions.aes— enable the Armv8-aaesandpmullcrypto extension. This also enables Advanced SIMD instructions.sha2— enable the Armv8-asha2crypto extension. This also enables Advanced SIMD instructions.sha3— enable thesha512andsha3crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported. 💀sm4— enable thesm3andsm4crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported. 💀profile— enable the Statistical Profiling extension. This one is only to enable the extension at the assembler level and does not affect code generation. ngl it just sits there looking pretty 🎀rng— enable the Armv8.5-a Random Number instructions. Again, only to enable the extension at the assembler level, no effect on code generation.memtag— enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.sb— enable the Armv8-a Speculation Barrier instruction. Only to enable the extension at the assembler level, doesn't affect code generation. Enabled by default for-march=armv8.5-a.ssbs— enable the Armv8-a Speculative Store Bypass Safe instruction. Only to enable the extension at the assembler level, doesn't affect code generation. Enabled by default for-march=armv8.5-a.predres— enable the Armv8-a Execution and Data Prediction Restriction instructions. Only to enable the extension at the assembler level, doesn't affect code generation. Enabled by default for-march=armv8.5-a.sve2— enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions. 🔥sve2-bitperm— enable SVE2 bitperm instructions. This also enables SVE2 instructions.sve2-sm4— enable SVE2sm4instructions. This also enables SVE2 instructions.sve2-aes— enable SVE2aesinstructions. This also enables SVE2 instructions.sve2-sha3— enable SVE2sha3instructions. This also enables SVE2 instructions.sve2p1— enable SVE2.1 instructions. This also enables SVE2 instructions.tme— enable the Transactional Memory Extension.i8mm— enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for-march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.f32mm— enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.f64mm— enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.bf16— enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for-march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.ls64— enable the 64-byte atomic load and store instructions for accelerators. 💅mops— enable the instructions to accelerate memory operations likememcpy,memmove,memset. Enabled by default for-march=armv8.8-a. ok that's actually so useful 🌈flagm— enable the Flag Manipulation instructions Extension.flagm2— enable the FlagM2 flag conversion instructions.pauth— enable the Pointer Authentication Extension.cssc— enable the Common Short Sequence Compression instructions.sme— enable the Scalable Matrix Extension. This is only supported when SVE2 is also enabled. 👀sme-i16i64— enable theFEAT_SME_I16I64extension to SME. This also enables SME instructions.sme-f64f64— enable theFEAT_SME_F64F64extension to SME. This also enables SME instructions.sme2— enable the Scalable Matrix Extension 2. This also enables SME instructions.sme-b16b16— enable theFEAT_SME_B16B16extension to SME. This also enables SME2 andSVE_B16B16instructions.sme-f16f16— enable theFEAT_SME_F16F16extension to SME. This also enables SME2 instructions.sme2p1— enable the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.fcma— enable the complex number SIMD extensions.jscvt— enable thefjcvtzsJavaScript conversion instruction. 🍓frintts— enable floating-point round to integral value instructions.wfxt— enablewfetandwfitinstructions.xs— enable the XS memory attribute extension.lse128— enable the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.d128— enable support for 128-bit system register read/write instructions. This also enables the LSE128 extension.gcs— enable support for Armv9.4-a Guarded Control Stack extension.the— enable support for Armv8.9-a/9.4-a translation hardening extension.rcpc2— enable the RCpc2 extension.rcpc3— enable the RCpc3 (Release Consistency) extension.fp8— enable the fp8 (8-bit floating point) extension.fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension.ssve-fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.ssve-fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.ssve-fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.faminmax— enable the Floating Point Absolute Maximum/Minimum extension.lut— enable the Lookup Table extension.cpa— enable the Checked Pointer Arithmetic instructions.sve-b16b16— enable the SVE non-widening brain floating-point (bf16) extension. This only has an effect whensve2orsme2are also enabled.
ok so the tea on the implications ☕ Featurecryptoimpliesaes,sha2, andsimd, which impliesfp. Conversely,nofpimpliesnosimd, which impliesnocrypto,noaesandnosha2. it's giving domino effect 💅
🧚 Adapteva Epiphany Options 🧚
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file— don't allocate any register in the ranger32…r63. That allows code to run on hardware variants that lack these registers. cute workaround ngl 🎀-mprefer-short-insn-regs— tell the register allocator to prefer registers that let it spit out short instructions. Lowkey a tradeoff tho: this can bump up the instruction count, so overall code size might go down or up. It's giving chaos, not guarantees 💅-mbranch-cost=num— set the cost of branches to roughlynum"simple" instructions. Heads up bestie: this is only a heuristic, so don't come for me if it doesn't give consistent results across releases 😭-mcmove— enables generation of conditional moves. Slay.-mnops=num— emitnumNOPs before every other generated instruction. Yes, on purpose. For science 🧚-mno-soft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. Faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers get compared such that their difference is calculated as zero. 🙈 The default is-msoft-cmpsf, which uses slower but IEEE-compliant software comparisons.
fr fr, if NaNs are anywhere near your code, maybe don't.
-mstack-offset=num— sets the offset between the top of the stack and the stack pointer. Like, a value of8means the eight bytes in the rangesp+0...sp+7can be used by leaf functions without stack allocation. Values other than8or16are untested and unlikely to work 💀 Also note this option changes the ABI — compiling a program with a different stack offset than the libraries were compiled with generally does not work. It can be useful if you wanna evaluate whether a different stack offset would give you better code, but to actually build working programs with a different stack offset, the move is to configure the toolchain with the appropriate--with-stack-offset=numoption. iykyk-mno-round-nearest— makes the scheduler assume the rounding mode has been set to truncating. The default is-mround-nearest.-mlong-calls— unless an attribute says otherwise, assume all calls might be beyond the offset range of theb/blinstructions, so load the function address into a register before doing an (otherwise direct) call. This is the default, no cap.-mshort-calls— unless an attribute says otherwise, assume all direct calls are in range of theb/blinstructions, so use those bad boys for direct calls. The default is-mlong-calls.-msmall16— assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses where-mlong-callssemantics are in effect.-mfp-mode=mode— sets the prevailing mode of the floating-point unit. This determines the floating-point mode that's provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. Obsessed honestly ✨
mode can be set to one of the following values:
caller— any mode at function entry is valid, and it's retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to drop into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.truncate— the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.round-nearest— the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.int— the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mno-split-lohi-mno-postinc-mno-postmodify— code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults aremsplit-lohi,-mpost-inc, and-mpost-modify.-mnovect-double— changes the preferred SIMD mode to SImode. The default is-mvect-double, which uses DImode as the preferred SIMD mode.-max-vect-align=num— the maximum alignment for SIMD vector mode types.nummay be4or8. The default is8. Real talk tho: this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. ⚠️-msplit-vecmove-early— split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case 😭-m1reg-reg— specify a register to hold the constant-1, which makes loading small negative constants and certain bitmasks faster. Allowable values forregarer43andr63, which specify use of that register as a fixed register, andnone, which means no register is used for this purpose. The default is-m1reg-none.
🌈 AMD GCN Options 🌈
These options are defined specifically for the AMD GCN port. GPU girlies rise 🫶
-march=gpu-mtune=gpu— set the architecture type or tuning forgpu. Supported values forgpuare:gfx900— compile for GCN5 Vega 10 devices (gfx900).gfx902— compile for GCN5 Vegagfx902devices. (Experimental)gfx904— compile for GCN5 Vegagfx904devices. (Experimental)gfx906— compile for GCN5 Vega 20 devices (gfx906).gfx908— compile for CDNA1 Instinct MI100 series devices (gfx908).gfx909— compile for GCN5 Vegagfx909devices. (Experimental)gfx90a— compile for CDNA2 Instinct MI200 series devices (gfx90a).gfx90c— compile for GCN5 Vega 7 devices (gfx90c).gfx9-generic— compile generic code for Vega devices, executable on the following subset of GFX9 devices:gfx900,gfx902,gfx904,gfx906,gfx909andgfx90c. (Experimental)gfx1030— compile for RDNA2gfx1030devices (GFX10 series).gfx1031— compile for RDNA2gfx1031devices (GFX10 series). (Experimental)gfx1032— compile for RDNA2gfx1032devices (GFX10 series). (Experimental)gfx1033— compile for RDNA2gfx1033devices (GFX10 series). (Experimental)gfx1034— compile for RDNA2gfx1034devices (GFX10 series). (Experimental)gfx1035— compile for RDNA2gfx1035devices (GFX10 series). (Experimental)gfx1036— compile for RDNA2gfx1036devices (GFX10 series).gfx10-3-generic— compile generic code for GFX10-3 devices, executable ongfx1030,gfx1031,gfx1032,gfx1033,gfx1034,gfx1035, andgfx1036. (Experimental)gfx1100— compile for RDNA3gfx1100devices (GFX11 series).gfx1101— compile for RDNA3gfx1101devices (GFX11 series). (Experimental)gfx1102— compile for RDNA3gfx1102devices (GFX11 series). (Experimental)gfx1103— compile for RDNA3gfx1103devices (GFX11 series).gfx1150— compile for RDNA3gfx1150devices (GFX11 series). (Experimental)gfx1151— compile for RDNA3gfx1151devices (GFX11 series). (Experimental)gfx1152— compile for RDNA3gfx1152devices (GFX11 series). (Experimental)gfx1153— compile for RDNA3gfx1153devices (GFX11 series). (Experimental)gfx11-generic— compile generic code for GFX11 devices, executable ongfx1100,gfx1101,gfx1102,gfx1103,gfx1150,gfx1151,gfx1152, andgfx1153. (Experimental)
-msram-ecc=on-msram-ecc=off-msram-ecc=any— compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default isany, for devices that support it.-mstack-size=bytes— specify how many bytes of stack space will be requested for each GPU thread (wave-front). ⚠️ Beware that there may be many threads and limited memory available. The size of the stack allocation may also have an impact on run-time performance. The default is32KBwhen using OpenACC or OpenMP, and1MBotherwise.-mxnack=on-mxnack=off-mxnack=any— compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is-mxnack=anyon devices that support Unified Shared Memory, and-mxnack=nootherwise.
🦋 ARC Options 🦋
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter— generate instructions supported by barrel shifter. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect.-mjli-always— force to call a function usingjli_sinstruction. This option is valid only for ARCv2 architecture. Periodt.-mcpu=cpu— set architecture type, register usage, and instruction scheduling parameters forcpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values forcpuare:arc600— compile for ARC600. Aliases:-mA6,-mARC600.arc601— compile for ARC601. Alias:-mARC601.arc700— compile for ARC700. Aliases:-mA7,-mARC700. This is the default when configured with--with-cpu=arc700.arcem— compile for ARC EM.archs— compile for ARC HS.em— compile for ARC EM CPU with no hardware extensions.em4— compile for ARC EM4 CPU.em4_dmips— compile for ARC EM4 DMIPS CPU.em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions.hs34— compile for ARC HS34 CPU.
hs38— compile for the ARC HS38 CPU 💅hs38_linux— compile for ARC HS38 CPU with all the hardware extensions turned on ✨hs4x— compile for the ARC HS4x CPU 🦋hs4xd— compile for the ARC HS4xD CPU 🌸hs4x_rel31— compile for ARC HS4x CPU release 3.10a 🎀arc600_norm— compile for ARC 600 CPU with "norm" instructions enabled 👑arc600_mul32x16— compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled 🔥arc600_mul64— compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled 💜arc601_norm— compile for ARC 601 CPU with "norm" instructions enabled 🧚arc601_mul32x16— compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled ⭐arc601_mul64— compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled 🌈nps400— compile for ARC 700 on the NPS400 chip 🍓em_mini— compile for ARC EM minimalist configuration, which features a reduced register set 🙈
-mdpfp,-mdpfp-compact— generate double-precision FPX instructions, tuned for the compact implementation 💫-mdpfp-fast— generate double-precision FPX instructions, tuned for the fast implementation ⚡-mno-dpfp-lrsr— stop "lr" and "sr" instructions from using FPX extension aux registers, periodt 🚫-mea— generate extended arithmetic instructions. rn only "divaw", "adds", "subs", and "sat16" are supported. Only valid for-mcpu=ARC700💅-mno-mpy— don't generate "mpy"-family instructions for ARC700. heads up bestie, this option is deprecated 💀-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions ✨-mmul64— generate "mul64" and "mulu64" instructions. Only valid for-mcpu=ARC600🎀-mnorm— generate "norm" instructions. This is the default if-mcpu=ARC700is in effect 👑-mspfp,-mspfp-compact— generate single-precision FPX instructions, tuned for the compact implementation 🌸-mspfp-fast— generate single-precision FPX instructions, tuned for the fast implementation 🦋-msimd— enable generation of ARC SIMD instructions via target-specific builtins. Only valid for-mcpu=ARC700🔥-msoft-float— this option is ignored; it's only here for compatibility purposes 🙃 Software floating-point code is emitted by default, and you can override that default with the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision.-mswap— generate "swap" instructions 🔄-matomic— this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores ⚠️-mdiv-rem— enable "div" and "rem" instructions for ARCv2 cores ➗-mcode-density— enable code density instructions for ARC EM. This option is on by default for ARC HS 🧚-mll64— enable double load/store operations for ARC HS cores 💜- *
-mtp-regno=regno* — specify the thread pointer register number 🧵 - *
-mmpy-option=multo* — compile ARCv2 code with a multiplier design option. You can specify the option using either a string or a numeric value for multo.wlh1is the default value. The recognized values are 👇0,none— no multiplier available 🚫1,w— 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw" ✨2,wlh1— 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" 🌟3,wlh2— 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" 💫4,wlh3— two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" 🎀5,wlh4— one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" 🌸6,wlh5— one 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s" 🦋7,plus_dmpy— ARC HS SIMD support 🔥8,plus_macd— ARC HS SIMD support 🔥9,plus_qmacw— ARC HS SIMD support 🔥
This option is only available for ARCv2 cores 👑
- *
-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. The supported values for fpu are 🫶fpus— enables support for single-precision floating-point hardware extensions ✨fpud— enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM ⚠️fpuda— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM 💫fpuda_div— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM ➗fpuda_fma— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM ✖️fpuda_all— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM 🌈fpus_div— enables support for single-precision floating-point, square-root and divide hardware extensions 🌸fpud_div— enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes optionfpus_div. Not available for ARC EM ⚠️fpus_fma— enables support for single-precision floating-point and fused multiply and add hardware extensions 🔥fpud_fma— enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes optionfpus_fma. Not available for ARC EM ⚠️fpus_all— enables support for all single-precision floating-point hardware extensions ⭐fpud_all— enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM ⚠️
- *
-mirq-ctrl-saved=register-range*, blink, lp_count — specifies the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. The register range always starts with "r0", and the upper limit is the "fp" register. blink and lp_count are optional. This option is only valid for ARC EM and ARC HS cores 💜 - *
-mrgf-banked-regs=number* — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. Fast interrupts are interrupts with the highest priority level P0. These interrupts save only the PC and STATUS32 registers to avoid memory transactions during the interrupt entry and exit sequences. Use this option when you're using fast interrupts in an ARC V2 family processor. Permitted values are4,8,16, and32🎯 -mlpc-width=width — specify the width of the "lp_count" register. Valid values for width are8,16,20,24,28and32bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler does *not attempt to transform loops in your program to use the zero-delay loop mechanism unless it's known that the "lp_count" register can hold the required loop-counter value. Depending on the width you specify, the compiler and run-time library might keep using the loop mechanism for various needs. This option defines the macro__ARC_LPC_WIDTH__with the value of *width 📏-mrf16— this option tells the compiler to generate code for a 16-entry register file. It also defines the__ARC_RF16__preprocessor macro 🧮-mbranch-index— enable use of "bi" or "bih" instructions to implement jump tables 🚀
The following options get passed straight through to the assembler, and they also define preprocessor macro symbols 👇
-mdsp-packa— passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol__Xdsp_packa. heads up, this option is deprecated 💀-mdvbf— passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol__Xdvbf. This option is deprecated 💀-mlock— passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol__Xlock🔒-mmac-d16— passed down to the assembler. Also sets the preprocessor symbol__Xxmac_d16. This option is deprecated 💀-mmac-24— passed down to the assembler. Also sets the preprocessor symbol__Xxmac_24. This option is deprecated 💀-mrtsc— passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol__Xrtsc. This option is deprecated 💀-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol__Xswape🔄-mtelephony— passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol__Xtelephony. This option is deprecated 💀-mxy— hands this down to the assembler so the XY memory extension gets enabled. Also sets the preprocessor symbol__Xxy. Two-for-one, love that for us ✨
🎀 The following options control how the assembly code is annotated 🎀
-misize— annotates assembler instructions with estimated addresses. cute little address guesses, bless 💅-mannotate-align— literally does nothing. it's just vibes, preserved for backward compatibility 🙈
🔗 The following options are passed through to the linker 🔗
-marclinux— gets passed through to the linker to specify use of thearclinuxemulation. This one's on by default in tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets when profiling is not requested. 👑-marclinux_prof— passed through to the linker, specifying use of thearclinux_profemulation. Enabled by default in tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets when profiling is requested. the profiling girlies get their own emulation, periodt 💖
🧠 The following options control the semantics of generated code 🧠
-mlong-calls— generates calls as register indirect calls, which means you get access to the full 32-bit address range. go big or go home fr fr 🔥-mmedium-calls— don't use less than a 25-bit addressing range for calls — that's the offset you actually get for an unconditional branch-and-link instruction. Conditional execution of function calls gets suppressed here, so we can use that 25-bit range instead of the 21-bit range you'd get with conditional branch-and-link. This is the default for tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets. 💫-G num— puts definitions of externally-visible data into a small data section if that data isn't bigger than num bytes. Default value of num is4for any ARC configuration, or8when we've got double load/store operations. numbers matter bestie, don't sleep on em 🍓-mno-sdata— do not generate sdata references. Default for tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets. no sdata for you 🚫-mvolatile-cache— uses ordinarily cached memory accesses for volatile references. This is the default. 💅-mno-volatile-cache— enables cache bypass for volatile references. sneaky lil bypass, iconic 🌈
🦋 The following options fine tune code generation 🦋
-malign-call— does nothing. preserved for backward compatibility, she's just here for the nostalgia 😭-mauto-modify-reg— enables the use of pre/post modify with register displacement. fancy register glow-up ✨-mbbit-peephole— does nothing. preserved for backward compatibility. another one that's just vibing 🙈-mno-brcc— disables a target-specific pass inarc_reorgthat generates compare-and-branch (brcc) instructions. Heads up though: it has no effect on generation of these instructions when the combiner pass is driving them. so like, partial power only 💀-mcase-vector-pcrel— uses PC-relative switch case tables so the case table can get shortened. This is the default for-Os. 🎀-mcompact-casesi— enables the compactcasesipattern. Default for-Os, and only available for ARCv1 cores. Heads up: this option is deprecated. ⚠️-mno-cond-exec— disables the ARCompact-specific pass that generates conditional execution instructions. Okay so real talk, because this one needs context: due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lacking — so the ARC port kept a special pass around that hunts for more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have all been done. This pass generally (but not always!) improves performance and code size, at the cost of extra compilation time — which is why there's an option to switch it off. And if you've got a problem with call instructions blowing past their allowable offset range because they got conditionalized, you should consider using-mmedium-callsinstead. 👑 full lore delivered-mearly-cbranchsi— enables pre-reload use of thecbranchsipattern. early bird behavior 🦋-mexpand-adddi— expandsadddi3andsubdi3at RTL generation time intoadd.f,adcetc. This option is deprecated. 🚨-mindexed-loads— enables the use of indexed loads. Just know this can be problematic, because some optimizers then go ahead and assume that indexed stores exist too — which is not the case. awkward 😭-mlra— does nothing. preserved for backward compatibility. the void stares back 💀-mlra-priority-none— don't indicate any priority for target registers. no thoughts, no priorities 🫶-mlra-priority-compact— indicate target register priority forr0..r3/r12..r15. 🌸-mlra-priority-noncompact— reduce target register priority forr0..r3/r12..r15. 📉-mmillicode— when optimizing for size (with-Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by using a call to a special function inlibgcc— this is called a millicode call. Since these calls can pose performance issues, and/or cause linking issues when you're linking in a nonstandard way, this option exists to turn millicode call generation on or off. 🔥-mcode-density-frame— lets the compiler emitenterandleaveinstructions. Important: these instructions are only valid for CPUs with the code-density feature. ⚠️ know your hardware bestie-mmixed-code— does nothing. preserved for backward compatibility. 🎀-mq-class— this option is deprecated. Enables q instruction alternatives. This is the default for-Os. 💅-mRcq— does nothing. preserved for backward compatibility. 🙈-mRcw— does nothing. preserved for backward compatibility. yes, another one. they're twins ✨-msize-level=level— fine-tunes size optimization with regards to instruction lengths and alignment. The recognized values for level are:0— no size optimization. This level is deprecated and treated like1.1— short instructions are used opportunistically.2— in addition, alignment of loops and of code after barriers are dropped.3— in addition, optional data alignment is dropped, and the optionOsis enabled.
This defaults to 3 when -Os is in effect. Otherwise, if this isn't set at all, the behavior is equivalent to level 1. 🎚️
-mtune=cpu— sets instruction scheduling parameters for cpu, overriding anything implied by-mcpu=. Supported values for cpu are:ARC600— tune for ARC600 CPU.ARC601— tune for ARC601 CPU.ARC700— tune for ARC700 CPU with standard multiplier block.ARC700-xmac— tune for ARC700 CPU with XMAC block.ARC725D— tune for ARC725D CPU.ARC750D— tune for ARC750D CPU.core3— tune for ARCv2 core3 type CPU. This option enables usage of thedbnzinstruction.release31a— tune for ARC4x release 3.10a.
🧚 the whole cpu fam, all invited to the party
-mmultcost=num— the cost to assume for a multiply instruction, with4being equal to a normal instruction. 💰-munalign-prob-threshold=probability— does nothing. preserved for backward compatibility. ⭐
💀 The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release 💀
nesties this is your warning era — these are on the way out, so don't fall in love 😭
-margonaut— obsolete FPX. old. ancient. leaving. 🦕-mbig-endian,-EB— compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to buildarceb-elf32andarceb-linux-uclibctargets, for which big endian is the default. 📚-mlittle-endian,-EL— compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to buildarc-elf32andarc-linux-uclibctargets, for which little endian is the default. 🌸-mbarrel_shifter— replaced by-mbarrel-shifter. (undescore to dash glow-up, we love a rebrand) ✨-mdpfp_compact— replaced by-mdpfp-compact. 💅-mdpfp_fast— replaced by-mdpfp-fast. 💅-mdsp_packa— replaced by-mdsp-packa. 💅-mEA— replaced by-mea. 💅-mmac_24— replaced by-mmac-24. 💅-mmac_d16— replaced by-mmac-d16. 💅-mspfp_compact— replaced by-mspfp-compact. 💅-mspfp_fast— replaced by-mspfp-fast. 💅-mtune=cpu— the valuesarc600,arc601,arc700andarc700-xmacfor cpu are replaced byARC600,ARC601,ARC700andARC700-xmacrespectively. (they got capital letters, she's thriving now 👑)-multcost=num— replaced by-mmultcost. 🫶
🌈 ARM Options 🌈
These -m options are defined for the ARM port. ok ARM girlies, this is your section, come get your food 🍓
-mabi=name— generate code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcs,aapcs-linuxandiwmmxt. pick your fighter 💅-mapcs-frame— generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if that's not strictly necessary for the code to execute correctly. If you specify-fomit-frame-pointeralongside this option, stack frames won't be generated for leaf functions. The default is-mno-apcs-frame. This option is deprecated. ⚠️-mapcs— this is a synonym for-mapcs-frameand is deprecated. literally the same girl, different name 😭-mthumb-interwork— generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets can't be reliably used inside one program. The default is-mno-thumb-interwork, since specifying-mthumb-interworkgenerates slightly larger code. Heads up: in AAPCS configurations this option is meaningless. 🦋-mno-sched-prolog— prevents reordering of instructions in the function prologue, and prevents merging those instructions with the instructions in the function's body. What that buys you: every function starts with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that information can be used to locate the start of functions inside an executable piece of code. The default is-msched-prolog. 🔍-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard. Let's break it down:softmakes GCC generate output containing library calls for floating-point operations.softfpallows generation of code that uses hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Also, real talk: the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. no mixing, no matching, i don't make the rules 💀
-mgeneral-regs-only— ok so this one tells the compiler to only ever touch the general-purpose registers 💅. It blocks it from reaching for floating-point and Advanced SIMD registers, but bestie listen — it puts zero restrictions on the assembler. That part's not its business.-mlittle-endian— generate code for a processor vibing in little-endian mode. This is the default for all standard configurations, so yeah, it's the main character here 🎀.-mbig-endian— generate code for a processor running in big-endian mode. The default, tho? Compiling for a little-endian processor. So you gotta ask for this one ✨.-mbe8,-mbe32— when you're linking a big-endian image, these pick between BE8 and BE32 formats. For little-endian images the option does literally nothing and gets ignored, no cap 🙈. The default depends on the target architecture you selected: ARMv6 and later architectures default to BE8, older ones default to BE32. Also — heads up — BE32 format has been deprecated by ARM, so it's giving legacy 💀.-march=name[+extension...]— this names the target ARM architecture. GCC uses that name to figure out what kind of instructions it's allowed to emit when it generates assembly code. You can use it alongside-mcpu=or instead of it, dealer's choice 🦋.
The permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.
Also, these architectures are recognized but their support is deprecated because they lack support for the Thumb execution state: armv4. Like, they still work, but 😬.
Lots of these architectures support extensions, and you add them by sticking +extension onto the end of the architecture name. Extension options get processed in order and the capabilities accumulate — it's additive, slay ✨. An extension will also turn on any base extensions it depends on. Example: the +crypto extension will always enable the +simd extension. The one exception to this whole additive construction is extensions prefixed with +no... — those disable the specified option and any other extensions that might depend on that extension being there.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4, because the +simd option gets entirely disabled by the +nofp option that follows it. Order matters, bestie 🔥.
Most extension names are generically named but their effect depends on which architecture you applied them to. Example: +simd can be applied to both armv7-a and armv8-a architectures, but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a. Same name, different lore 🌈.
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions — sorry, nothing for them 💀.
armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
+fp— the VFPv2 floating-point instructions. The extension+vfpv2can be used as an alias for this extension.+nofp— disable the floating-point instructions.
armv7
This is the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures 🫶.
+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but it's compatible with both the ARMv7-A and ARMv7-R architectures.+nofp— disable the floating-point instructions.
armv7-a
+mp— the multiprocessing extension.+sec— the security extension.+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+simd— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can be used as aliases for this extension.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers.+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+neon-vfpv4— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.+nosimd— disable the Advanced SIMD instructions (does not disable floating point).+nofp— disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture, with support for virtualization 👑
+fp— the VFPv4 floating-point instructions, with 16 double-precision registers. The extension+vfpv4-d16can be used as an alias for this extension.+simd— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension+neon-vfpv4can be used as an alias for this extension.+vfpv3-d16— the VFPv3 floating-point instructions, with 16 double-precision registers.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers.+neon— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension+neon-vfpv3can be used as an alias for this extension.+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+nosimd— disable the Advanced SIMD instructions (does not disable floating point).+nofp— disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc— the Cyclic Redundancy Check (CRC) instructions.+simd— the ARMv8-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions.+nocrypto— disable the cryptographic instructions.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+nocrypto— disable the cryptographic instructions.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.2-a, armv8.3-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.+fp16fml— the half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+dotprod— enable the Dot Product extension. This also enables Advanced SIMD instructions.+nocrypto— disable the cryptographic extension.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension ✨+crypto— the cryptographic instructions. Turning this on also unlocks the Advanced SIMD and floating-point instructions and the Dot Product extension, bestie, it's a whole bundle 🎀+nocrypto— nope, no cryptographic extension for you 🙈+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. All of it. Gone 💀+sb— the Speculation Barrier Instruction+predres— the Execution and Data Prediction Restriction Instructions+i8mm— the 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions 🧚+bf16— the Brain half-precision floating-point instructions. This one also enables Advanced SIMD and floating-point instructions ✨
armv8.5-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension 🌸+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension+nocrypto— disable the cryptographic extension+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions+i8mm— the 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions+bf16— the Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions
armv8.6-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension 🦋+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension+nocrypto— disable the cryptographic extension+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions+i8mm— the 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions+bf16— the Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions
armv7-r
+fp.sp— the single-precision VFPv3 floating-point instructions. You can also use the extension+vfpv3xdas an alias for this one, iykyk 👑+fp— the VFPv3 floating-point instructions with 16 double-precision registers. The extension+vfpv3-d16works as an alias for this one too+vfpv3xd-d16-fp16— the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations+vfpv3-d16-fp16— the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations+nofp— disable the floating-point extension+idiv— the ARM-state integer division instructions+noidiv— disable the ARM-state integer division extension
armv7e-m
+fp— the single-precision VFPv4 floating-point instructions+fpv5— the single-precision FPv5 floating-point instructions+fp.dp— the single- and double-precision FPv5 floating-point instructions+nofp— disable the floating-point extensions
armv8.1-m.main
+dsp— the DSP instructions+mve— the M-Profile Vector Extension (MVE) integer instructions+mve.fp— the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions+fp— the single-precision floating-point instructions+fp.dp— the single- and double-precision floating-point instructions+nofp— disable the floating-point extension+cdecp0,+cdecp1, ... ,+cdecp7— enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7 🎀+pacbti— enable the Pointer Authentication and Branch Target Identification Extension 💅
armv8-m.main
+dsp— the DSP instructions+nodsp— disable the DSP extension+fp— the single-precision floating-point instructions+fp.dp— the single- and double-precision floating-point instructions+nofp— disable the floating-point extension+cdecp0,+cdecp1, ... ,+cdecp7— enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7
armv8-r
+crc— the Cyclic Redundancy Check (CRC) instructions+fp.sp— the single-precision FPv5 floating-point instructions+simd— the ARMv8-A Advanced SIMD and floating-point instructions+crypto— the cryptographic instructions+nocrypto— disable the cryptographic instructions+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions
So like, -march=native tells the compiler to auto-detect the architecture of the build computer, bestie 🖥️ At the moment this feature is only supported on GNU/Linux, and not all architectures are recognized. And if the auto-detect doesn't work out? The option literally has no effect. It's giving "no thoughts, head empty" 😭
And -march=unset makes the compiler ignore any -march=... options that show up earlier on the command line and behave like the option was never passed at all. Super useful for dodging warnings about conflicting CPU and architecture options when the two produce different architecture specifications. Thank u, next to those warnings 🙈
🌈 -mtune=name
Okay so this one specifies the name of the target ARM processor that GCC should tune the performance of the code for 🎯 With some ARM implementations you genuinely get better performance by using this option, no cap.
Permissible names are:
arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s,
arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76,
cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710,
ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52,
cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4,
cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55,
cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1,
marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2,
ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1.✨ Bonus round: this option can also specify that GCC should tune the performance of the code for a big.LITTLE system. Those permissible names are:
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55.-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch 🫶 The whole goal is generating code that runs well on the current most popular processors — balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. Fair and balanced, iconic behavior. Heads up though: the effects of this option may change in future GCC versions as CPU models come and go 💀
-mtune permits the same extension options as -mcpu, but — plot twist — the extension options do not affect the tuning of the generated code 🙈
-mtune=native makes the compiler auto-detect the CPU of the build computer. At present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful, the option has no effect. Same energy as before, fr fr.
👑 -mcpu=name[+extension...]
This specifies the name of the target ARM processor 🎀 GCC uses this name to derive the name of the target ARM architecture (as if it had been specified by -march) and the ARM processor type to tune for performance (as if it had been specified by -mtune).
And here's the tea: where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. So -mcpu is like, the vibe-setter, but -march and -mtune are the main characters if they show up 💅
Like fr, so many CPUs out here have optional architectural extensions, and normally those extensions are just on by default ✨. But if there are implementations floating around that don't have the extension? That's when you use the extension syntax to switch off the ones that got left out. And for floating-point and Advanced SIMD (Neon) instructions, bestie, you gotta think about -mfloat-abi and -mfpu too: floating-point and Advanced SIMD instructions only get used if -mfloat-abi is not set to soft, and any -mfpu setting other than auto overrides whatever floating-point and SIMD extension instructions were available.
Take cortex-a9 — it comes in three major configurations: integer only, floating-point unit only, or floating-point and Advanced SIMD. Default is all the instructions enabled, but you can hit +nosimd and +nofp to disable just SIMD, or both SIMD and floating-point instructions respectively 🎀.
Permissible names for this option are literally the same as for -mtune.
The extension options common to the listed CPUs are:
+nodsp— disables the DSP instructions oncortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nopacbti— disables the Pointer Authentication and Branch Target Identification Extension oncortex-m52andcortex-m85.+nomve— disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nomve.fp— disables the M-Profile Vector Extension (MVE) single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+cdecp0,+cdecp1, ... ,+cdecp7— enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7, oncortex-m52,cortex-m55andstar-mc1.+nofp— disables the floating-point instructions onarm9e,arm946e-s,arm966e-s,arm968e-s,arm10e,arm1020e,arm1022e,arm926ej-s,arm1026ej-s,cortex-r5,cortex-r7,cortex-r8,cortex-m4,cortex-m7,cortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Disables the floating-point and SIMD instructions ongeneric-armv7-a,cortex-a5,cortex-a7,cortex-a8,cortex-a9,cortex-a12,cortex-a15,cortex-a17,cortex-a15.cortex-a7,cortex-a17.cortex-a7,cortex-a32,cortex-a35,cortex-a53andcortex-a55.+nofp.dp— disables the double-precision component of the floating-point instructions oncortex-r5,cortex-r7,cortex-r8,cortex-r52,cortex-r52plusandcortex-m7.+nosimd— disables the SIMD (but not floating-point) instructions ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— enables the cryptographic instructions oncortex-a32,cortex-a35,cortex-a53,cortex-a55,cortex-a57,cortex-a72,cortex-a73,cortex-a75,exynos-m1,xgene1,cortex-a57.cortex-a53,cortex-a72.cortex-a53,cortex-a73.cortex-a35,cortex-a73.cortex-a53andcortex-a75.cortex-a55.
Also, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers 💅. It supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. Meanings are the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. Peep -mtune for more on that 👀.
-mcpu=native makes the compiler auto-detect the CPU of the build computer. At present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect doesn't work out, the option just has no effect, no drama.
-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed. So useful for avoiding warnings about conflicting CPU and architecture options when the two produce different architecture specifications 🫶.
-mfpu=name — this specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are:auto,vfpv2,vfpv3,vfpv3-fp16,vfpv3-d16,vfpv3-d16-fp16,vfpv3xd,vfpv3xd-fp16,neon-vfpv3,neon-fp16,vfpv4,vfpv4-d16,fpv4-sp-d16,neon-vfpv4,fpv5-d16,fpv5-sp-d16,fp-armv8,neon-fp-armv8andcrypto-neon-fp-armv8. Note thatneonis an alias forneon-vfpv3andvfpis an alias forvfpv2.
The setting auto is the default and is special ✨. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero 😭), so using NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
-mfp16-format=name — specify the format of the__fp16half-precision floating-point type. Permissible names arenone,ieee, andalternative; the default isnone, in which case the__fp16type is not defined.-mstructure-size-boundary=n — the sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option. Permissible values are8,32and64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is8. A value of64is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible 🚨. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.
This option is deprecated. RIP to it, honestly 💀.
-mabort-on-noreturn— generate a call to the functionabortat the end of anoreturnfunction. It gets executed if the function tries to return.-mlong-calls-mno-long-calls— tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers 👑.
-msingle-pic-base— treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.-mpic-register=reg — specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default isR9if target is EABI based or stack-checking is enabled, otherwise the default isR10.-mpic-data-is-text-relative— assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable-msingle-pic-baseby default.-mpoke-function-name— write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000) 🔥.
-mthumb-marm— select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the--with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas 🦋.
-mflip-thumb— flips ARM/Thumb modes on alternating functions 🔄 ngl this one exists purely so the compiler devs can regression-test mixed Thumb/ARM codegen — it is not meant for ordinary code compiling, bestie, pls don't ship it 🙈-mtpcs-frame— generates a stack frame that's compliant with the Thumb Procedure Call Standard for all non-leaf functions (a leaf function is one that doesn't call any other functions 🌱). The default is-mno-tpcs-frame.-mtpcs-leaf-frame— same Thumb Procedure Call Standard stack-frame deal, but for all leaf functions 🌱 (again: a leaf function is one that doesn't call any other functions). The default is-mno-apcs-leaf-frame.-mcallee-super-interworking— gives every externally visible function in the file you're compiling an ARM instruction set header that switches to Thumb mode before running the rest of the function ✨ so those functions can be called from non-interworking code. This option is not valid in AAPCS configurations, because interworking is enabled by default there 💅-mcaller-super-interworking— lets calls through function pointers (virtual functions included) execute correctly no matter whether the target code was compiled for interworking or not 🫶. There's a small overhead in the cost of executing a function pointer if you turn this on. Also not valid in AAPCS configurations, because interworking is enabled by default there already 💅- *
-mtp=name* — sets the access model for the thread local storage pointer. Thesoftmodel generates calls to"__aeabi_read_tp". The other accepted models aretpidrurw,tpidruroandtpidrprw, which fetch the thread pointer straight from the corresponding system register (supported from the arm6k architecture and later ⭐). Those system registers get accessed through the CP15 co-processor interface, and the argumentcp15is also accepted as a convenience alias oftpidruro. The argumentautouses the best available method for the selected processor. Default setting isauto👑 - *
-mtls-dialect=dialect* — picks the dialect for accessing thread local storage. Two dialects are supported —gnuandgnu2🎀. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect selects the GNU descriptor scheme, which gives better performance for shared libraries 🔥. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. -mword-relocations— only generate absolute relocations on word-sized values (i.e.R_ARM_ABS32). It's enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when-fpicor-fPICis specified ✨. Heads up: this option conflicts with-mslow-flash-data💀-mfix-cortex-m3-ldrd— some Cortex-M3 cores can cause data corruption whenldrdinstructions with overlapping destination and base registers are used 😭 this option avoids generating those instructions. Enabled by default when-mcpu=cortex-m3is specified.-mfix-cortex-a57-aes-1742098,-mno-fix-cortex-a57-aes-1742098,-mfix-cortex-a72-aes-1655431,-mno-fix-cortex-a72-aes-1655431— enable (or disable) the mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions 🔐. Enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified.-munaligned-access,-mno-unaligned-access— enables (or disables) reading and writing of 16- and 32-bit values from addresses that aren't 16- or 32-bit aligned 🌀. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access isn't enabled, words in packed data structures get accessed a byte at a time 🐌
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending on how you set this option. And if unaligned access is enabled, the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined ✨
-mneon-for-64bits— this option is deprecated and has no effect 💀 literally a ghost.-mslow-flash-data— assumes loading data from flash is slower than fetching instructions, so literal load gets minimized for better performance 🐢. Only supported when compiling for ARMv7 M-profile, and it's off by default. It conflicts with-mword-relocations⚔️-masm-syntax-unified— assumes inline assembler is using unified asm syntax. Default is currently off, which implies divided syntax. This option has no impact on Thumb2 — but that may change in future GCC releases. Divided syntax should be considered deprecated 🫠-mrestrict-it— restricts generation of IT blocks so they conform to the rules of ARMv8-A 🎀 IT blocks can only contain a single 16-bit instruction from a select set of instructions. On by default for ARMv8-A Thumb mode.-mprint-tune-info— prints CPU tuning information as a comment in the assembler file 🖨️ this one's only for regression testing the compiler, not intended for ordinary use in compiling code. Disabled by default.-mverbose-cost-dump— enables verbose cost model dumping in the debug dump files 🔍 provided for use in debugging the compiler.-mpure-code— do not allow constant data to be placed in code sections 🚫 additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute"SHF_ARM_PURECODE". Only available when generating non-pic code for M-profile targets.-mcmse— generates secure code as per the"ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification"🔐 which lives at<https://developer.arm.com/documentation/ecm0359818/latest/>.-mfix-cmse-cve-2021-35465— mitigates a potential security issue with the"VLLDM"instruction in some M-profile devices when using CMSE (CVE-2021-365465) 😱. Enabled by default when the option-mcpu=is used with"cortex-m33","cortex-m35p","cortex-m52","cortex-m55","cortex-m85"or"star-mc1". You can disable the mitigation with-mno-fix-cmse-cve-2021-35465.- *
-mstack-protector-guard=guard* — generate stack protection code using a canary at guard 🐤. Supported locations areglobalfor a global canary, ortlsfor a canary reachable via the TLS register. And *-mstack-protector-guard-offset=offset* is for use with-fstack-protector-guard=tls, and not for use in user-land code 🚨 (no cap, don't do it there). -mfdpic,-mno-fdpic— selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions 🦋. When the compiler is configured for"arm-*-uclinuxfdpiceabi"targets, this option is on by default and implies-fPIEif none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed.
Note that static linking is not supported, because it would still involve the dynamic linker when the program self-relocates 😭. If that behavior is acceptable to you, use -static and -Wl,-dynamic-linker options.
The opposite -mno-fdpic option is useful (and required!) to build the Linux kernel using the same ("arm-*-uclinuxfdpiceabi") toolchain as the one used to build the userland programs 🐧👑
-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]— enables branch protection features (armv8.1-m.main only) 🛡️none— generate code without branch protection or return address signing.standard[+leaf]— generate code with all branch protection features enabled at their standard level.pac-ret[+leaf]— generate code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.leaf— when return address signing is enabled, also sign leaf functions even if they don't write the return address to memory 🌱+bti— add landing-pad instructions at the permitted targets of indirect branch instructions.
If the +pacbti architecture extension isn't enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space 💅. The generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be enabled at run time on processors that support the PACBTI extension.
Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti ⚠️
Any setting other than none is supported only on armv8-m.main or later.
The default is to generate code without branch protection or return address signing.
🌸 AVR Options 🌸
These options are defined for AVR implementations 🧚:
- *
-mmcu=mcu* — specify the AVR instruction set architecture (ISA) or device type 🎯 the default for this option is"avr2".
Here are the supported AVR devices and ISAs 👇 Note: a complete device support consists of startup code "crtmcu.o", a device header "avr/io*.h", a device library "libmcu.a" and a device-specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") file "specs-mcu". Only that last one is provided by the compiler, according to the supported "mcu"s below 🫶. The rest is supported by AVR-LibC ("https://github.com/avrdudes/avr-libc/"), or by means of "atpack" ("https://gcc.gnu.org/wiki/avr-gcc#atpack") files from the hardware manufacturer.
"avr2"— "Classic" devices with up to 8 KiB of program memory 🕹️ mcu ="attiny22","attiny26","at90s2313","at90s2323","at90s2333","at90s2343","at90s4414","at90s4433","at90s4434","at90c8534","at90s8515","at90s8535".
okay so the AVR device-family lore is still going, and bestie, this part is a whole parade of chip names 🌈 every single one matters, no cap.
avr25— “Classic” devices with up to 8 KiB of program memory and with theMOVWinstruction.mcu =attiny13,attiny13a,attiny24,attiny24a,attiny25,attiny261,attiny261a,attiny2313,attiny2313a,attiny43u,attiny44,attiny44a,attiny45,attiny48,attiny441,attiny461,attiny461a,attiny4313,attiny84,attiny84a,attiny85,attiny87,attiny88,attiny828,attiny841,attiny861,attiny861a,ata5272,ata6616c,at86rf401.avr3— “Classic” devices with 16 KiB up to 64 KiB of program memory.mcu =at76c711,at43usb355.avr31— “Classic” devices with 128 KiB of program memory.mcu =atmega103,at43usb320.avr35— “Classic” devices with 16 KiB up to 64 KiB of program memory and with theMOVWinstruction.mcu =attiny167,attiny1634,atmega8u2,atmega16u2,atmega32u2,ata5505,ata6617c,ata664251,at90usb82,at90usb162.avr4— “Enhanced” devices with up to 8 KiB of program memory.mcu =atmega48,atmega48a,atmega48p,atmega48pa,atmega48pb,atmega8,atmega8a,atmega8hva,atmega88,atmega88a,atmega88p,atmega88pa,atmega88pb,atmega8515,atmega8535,ata5795,ata6285,ata6286,ata6289,ata6612c,at90pwm1,at90pwm2,at90pwm2b,at90pwm3,at90pwm3b,at90pwm81.avr5— “Enhanced” devices with 16 KiB up to 64 KiB of program memory.mcu =atmega16,atmega16a,atmega16hva,atmega16hva2,atmega16hvb,atmega16hvbrevb,atmega16m1,atmega16u4,atmega161,atmega162,atmega163,atmega164a,atmega164p,atmega164pa,atmega165,atmega165a,atmega165p,atmega165pa,atmega168,atmega168a,atmega168p,atmega168pa,atmega168pb,atmega169,atmega169a,atmega169p,atmega169pa,atmega32,atmega32a,atmega32c1,atmega32hvb,atmega32hvbrevb,atmega32m1,atmega32u4,atmega32u6,atmega323,atmega324a,atmega324p,atmega324pa,atmega324pb,atmega325,atmega325a,atmega325p,atmega325pa,atmega328,atmega328p,atmega328pb,atmega329,atmega329a,atmega329p,atmega329pa,atmega3250,atmega3250a,atmega3250p,atmega3250pa,atmega3290,atmega3290a,atmega3290p,atmega3290pa,atmega406,atmega64,atmega64a,atmega64c1,atmega64hve,atmega64hve2,atmega64m1,atmega64rfr2,atmega640,atmega644,atmega644a,atmega644p,atmega644pa,atmega644rfr2,atmega645,atmega645a,atmega645p,atmega649,atmega649a,atmega649p,atmega6450,atmega6450a,atmega6450p,atmega6490,atmega6490a,atmega6490p,ata5790,ata5790n,ata5791,ata6613c,ata6614q,ata5782,ata5831,ata8210,ata8510,ata5787,ata5835,ata5700m322,ata5702m322,at90pwm161,at90pwm216,at90pwm316,at90can32,at90can64,at90scr100,at90usb646,at90usb647,at94k,m3000.avr51— “Enhanced” devices with 128 KiB of program memory.mcu =atmega128,atmega128a,atmega128rfa1,atmega128rfr2,atmega1280,atmega1281,atmega1284,atmega1284p,atmega1284rfr2,at90can128,at90usb1286,at90usb1287.avr6— “Enhanced” devices with 3-byte PC, i.e. with more than 128 KiB of program memory.mcu =atmega256rfr2,atmega2560,atmega2561,atmega2564rfr2.avrxmega2— “XMEGA” devices with more than 8 KiB and up to 64 KiB of program memory.mcu =atxmega8e5,atxmega16a4,atxmega16a4u,atxmega16c4,atxmega16d4,atxmega16e5,atxmega32a4,atxmega32a4u,atxmega32c3,atxmega32c4,atxmega32d3,atxmega32d4,atxmega32e5,avr64da28,avr64da28s,avr64da32,avr64da32s,avr64da48,avr64da48s,avr64da64,avr64da64s,avr64db28,avr64db32,avr64db48,avr64db64,avr64dd14,avr64dd20,avr64dd28,avr64dd32,avr64du28,avr64du32,avr64ea28,avr64ea32,avr64ea48,avr64sd28,avr64sd32,avr64sd48.avrxmega3— “XMEGA” devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space.mcu =attiny202,attiny204,attiny212,attiny214,attiny402,attiny404,attiny406,attiny412,attiny414,attiny416,attiny416auto,attiny417,attiny424,attiny426,attiny427,attiny804,attiny806,attiny807,attiny814,attiny816,attiny817,attiny824,attiny826,attiny827,attiny1604,attiny1606,attiny1607,attiny1614,attiny1616,attiny1617,attiny1624,attiny1626,attiny1627,attiny3214,attiny3216,attiny3217,attiny3224,attiny3226,attiny3227,atmega808,atmega809,atmega1608,atmega1609,atmega3208,atmega3209,atmega4808,atmega4809,avr16dd14,avr16dd20,avr16dd28,avr16dd32,avr16du14,avr16du20,avr16du28,avr16du32,avr16ea28,avr16ea32,avr16ea48,avr16eb14,avr16eb20,avr16eb28,avr16eb32,avr32da28,avr32da28s,avr32da32,avr32da32s,avr32da48,avr32da48s,avr32db28,avr32db32,avr32db48,avr32dd14,avr32dd20,avr32dd28,avr32dd32,avr32du14,avr32du20,avr32du28,avr32du32,avr32ea28,avr32ea32,avr32ea48,avr32sd20,avr32sd28,avr32sd32.avrxmega4— “XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory.mcu =atxmega64a3,atxmega64a3u,atxmega64a4u,atxmega64b1,atxmega64b3,atxmega64c3,atxmega64d3,atxmega64d4,avr128da28,avr128da28s,avr128da32,avr128da32s,avr128da48,avr128da48s,avr128da64,avr128da64s,avr128db28,avr128db32,avr128db48,avr128db64.avrxmega5— “XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM.mcu =atxmega64a1,atxmega64a1u.avrxmega6— “XMEGA” devices with more than 128 KiB of program memory.mcu =atxmega128a3,atxmega128a3u,atxmega128b1,atxmega128b3,atxmega128c3,atxmega128d3,atxmega128d4,atxmega192a3,atxmega192a3u,atxmega192c3,atxmega192d3,atxmega256a3,atxmega256a3b,atxmega256a3bu,atxmega256a3u,atxmega256c3,atxmega256d3,atxmega384c3,atxmega384d3.avrxmega7— “XMEGA” devices with more than 128 KiB of program memory and more than 64 KiB of RAM.mcu =atxmega128a1,atxmega128a1u,atxmega128a4u.avrtiny— “Reduced Tiny” Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory.mcu =attiny4,attiny5,attiny9,attiny10,attiny102,attiny104,attiny20,attiny40.avr1— This ISA is implemented by the minimal AVR core and supported for assembler only.mcu =attiny11,attiny12,attiny15,attiny28,at90s1200.
-mabsdata— Assume that all data in static storage can be accessed byLDS/STSinstructions. This option has only an effect on reduced Tiny devices likeATtiny40. See also theabsdataAVR Variable Attributes variable attribute.-mcvt— Use a compact vector table. Some devices support a CVT with only four entries:0=Reset,1=NMI,2=Prio1 IRQ,3=Prio0 IRQs. This option will link startup code fromcrtmcu-cvt.oinstead of the usualcrtmcu.o. Apart from providing a compact vector table, the startup code will set bitCPUINT_CTRLA.CPUINT_CVTwhich enables the CVT on the device.
When you do not want the startup code to set CPUINT_CTRLA.CPUINT_CVT, then you can satisfy symbol __init_cvt so that the respective code is no more pulled in from libmcu.a. For example, you can link with -Wl,--defsym,__init_cvt=0.
The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).
-mdouble=bits,-mlong-double=bits— Set the size (in bits) of thedoubleorlong doubletype, respectively. Possible values for bits are32and64. Whether or not a specific value for bits is allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and the same applies for the default values of the options.-mgas-isr-prologues— Interrupt service routines (ISRs) may use the__gcc_isrpseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributesno_gccisrfunction attribute. This feature is activated per default if optimization is on (but not with-Og,@pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).-mint8— Assumeintto be 8-bit integer. This affects the sizes of all types: acharis 1 byte, anintis 1 byte, alongis 2 bytes, andlong longis 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size. 💀-mmain-is-OS_task— Do not save registers inmain. The effect is the same like attaching attribute AVR Function AttributesOS_tasktomain. It is activated per default if optimization is on.-mno-call-main— Don’t runmainby means of:
XCALL main
XJMP exitInstead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option the user asserts that main will not return.
This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain.
-mno-interrupts— the code it generates does not play nice with hardware interrupts, bestie. The upside? Smaller code size. That's the trade, periodt.-mrelax— tries to swap aCALL(or resp.JMP) instruction for the shorterRCALL(resp.RJMP) whenever that's actually applicable. Setting-mrelaxjust sneaks the--mlink-relaxoption onto the assembler's command line and the--relaxoption onto the linker's command line. That's literally all it does 💅
Jump relaxing gets done by the linker, because jump offsets aren't known before the code is located. So the assembler code the compiler generates is exactly the same — but the instructions in the executable may differ from the instructions in the assembler code. Iconic linker behavior, ngl.
You must turn relaxing on if linker stubs are needed — see the section on EIND and linker stubs below, fr fr.
-mrodata-in-ram,-mno-rodata-in-ram— puts the.rodatasections for read-only data in RAM resp. in program memory. For most devices there's no choice at all, so this option acts kinda like an assertion.
Since v14 and for the AVR64* and AVR128* devices, .rodata sits in flash memory by default — provided the required GNU Binutils support (PR31124, https://sourceware.org/PR31124) is available. In that case, -mrodata-in-ram can be used to go back to the old layout with .rodata in RAM. Retro vibes 🎀
-mtiny-stack— only changes the lower 8 bits of the stack pointer. That's it. Tiny stack, tiny changes ✨-mfract-convert-truncate— lets you use truncation instead of rounding towards zero for fractional fixed-point types. No cap.-nodevicelib— doesn't link against AVR-LibC's device-specific librarylibmcu.a.
Heads up bestie: since AVR-LibC v2.3, that library contains code that is essential for a program to function correctly. Specifically it contains parts of the startup code like:__init_sp(https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with the symbol__stack,__init_cvt(https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with-mcvt,__call_main(https://github.com/avrdudes/avr-libc/issues/1012) to callmainandexit, and__do_flmap_init(https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to the symbol__flmap. So like… read that twice 😭
-nodevicespecs— don't add-specs=device-specs/specs-mcuto the compiler driver's command line. The user takes responsibility for supplying the sub-processes — compiler proper, assembler and linker — with appropriate command line options. Which means you have to supply your own private device specs file via-specs=path-to-specs-file. And there's no more need for the option-mmcu=mcuanymore either.
This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path pointing to a directory containing a folder named device-specs, which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. Whew. Ancestor behavior 💀
-Waddr-space-convert— warns about conversions between address spaces when the resulting address space isn't contained in the incoming address space. Vibe check failed → warning 🦋-Wmisspelled-isr— warns if the ISR is misspelled, i.e. without the__vectorprefix. Enabled by default. Not me letting you typo your interrupt handler 🙈
✨ AVR Optimization Options ✨
Okay so these following options are pure optimization options. But bestie, be aware: -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from above are only almost optimization options, since there are rare occasions where their different code generation actually matters. Sneaky.
-maccumulate-args— accumulates outgoing function arguments and acquires/releases the needed stack space for outgoing function arguments once, in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards. See also the-fdefer-popOptimize Options,,optimization option.
Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments don't need to be removed from the stack after such a function call.
This option can lead to reduced code size for functions that make several calls to functions which get their arguments on the stack — like calls to printf-like functions 🍓
- *
-mbranch-cost=cost* — sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is0. -mcall-prologues— function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. Reuse queen 👑-mfuse-add,-mno-fuse-add, *-mfuse-add=level* — optimizes indirect memory accesses on reduced Tiny devices. The default useslevel=1for optimizations-Ogand-O1, andlevel=2for higher optimizations. Valid values for level are0,1and2. Slay.-mfuse-move,-mno-fuse-move, *-mfuse-move=level* — runs a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default useslevel=3for optimization-O1, andlevel=23for higher optimizations. Valid values for level are in the range0 ... 23, which is a3:2:2:2mixed radix value. Each digit controls some aspect of the optimization. Math girlie hours 🔥-mfuse-move2— runs a post combine optimization pass that tries to fuse move instructions.-mstrict-X— uses the address registerXin the way the hardware proposes. Which meansXis only used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register may be used the same way as Y or Z, which then gets emulated by additional instructions. For example, loading a value with X+const addressing where a small non-negative const < 64 goes to a register Rn is performed as:
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-msplit-bit-shift— splits multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for-O2and higher, including-Osbut excluding-Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by-mfuse-move. Situational awareness 💅-msplit-ldst— splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for-O2and higher.-muse-nonzero-bits— enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for-O2and higher. Zero bits, nonzero relevance ✨
🦋 EIND and Devices with More Than 128 Ki Bytes of Flash 🦋
Okay so pointers in the implementation are 16 bits wide. The address of a function or label is represented as a word address, so indirect jumps and calls can target any code address in the range of 64 Ki words.
To make indirect jump work on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as the most significant part of the target address whenever the EICALL or EIJMP instructions are used. Main character register, honestly.
Indirect jumps and calls on these devices are handled as follows by the compiler, and they are subject to some limitations:
- The compiler never sets
EIND. - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order to emulate an indirect call/jump by means of aRETinstruction. - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis not saved/restored in function or interrupt service routine prologue/epilogue. - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option
-mrelaxand the linker option--relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 😭 - The default linker script is arranged for code with
EIND = 0. If code is supposed to work for a setup withEIND != 0, a custom linker script has to be used in order to place the sections whose name starts with.trampolinesinto the segment whereEINDpoints to. - The startup code from libgcc never sets
EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC onEIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/). - It is legitimate for user-specific startup code to set up
EINDearly, for example by means of initialization code located in section.init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that setsEINDto the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}The __trampolines_start symbol is defined in the linker script. Periodt.
- Stubs are generated automatically by the linker if the following two conditions are met:
- The address of a label is taken by means of the
gsmodifier (short for generate stubs), like so:
- The address of a label is taken by means of the
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))- The final location of that label is in a code segment outside the segment where the stubs are located.
- The compiler emits such
gsmodifiers for code labels in the following situations:- Taking address of a function or code label.
- Computed goto.
- If prologue-save function is used, see the
-mcall-prologuescommand-line option. - Switch/case dispatch tables. If you do not want such dispatch tables, you can specify the
-fno-jump-tablescommand-line option.
- C and C++ constructors/destructors called during startup/shutdown.
- If the tools hit a
gs()modifier explained above. - Jumping to non-symbolic addresses like so is not supported, bestie:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script.
🌀 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers 🌀
Okay so some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as high part of the address: The X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing.
- The startup code initializes the
RAMPspecial function registers with zero. - If a AVR Named Address Spaces,named address space other than generic or
__flashis used, thenRAMPZis set as needed before the operation. - If the device supports RAM larger than 64 KiB and the compiler needs to change
RAMPZto accomplish an operation,RAMPZis reset to zero after the operation. - If the device comes with a specific
RAMPregister, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. - RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the
RAMPregisters, you must reset it to zero after the access.
✨ AVR Built-in Macros ✨
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
__AVR_ARCH__— Build-in macro that resolves to a decimal number that identifies the architecture and depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", mcu="avr25", mcu="avr3", mcu="avr31", mcu="avr35", mcu="avr4", mcu="avr5", mcu="avr51", mcu="avr6", respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", mcu="avrxmega2", mcu="avrxmega3", mcu="avrxmega4", mcu="avrxmega5", mcu="avrxmega6", mcu="avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.
__AVR_Device__— Setting-mmcu=devicedefines this built-in macro which reflects the device's name. For example,-mmcu=atmega8defines the built-in macro__AVR_ATmega8__,-mmcu=attiny261adefines__AVR_ATtiny261A__, etc.
The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_DEVICE_NAME__— Setting-mmcu=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined to"atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_CVT__— The code is being compiled with option-mcvtto use a compact vector table.__AVR_XMEGA__— The device / architecture belongs to the XMEGA family of devices.__AVR_HAVE_ADIW__— The device has theADIWandSBIWinstructions.__AVR_HAVE_ELPM__— The device has theELPMinstruction.__AVR_HAVE_ELPMX__— The device has theELPM Rn,ZandELPM Rn,Z+instructions.__AVR_HAVE_LPMX__— The device has theLPM Rn,ZandLPM Rn,Z+instructions.__AVR_HAVE_MOVW__— The device has theMOVWinstruction to perform 16-bit register-register moves.__AVR_HAVE_MUL__— The device has a hardware multiplier.__AVR_HAVE_JMP_CALL__— The device has theJMPandCALLinstructions. This is the case for devices with more than 8 KiB of program memory.__AVR_HAVE_EIJMP_EICALL__,__AVR_3_BYTE_PC__— The device has theEIJMPandEICALLinstructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.__AVR_2_BYTE_PC__— The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.__AVR_HAVE_8BIT_SP__,__AVR_HAVE_16BIT_SP__— The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by-mtiny-stack.__AVR_HAVE_SPH__,__AVR_SP8__— The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by-mmcu=and in the cases of-mmcu=avr2and-mmcu=avr25also by-msp8.__AVR_HAVE_RAMPD__,__AVR_HAVE_RAMPX__,__AVR_HAVE_RAMPY__,__AVR_HAVE_RAMPZ__— The device has theRAMPD,RAMPX,RAMPY,RAMPZspecial function register, respectively.__NO_INTERRUPTS__— This macro reflects the-mno-interruptscommand-line option.__AVR_ERRATA_SKIP__,__AVR_ERRATA_SKIP_JMP_CALL__— Some AVR devices (AT90S8515,ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions areSBRS,SBRC,SBIS,SBICandCPSE. The second macro is only defined if__AVR_HAVE_JMP_CALL__is also set.__AVR_ISA_RMW__— The device has Read-Modify-Write instructions (XCH,LAC,LASandLAT).__AVR_SFR_OFFSET__=offset— Instructions that can address I/O special function registers directly likeIN,OUT,SBI, etc. may use a different address as if addressed by an instruction to access RAM likeLDorSTS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.__AVR_SHORT_CALLS__— The-mshort-callscommand line option is set.__AVR_PM_BASE_ADDRESS__=addr— Some devices support reading from flash memory by means ofLD*instructions. The flash memory is seen in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put.rodatainto RAM. This is handled by the default linker description file, and is currently available foravrtinyandavrxmega3. Even more convenient, there is no need to use address spaces like__flashor features like attributeprogmemandpgm_read_*.__AVR_HAVE_FLMAP__— This macro is defined provided the following conditions are met:- The device has the
NVMCTRL_CTRLB.FLMAPbitfield. This applies to theAVR64*andAVR128*devices. - It's not known at assembler-time which emulation will be used.
- The device has the
This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).
__AVR_RODATA_IN_RAM__— This macro is undefined when the code is compiled for a core architecture.
When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.
__WITH_AVRLIBC__— The compiler is configured to be used together with AVR-LibC. See the--with-avrlibcconfigure option.__HAVE_SIGNAL_N__— The compiler supports thesignal(num)andinterrupt(num)AVR Function Attributes,,function attributes with an argumentnumthat specifies the number of the interrupt service routine.__HAVE_DOUBLE_MULTILIB__— Defined if-mdouble=acts as a multilib option.__HAVE_DOUBLE32__,__HAVE_DOUBLE64__— Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option-mdouble=.__DEFAULT_DOUBLE__— The size in bits ofdoubleif-mdouble=is not set. To test the layout ofdoublein a program, use the built-in macro__SIZEOF_DOUBLE__.__HAVE_LONG_DOUBLE32__,__HAVE_LONG_DOUBLE64__,__HAVE_LONG_DOUBLE_MULTILIB__,__DEFAULT_LONG_DOUBLE__— Same as above, but forlong doubleinstead ofdouble.__WITH_DOUBLE_COMPARISON__— Reflects the--with-double-comparison={tristate|bool|libf7}configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to2or3.__WITH_LIBF7_LIBGCC__,__WITH_LIBF7_MATH__,__WITH_LIBF7_MATH_SYMBOLS__— Reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr).
🛠️ AVR Internal Options 🛠️
The following options are used internally by the compiler and to communicate between device specs files and the compiler proper. You don't need to set these options by hand, in particular they are not optimization options. Using these options in the wrong way may lead to sub-optimal or wrong code. They are documented for completeness, and in order to get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files.
-mn-flash=num— Assume that the flash memory has a size ofnumtimes 64 KiB. This determines which__flashNaddress spaces are available.-mflmap— The device has theFLMAPbit field located in special function registerNVMCTRL_CTRLB.-mrmw— Assume that the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT.-mshort-calls— bestie, this one assumes thatRJMPandRCALLcan target the whole program memory. It's used for multilib generation and selection for the devices from architectureavrxmega3. 💅-mskip-bug— generates code without skips (CPSE,SBRS,SBRC,SBIS,SBIC) over 32-bit instructions.-msp8— treats the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero 🫶. This option is used by the compiler to select and build multilibs for architecturesavr2andavr25. Those architectures mix devices with and withoutSPH.
🖤 Blackfin Options 🖤
-mcpu=cpu[-sirevision]— tells GCC the name of the target Blackfin processor. Currently, cpu can be one of:bf512,bf514,bf516,bf518,bf522,bf523,bf524,bf525,bf526,bf527,bf531,bf532,bf533,bf534,bf536,bf537,bf538,bf539,bf542,bf544,bf547,bf548,bf549,bf542m,bf544m,bf547m,bf548m,bf549m,bf561,bf592.
The optional sirevision specifies the silicon revision of the target Blackfin processor 👑. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default. 😌
Heads up bestie — support forbf561is incomplete. Forbf561, only the preprocessor macro is defined. 💀
-msim— says the program will be run on the simulator, which causes the simulator BSP provided by libgloss to be linked in. This option has effect only for thebfin-elftoolchain. Certain other options, such as-mid-shared-libraryand-mfdpic, imply-msim. 🧚-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. little win, slay ✨-mspecld-anomaly— when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used,__WORKAROUND_SPECULATIVE_LOADSis defined.-mno-specld-anomaly— don't generate extra code to prevent speculative loads from occurring.-mcsync-anomaly— when enabled, the compiler ensures that the generated code does not containCSYNCorSSYNCinstructions too soon after conditional branches. If this option is used,__WORKAROUND_SPECULATIVE_SYNCSis defined.-mno-csync-anomaly— don't generate extra code to preventCSYNCorSSYNCinstructions from occurring too soon after a conditional branch.-mlow64k— when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. tiny program era 🌸-mno-low64k— assume that the program is arbitrarily large. This is the default.-mstack-check-l1— do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.-mid-shared-library— generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies-fPIC. With abfin-elftarget, this option implies-msim. 🦋-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used. This is the default.-mleaf-id-shared-library— generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. ⭐-mno-leaf-id-shared-library— do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns.-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.-msep-data— generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.-mno-sep-data— generate code that assumes that the data segment follows the text segment. This is the default.-mlong-calls,-mno-long-calls— tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction. 😭
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mfast-fp— link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. 🔥-minline-plt— enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without-mfdpic.-mmulticore— build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro__BFIN_MULTICORE. It can only be used with-mcpu=bf561[-sirevision]. 👑
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main.
If this option is not used, the single-core application programming model is used.
-mcorea— build a standalone application for Core A ofBF561when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro__BFIN_COREAis defined. This option can only be used in conjunction with-mmulticore.-mcoreb— build a standalone application for Core B ofBF561when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro__BFIN_COREBis defined. When this option is used,coreb_mainshould be used instead ofmain. This option can only be used in conjunction with-mmulticore.-msdram— build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro__BFIN_SDRAMis defined. The loader should initialize SDRAM before loading the application. 🍓-micplb— assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
🌈 C6X Options 🌈
-march=name— specifies the name of the target architecture 🎀. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are:c62x,c64x,c64x+,c67x,c67x+,c674x.-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default.-msim— choose startup files and linker script suitable for the simulator. 🧚-msdata=default— put small global and static data in the.neardatasection, which is pointed to by registerB14. Put small uninitialized global and static data in the.bsssection, which is adjacent to the.neardatasection. Put small read-only data into the.rodatasection. The corresponding sections used for large pieces of data are.fardata,.farand.const. 💖-msdata=all— put all data, not just small objects, into the sections reserved for small data, and use addressing relative to theB14register to access them. go big or go home ✨-msdata=none— make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the.fardatasection, and all uninitialized data in the.farsection. Put all constant data into the.constsection.
💜 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 arev3,v8andv10for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0.-mtune=architecture-type— tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for-march=architecture-type.-mmax-stack-frame=n— warn when the stack frame of a function exceeds n bytes. 🫶-metrax4,-metrax100— the options-metrax4and-metrax100are synonyms for-march=v3and-march=v8respectively.-mmul-bug-workaround,-mno-mul-bug-workaround— work around a bug in themulsandmuluinstructions for CPU models where it applies. This option is disabled by default. 😭 iconic of it to be off by default honestly-mpdebug— turns on the CRIS-specific verbose debug tea ☕ in the assembly output. It also has a side quest: it switches off the#NO_APPformatted-code indicator that gets handed to the assembler at the top of the assembly file.-mcc-init— no reusing condition-code results from whatever instruction came before, bestie. Compare and test instructions get emitted before condition codes are used, every single time. Fresh vibes only.-mno-side-effects— don't emit instructions with side effects in addressing modes other than post-increment. That's it, that's the whole rule 💅-mstack-align,-mno-stack-align,-mdata-align,-mno-data-align,-mconst-align,-mno-const-align— these bad boys (and theirno-twins) arrange — or eliminate the arrangement of, theno-versions being the eliminators — the stack frame, individual data, and constants so they're aligned for the maximum single data access size of whichever CPU model you picked. Default is to arrange for 32-bit alignment. ABI details like structure layout? Untouched by these. Not affected, not invited 🫶-m32-bit,-m16-bit,-m8-bit— basically the same energy as the stack-/data-/const-align options above, except these arrange for the stack frame, writable data and constants to all be 32-bit, 16-bit, or 8-bit aligned respectively. Default: 32-bit alignment. Slay.-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame are just… omitted 😭 and no return instructions or return sequences get generated in the code.
⚠️ Only use this one together with visual inspection of the compiled code, fr fr. No warnings, no errors — nothing gets generated when call-saved registers must be saved or when storage for local variables needs to be allocated. It's giving silent chaos. 💀
-melf— legacy no-op option. It's giving "still showing up to the party but doing absolutely nothing" 🎀-sim— arranges to link with input-output functions from a simulator library. Code, initialized data, and zero-initialized data all get allocated consecutively.-sim2— like-sim, but it passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Fancy addresses, no cap ✨
🌈 C-SKY Options 🌈
GCC supports these options when compiling for C-SKY V2 processors. Let's gooo 🦋
-march=arch— specify the C-SKY target architecture. Valid values for arch are:ck801,ck802,ck803,ck807, andck810. The default isck810. Iconic lineup tbh 👑-mcpu=cpu— specify the C-SKY target processor. Valid values for cpu are:ck801,ck801t,ck802,ck802t,ck802j,ck803,ck803h,ck803t,ck803ht,ck803f,ck803fh,ck803e,ck803eh,ck803et,ck803eht,ck803ef,ck803efh,ck803ft,ck803eft,ck803efht,ck803r1,ck803hr1,ck803tr1,ck803htr1,ck803fr1,ck803fhr1,ck803er1,ck803ehr1,ck803etr1,ck803ehtr1,ck803efr1,ck803efhr1,ck803ftr1,ck803eftr1,ck803efhtr1,ck803s,ck803st,ck803se,ck803sf,ck803sef,ck803seft,ck807e,ck807ef,ck807,ck807f,ck810e,ck810et,ck810ef,ck810eft,ck810,ck810v,ck810f,ck810t,ck810fv,ck810tv,ck810ft, andck810ftv. That's the whole roster, don't @ me 😮💨-mbig-endian,-EB,-mlittle-endian,-EL— select big- or little-endian code. The default is little-endian. Tiny endian rights 💜-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard.- Specifying
softcauses GCC to generate output containing library calls for floating-point operations. softfpallows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
- Specifying
The default depends on the specific target configuration.
🚨 Note that the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. Mixing them is not the vibe check you want. 💀
-mhard-float,-msoft-float— select hardware or software floating-point implementations. The default is soft float.-mdouble-float,-mno-double-float— when-mhard-floatis in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, enable generation of"frecipd","fsqrtd", and"fdivd"instructions. This is the default except when compiling for CK803.-mfpu=fpu— select the floating-point processor. This option can only be used with-mhard-float. Values for fpu arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_divd(-mdouble-float -mdivdu). Copy-paste those exactly, bestie, they're specific 👀-melrw,-mno-elrw— enable the extended"lrw"instruction. This option defaults to on for CK801 and off otherwise.-mistack,-mno-istack— enable interrupt stack instructions; the default is off. The-mistackoption is required to handle the"interrupt"and"isr"function attributes. So like… you literally need it for those 🫡-mmp— enable multiprocessor instructions; the default is off.-mcp— enable coprocessor instructions; the default is off.-mcache— enable coprocessor instructions; the default is off. (Yes, the text is the same as-mcp— that's what it says, and I'm not out here rewriting the manpage's lore 🙈)-msecurity— enable C-SKY security instructions; the default is off.-mtrust— enable C-SKY trust instructions; the default is off.-mdsp,-medsp,-mvdsp— enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off.-mdiv,-mno-div— generate divide instructions. Default is off.-msmart,-mno-smart— generate code for Smart Mode, using only registers numbered0-7to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off. Big brain energy 🧠✨-mhigh-registers,-mno-high-registers— generate code using the high registers numbered16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors.-manchor,-mno-anchor— generate code using global anchor symbol addresses.-mpushpop,-mno-pushpop— generate code using"push"and"pop"instructions. This option defaults to on. Push pop princess 👑-mmultiple-stld,-mstm,-mno-multiple-stld,-mno-stm— generate code using"stm"and"ldm"instructions. This option isn't supported on CK801 but is enabled by default on other processors.-mconstpool,-mno-constpool— create constant pools in the compiler instead of deferring it to the assembler. This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors.-mstack-size,-mno-stack-size— emit".stack_size"directives for each function in the assembly output. This option defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.-mbranch-cost=n— set the branch costs to roughlyninstructions. The default is1.-msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences.
⚠️ Heads up: using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. Cute option, chaotic consequences 😭
-msim— links the librarylibsemi.awhich is in compatible with simulator. Applicable to ELF compiler only. (That's the original's wording, word for word — I don't make the rules 🎀)
🌸 Cygwin and MinGW Options 🌸
These additional options are available for Microsoft Windows targets. Windows girlies, this one's for you 🪟💖
-mconsole— this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets.-mcrtdll=library— preprocess, compile or link with specified C RunTime DLL library. This option adjusts the predefined macros"__CRTDLL__","__MSVCRT__","_UCRT"and"__MSVCRT_VERSION__"for the specified CRT library, chooses the start file for the CRT library, and links with the CRT library.
Recognized CRT library names for the preprocessor are: "crtdll*", "msvcrt10*", "msvcrt20*", "msvcrt40*", "msvcr40*", "msvcrtd*", "msvcrt-os*", "msvcr70*", "msvcr71*", "msvcr80*", "msvcr90*", "msvcr100*", "msvcr110*", "msvcr120*" and "ucrt*".
If this option is not specified then the default MinGW import library "msvcrt" is used for linking and no other adjustment for the preprocessor is done.
MinGW import library "msvcrt" is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation.
MinGW import library "msvcrt-os" is for the Windows system CRT DLL library "msvcrt.dll" and in most cases is the default MinGW import library.
Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT.
🚨 This option is for experimental and testing purposes only. This option is available for MinGW targets.
-mdll— this option is available for Cygwin and MinGW targets. It specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point. It's giving main character library energy 👑-mnop-fun-dllimport— this option is available for Cygwin and MinGW targets. It specifies that the"dllimport"attribute should be ignored.-mthreads— this option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used. Threads, but make it MinGW 💅-municode— this option is available for MinGW-w64 targets. It causes the"UNICODE"preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. Unicode ate, no crumbs 🌈-mwin32— this option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.-mwindows— this option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.-fno-set-stack-executable— this option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there theUser32API, which is used to set executable privileges, isn't available. Kernel-mode problems require kernel-mode solutions 🫶-fwritable-relocated-rdata— this one's for MinGW and Cygwin targets only, bestie. It says: take the relocated-data chillin' in the read-only section and yeet it into the".data"section instead. Why? Because older runtimes are literally not supporting modification of".rdata"sections for pseudo-relocation, so this is a necessary workaround. 💅-mpe-aligned-commons— Cygwin and MinGW targets again ✨. This tells GCC to use the GNU extension to the PE file format that permits the correct alignment of COMMON variables when it's generating code. And plot twist: it's enabled by default if GCC detects that the target assembler found during configuration supports the feature. Lucky you 🍀.
See also under x86 Options for standard options (iykyk, go peek 👀).
🍎 Darwin Options 🍎
These options are defined for all architectures running the Darwin operating system, no cap.
FSF GCC on Darwin does not create "fat" object files — it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo. Teamwork fr fr 🫶
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. Slay, override ✨
The Darwin tools vary in their behavior when presented with an ISA mismatch, and honestly it's giving chaotic energy:
- The assembler,
as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in appc750object file. Periodt. 🚫 - The linker for shared libraries,
/usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put appc970object file in appc7400library). It said nope 💀 - The linker for executables,
ld, quietly gives the executable the most restrictive subtype of any of its input files. Sneaky but valid 🙈 -Fdir— add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by-Ioptions and are scanned in a left-to-right order. Okay so let me explain the whole framework lore, because it's a lot:- A framework directory is a directory with frameworks in it.
- A framework is a directory with a
Headersand/orPrivateHeadersdirectory 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
Headersbeing searched first. - A subframework is a framework directory that is in a framework's
Frameworksdirectory. - Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header.
- Two subframeworks are siblings if they occur in the same framework.
- A subframework should not have the same name as a framework; a warning is issued if this is violated ⚠️.
- Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. Manifesting it 🕯️✨
- The standard frameworks can be found in
/System/Library/Frameworksand/Library/Frameworks. - An example include looks like
#include <Framework/header.h>, whereFrameworkdenotes the name of the framework andheader.his found in thePrivateHeadersorHeadersdirectory.
-iframeworkdir— like-Fexcept the directory is a treated as a system directory. The main difference between this-iframeworkand-Fis that with-iframeworkthe compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages. C family only, bestie 💅-gused— emit debugging information for symbols that are used. For stabs debugging format, this enables-feliminate-unused-debug-symbols. This is by default ON ✅.-gfull— emit debugging information for all symbols and types. No filter, no skips, full send 🔥.-fconstant-cfstrings— the-fconstant-cfstringsis an alias for-mconstant-cfstrings. Same girl, different name 🎀.-mconstant-cfstrings— when the NeXT runtime is being used (the default on these systems), override any-fconstant-string-classsetting and cause"@"...""literals to be laid out as constant CoreFoundation strings.-mmacosx-version-min=version— the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include12,10.12, and10.5.8. If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible. Flexible icon 💫-mkernel— enable kernel development mode. The-mkerneloption sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weakand-fno-rttiwhere applicable. This mode also sets-mno-altivec,-msoft-float,-fno-builtinand-mlong-branchfor PowerPC targets. It's a whole bundle, like getting the full set in the group chat 👑-mone-byte-bool— override the defaults forboolso thatsizeof(bool)==1. By defaultsizeof(bool)is4when compiling for Darwin/PowerPC and1when compiling for Darwin/x86, so this option has no effect on x86. Wild, I know 😭
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. Not a vibe to do casually, ok? 💀
-mfix-and-continue,-ffix-and-continue,-findirect-data— generate code suitable for fast turnaround development, such as to allow GDB to dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare provided for backwards compatibility. Legacy icons but still valid 🫶-all_load— loads all members of static archive libraries. Seeman ld(1)for more information.-arch_errors_fatal— cause the errors having to do with files that have the wrong architecture to be fatal. Dramatic and correct 💀-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. Seeman ld(1)for more information.-bundle_loaderexecutable — this option specifies the executable that will load the build output file being linked. Seeman ld(1)for more information.-dynamiclib— when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand. Glow up from exe to lib ✨-force_cpusubtype_ALL— this causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.-nodefaultrpaths— do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds-nodefaultrpathsto the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. So like, you can turn it off, but you probably shouldn't, bestie 🥴
Okay and now the big slay list of linker pass-throughs 👇 these options are passed to the Darwin linker, and the Darwin linker man page describes them in detail. Every single one, verbatim, no edits:
-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(yes, some of them show up twice in there — that's literally how the page has it, I'm not touching it 🙈)
💜 DEC Alpha Options 💜
These -m options are defined for the DEC Alpha implementations, and we're giving retro-tech main character energy rn ⭐
-mno-soft-float,-msoft-float— use (do not use) the hardware floating-point instructions for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers. Yeah, they gotta have them even if they're not using them. Irony 💀-mfp-reg,-mno-fp-regs— generate code that uses (does not use) the floating-point register set.-mno-fp-regsimplies-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$0instead of$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-regsmust also be compiled with that option. Matching set or it breaks, no exceptions 💅 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. Efficient queen behavior 👑-mieee— so the Alpha architecture ships floating-point hardware that's hyper-optimized for speed, and it's mostly IEEE-compliant, but bestie... "mostly" is doing a LOT of heavy lifting there 😭. For full compliance you need software to come help out. This option makes GCC generate code that's fully IEEE-compliant except the inexact-flag isn't maintained (more on that below). When it's turned on, the preprocessor macro_IEEE_FPgets defined during compilation. The resulting code is less efficient, yeah, but it can correctly handle denormalized numbers and those feral exceptional IEEE values like not-a-number and plus/minus infinity 💅. Other Alpha compilers call this option-ieee_with_no_inexact.-mieee-with-inexact— this is like-mieeebut the generated code also maintains the IEEE inexact-flag. Turning this on makes the generated code implement fully-compliant IEEE math, no cap ✨. On top of_IEEE_FP, it also defines_IEEE_FP_EXACTas a preprocessor macro. Heads up though: on some Alpha implementations the resulting code may run significantly slower than the code generated by default 🔥. And since there's very little code out there that actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option-ieee_with_inexact.-mfp-trap-mode=trap-mode— controls which 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 enabled are the ones that can't be disabled in software (e.g., division by zero trap).u— on top of the traps enabled byn, underflow traps are enabled as well.su— likeu, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for the deets).sui— likesu, 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 the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies thefpcr,dcorresponds to round towards plus infinity 🌀.
-mtrap-precision=trap-precision— okay so in the Alpha architecture, floating-point traps are imprecise 😭. That means without software assistance it's literally impossible to recover from a float trap, and program execution normally needs to be terminated. GCC can generate code that helps operating system trap handlers figure out the exact location that caused a floating-point trap. Depending on what an application needs, you can select different levels of precision:p— program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.f— function precision. The trap handler can determine the function that caused a floating-point exception.i— instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant— marks the generated code as IEEE conformant ✨. > Bestie, do NOT use this option unless you also specify-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file. That's it, that's the whole vibe.-mbuild-constants— normally GCC looks at a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). You typically use this option to build a shared library dynamic loader 🫶 — itself being a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment.-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— these 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 the-mcpu=option, or that of the CPU on which GCC was built if none is specified 👑-msafe-bwa,-mno-safe-bwa— indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.-msafe-partial,-mno-safe-partial— indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses 💀 — including piecemeal accesses to unaligned data, as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that do not respectively start and end on an aligned 64-bit data boundary.-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, and using those macros doesn't allow optimal instruction scheduling 😔. GNU binutils as of version2.12supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.-msmall-data,-mlarge-data— when-mexplicit-relocsis in effect, static data is accessed via gp-relative relocations. When-msmall-datais used, objects 8 bytes long or smaller are placed in a small data area (the.sdataand.sbsssections) and are accessed via 16-bit relocations off of the$gpregister. This limits the size of the small data area to 64KB, but lets the variables be directly accessed via a single instruction ⚡. The default is-mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must usemallocormmapto allocate the data in the heap instead of in the program's data segment. When generating code for shared libraries,-fpicimplies-msmall-dataand-fPICimplies-mlarge-data🎀-msmall-text,-mlarge-text— when-msmall-textis used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When-msmall-datais used, the compiler can assume that all local symbols share the same$gpvalue, and thus reduce the number of instructions required for a function call from 4 to 1 🔥. The default is-mlarge-text.-mcpu=cpu_type— sets 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 chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built 💅
Supported values for cpu_type are:
ev4,ev45,21064— schedules as an EV4 and has no instruction set extensions.ev5,21164— schedules as an EV5 and has no instruction set extensions.ev56,21164a— schedules as an EV5 and supports the BWX extension.pca56,21164pc,21164PC— schedules as an EV5 and supports the BWX and MAX extensions.ev6,21264— schedules as an EV6 and supports the BWX, FIX, and MAX extensions.ev67,21264a— schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor 🙈
-mtune=cpu_type— sets only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed. Native toolchains also support the valuenative, which selects the best architecture option for the host processor.-mtune=nativehas no effect if GCC does not recognize the processor.-mmemory-latency=time— sets the latency the scheduler should assume for typical memory references as seen by the application. This number is super 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 thatL3is only valid for EV5 👑
✨ eBPF Options ✨
-mframe-limit=bytes— specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted 🐧-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default 💖-mjmpext,-mno-jmpext— turns generation of the extra conditional-branch instructions on or off. It's already on for CPU v2 and up ✨-mjmp32,-mno-jmp32— turns generation of 32-bit jump instructions on or off. On for CPU v3 and above 💅-malu32,-mno-alu32— turns generation of 32-bit ALU instructions on or off. On for CPU v3 and above 🎀-mv3-atomics,-mno-v3-atomics— turns the general atomic operation instructions (the ones that showed up in CPU v3) on or off. On for CPU v3 and above ✨-mbswap,-mno-bswap— turns byte swap instructions on or off. On for CPU v4 and above 🦋-msdiv,-mno-sdiv— turns signed division and modulus instructions on or off. On for CPU v4 and above 🌸-msmov,-mno-smov— turns sign-extending move and memory load instructions on or off. On for CPU v4 and above 💖-mcpu=version— tells the compiler which version of the eBPF ISA to target. Heads up bestie: newer versions might not be supported by every kernel 😭 The default isv4.
The supported values for version are:
v1— the first stable eBPF ISA, no special features or extensions, just vibes.v2— supports the jump extensions, like-mjmpext.v3— everything from v2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32 - general atomic operations, as in
-mv3-atomics
- 32-bit jump operations, as in
v4— everything from v3, plus:- byte swap instructions, as in
-mbswap - signed division and modulus instructions, as in
-msdiv - sign-extending move and memory load instructions, as in
-msmov
- byte swap instructions, as in
-mco-re— enables BPF Compile Once - Run Everywhere (CO-RE) support. It requires-gbtf, and-gbtfimplies it 🔥-mno-co-re— disables BPF Compile Once - Run Everywhere (CO-RE) support. FYI it's enabled by default when you're generating BTF debug info for the BPF target 💅-mxbpf— generates code for an expanded version of BPF that relaxes some of the restrictions the BPF architecture imposes:- saves and restores callee-saved registers at function entry and exit, respectively ✨
-masm=dialect— outputs assembly instructions using the eBPF dialect you picked. The default ispseudoc.
Supported values for dialect are:
normal— outputs normal assembly dialect.pseudoc— outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes— sets a size threshold in bytes at or below whichmemmove,memcpyandmemsetare always expanded inline. Operations with sizes bigger than this threshold would have to be implemented with a library call instead of being expanded inline — but since BPF doesn't allow libcalls, going over this threshold makes the compile blow up at compile time 💀 The default is1024bytes.
🌈 FR30 Options 🌈
These options are defined specifically for the FR30 port, iykyk.
-msmall-model— use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. No cap, that's the tradeoff 👑-mno-lsim— assumes runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line ✨
🦋 FT32 Options 🦋
These options are defined specifically for the FT32 port, bestie.
-msim— says the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions are needed 💀-mlra— does literally nothing. It's preserved for backward compatibility 🙈-mnodiv— don't usedivandmodinstructions.-mft32b— enable use of the extended instructions of the FT32B processor ⭐-mcompress— compress all code using the Ft32B code compression scheme 🎀-mnopm— don't generate code that reads program memory.
💜 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— don't try to dynamically allocate condition code registers, only use"icc0"and"fcc0"🫶-mdword— change the ABI to use double word insns.-mno-dword— don't use double word instructions.-mdouble— use floating-point double instructions.-mno-double— don't use floating-point double instructions.-mmedia— use media instructions.-mno-media— don't use media instructions.-mmuladd— use multiply and add/subtract instructions.-mno-muladd— don't use multiply and add/subtract instructions.-mfdpic— selects the FDPIC ABI, which uses function descriptors to represent pointers to functions. With no PIC/PIE-related options, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with-fPICor-fPIE, GOT offsets are computed with 32 bits. With abfin-elftarget, this option implies-msim🔥-minline-plt— enables inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without-mfdpic. It's on by default if you're optimizing for speed and compiling for shared libraries (that's-fPICor-fpic), or when an optimization option like-O3or above is on the command line ✨-mTLS— assume a large TLS segment when generating thread-local code.-mtls— don't 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's known to be in read-only sections. It's on by default, except for-fpicor-fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4 😭 With-fPICor-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 isn't,-mno-gprel-rocan 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-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly, fr fr 🫶-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 lets the functions 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— don't pack VLIW instructions.-mno-eflags— don't mark ABI switches ine_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— don't 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's enabled by default ✨-mno-optimize-membar— this switch disables the automatic removal of redundant"membar"instructions from the generated code.-mtomcat-stats— causegasto print out tomcat statistics 🐱-mcpu=cpu— picks the processor type your code is gonna be generated for. Your options arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple. choose wisely bestie 💅
✨ GNU/Linux Options ✨
These -m options are defined for GNU/Linux targets, so if you're on Linux-land, this is your aisle:
-mglibc— use the GNU C library. This is the default except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets. the OG libc, periodt 🎀-muclibc— use uClibc C library. This is the default on*-*-linux-*uclibc*targets.-mmusl— use the musl C library. This is the default on*-*-linux-*musl*targets. tiny and adorable, ngl 🦋-mbionic— use Bionic C library. This is the default on*-*-linux-*android*targets.-mandroid— compile code compatible with the Android platform. This is the default on*-*-linux-*android*targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined. she does it ALL 👑
-tno-android-cc— disable the compilation effects of-mandroid, i.e., do not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— disable the linking effects of-mandroid, i.e., pass standard Linux linking options to the linker.
🌸 H8/300 Options 🌸
These -m options are defined for the H8/300 implementations:
-mrelax— shorten some address references at link time, when possible; uses the linker option-relax. little glow-up for your addresses ✨-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-mhor-ms. no solo runs, she needs a buddy 🫶-ms2600— generate code for the H8S/2600. This switch must be used with-ms.-mexr— extended registers are stored on stack before execution of function with monitor attribute. Default option is-mexr. This option is valid only for H8S targets.-mno-exr— extended registers are not stored on stack before execution of function with monitor attribute. Default option is-mno-exr. This option is valid only for H8S targets.
ok wait, both of those say "Default option is" — that's literally what the page says, don't come for me 😭 it's giving "pick one" energy.
-mint32— makeintdata 32 bits by default.-malign-300— on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries.-malign-300causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300. she said "nope, we're doing 2-byte now" 💀
🔥 HPPA Options 🔥
These -m options are defined for the HPPA family of computers. vintage icon behavior, honestly 🎀
-march=architecture-type— generate code for the specified architecture. The choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. Refer to/usr/lib/sched.modelson an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around. one-way street bestie 🚦-mpa-risc-1-0,-mpa-risc-1-1,-mpa-risc-2-0— synonyms for-march=1.0,-march=1.1, and-march=2.0respectively. same fit, different label 👑-matomic-libcalls— generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations provided they are implemented as a single access.
The PA-RISC architecture does not support any atomic operations in hardware except for the ldcw instruction. Thus, all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.
This option generates __atomic_exchange calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets. atomic but make it ✨libraried✨
-mbig-switch— does nothing. Preserved for backward compatibility. she's just here for the vibes, an absolute decoration 💀💅-mcaller-copies— the caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
careful bestie, this one's a little spicy 🌶️ not compatible with the default 32-bit runtime — noted.
-mcoherent-ldcw— useldcw/ldcdcoherent cache-control hint.-mdisable-fpregs— disable floating-point registers. Equivalent to-msoft-float.-mdisable-indexing— prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.-mfast-indirect-calls— generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested functions. sorry babe, it's giving conditional love 😭
-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. hands off, those are TAKEN 🫶-mgas— enable the use of assembler directives only GAS understands.-mgnu-ld— use options specific to GNU ld. This passes-sharedto ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mhp-ld— use options specific to HP ld. This passes-bto ld when building a shared library and passes+Accept TypeMismatchto ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mlinker-opt— enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. trading debug for speed, a bold move 😭-mlong-calls— generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. she takes the scenic route 🚗💨
-mlong-load-store— generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the+koption to the HP compilers. exactly three, she counted ⭐-mjump-in-delay— this option is ignored and provided for compatibility purposes only. ghost option, she's not even here 👻-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.
-mordered— assume memory references are ordered and barriers are not needed.-mportable-runtime— use the portable calling conventions proposed by HP for ELF systems.-mschedule=cpu-type— schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are7007100,7100LC,7200,7300and8000. Refer to/usr/lib/sched.modelson an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is8000. bestie's on a schedule fr 📅-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.-msoft-float— generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. all-or-nothing energy, no half measures 💅
-msoft-mult— use software integer multiplication.
This disables the use of the xmpyu instruction. bye bye hardware mult, it's software hours now 😭
-munix=unix-std— okay bestie, this one generates compiler predefines and picks a startfile for the UNIX standard you name. The choices for unix-std are93,95, and98. Availability tea:93is supported on all HP-UX versions,95is available on HP-UX 10.10 and later, and98is available on HP-UX 11.11 and later. The default values are93for HP-UX 10.00,95for HP-UX 10.10 through to 11.00, and98for HP-UX 11.11 and later.-munix=93provides the same predefines as GCC 3.3 and 3.4.-munix=95provides additional predefines for"XOPEN_UNIX"and"_XOPEN_SOURCE_EXTENDED", and the startfileunix95.o.-munix=98provides additional predefines for"_XOPEN_UNIX","_XOPEN_SOURCE_EXTENDED","_INCLUDE__STDC_A1_SOURCE"and"_INCLUDE_XOPEN_SOURCE_500", and the startfileunix98.o.
⚠️ Important note, and I mean it: 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— suppresses the generation of link options to searchlibdld.slwhen the-staticoption is specified on HP-UX 10 and later.-static— so the HP-UX implementation ofsetlocalein libc has a dependency onlibdld.sl, and there isn't an archive version oflibdld.sl. Thus, when the-staticoption 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 withlibdld.slwhen the-staticoption is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The-nolibdldoption can be used to prevent the GCC driver from adding these link options.-threads— adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.
🔥 IA-64 Options 🔥
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian— generate code for a big-endian target. This is the default for HP-UX.-mlittle-endian— generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.-mgnu-as,-mno-gnu-as— generate (or don't) code for the GNU assembler. This is the default.-mgnu-ld,-mno-gnu-ld— generate (or don't) code for the GNU linker. This is the default.-mno-pic— generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.-mvolatile-asm-stop,-mno-volatile-asm-stop— generate (or don't) a stop bit immediately before and after volatile asm statements.-mregister-names,-mno-register-names— generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable.-mno-sdata,-msdata— disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.-mconstant-gp— generate code that uses a single constant global pointer value. This is useful when compiling kernel code.-mauto-pic— generate code that is self-relocatable. This implies-mconstant-gp. This is useful when compiling firmware code.-minline-float-divide-min-latency— generate code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generate code for inline divides of floating-point values using the maximum throughput algorithm.-mno-inline-float-divide— do not generate inline code for divides of floating-point values.-minline-int-divide-min-latency— generate code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generate code for inline divides of integer values using the maximum throughput algorithm.-mno-inline-int-divide— do not generate inline code for divides of integer values.-minline-sqrt-min-latency— generate code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generate code for inline square roots using the maximum throughput algorithm.-mno-inline-sqrt— do not generate inline code forsqrt.-mfused-madd,-mno-fused-madd— do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.-mno-dwarf2-asm,-mdwarf2-asm— don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.-mearly-stop-bits,-mno-early-stop-bits— allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.-mtls-size=tls-size— specify bit size of immediate TLS offsets. Valid values are14,22, and64.-mtune=cpu-type— tune the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley.-milp32,-mlp64— generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.-mno-sched-br-data-spec,-msched-br-data-spec— (dis/en)able data speculative scheduling before reload. This results in generation ofld.ainstructions and the corresponding check instructions (ld.c/chk.a). The default setting is disabled.-msched-ar-data-spec,-mno-sched-ar-data-spec— (en/dis)able data speculative scheduling after reload. This results in generation ofld.ainstructions and the corresponding check instructions (ld.c/chk.a). The default setting is enabled.-mno-sched-control-spec,-msched-control-spec— (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of theld.sinstructions and the corresponding check instructionschk.s. The default setting is disabled.-msched-br-in-data-spec,-mno-sched-br-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with-msched-br-data-specenabled. The default setting is enabled.-msched-ar-in-data-spec,-mno-sched-ar-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with-msched-ar-data-specenabled. The default setting is enabled.-msched-in-control-spec,-mno-sched-in-control-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with-msched-control-specenabled. The default setting is enabled.-mno-sched-prefer-non-data-spec-insns,-msched-prefer-non-data-spec-insns— if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled.-mno-sched-prefer-non-control-spec-insns,-msched-prefer-non-control-spec-insns— if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled.-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.-msched-spec-ldc— use a simple data speculation check. This option is on by default.-msched-control-spec-ldc— use a simple check for control speculation. This option is on by default.-msched-stop-bits-after-every-cycle— place a stop bit after every cycle when scheduling. This option is on by default.-msched-fp-mem-deps-zero-cost— assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.-msel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. This flag is disabled by default.-msched-max-memory-insns=max-insns— limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is1.-msched-max-memory-insns-hard-limit— makes the limit specified bymsched-max-memory-insnsa hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is"soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.
🧚 LM32 Options 🧚
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled— enable barrel-shift instructions.-mdivide-enabled— enable divide and modulus instructions.-mmultiply-enabled— enable multiply instructions.-msign-extend-enabled— enable sign extend instructions.-muser-enabled— enable user-defined instructions.
🦋 LoongArch Options 🦋
These command-line options are defined for LoongArch targets:
-march=arch-type— bestie, this tells GCC which machine type to generate instructions for. Heads up tho:-march=arch-typelets GCC spit out code that straight up will not run at all on processors other than the one you named. It's giving exclusive 💅
The choices for arch-type are:
native— the local processor type, detected by the native compiler.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464-based processor with LSX, LASX.la664— LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.la64v1.0— LoongArch64 ISA version 1.0.la64v1.1— LoongArch64 ISA version 1.1.
More info about LoongArch ISA versions lives at <https://github.com/loongson/la-toolchain-conventions>. iykyk 🦋
-mtune=tune-type— optimizes the generated code for the given processor target. The choices for tune-type are:native— local processor type detected by the native compiler.generic— generic LoongArch processor.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464 core.la664— LoongArch LA664 core.
-mabi=base-abi-type— generates code for the specified calling convention. base-abi-type can be one of:lp64d— uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits. 💖lp64f— uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64s— uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.
-mfpu=fpu-type— generates code for the specified FPU type, which can be one of:64— allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.32— allow the use of hardware floating-point instructions for 32-bit operations.none,0— prevent the use of hardware floating-point instructions. 🚫
-msimd=simd-type— enables generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:lasx— enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).lsx— enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).none— no LoongArch SIMD instruction may be generated.
-msoft-float— forces-mfpu=noneand prevents the use of floating-point registers for parameter passing. ⚠️ This option may change the target ABI.-msingle-float— forces-mfpu=32and allows the use of 32-bit floating-point registers for parameter passing. This option may change the target ABI too, just so you know.-mdouble-float— forces-mfpu=64and allows the use of 32/64-bit floating-point registers for parameter passing. Yep, this one may change the target ABI as well.-mlasx,-mno-lasx,-mlsx,-mno-lsx— these incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. Enabling LASX withmlasxautomatically enables LSX, and diabling LSX withmno-lsxautomatically disables LASX. These are driver-only options: they act upon the finalmsimdconfiguration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalizedmsimdoption that gets passed to the compiler proper. Order matters, bestie — no cap. ✨-mbranch-cost=n— sets the cost of branches to roughly n instructions.-maddr-reg-reg-cost=n— sets the cost ofADDRESS_REG_REGto the value calculated by n.-mcheck-zero-division,-mno-check-zero-divison— trap (do not trap) on integer division by zero. The default is-mcheck-zero-divisionfor-O0or-Og, and-mno-check-zero-divisionfor other optimization levels. 👑-mcond-move-int,-mno-cond-move-int— conditional moves for integral data in general-purpose registers are enabled (disabled). The default is-mcond-move-int.-mcond-move-float,-mno-cond-move-float— conditional moves for floating-point registers are enabled (disabled). The default is-mcond-move-float.-mmemcpy,-mno-memcpy— force (do not force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which lets GCC inline most constant-sized copies. Setting the optimization level to-Osalso forces the use ofmemcpy, but-mno-memcpymay override this behavior if explicitly specified — regardless of the order these options appear on the command line. Sneaky little exception, love that for us 💅-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture spec. The default is-mno-strict-align.-G num— puts global and static data smaller than num bytes into a small data section. The default value is0.-mmax-inline-memcpy-size=n— inlines all block moves (such as calls tomemcpyor structure copies) less than or equal to n bytes. The default value of n is1024.-mcmodel=code-model— sets the code model to one of:tiny-static— (Not implemented yet) 🙈tiny— (Not implemented yet)normal— the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space.medium— the text segment and data segment must be within 2GB addressing space.large— (Not implemented yet)extreme— this mode does not limit the size of the code segment and data segment. The-mcmodel=extremeoption is incompatible with-fpltand/or-mexplicit-relocs=none. ⚠️
The default code model is normal.
-mexplicit-relocs=style— sets when to use assembler relocation operators when dealing with symbolic addresses. The alternative is using assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With-mexplicit-relocs=nonethe assembler macros are always used; with-mexplicit-relocs=alwaysthe assembler relocation operators are always used; with-mexplicit-relocs=autothe compiler uses relocation operators where linker relaxation is impossible (to improve code quality) and macros everywhere else. The default value is determined by the assembler capability detected during GCC build-time and the setting of-mrelax:-mexplicit-relocs=noneif the assembler doesn't support relocation operators at all,-mexplicit-relocs=alwaysif the assembler supports relocation operators but-mrelaxisn't enabled, and-mexplicit-relocs=autoif the assembler supports relocation operators and-mrelaxis enabled. 🧚-mexplicit-relocs— an alias of-mexplicit-relocs=always, kept around for backward compatibility.-mno-explicit-relocs— an alias of-mexplicit-relocs=nonefor backward compatibility. 🫶-mdirect-extern-access,-mno-direct-extern-access— do not use or use GOT to access external symbols. The default is-mno-direct-extern-access: GOT is used for external symbols with default visibility, but not used for other external symbols.
With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link is performed — like firmwares, OS kernels, executables linked with -static or -static-pie. And just so we're clear: -mdirect-extern-access is not compatible with -fPIC or -fpic. 💀
-mrelax,-mno-relax— take (do not take) advantage of linker relaxations. If-mpass-mrelax-to-asis enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mrelaxif the assembler supports both the-mrelaxoption and conditional branch relaxation (this one's required — otherwise the.aligndirectives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow 😭), and-mno-relaxotherwise.-mpass-mrelax-to-as,-mno-pass-mrelax-to-as— pass (do not pass) the-mrelaxor-mno-relaxoption to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mpass-mrelax-to-asif the assembler supports the-mrelaxoption,-mno-pass-mrelax-to-asotherwise. This option is mostly useful for debugging, or for interoperation with assemblers different from the build-time one. 🔧-mrecip— this one enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. This option is off by default. Before you can use this option, you must sure the target CPU supportsfrecipeandfrsqrteinstructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of1.0equals0.99999994). Bestie, read that twice — precision go down. 🙈-mrecip=opt— controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.rsqrt— enable the approximation for scalar reciprocal square root. 🍓
vec-rsqrt— turns on the approximation for vectorized reciprocal square root 💫
So, like, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root.
-mfrecipe,-mno-frecipe— use (or don’t use)frecipe.{s/d}andfrsqrte.{s/d}instructions. When you build with-march=la664, it’s enabled by default. The default is-mno-frecipe.-mdiv32,-mno-div32— use (or don’t use)div.w[u]andmod.w[u]instructions with input not sign-extended. When you build with-march=la664, it’s enabled by default. The default is-mno-div32.-mlam-bh,-mno-lam-bh— use (or don’t use)am{swap/add}[_db].{b/h}instructions. When you build with-march=la664, it’s enabled by default. The default is-mno-lam-bh.-mlamcas,-mno-lamcas— use (or don’t use)amcas[_db].{b/h/w/d}instructions. When you build with-march=la664, it’s enabled by default. The default is-mno-lamcas.-mld-seq-sa,-mno-ld-seq-sa— whether a same-address load-load barrier (dbar 0x700) is needed. When you build with-march=la664, it’s enabled by default. The default is-mno-ld-seq-sa; the load-load barrier is needed.-mtls-dialect=opt — controls which TLS dialect may be used for general dynamic and local dynamic TLS models.trad— Use traditional TLS. This is the default.desc— Use TLS descriptors.
-mannotate-tablejump,-mno-annotate-tablejump— create an annotation section.discard.tablejump_annotateto correlate thejirlinstruction and the jump table when a jump table is used to optimize theswitchstatement. Some external tools, for exampleobjtoolof the Linux kernel building system, need the annotation to analyze the control flow. The default is-mno-annotate-tablejump.--param loongarch-vect-unroll-limit=n — the vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value issix.
🌸 M32C Options 🌸
-mcpu=name — select the CPU for which code is generated. name may be one ofr8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series.-msim— specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O.
🚨 Bestie, 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. Periodt.
-memregs=number — specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC’s ability to fit the code into available registers, and the performance penalty of using memory instead of registers.
💀 Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC’s default runtime libraries.
🦋 M32R/D Options 🦋
These -m options are defined for Renesas M32R/D architectures:
-m32r2— generate code for the M32R/2.-m32rx— generate code for the M32R/X.-m32r— generate code for the M32R. This is the default.-mmodel=small— assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with theld24instruction), and assume all subroutines are reachable with theblinstruction. This is the default. The addressability of a particular object can be set with themodelattribute.-mmodel=medium— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume all subroutines are reachable with theblinstruction.-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume subroutines may not be reachable with theblinstruction (the compiler generates the much slowerseth/add3/jlinstruction sequence).-msdata=none— disable use of the small data area. Variables are put into one of.data,.bss, or.rodata(unless thesectionattribute has been specified). This is the default. The small data area consists of sections.sdataand.sbss. Objects may be explicitly put in the small data area with thesectionattribute 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.-Gnum — 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 is8. The-msdataoption must be set to one ofsdataorusefor this option to have any effect. All modules should be compiled with the same-Gnum value. Compiling with different values of num may or may not work; if it doesn’t the linker gives an error message---incorrect code is not generated.-mdebug— makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.-malign-loops— align all loops to a 32-byte boundary.-mno-align-loops— do not enforce a 32-byte alignment for loops. This is the default.-missue-rate=number — issue number instructions per cycle. number can only be1or2.-mbranch-cost=number — number can only be1or2. If it is1then branches are preferred over conditional code, if it is2, then the opposite applies.-mflush-trap=number — specifies the trap number to use to flush the cache. The default is12. Valid numbers are between0and15inclusive.-mno-flush-trap— specifies that the cache cannot be flushed by using a trap.-mflush-func=name — specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available.-mno-flush-func— indicates that there is no OS function for flushing the cache.
🔥 M680x0 Options 🔥
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
-march=arch — generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale’s ISA classification and the permissible values are:isaa,isaaplus,isabandisac. GCC defines a macro__mcfarch__whenever it is generating code for a ColdFire target. The arch in this macro is one of the-marcharguments given above. When used together,-marchand-mtuneselect code that runs on a family of similar processors but that is optimized for a particular microarchitecture.-mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485-mcpu= cpu overrides -march= arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above.
-mtune=tune — tune the code for a particular microarchitecture within the constraints set by-marchand-mcpu. The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e. You can also use-mtune=68020-40for code that needs to run relatively well on68020,68030and68040targets.-mtune=68020-60is similar but includes68060targets as well. These two options select the same tuning decisions as-m68020-40and-m68020-60respectively. GCC defines the macros__mcarchand__mcarch__when tuning for680x0architecture arch. It also definesmcarchunless either-ansior a non-GNU-stdoption is used. If GCC is tuning for a range of architectures, as selected by-mtune=68020-40or-mtune=68020-60, it defines the macros for every architecture in the range. GCC also defines the macro__muarch__when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.-m68000,-mc68000— generate output for a68000. This is the default when the compiler is configured for68000-based systems. It is equivalent to-march=68000. Use this option for microcontrollers with a68000orEC000core, including the68008,68302,68306,68307,68322,68328and68356.-m68010— generate output for a68010. This is the default when the compiler is configured for68010-based systems. It is equivalent to-march=68010.-m68020,-mc68020— bestie we're making output for a 68020 ✨ this is literally the default when the compiler is configured for 68020-based systems, and it's equivalent to-march=68020.-m68030— output for a 68030 🦋 default when the compiler is configured for 68030-based systems, equivalent to-march=68030.-m68040— output for a 68040 💅 default when the compiler is configured for 68040-based systems, equivalent to-march=68040.
Okay so important bit: this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use it if your 68040 does not have code to emulate those instructions 👑
-m68060— output for a 68060 🌸 default when the compiler is configured for 68060-based systems, equivalent to-march=68060.
Same deal but bigger: this one inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 does not have code to emulate those instructions 💀
-mcpu32— output for a CPU32 🎀 default when the compiler is configured for CPU32-based systems, equivalent to-march=cpu32.
Use this one for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360 🫶
-m5200— output for a 520X ColdFire CPU 🔥 default when the compiler is configured for 520X-based systems. It's equivalent to-mcpu=5206, and ngl it's now deprecated in favor of that option.
Use it for microcontrollers with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e— output for a 5206e ColdFire CPU ✨ the option is now deprecated in favor of the equivalent-mcpu=5206e.-m528x— output for a member of the ColdFire 528X family 💜 the option is now deprecated in favor of the equivalent-mcpu=528x.-m5307— output for a ColdFire 5307 CPU 🌈 the option is now deprecated in favor of the equivalent-mcpu=5307.-m5407— output for a ColdFire 5407 CPU ⭐ the option is now deprecated in favor of the equivalent-mcpu=5407.-mcfv4e— output for a ColdFire V4e family CPU (e.g.547x/548x) 💖 this includes use of hardware floating-point instructions. The option is equivalent to-mcpu=547x, and is now deprecated in favor of that option.-m68020-40— output for a 68040, but without using any of the new instructions 🦋 this results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40 💅
-m68020-60— output for a 68060, without using any of the new instructions ✨ this results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60 🎀
-mhard-float,-m68881— generate floating-point instructions 💖 this is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro"__HAVE_68881__"on M680x0 targets and"__mcffpu__"on ColdFire targets.-msoft-float— do not generate floating-point instructions; use library calls instead 🌸 this is the default for 68000, 68010, and 68832 targets. It's also the default for ColdFire devices that have no FPU.-mdiv,-mno-div— generate (do not generate) ColdFire hardware divide and remainder instructions 👑 if-marchis used without-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by-mcpu). For example, the default is "off" for-mcpu=5206and "on" for-mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled 🔥
-mshort— consider type"int"to be 16 bits wide, like"short int"💅 additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.-mno-short— do not consider type"int"to be 16 bits wide 🦋 this is the default.-mnobitfield,-mno-bitfield— do not use the bit-field instructions 🙈 the-m68000,-mcpu32and-m5200options imply-mnobitfield.-mbitfield— do use the bit-field instructions ✨ the-m68020option implies-mbitfield. This is the default if you use a configuration designed for a 68020.-mrtd— use a different function-calling convention, where 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's no need to pop the arguments there.
Okay so real talk, this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler 😭
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code is generated for calls to those functions 💀
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) 🙈
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int,-mno-align-int— control whether GCC aligns"int","long","long long","float","double", and"long double"variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int) 🎀 aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
⚠️ Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k 😭
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table 💜 at present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align,-mstrict-align— do not (do) assume that unaligned memory references are handled by the system 🦋-msep-data— generate code that allows the data segment to be located in a different area of memory from the text segment ✨ this allows for execute-in-place in an environment without virtual memory management. This option implies-fPIC.-mno-sep-data— generate code that assumes that the data segment follows the text segment 🌸 this is the default.-mid-shared-library— generate code that supports shared libraries via the library ID method 👑 this allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies-fPIC.-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used 💅 this is the default.-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled 🔥 specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.-mxgot,-mno-xgot— when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries 💀 this code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;-fPICsuffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobarIf this happens, you should recompile your code with -mxgot 🫶 it should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries 🌈 if you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do 👀
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets— use 32-bit offsets in"switch"tables ⭐ the default is to use 16-bit offsets.
🧚 MCore Options 🧚
These are the -m options defined for the Motorola M*Core processors 💖
-mhardlit,-mno-hardlit— inline constants into the code stream if it can be done in two instructions or less ✨-mdiv,-mno-div— use the divide instruction. (Enabled by default.) 💅-mrelax-immediate,-mno-relax-immediate— allow arbitrary-sized immediates in bit operations 🦋-mwide-bitfields,-mno-wide-bitfields— always treat bit-fields as"int"-sized 🎀-m4byte-functions,-mno-4byte-functions— force all functions to be aligned to a 4-byte boundary 🌸-mcallgraph-data,-mno-callgraph-data— emit callgraph information 👑-mslow-bytes,-mno-slow-bytes— prefer word access when reading byte quantities 💜-mlittle-endian,-mbig-endian— generate code for a little-endian target 🔥-m210,-m340— generate code for the 210 processor ⭐-mno-lsim— assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line 🫶-mstack-increment=size— caps how big a single stack-increment operation is allowed to get. Bigger values can genuinely speed up programs whose functions need a lot of stack space… but they can also yeet you straight into a segmentation fault if the stack gets extended too far 😭. Default is0x1000. Tune responsibly, bestie.
🦋 MicroBlaze Options 🦋
-msoft-float— emulate floating point in software. This is the default, no cap.-mhard-float— use actual hardware floating-point instructions. Glow up ✨-mmemcpy— don't optimize block moves, just usememcpy. Let the library girlies handle it.-mno-clearbss— ⚠️ deprecated!! Use-fno-zero-initialized-in-bssinstead. She's old news, respectfully.-mcpu=cpu-type— use the features of, and schedule code for, the CPU you name. Supported values are in the formatvX.YY.Z, whereXis the major version,YYis the minor version, andZis compatibility code. Example values arev3.00.a,v4.00.b,v5.00.a,v5.00.b,v6.00.a. Versions, but make it ✨aesthetic✨.-mxl-soft-mul— software multiply emulation. Default behavior, she's the baseline 💅-mxl-soft-div— software emulation for divides. Also the default. We love a consistent queen.-mxl-barrel-shift— use the hardware barrel shifter. Spin spin spin 🌪️-mxl-pattern-compare— use pattern compare instructions. Cute AND useful.-msmall-divides— use table-lookup optimization for small signed integer divisions. Tiny math, big brain energy 🧠-mxl-stack-check— ⚠️ deprecated!! Use-fstack-checkinstead. Same vibe, new name.-mxl-gp-opt— use GP-relative.sdata/.sbsssections. Fancy addressing, we stan.-mxl-multiply-high— use multiply-high instructions for the high part of a 32x32 multiply. Half the bits, all the drama.-mxl-float-convert— use hardware floating-point conversion instructions. Converting, but make it hardware 💅-mxl-float-sqrt— use the hardware floating-point square root instruction. Math but fast, periodt.-mbig-endian— generate code for a big-endian target. Big-endian girlies rise 🫶-mlittle-endian— generate code for a little-endian target. Little-endian girlies, your moment 🌸-mxl-reorder— use reorder instructions (swap and byte-reversed load/store). Shuffle it, slay it.-mxl-mode-app-model— pick your application model, app-model. The valid models are:executable— a normal executable (the default), uses startup codecrt0.o. The classic, the icon, the blueprint.xmdstub— for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent calledxmdstub. This uses startup filecrt1.oand sets the start address of the program to0x800. Debugger-coded, lowkey iconic.bootstrap— for applications that are loaded using a bootloader. This model uses startup filecrt2.o, which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than to the application. Bootloader does the intro, app does the main character moment ✨novectors— for applications that don't require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model usescrt3.oas a startup file. No vectors, no problems.
Also: option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. Kept around for the nostalgia 🎀
-mpic-data-is-text-relative— assume the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing isn't supported. Fixed-distance besties fr fr.
🧚 MIPS Options 🧚
-EB— generate big-endian code. Big-endian supremacy 🫶-EL— generate little-endian code. This is the default formips*el-*-*configurations. Little-endian girlies, you're the default bestie 💖-march=arch— generate code that runs on arch, which can be the name of a generic MIPS ISA or the name of a particular processor. The ISA names are:mips1,mips2,mips3,mips4,mips32,mips32r2,mips32r3,mips32r5,mips32r6,mips64,mips64r2,mips64r3,mips64r5andmips64r6. The processor names are:4kc,4km,4kp,4ksc,4kec,4kem,4kep,4ksd,5kc,5kf,20kc,24kc,24kf2_1,24kf1_1,24kec,24kef2_1,24kef1_1,34kc,34kf2_1,34kf1_1,34kn,74kc,74kf2_1,74kf1_1,74kf3_2,1004kc,1004kf2_1,1004kf1_1,i6400,i6500,interaptiv,loongson2e,loongson2f,loongson3a,gs464,gs464e,gs264e,m4k,m14k,m14kc,m14ke,m14kec,m5100,m5101,octeon,octeon+,octeon2,octeon3,orion,p5600,p6600,r2000,r3000,r3900,r4000,r4400,r4600,r4650,r4700,r5900,r6000,r8000,rm7000,rm9000,r10000,r12000,r14000,r16000,sb1,sr71000,vr4100,vr4111,vr4120,vr4130,vr4300,vr5000,vr5400,vr5500,xlrandxlp. That's a whole roster, bestie, we're not skipping a single one 💅
The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC doesn't recognize the processor. Rude, but okay 🙈
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r. Shortcuts, but make them legal.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1. Clock math, but it's giving ✨legacy support✨.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of the target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given. Full names only, she does not do nicknames 😌
-mtune=arch— optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for-march. Same roster, different job 💅
When this option isn't used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it's possible to generate code that runs on a family of processors but optimizes the code for one particular member of that family. Group-chat compatible but BFF-optimized 🫶
-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. Shortcut queen.-mips2— equivalent to-march=mips2.-mips3— equivalent to-march=mips3.-mips4— equivalent to-march=mips4.-mips32— equivalent to-march=mips32.-mips32r3— equivalent to-march=mips32r3.-mips32r5— equivalent to-march=mips32r5.-mips32r6— equivalent to-march=mips32r6.-mips64— equivalent to-march=mips64.-mips64r2— equivalent to-march=mips64r2.-mips64r3— equivalent to-march=mips64r3.-mips64r5— equivalent to-march=mips64r5.-mips64r6— equivalent to-march=mips64r6.-mips16,-mno-mips16— generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. On/off, but make it architecture-aware ✨
MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes. Per-function vibes, iykyk.
-mmips16e2,-mno-mips16e2— use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the-mips16option such that it targets the MIPS16e2 ASE. She upgrades the vibe of her sister option 💅-mflip-mips16— generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code. It's a chaos-mode testing tool, bestie, don't ship it 😭-minterlink-compressed,-mno-interlink-compressed— require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed. No shortcuts across the aisle, periodt.
-minterlink-mips16,-mno-interlink-mips16— aliases of-minterlink-compressedand-mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. Legacy icons still slaying 🎀-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generate code for the given ABI. Pick your lane, bestie.
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 <https://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. Specific girlies only, no exceptions 🫶
The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. Same rules, bigger registers, we love an upgrade that doesn't break the dress code 💅
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both — pick a side, bestie 🙈. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. Interlink and thrive, no mode-switching required ✨
-mabicalls,-mno-abicalls— generate (do not generate) code that is suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems. Dynamic object friendly, as she should be 💖-mshared,-mno-shared— bestie, this one's about generating code that's fully position-independent (or, like, not) so it can be linked into shared libraries ✨. Heads up tho: it only does anything for-mabicalls💅.
So all -mabicalls code has traditionally been position-independent anyway, no matter what you pass with -fPIC and -fpic 🌸. But plot twist: as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols 🫶. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions — and that mode is what -mno-shared picks 👑
Ok but real talk: -mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker 😭. The good news? It does not affect the ABI of the final executable — it only affects the ABI of relocatable objects 🎀. And using -mno-shared generally makes executables both smaller and quicker 🔥.
-mshared is the default 💖.
-mplt,-mno-plt— assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations 🦋. This option only affects-mno-shared-mabicalls. For the n64 ABI it has no effect without-msym32🙈.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise 💅.
-mxgot,-mno-xgot— lift (or don't lift) the usual restrictions on the size of the global offset table ⭐.
GCC normally uses a single instruction to load values from the GOT. Relatively efficient, sure, but it only works if the GOT is smaller than about 64k 😬. Anything bigger and the linker hits you with an error like:
relocation truncated to fit: R_MIPS_GOT16 foobarIf that happens, recompile your code with -mxgot 💖. It works with very large GOTs, but the code is also less efficient, since it takes three instructions to fetch the value of a global symbol 🐌.
Note that some linkers can create multiple GOTs. If yours can, you should only need -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 general-purpose registers are 32 bits wide 💅.-mgp64— assume general-purpose registers are 64 bits wide 👑-mfp32— assume floating-point registers are 32 bits wide 🎀.-mfp64— assume floating-point registers are 64 bits wide ✨.-mfpxx— do not assume the width of floating-point registers. mysterious. iconic. we simply don't know 💅.-mhard-float— use floating-point coprocessor instructions, periodt 🔥.-msoft-float— do not use floating-point coprocessor instructions; implement floating-point calculations using library calls instead 🫶.-mno-float— equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations 🚨. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-pointprintfformats 😭). If code compiled with-mno-floataccidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure 💀. you've been warned bestie.-msingle-float— assume the floating-point coprocessor only supports single-precision operations 💅.-mdouble-float— assume the floating-point coprocessor supports double-precision operations ✨. This is the default 👑.-modd-spreg,-mno-odd-spreg— enable the use of odd-numbered single-precision floating-point registers for the o32 ABI 🎀. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI,-mno-odd-spregis set by default 🌸.-mabs=2008,-mabs=legacy— these control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with theabs.fmtandneg.fmtmachine instructions 💅.
By default, or when -mabs=legacy is used, the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN 🙈. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — unless the -ffinite-math-only option has also been specified 🫶.
The -mabs=2008 option selects the IEEE 754-2008 treatment ✨. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN 🌈. These instructions are therefore always used for the respective operations 👑.
-mnan=2008,-mnan=legacy— these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data 🎀.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1 💅.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0 🌸. yes, they literally just swapped the bits. chaos. love that for us.
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008 ⭐.
-mllsc,-mno-llsc— use (or don't use)ll,sc, andsyncinstructions to implement atomic memory built-in functions 🔥. When neither option is specified, GCC uses the instructions if the target architecture supports them 💅.
-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs 🦋. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details 👑
-mdsp,-mno-dsp— use (or don't use) revision 1 of the MIPS DSP ASE ✨. This option defines the preprocessor macro__mips_dsp. It also defines__mips_dsp_revto 1 🎀.-mdspr2,-mno-dspr2— use (or don't use) revision 2 of the MIPS DSP ASE 🌸. This option defines the preprocessor macros__mips_dspand__mips_dspr2. It also defines__mips_dsp_revto 2 💅.-msmartmips,-mno-smartmips— use (or don't use) the MIPS SmartMIPS ASE 👑.-mpaired-single,-mno-paired-single— use (or don't use) paired-single floating-point instructions 🫶. This option requires hardware floating-point support to be enabled 🚨.-mdmx,-mno-mdmx— use (or don't use) MIPS Digital Media Extension instructions 🎀. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled 💅.-mips3d,-mno-mips3d— use (or don't use) the MIPS-3D ASE ⭐. The option-mips3dimplies-mpaired-single😌.-mmicromips,-mno-micromips— generate (or don't generate) microMIPS code ✨.
MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes 🎀.
-mmt,-mno-mt— use (or don't use) MT Multithreading instructions 🦋.-mmcu,-mno-mcu— use (or don't use) the MIPS MCU ASE instructions 🌸.-meva,-mno-eva— use (or don't use) the MIPS Enhanced Virtual Addressing instructions 💜.-mvirt,-mno-virt— use (or don't use) the MIPS Virtualization (VZ) instructions 👑-mxpa,-mno-xpa— use (or don't use) the MIPS eXtended Physical Address (XPA) instructions 🔥.-mcrc,-mno-crc— use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions 💅.-mginv,-mno-ginv— use (or don't use) the MIPS Global INValidate (GINV) instructions ✨.-mloongson-mmi,-mno-loongson-mmi— use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI) 🎀.-mloongson-ext,-mno-loongson-ext— use (or don't use) the MIPS Loongson EXTensions (EXT) instructions 🌸.-mloongson-ext2,-mno-loongson-ext2— use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions 🦋.-mlong64— forcelongtypes to be 64 bits wide 💪. See-mlong32for an explanation of the default and the way that the pointer size is determined 👀.-mlong32— forcelong,int, and pointer types to be 32 bits wide 🎀.
The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs ✨. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller 💅.
-msym32,-mno-sym32— assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI 🫶. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses 🔥.-Gnum — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes 🌸. GCC can then generate more efficient accesses to the data; see-mgpoptfor details 👀.
The default -G option depends on the configuration 🎀.
-mlocal-sdata,-mno-local-sdata— extend (or don't extend) the-Gbehavior to local data too, such as to static variables in C 💅.-mlocal-sdatais the default for all configurations ✨.
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata 🫶. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program 🌈.
-mextern-sdata,-mno-extern-sdata— assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the-Glimit 👑.-mextern-sdatais the default for all configurations 💖.
Okay so if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that’s no bigger than num bytes, then bestie, you must make sure Var is placed in a small data section. If Var is defined by another module, you have to either compile that module with a high-enough -G setting or attach a "section" attribute to Var’s definition. And if Var is common, you must link the application with a high-enough -G setting. No cap.
The easiest way to satisfy these restrictions is to compile and link every module with the same -G option. But maybe you want to build a library that supports several different small data limits — iconic, honestly. You can do that by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 💅
-mgpopt,-mno-gpopt— Use (or don’t use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-sdataand-mextern-sdata.-mgpoptis the default for all configurations.
-mno-gpopt is useful when the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In those situations, the boot monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data,-mno-embedded-data— Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.-muninit-const-in-rodata,-mno-uninit-const-in-rodata— Put uninitializedconstvariables in the read-only data section. This option is only meaningful in conjunction with-membedded-data.-mcode-readable=setting— Specify whether GCC may generate code that reads from executable sections. There are three possible settings:-mcode-readable=yes— Instructions may freely access executable sections. This is the default setting.-mcode-readable=pcrel— MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.-mcode-readable=no— Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses,-mno-split-addresses— Enable (disable) use of the%hi()and%lo()assembler relocation operators. This option has been superseded by-mexplicit-relocsbut is retained for backwards compatibility.-mexplicit-relocs=none,-mexplicit-relocs=base,-mexplicit-relocs=pcrel,-mexplicit-relocs,-mno-explicit-relocs— These options control whether explicit relocs (such as%gp_rel) are used. The default value depends on the version of GAS when GCC itself was built.
The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo.
-mcheck-zero-division,-mno-check-zero-division— Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps,-mdivide-breaks— MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use-mdivide-trapsto allow conditional traps on architectures that support them and-mdivide-breaksto force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mload-store-pairs,-mno-load-store-pairs— Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding.-mstrict-align,-mno-strict-align,-munaligned-access,-mno-unaligned-access— Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So-mstrict-alignmay be needed by kernel. The options-munaligned-accessand-mno-unaligned-accessare obsoleted, and only for backward-compatible.-mmemcpy,-mno-memcpy— Force (do not force) the use ofmemcpyfor 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 thejalinstruction. Calling functions usingjalis 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 themad,maduandmulinstructions, as provided by the R4650 ISA.-mimadd,-mno-imadd— Enable (disable) use of themaddandmsubinteger instructions. The default is-mimaddon architectures that supportmaddandmsubexcept for the 74k architecture where it was found to generate slower code.-mfused-madd,-mno-fused-madd— Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.
-nocpp— Tell the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them.-mfix-24k,-mno-fix-24k— Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC.-mfix-r4000,-mno-fix-r4000— Work around certain R4000 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400,-mno-fix-r4400— Work around certain R4400 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— Work around certain R10000 errata:ll/scsequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900,-mno-fix-r5900— Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer and always schedule anopinstruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.-mfix-rm7000,-mno-fix-rm7000— Work around the RM7000dmult/dmultuerrata. The workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120,-mno-fix-vr4120— Work around certain VR4120 errata:dmultudoes not always produce the correct result.divandddivdo 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 configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.
-mfix-vr4130— Work around the VR4130mflo/mfhierrata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids usingmfloandmfhiif the VR4130macc,macchi,dmaccanddmacchiinstructions are available instead.-mfix-sb1,-mno-fix-sb1— Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2"F1"and"F2"floating-point errata.)-mr10k-cache-barrier=setting— Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects.
Okay so this part is the R10K cache-barrier situation, and it is very specific but also very real. This problem only affects kernel stores and, depending on the system, kernel loads. Example time: a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. 😭 See the R10K processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:
- the memory occupied by the current function's stack frame;
- the memory occupied by an incoming stack argument;
- the memory occupied by an object with a link-time-constant address.
🚨 No cap, bestie, it is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store— insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=store— insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=none— disable the insertion of cache barriers. This is the default setting. Periodt.-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_funcor__cpu_flush.-mbranch-cost=num— set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the-mtunesetting.-mbranch-likely,-mno-branch-likely— enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.
These options control which form of branches will be generated. The default is -mcompact-branches=optimal.
-mcompact-branches=never— ensures that compact branch instructions will never be generated.-mcompact-branches=optimal— the default. It will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available.-mcompact-branches=always— ensures that a compact branch instruction will be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.
If -mcompact-branches=always is used for MIPS16/microMIPS targets, it will be just ignored now. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions.
-mfp-exceptions,-mno-fp-exceptions— specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled. For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe.-mvr4130-align,-mno-vr4130-align— the VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel. This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level-O3.-msynci,-mno-synci— enable (disable) generation ofsynciinstructions on architectures that support it. Thesynciinstructions (if enabled) are generated when__builtin___clear_cacheis compiled. This option defaults to-mno-synci, but the default can be overridden by configuring GCC with--with-synci. When compiling code for single processor systems, it is generally safe to usesynci. However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior.-mrelax-pic-calls,-mno-relax-pic-calls— try to turn PIC calls that are normally dispatched via register$25into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.-mrelax-pic-callsis the default if GCC was configured to use an assembler and a linker that support the.relocassembly directive and-mexplicit-relocsis in effect. With-mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler.-mmcount-ra-address,-mno-mcount-ra-address— emit (do not emit) code that allows_mcountto modify the calling function's return address. When enabled, this option extends the usual_mcountinterface with a new ra-address parameter, which has typeintptr_t *and is passed in register$12._mcountcan then modify the return address by doing both of the following:- Returning the new address in register
$31. - Storing the new address in
*ra-address, if ra-address is nonnull.
- Returning the new address in register
The default is -mno-mcount-ra-address.
-mframe-header-opt,-mno-frame-header-opt— enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused. This optimization is off by default at all optimization levels.-mlxc1-sxc1,-mno-lxc1-sxc1— when applicable, enable (disable) the generation oflwxc1,swxc1,ldxc1,sdxc1instructions. Enabled by default.-mmadd4,-mno-madd4— when applicable, enable (disable) the generation of 4-operandmadd.s,madd.dand related instructions. Enabled by default.
MMIX Options 🦋✨
These options are defined for the MMIX, iykyk:
-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 therEepsilon register.-mabi=mmixware,-mabi=gnu— generate code that passes function parameters and return values that (in the called function) are seen as registers$0and up, as opposed to the GNU ABI which uses global registers$231and 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 thePREFIXassembly directive.-melf— generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.-mbranch-predict,-mno-branch-predict— use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.-mbase-addresses,-mno-base-addresses— generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require-mno-base-addresses.-msingle-exit,-mno-single-exit— force (do not force) generated code to have a single exit point in each function.
MN10300 Options 🎀
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug— generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.-mno-mult-bug— do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.-mam33— generate code using features specific to the AM33 processor.-mno-am33— do not generate code using features specific to the AM33 processor. This is the default. 🫶-mam33-2— generate code using features specific to the AM33/2.0 processor. ✨-mam34— generate code using features specific to the AM34 processor. 💅-mtune=cpu-type— use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one ofmn10300,am33,am33-2oram34. 🎀-mreturn-pointer-on-d0— when generating a function that returns a pointer, return the pointer in botha0andd0. Otherwise, the pointer is returned only ina0, and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use-mno-return-pointer-on-d0to disable it. 🦋-mno-crt0— do not link in the C run-time initialization object file. 🍓-mrelax— tell 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.
⚠️ Heads up bestie: this option makes symbolic debugging impossible. 😭
-mliw— allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro"__LIW__". 💖-mno-liw— do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro"__NO_LIW__". 🌸-msetlb— allow the compiler to generate theSETLBandLccinstructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro"__SETLB__". ⭐-mno-setlb— do not allow the compiler to generateSETLBorLccinstructions. This option defines the preprocessor macro"__NO_SETLB__". 💀
🦋 Moxie Options 🦋
-meb— generate big-endian code. This is the default formoxie-*-*configurations. 👑-mel— generate little-endian code. 🎀-mmul.x— generatemul.xandumul.xinstructions. This is the default formoxiebox-*-*configurations. 🔥-mno-crt0— do not link in the C run-time initialization object file. 🫶
🍓 MSP430 Options 🍓
These options are defined for the MSP430:
-masm-hex— force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes. ✨-mmcu=— select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with__. This in turn is used by themsp430.hheader file to select an MCU-specific supplementary header file. The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name ofmsp430can also be used to select the 430 ISA. Similarly the genericmsp430xMCU name selects the 430X ISA. In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with.ldappended. Thus specifying-mmcu=xxxon the gcc command line defines the C preprocessor symbol"__XXX__"and cause the linker to search for a script calledxxx.ld. The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an externaldevices.csvfile can be used to extend device support beyond those that have been hard-coded. GCC searches for thedevices.csvfile using the following methods in the given precedence order, where the first method takes precedence over the second which takes precedence over the third. First: include path specified with"-I"and"-L"—devices.csvis searched for in each of the directories specified by include paths and linker library search paths. Second: path specified by the environment variableMSP430_GCC_INCLUDE_DIR— define the value of the global environment variableMSP430_GCC_INCLUDE_DIRto the full path to the directory containingdevices.csv, and GCC will search this directory fordevices.csv. Ifdevices.csvis found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying"-I"and"-L"on the command line. Third: themsp430-elf{,bare}/include/devicesdirectory — finally, GCC examinesmsp430-elf{,bare}/include/devicesfrom the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copieddevices.csvthere, then the MCU data is read. As above, this directory is also registered as an include path and linker library path. If none of the above search methods finddevices.csv, then the hard-coded MCU data is used. 💅-mwarn-mcu,-mno-warn-mcu— this option enables or disables warnings about conflicts between the MCU name specified by the-mmcuoption and the ISA set by the-mcpuoption and/or the hardware multiply support set by the-mhwmultoption. It also toggles warnings about unrecognized MCU names. This option is on by default. 🌈-mcpu=— specifies the ISA to use. Accepted values aremsp430,msp430xandmsp430xv2. This option is deprecated. The-mmcu=option should be used to select the ISA. 🙈-msim— link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the-mmcu=option. 🧚-mlarge— use large-model addressing (20-bit pointers, 20-bitsize_t). 💜-msmall— use small-model addressing (16-bit pointers, 16-bitsize_t). 🎀-mrelax— this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. ⭐mhwmult=— describes the type of hardware multiply supported by the target. Accepted values arenonefor no hardware multiply,16bitfor the original 16-bit-only multiply supported by early MCUs.32bitfor the 16/32-bit multiply supported by later MCUs andf5seriesfor the 16/32-bit multiply supported by F5-series MCUs. A value ofautocan also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the-mmcuoption. If no-mmcuoption is specified or if the MCU name is not recognized then no hardware multiply support is assumed."auto"is the default setting. Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at-O3or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code. The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 🔥-minrt— enable the use of a minimum runtime environment - no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🫶-mtiny-printf— enable reduced code sizeprintfandputslibrary functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications. Support for streams has been removed and the string to be printed will always be sent to stdout via thewritesyscall. The string is not buffered before it is sent to write. This option requires Newlib Nano IO, so GCC must be configured with--enable-newlib-nano-formatted-io. 😭-mmax-inline-shift=— this option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, anmspabihelper function is used instead. The default value is4. This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts>1on the 430 ISA). Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. 🌸-mcode-region=,-mdata-region=— these options tell the compiler where to place functions and data that do not have one of the"lower","upper","either"or"section"attributes. Possible values are"lower","upper","either"or"any". The first three behave like the corresponding attribute. The fourth possible value -"any"- is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (".text",".data", etc) to the memory regions. 💖-msilicon-errata=— this option passes on a request to assembler to enable the fixes for the named silicon errata. 🦋-msilicon-errata-warn=— this option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied. ⭐-mwarn-devices-csv,-mno-warn-devices-csv— warn ifdevices.csvis not found or there are problem parsing it (default: on). 💀
🌈 NDS32 Options 🌈
These options are defined for NDS32 implementations:
-mbig-endian— generate code in big-endian mode. 👑-mlittle-endian— generate code in little-endian mode. 🎀-mreduced-regs— use reduced-set registers for register allocation. 🍓-mfull-regs— use full-set registers for register allocation. 💅-mcmov— generate conditional move instructions. ✨-mno-cmov— do not generate conditional move instructions. 🌸-mext-perf— generate performance extension instructions. 🔥-mno-ext-perf— do not generate performance extension instructions. 🫶-mext-perf2— generate performance extension 2 instructions. ⭐-mno-ext-perf2— do not generate performance extension 2 instructions. 🧚-mext-string— generate string extension instructions. 💜-mno-ext-string— do not generate string extension instructions. 🌈-mv3push— generate v3push25/pop25instructions. 💖-mno-v3push— nope, no v3 push25/pop25 instructions for you. We're not generating those.-m16-bit— yes babe, generate 16-bit instructions ✨-mno-16-bit— okay actually no, skip the 16-bit instructions.-misr-vector-size=num— sets how big each interrupt vector is. It's gotta be4or16, no in-between, the compiler WILL side-eye anything else 💅-mcache-block-size=num— sets the size of each cache block. Rules: it has to be a power of 2, and it has to sit between4and512. That's the whole vibe check.-march=arch— tells GCC the name of the target architecture. Slay, be specific.-mcmodel=code-model— picks the code model, and there are three flavors:small— every data and read-only data segment has to live inside a 512KB addressing space, and the text segment has to stay within 16MB addressing space. cozy.medium— the data segment has to be within 512KB, but the read-only data segment gets to sprawl across 4GB addressing space. Text segment should still be within that 16MB addressing space though. main character data, supporting-character text.large— everything, text and data both, can be within 4GB addressing space. go big or go home 🫶
-mctor-dtor— turns on the constructor/destructor feature. yes queen, run your setup code.-mrelax— gives the linker a little nudge to relax instructions. self-care for your instruction stream 💆
🦋 Nvidia PTX Options 🦋
These ones are defined for Nvidia PTX, bestie:
-m64— literally ignored 😭 but it's preserved for backward compatibility, so don't come for it. Only the 64-bit ABI is supported anyway.-march=architecture-string— generates code for the PTX ISA target architecture you name. The valid architecture strings aresm_30,sm_35,sm_37,sm_52,sm_53,sm_61,sm_70,sm_75,sm_80, andsm_89. What's the default? Depends on how the compiler was configured — go peep--with-arch.
This option sets the value of the preprocessor macro __PTX_SM__. Like, for sm_35, it has the value 350. cute lil number.
-misa=architecture-string— it's just an alias of-march=. same thing, different name, we love a rebrand 💅-march-map=architecture-string— picks the closest available-march=value that isn't more capable. So for-march-map=sm_50you get-march=sm_37, and for-march-map=sm_53you get-march=sm_53. it's giving "aim slightly lower, queen".-mptx=version-string— generates code for the PTX ISA version you specify. Valid version strings:3.1,4.1,4.2,5.0,6.0,6.3,7.0,7.3, and7.8. The default PTX ISA version is whatever one added support for the PTX ISA target architecture you selected (peep-march=), but at least6.3, or7.3for-march=sm_52and higher. no cap, it just knows.
This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__. For example for 3.1 the macros get 3 and 1 respectively. numbers assigned, iconic.
-mmainkernel— links in code for a__mainkernel. This is for stand-alone execution instead of offloading execution. different vibe entirely.-moptimize— applies partitioned execution optimizations. This is already the default whenever any level of optimization is selected, so like, it's already on. 🫶-msoft-stack,-mno-soft-stack— okay this one's a whole thing, buckle up:
With -mno-soft-stack (which is the default, unless -mgomp has been specified), you get PTX "native" stacks — meaning the compiler generates code that uses .local memory or PTX "alloca" directly for stack storage. Heads up though: unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with alloca are not supported. so, lowkey limited.
With -msoft-stack (implied by -mgomp), the compiler generates code that does not use .local memory or PTX alloca directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or alloca), and when global memory is used for underlying storage, it makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is what's used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler. So if you actually want code suitable for linking into programs using OpenMP offloading, use option -mgomp, bestie. the right tool, etc. ✨
-muniform-simt— switches to a code generation variant that lets every thread in each warp execute, while keeping memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc,free,vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. (main character energy for everyone, honestly.)
The shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y with the current lane index to compute the master lane index. math girlies rise 🧮
-mgomp— generates code for use in OpenMP offloading. It enables the-msoft-stackand-muniform-simtoptions, and selects the corresponding multilib variant. one flag, three jobs, we stan.
🌸 OpenRISC Options 🌸
Defined for OpenRISC, here we go:
-mboard=name— configures a board specific runtime. This gets passed to the linker for newlib board library linking. The default isor1ksim.-mnewlib— this option is ignored 🙈 it's for compatibility purposes only. It used to select linker and preprocessor options for use with newlib. retired queen, still showing up to the party.-msoft-div,-mhard-div— picks software or hardware divide (l.div,l.divu) instructions. The default is hardware divide.-msoft-mul,-mhard-mul— picks software or hardware multiply (l.mul,l.muli) instructions. The default is hardware multiply.-msoft-float,-mhard-float— picks software or hardware for floating point operations. The default here is software.-mdouble-float— when-mhard-floatis selected, this enables generation of double-precision floating point instructions. By default, functions from libgcc are used to do the double-precision floating point operations. so it's a "unlock the good version" flag.-munordered-float— when-mhard-floatis selected, this enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default, libgcc functions handle the unordered floating point compare and set flag operations.-mcmov— enables generation of conditional move (l.cmov) instructions. By default, the equivalent gets generated using set and branch. same results, more steps.-mror— enables generation of rotate right (l.ror) instructions. By default libgcc functions do the rotate right operations.-mrori— enables generation of rotate right with immediate (l.rori) instructions. By default libgcc functions do the rotate right with immediate operations.-msext— enables generation of sign extension (l.ext*) instructions. By default, memory loads are used to perform sign extension.-msfimm— enables generation of compare and set flag with immediate (l.sf*i) instructions. By default extra instructions get generated to store the immediate to a register first. inefficient but valid 💀-mshftimm— enables generation of shift with immediate (l.srai,l.srli,l.slli) instructions. By default extra instructions get generated to store the immediate to a register first.-mcmodel=small— generates OpenRISC code for the small model: the GOT is limited to 64k. This is the default model, fyi.-mcmodel=large— generates OpenRISC code for the large model: the GOT may grow up to 4G in size. glow-up unlocked 🌈
💜 PDP-11 Options 💜
Defined for the PDP-11, vintage icon behavior:
-mfpu— uses hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported, just so we're clear.) Implies-m45.-msoft-float— do not use hardware floating point. software girlies only.-mac0— returns floating-point results inac0(fr0in Unix assembler syntax).-mno-ac0— returns floating-point results in memory. This is the default.-m40— generates code for a PDP-11/40. Implies-msoft-float -mno-split.-m45— generates code for a PDP-11/45. This is the default.-m10— generates code for a PDP-11/10. Implies-msoft-float -mno-split.-mint16,-mno-int32— uses 16-bitint. This is the default.-mint32,-mno-int16— uses 32-bitint.-msplit— the target has split instruction and data space. Implies-m45.-munix-asm— uses Unix assembler syntax.-mdec-asm— uses DEC assembler syntax.-mgnu-asm— uses GNU assembler syntax. This is the default.-mlra— uses the new LRA register allocator. By default, the oldreloadallocator is used. upgrade time 🎀
👑 PowerPC Options 👑
These are listed under
🍓 PRU Options 🍓
These command-line options are defined for the PRU target:
-minrt— links with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features get removed. trade-offs, bestie.
This option disables support for static initializers and constructors. Beware: the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features. no take-backs.
The minimal startup code would not pass argc and argv arguments to main, so main has to be declared as int main (void). This is already the norm for most firmware projects, so like, you're probably fine.
-mmcu=mcu— specifies the PRU hardware variant to use. A correspondingly named spec file gets loaded, passing the memory region sizes to the linker and defining hardware-specific C macros.
Heads up: Newlib provides only the sim spec, which is intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package. go get it ✨
-mno-relax— makes GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption. choose chaos.-mloop— allows (or does not allow) GCC to use the LOOP instruction. yes loop, no loop, that's the whole setting.-mabi=variant— specifies the ABI variant to output code for.-mabi=tiselects the unmodified TI ABI, while-mabi=gnuselects a GNU variant that copes more naturally with certain GCC assumptions. The differences are:- Function Pointer Size — the TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers. so, small pointer energy vs. chunky pointer energy 💅
Optional Return Value Pointer ✨
Okay so picture this, bestie — when a function returns something bigger than 64 bits, it gets passed back using a hidden pointer as the very first argument of the function. Sneaky but iconic. 🎁 But the TI ABI is out here like, "um, actually, that pointer can be NULL if the caller isn't using the returned value." Meanwhile GNU is just always passing and expecting a valid return value pointer. No flex, just facts. 💅
The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. And because of that 😭, the standard C library cannot be built and it is omitted when linking with -mabi=ti. It's giving "we tried" energy.
Also relaxation is a GNU feature, and for safety reasons it is disabled when using -mabi=ti. Why? Because the TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions. Periodt. 💀
RISC-V Options 🦋
These command-line options are defined for RISC-V targets — let's get into it! ✨
-mbranch-cost=n— Set the cost of branches to roughly n instructions.-mplt,-mno-plt— When generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is-mplt.-mabi=ABI-string— Specify the integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example,-march=rv64ifd -mabi=lp64dmeans that long and pointers are 64-bit (implicitly definingintto be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with-march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or-march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.
The default for this argument is system dependent, so if you want a specific calling convention, specify one explicitly, bestie. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well specified at present, and are subject to change. ⚠️
-mfdiv,-mno-fdiv— Do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.-mfence-tso,-mno-fence-tso— Do or don't use thefence.tsoinstruction, which is unimplemented on some processors (including those from T-Head). If thefence.tsoinstruction is not available, then a stronger fence is used instead.-mdiv,-mno-div— Do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.-misa-spec=ISA-spec-string— Specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are:2.2— produce code conforming to version 2.2.20190608— produce code conforming to version 20190608.20191213— produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.
-march=ISA-string— Generate code for the given RISC-V ISA (e.g.rv64im). ISA strings must be lower-case. Examples includerv64i,rv32g,rv32e, andrv32imaf. Additionally, a special value help (-march=help) is accepted to list all supported extensions.
The syntax of the ISA string is defined as follows:
- The string must start with
rv32orrv64, followed byi,e, org, referred to as the base ISA. - The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g.
zba) and single-letter (e.g.v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores. - An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
- Extension names may include an optional version number, following the syntax
<major>p<minor>or<major>, (e.g.m2p1orm2).
Supported extensions are listed below — here's the whole menu, formatted as extension name : supported version : description, and we are not skipping a single one 🍓:
i— supported version:2.0,2.1; base integer extension.e— supported version:2.0; reduced base integer extension.g— supported version:-; general-purpose computing base extension,gwill expand toi,m,a,f,d,zicsrandzifencei.m— supported version:2.0; integer multiplication and division extension.a— supported version:2.0,2.1; atomic extension.f— supported version:2.0,2.2; single-precision floating-point extension.d— supported version:2.0,2.2; double-precision floating-point extension.c— supported version:2.0; compressed extension.h— supported version:1.0; hypervisor extension.v— supported version:1.0; vector extension.zicsr— supported version:2.0; control and status register access extension.zifencei— supported version:2.0; instruction-fetch fence extension.zicond— supported version:1.0; integer conditional operations extension.za64rs— supported version:1.0; reservation set size of 64 bytes.za128rs— supported version:1.0; reservation set size of 128 bytes.zawrs— supported version:1.0; wait-on-reservation-set extension.zba— supported version:1.0; address calculation extension.zbb— supported version:1.0; basic bit manipulation extension.zbc— supported version:1.0; carry-less multiplication extension.zbs— supported version:1.0; single-bit operation extension.zfinx— supported version:1.0; single-precision floating-point in integer registers extension.zdinx— supported version:1.0; double-precision floating-point in integer registers extension.zhinx— supported version:1.0; half-precision floating-point in integer registers extension.zhinxmin— supported version:1.0; minimal half-precision floating-point in integer registers extension.zbkb— supported version:1.0; cryptography bit-manipulation extension.zbkc— supported version:1.0; cryptography carry-less multiply extension.zbkx— supported version:1.0; cryptography crossbar permutation extension.zkne— supported version:1.0; AES Encryption extension.zknd— supported version:1.0; AES Decryption extension.zknh— supported version:1.0; hash function extension.zkr— supported version:1.0; entropy source extension.zksed— supported version:1.0; SM4 block cipher extension.zksh— supported version:1.0; SM3 hash function extension.zkt— supported version:1.0; data independent execution latency extension.zk— supported version:1.0; standard scalar cryptography extension.zkn— supported version:1.0; NIST algorithm suite extension.zks— supported version:1.0; ShangMi algorithm suite extension.zihintntl— supported version:1.0; non-temporal locality hints extension.zihintpause— supported version:1.0; pause hint extension.zicboz— supported version:1.0; cache-block zero extension.zicbom— supported version:1.0; cache-block management extension.zicbop— supported version:1.0; cache-block prefetch extension.zic64b— supported version:1.0; cache block size isf 64 bytes (yes, "isf" — we don't fix it, we respect it 💀).ziccamoa— supported version:1.0; main memory supports all atomics in A.ziccif— supported version:1.0; main memory supports instruction fetch with atomicity requirement.zicclsm— supported version:1.0; main memory supports misaligned loads/stores.ziccrse— supported version:1.0; main memory supports forward progress on LR/SC sequences.zicntr— supported version:2.0; standard extension for base counters and timers.zihpm— supported version:2.0; standard extension for hardware performance counters.ztso— supported version:1.0; total store ordering extension.zve32x— supported version:1.0; vector extensions for embedded processors.zve32f— supported version:1.0; vector extensions for embedded processors.zve64x— supported version:1.0; vector extensions for embedded processors.zve64f— supported version:1.0; vector extensions for embedded processors.zve64d— supported version:1.0; vector extensions for embedded processors.zvl32b— supported version:1.0; minimum vector length standard extensions.zvl64b— supported version:1.0; minimum vector length standard extensions.zvl128b— supported version:1.0; minimum vector length standard extensions.zvl256b— supported version:1.0; minimum vector length standard extensions.zvl512b— supported version:1.0; minimum vector length standard extensions.zvl1024b— supported version:1.0; minimum vector length standard extensions.zvl2048b— supported version:1.0; minimum vector length standard extensions.zvl4096b— supported version:1.0; minimum vector length standard extensions.zvbb— supported version:1.0; vector basic bit-manipulation extension.zvbc— supported version:1.0; vector carryless multiplication extension.zvkb— supported version:1.0; vector cryptography bit-manipulation extension.zvkg— supported version:1.0; vector GCM/GMAC extension.zvkned— supported version:1.0; vector AES block cipher extension.zvknha— supported version:1.0; vector SHA-2 secure hash extension.zvknhb— supported version:1.0; vector SHA-2 secure hash extension.zvksed— supported version:1.0; vector SM4 Block Cipher extension.zvksh— supported version:1.0; vector SM3 Secure Hash extension.zvkn— supported version:1.0; vector NIST Algorithm Suite extension,zvknwill expand tozvkned,zvknhb,zvkbandzvkt.zvknc— supported version:1.0; vector NIST Algorithm Suite with carryless multiply extension,zvkncwill expand tozvknandzvbc.
zvkng— @tab 1.0 @tab Vector NIST Algorithm Suite with GCM extension,zvkngwill expand tozvknandzvkg. 🎀zvks— @tab 1.0 @tab Vector ShangMi algorithm suite extension,zvkswill expand tozvksed,zvksh,zvkbandzvkt. 💜zvksc— @tab 1.0 @tab Vector ShangMi algorithm suite with carryless multiplication extension,zvkscwill expand tozvksandzvbc. ✨zvksg— @tab 1.0 @tab Vector ShangMi algorithm suite with GCM extension,zvksgwill expand tozvksandzvkg. 🌸zvkt— @tab 1.0 @tab Vector data independent execution latency extension. 🦋zfh— @tab 1.0 @tab Half-precision floating-point extension.zfhmin— @tab 1.0 @tab Minimal half-precision floating-point extension.zvfh— @tab 1.0 @tab Vector half-precision floating-point extension.zvfhmin— @tab 1.0 @tab Vector minimal half-precision floating-point extension.zvfbfmin— @tab 1.0 @tab Vector BF16 converts extension.zfa— @tab 1.0 @tab Additional floating-point extension.zmmul— @tab 1.0 @tab Integer multiplication extension.zca— @tab 1.0 @tab Integer compressed instruction extension.zcf— @tab 1.0 @tab Compressed single-precision floating point loads and stores extension. 🍓zcd— @tab 1.0 @tab Compressed double-precision floating point loads and stores extension.zcb— @tab 1.0 @tab Simple compressed instruction extension.zce— @tab 1.0 @tab Compressed instruction extensions for embedded processors.zcmp— @tab 1.0 @tab Compressed push pop extension.zcmt— @tab 1.0 @tab Table jump instruction extension. 💫smaia— @tab 1.0 @tab Advanced interrupt architecture extension.smepmp— @tab 1.0 @tab PMP Enhancements for memory access and execution prevention on Machine mode.smstateen— @tab 1.0 @tab State enable extension.ssaia— @tab 1.0 @tab Advanced interrupt architecture extension for supervisor-mode.sscofpmf— @tab 1.0 @tab Count overflow & filtering extension.ssstateen— @tab 1.0 @tab State-enable extension for supervisor-mode.sstc— @tab 1.0 @tab Supervisor-mode timer interrupts extension.svinval— @tab 1.0 @tab Fine-grained address-translation cache invalidation extension. 🧚svnapot— @tab 1.0 @tab NAPOT translation contiguity extension.svpbmt— @tab 1.0 @tab Page-based memory types extension.xcvmac— @tab 1.0 @tab Core-V multiply-accumulate extension.xcvalu— @tab 1.0 @tab Core-V miscellaneous ALU extension.xcvelw— @tab 1.0 @tab Core-V event load word extension.xtheadba— @tab 1.0 @tab T-head address calculation extension.xtheadbb— @tab 1.0 @tab T-head basic bit-manipulation extension. 🔥xtheadbs— @tab 1.0 @tab T-head single-bit instructions extension.xtheadcmo— @tab 1.0 @tab T-head cache management operations extension.xtheadcondmov— @tab 1.0 @tab T-head conditional move extension.xtheadfmemidx— @tab 1.0 @tab T-head indexed memory operations for floating-point registers extension.xtheadfmv— @tab 1.0 @tab T-head double floating-point high-bit data transmission extension.xtheadint— @tab 1.0 @tab T-head acceleration interruption extension.xtheadmac— @tab 1.0 @tab T-head multiply-accumulate extension.xtheadmemidx— @tab 1.0 @tab T-head indexed memory operation extension.xtheadmempair— @tab 1.0 @tab T-head two-GPR memory operation extension.xtheadsync— @tab 1.0 @tab T-head multi-core synchronization extension. 🌈xventanacondops— @tab 1.0 @tab Ventana integer conditional operations extension. 👑
Okay so when -march= is not specified, use the setting from -mcpu.
And if both -march and -mcpu= are not specified, the default for this argument is system dependent — so bestie, if you want a specific architecture extensions, specify one explicitly, fr fr. 🙈
Also: when the RISC-V specifications define an extension as depending on other extensions, GCC will implicitly add the dependent extensions to the enabled extension set if they weren't added explicitly. No cap, it just does that for you. 💅
-mcpu=processor-string— Use architecture of and optimize the output for the given processor, specified by particular CPU name. The permissible values for this option are literally:sifive-e20,sifive-e21,sifive-e24,sifive-e31,sifive-e34,sifive-e76,sifive-s21,sifive-s51,sifive-s54,sifive-s76,sifive-u54,sifive-u74,sifive-x280,sifive-xp450,sifive-x670,thead-c906,tt-ascalon-d8,xiangshan-nanhu. 🎀
Note that -mcpu does not override -march or -mtune. Like, it's just not that girl.
-mtune=processor-string— Optimize the output for the given processor, specified by microarchitecture or particular CPU name. The permissible values here are:generic-ooo,rocket,sifive-3-series,sifive-5-series,sifive-7-series,size,sifive-p400-series,sifive-p600-series, and all valid options for-mcpu=. ✨
When -mtune= is not specified, use the setting from -mcpu — and the default is rocket if both are not specified. 🦋
So the size choice is not intended for use by end-users, okay? This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but it does not override the pipeline info. This helps reduce code size while still giving good performance.
-mpreferred-stack-boundary=num— Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is4(16 bytes or 128-bits). 💜
⚠️ 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. Like, all of them. No mixing. 😭
-msmall-data-limit=n— Put global and static data smaller than n bytes into a special section (on some targets).-msave-restore,-mno-save-restore— Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues. 🌸-mmovcc,-mno-movcc— Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable. 🔥-minline-atomics,-mno-inline-atomics— Do or don't use smaller but slower subword atomic emulation code that useslibatomicfunction calls. The default is to use fast inline subword atomics that do not requirelibatomic. 💫-minline-strlen,-mno-inline-strlen— Do or do not attempt to inlinestrlencalls if possible. Inlining will only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to not inlinestrlencalls. 🫶-minline-strcmp,-mno-inline-strcmp— Do or do not attempt to inlinestrcmpcalls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inlinestrcmpcalls.
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64. 👑
-minline-strncmp,-mno-inline-strncmp— Do or do not attempt to inlinestrncmpcalls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inlinestrncmpcalls. 🌈
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64. (Yes, same param, it's giving shared responsibility.)
-mshorten-memrefs,-mno-shorten-memrefs— Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. 🎀-mstrict-align,-mno-strict-align— Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not. 🦋-mscalar-strict-align,-mno-scalar-strict-align— Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not. This is an alias for-mstrict-align. 🍓-mvector-strict-align,-mno-vector-strict-align— Do not or do generate unaligned vector memory accesses. The default is set to off unless the processor we are optimizing for explicitly supports element-misaligned vector memory access. ✨
-mcmodel=medlow— Generate code for the medium-low code model. The program and its statically defined symbols must lie within a single2 GiBaddress range and must lie between absolute addresses-2 GiBand+2 GiB. Programs can be statically or dynamically linked. This is the default code model unless GCC has been configured with--with-cmodel=specifying a different default code model. 💜-mcmodel=medany— Generate code for the medium-any code model. The program and its statically defined symbols must be within any single2 GiBaddress range. Programs can be statically or dynamically linked. 🌸
The code generated by the medium-any code model is position-independent, but is not guaranteed to function correctly when linked into position-independent executables or libraries. Like, it's giving "works until it doesn't" — be careful. 😭
-mcmodel=large— Generate code for a large code model, which has no restrictions on size or placement of symbols. Main character energy, no limits. 👑🔥-mexplicit-relocs,-mno-exlicit-relocs— Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. 🫶 (And yes, the second one is spelled-mno-exlicit-relocsin the source — copy it exactly, bestie, don't "fix" it.)-mrelax,-mno-relax— Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations. 💅✨-mriscv-attribute,-mno-riscv-attribute— emit (or don't emit) the RISC-V attribute so extra info gets recorded into ELF objects. Heads up bestie: this feature needs at least binutils 2.32. 💅-mcsr-check,-mno-csr-check— turns CSR checking on or off. That's it, that's the vibe. ✨-malign-data=type— controls how GCC aligns variables and constants of array, structure, or union types. Supported values for type arexlen, which uses x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default. 🎀-mbig-endian— generate big-endian code. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.-mlittle-endian— generate little-endian code. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*but not ariscv64be-*-*orriscv32be-*-*target.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary ortlsfor a per-thread canary in the TLS block. 🦋
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. no cap.
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.-mtls-dialect=trad— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
🌸 RL78 Options 🌸
-msim— links in additional target libraries to support operation within a simulator. sim girlies rise 🫶-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— specifies the type of hardware multiplication and division support to be used. The simplest is"none", which uses software for both multiplication and division. This is the default. The"g13"value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The"g14"value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value"rl78"is an alias for"g14"and the value"mg10"is an alias for"none". 😭 yes,"mg10"— that's literally what the page says, don't come for me.
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__". ✨
-mcpu=g10,-mcpu=g13,-mcpu=g14,-mcpu=rl78— specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.
If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether. 🔥
Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support. 💀 classic backwards-compat behavior.
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10,-mg13,-mg14,-mrl78— these are aliases for the corresponding-mcpu=option. They are provided for backwards compatibility. 🎀-mallregs— allow the compiler to use all of the available registers. By default registers"r24..r31"are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. main character registers, honestly.-m64bit-doubles,-m32bit-doubles— make the"double"data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option-mg13needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function. 🧚
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt,-mno-powerpc-gpopt,-mpowerpc-gfxopt,-mno-powerpc-gfxopt,-mpowerpc64,-mno-powerpc64,-mmfcrf,-mno-mfcrf,-mpopcntb,-mno-popcntb,-mpopcntd,-mno-popcntd,-mfprnd,-mno-fprnd,-mcmpb,-mno-cmpb,-mhard-dfp,-mno-hard-dfp— you use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the-mcpu=cpu_typeoverrides the specification of these options. We recommend you use the-mcpu=cpu_typeoption rather than the options listed above. 💅 like, just use-mcpu, bestie.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors. 🔥
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type— set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are401,403,405,405fp,440,440fp,464,464fp,476,476fp,505,601,602,603,603e,604,604e,620,630,740,7400,7450,750,801,821,823,860,970,8540,a2,e300c2,e300c3,e500mc,e500mc64,e5500,e6500,ec603e,G3,G4,G5,titan,power3,power4,power5,power5+,power6,power6x,power7,power8,power9,power10,power11,powerpc,powerpc64,powerpc64le,rs64, andnative. 🦋
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu_type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor. 🙈
The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others. 💀
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protectThe particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec. 🎀
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment. 👑
-mtune=cpu_type— set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type or register usage, as-mcpu=cpu_typedoes. The same values for cpu_type are used for-mtuneas for-mcpu. If both are specified, the code generated uses the architecture and registers set by-mcpu, but the scheduling parameters set by-mtune. 🌈-mcmodel=small— generate PowerPC64 code for the small model: The TOC is limited to 64k. ✨-mcmodel=medium— bestie this one's for PowerPC64: generate code for the medium model, where the TOC plus all your other static data can be up to a total of 4G in size. Fun fact: this is the default for 64-bit Linux. ✨-mcmodel=large— the large model era 💅. The TOC itself may be up to 4G, and everything else (other data, code) is only limited by the 64-bit address space. Big main character energy.-maltivec,-mno-altivec— generate code that uses (or does not use) AltiVec instructions, and also flip on the built-in functions that let you poke the AltiVec instruction set directly. You may also need to set-mabi=altivecto adjust the current ABI with the AltiVec ABI enhancements. 🦋 When-maltivecis used, the element order for AltiVec intrinsics likevec_splat,vec_extract, andvec_insertmatches array element order for the target's endianness. Translation: element zero is the leftmost element in a vector register on big-endian, and the rightmost element on little-endian. iykyk 🌸-mvrsave,-mno-vrsave— generate VRSAVE instructions whenever you're generating AltiVec code. 🎀-msecure-plt— generate code that letsldandld.sobuild executables and shared libraries with non-executable".plt"and".got"sections. This is a PowerPC 32-bit SYSV ABI option. security slay 🔥-mbss-plt— generate code that uses a BSS".plt"section thatld.sofills in, and it requires".plt"and".got"sections that are both writable and executable. Also a PowerPC 32-bit SYSV ABI option. > heads up bestie, writable and executable is a whole vibe — know what you're signing up for 💀-msplit-patch-nops— when you're adding NOPs for a patchable area via the-fpatchable-function-entryoption, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. This makes the sequence of NOPs not consecutive when a global entry point is generated. 🧚 Without this option, the NOPs are emitted directly before and after the local entry point — so they're consecutive, but it moves the global and local entry points further apart. If only a single entry point is generated, this option has no effect at all. no cap.-misel,-mno-isel— this switch enables or disables the generation of ISEL instructions. that's it, that's the tweet 💅-mvsx,-mno-vsx— generate code that uses (or does not use) vector/scalar (VSX) instructions, and also enable the built-in functions that give you more direct access to the VSX instruction set. ⭐-mcrypto,-mno-crypto— enable (or disable) the built-in functions that allow direct access to the cryptographic instructions added in version 2.07 of the PowerPC ISA. 🔐-mhtm,-mno-htm— enable (or disable) the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions, also added in version 2.07 of the PowerPC ISA. 🫶-mpower8-fusion,-mno-power8-fusion— generate code that keeps (or doesn't keep) some integer operations adjacent so the instructions can be fused together on power8 and later processors. fusion core duo vibes 🔥-mquad-memory,-mno-quad-memory— generate code that uses (or doesn't use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode. 👑-mquad-memory-atomic,-mno-quad-memory-atomic— generate code that uses (or doesn't use) the atomic quad word memory instructions. The-mquad-memory-atomicoption also requires use of 64-bit mode. ⚛️-mfloat128,-mno-float128— enable/disable the__float128keyword for IEEE 128-bit floating point, and then use either software emulation for IEEE 128-bit floating point or hardware instructions. 🍓 The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. Also, IEEE 128-bit floating point is only supported on Linux. The default for-mfloat128is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems. If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise — if you don't specify ISA 3.0 instructions, or you're targeting a 32-bit big endian system — IEEE 128-bit floating point gets done with software emulation. 😭-mfloat128-hardware,-mno-float128-hardware— enable/disable using ISA 3.0 hardware instructions to support the__float128data type. 🌈 The default for-mfloat128-hardwareis enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems. periodt.-m32,-m64— generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment setsint,longand pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment setsintto 32 bits andlongand 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. Let's break it down bestie:-mfull-tocis selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC — however, only 16,384 entries are available in the TOC. 😬 If you get a linker error message saying you overflowed the available TOC space, you can reduce the amount of TOC space used with the-mno-fp-in-tocand-mno-sum-in-tocoptions.-mno-fp-in-tocprevents GCC from putting floating-point constants in the TOC, and-mno-sum-in-tocforces GCC to generate code that calculates 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 one makes GCC 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-tocinstead. This option causes GCC to make only one TOC entry for every file. When you use it, GCC produces code that is slower and larger but uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 💀-maix64,-maix32— enable the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bitlongtype, and the infrastructure needed to support them. Specifying-maix64implies-mpowerpc64, while-maix32disables 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 the AIX-compatible ABI. This passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It also means: don't assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double, and use XL symbol names for long double support routines. 🧚 Story time: the AIX calling convention was extended but not initially documented, to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — it's only necessary when calling subroutines compiled by IBM XL compilers without optimization. ngl that's a lot 💅-mpe— support the IBM RS/6000 SP Parallel Environment (PE). This links an application written to use message passing with special startup code so the application can 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-mpeoption and the-pthreadoption are incompatible. 😭 #theycannevermakemebestieswiththreads-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of larger types (like floating-point doubles) onto their natural size-based boundary. Meanwhile-malign-powertells 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-poweris not supported. 🙈-msoft-float,-mhard-float— generate code that does not use (or does use) the floating-point register set. Software floating-point emulation is provided if you use the-msoft-floatoption — and pass the option to GCC when linking. 🫶-mmultiple,-mno-multiple— generate code that uses (or doesn't 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-mmultipleon little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode. The exceptions arePPC740andPPC750, which permit these instructions in little-endian mode. 💀-mupdate,-mno-update— generate code that uses (or doesn't 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's a small window between when the stack pointer is updated and when the address of the previous frame is stored — which means code that walks the stack frame across interrupts or signals may get corrupted data. 😬 bestie, be careful.-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generate code that tries to avoid (or not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, like when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. 🔥
-mfused-madd,-mno-fused-madd— bestie, this is the "do we fuse the math or not" switch 💅 These generate code that uses (or does not use) the floating-point multiply and accumulate instructions, and they're generated by default if hardware floating point is used. Plot twist tho: the machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. So like, they're aliases now. iykyk ✨-mmulhw,-mno-mulhw— generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors 🔥-mdlmzb,-mno-dlmzb— generate code that uses (does not use) the string-searchdlmzbinstruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors 🎀-mno-bit-align,-mbit-align— on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. Okay so fr fr, here's the glow-up example: by default a structure containing nothing but 8unsignedbit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using-mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. Tiny! ✨-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems, do not (do) assume that unaligned memory references are handled by the system 🙈-mrelocatable,-mno-relocatable— generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of.got2and 4-byte locations listed in the.fixupsection, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with-mrelocatableor-mrelocatable-lib. Also-mrelocatablecode aligns the stack to an 8-byte boundary 🫶-mrelocatable-lib,-mno-relocatable-lib— like-mrelocatable,-mrelocatable-libgenerates a.fixupsection to allow static executables to be relocated at run time, but-mrelocatable-libdoes not use the smaller stack alignment of-mrelocatable. Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. Mix and match, we love a flexible icon 🌈-mno-toc,-mtoc— on System V.4 and embedded PowerPC systems, do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program 💅-mlittle,-mlittle-endian— on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The-mlittle-endianoption 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-endianoption is the same as-mbig⭐-mdynamic-no-pic— on Darwin / macOS systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries 🍓-msingle-pic-base— treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. So that's on the runtime, not on us 👑-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 value0,1, or2to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions 💖-msched-costly-dep=dependence_type— this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:no— no dependence is costly.all— all dependences are costly.true_store_to_load— a true dependence from store to load is costly.store_to_load— any dependence from store to load is costly.- number — any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme— this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:no— don't insert NOPs.pad— pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.regroup_exact— insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.- number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
-mcall-sysv— on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC usingpowerpc-*-eabiaix📜-mcall-sysv-eabi,-mcall-eabi— specify both-mcall-sysvand-meabioptions. Two for one, we stan a bundle 💅-mcall-sysv-noeabi— specify both-mcall-sysvand-mno-eabioptions.-mcall-aixdesc— on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.-mcall-linux— on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system 🐧-mcall-freebsd— on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system 😈-mcall-netbsd— on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system 🎏-mcall-openbsd— on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system 🐡-mtraceback=traceback_type— select the type of traceback table. Valid values for traceback_type arefull,part, andno🦋-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). Small structs get the VIP register treatment ✨-mabi=abi-type— extend the current ABI with a particular extension, or remove such extension. Valid values are:altivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX:vec-extabi,vec-default🎀-mabi=ibmlongdouble— change the current ABI to use IBM extended-precisionlong double. This is not likely to work if your system defaults to using IEEE extended-precisionlong double💀 If you change thelong doubletype from IEEE extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.-mabi=ieeelongdouble— change the current ABI to use IEEE extended-precisionlong double. This is not likely to work if your system defaults to using IBM extended-precisionlong double😭 If you change thelong doubletype from IBM extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.-mabi=elfv1— change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. > Overriding the default ABI requires special system support and is likely to fail in spectacular ways. Not me warning you twice 💀🔥-mabi=elfv2— change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. > Overriding the default ABI requires special system support and is likely to fail in spectacular ways. Same energy, different endianness 💅-mgnu-attribute,-mno-gnu-attribute— emit.gnu_attributeassembly directives to set tag/value pairs in a.gnu.attributessection that specify ABI variations in function parameters or return values 🏷️-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments. With-mprototype, only calls to prototyped variable argument functions set or clear the bit. Bit 6 ofCRcatching strays fr fr 😭-msim— on embedded PowerPC systems, assume that the startup module is calledsim-crt0.oand that the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations 🌸-mmvme— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a.-mads— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a.-myellowknife— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a💛 (yellowknife is such a main character name ngl)-mvxworks— on System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system 🧚-memb— on embedded PowerPC systems, set thePPC_EMBbit 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. Okay so here's the breakdown, bestie:- Selecting
-meabimeans that the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and the-msdataoption can use bothr2andr13to point to two separate small data areas. - Selecting
-mno-eabimeans that the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and the-msdataoption only usesr13to point to a single small data area.
- Selecting
The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options ✨
-msdata=eabi— okay so on System V.4 and embedded PowerPC systems, this one puts small initializedconstglobal and static data into the.sdata2section, which registerr2points to. Then small initialized non-constglobals and statics go in.sdata, pointed to by registerr13. And small uninitialized globals and statics go in.sbss, which sits right next to.sdata. Heads up though:-msdata=eabiis incompatible with-mrelocatable💀 and it also sets-membfor you. iykyk.-msdata=sysv— on System V.4 and embedded PowerPC systems, this drops small global and static data into.sdata(pointed to byr13), and small uninitialized global and static data into.sbss, right next to.sdata. Also incompatible with-mrelocatable. No cap.-msdata=default,-msdata— on System V.4 and embedded PowerPC systems: if-meabiis in play, it compiles exactly like-msdata=eabi; otherwise it compiles like-msdata=sysv. Vibes-based delegation, honestly.-msdata=data— on System V.4 and embedded PowerPC systems, this puts small global data in.sdataand small uninitialized global data in.sbss. But it does not use registerr13to address small data, bestie. This is the default behavior unless you're using other-msdataoptions.-msdata=none,-mno-sdata— on embedded PowerPC systems, this puts all initialized global and static data in.data, and all uninitialized data in.bss. Simple. Clean. Iconic.-mreadonly-in-sdata— puts read-only objects in.sdatatoo. This is the default, so slay, nothing to do.- *
-mblock-move-inline-limit=num* — inlines all block moves (think calls tomemcpyor structure copies) that are less than or equal to num bytes. The minimum num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. Default is target-specific. 🎀 -mblock-compare-inline-limit=num — generates non-looping inline code for all block compares (like calls tomemcmpor structure compares) less than or equal to num bytes. If num is 0, *all inline expansion (non-loop *and loop) of block compare gets disabled. Default is target-specific.- *
-mblock-compare-inline-loop-limit=num* — generates an inline expansion using loop code for all block compares less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared beforememcmpis called to handle the rest of the block. Default is target-specific. 💅 - *
-mstring-compare-inline-limit=num* — compares at most num string bytes with inline code. If the difference or end of string isn't found by the end of the inline compare, a call tostrcmporstrncmptakes care of the rest. Default is 64 bytes. Literally that's it. -Gnum — on embedded PowerPC systems, this puts 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-Gnum switch also gets passed to the linker, and all modules should be compiled with the same-Gnum value — so like, keep it consistent, bestie, or things get weird. 🙈-mregnames,-mno-regnames— on System V.4 and embedded PowerPC systems, these do (or do not) emit register names in the assembly language output using symbolic forms. Cute either way.-mlongcall,-mno-longcall— by default, GCC assumes all calls are far away, so a longer and more expensive calling sequence is required. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't possibly be that far away. You can override this setting with theshortcallfunction attribute, or with#pragma longcall(0).
Some linkers can detect out-of-range calls and generate glue code on the fly — on those systems, long calls are unnecessary and just make slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. There are plans to add this feature to the GNU linker for 32-bit PowerPC systems too.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (peep -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls, periodt.
On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). Those two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise the linker generates bl L42 to call the branch island. The branch island gets appended to the body of the calling function and computes the full 32-bit address of the callee, then jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler to emit the glue for every direct call, and the Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. 👑
-mpltseq,-mno-pltseq— implement (or don't implement)-fno-pltand long calls using an inline PLT call sequence that supports lazy linking and long calls to functions indlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with--enable-secureplt. Warning though:-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together. 🫶-mtls-markers,-mno-tls-markers— mark (or don't mark) calls to__tls_get_addrwith a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. Literally teamwork.-mrecip,-mno-recip— enables use of the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps to bump precision, instead of doing a divide (or square root and divide) for floating-point arguments. You should use-ffast-mathwhen you use-mrecip(or at least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math). Real talk though: while the throughput of the sequence is generally higher than the non-reciprocal instruction's throughput, the precision can drop by up to 2 ulp (like, the inverse of 1.0 equals 0.99999994) for reciprocal square roots. 💀- *
-mrecip=opt* — this controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the reciprocal approximation instructions for both single and double precision.divf— enable the single-precision reciprocal approximation instructions.divd— enable the double-precision reciprocal approximation instructions.rsqrt— enable the reciprocal square root approximation instructions for both single and double precision.rsqrtf— enable the single-precision reciprocal square root approximation instructions.rsqrtd— enable the double-precision reciprocal square root approximation instructions.
So for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions except the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions, which handle the double-precision reciprocal square root calculations. Selective queen behavior. ✨
-mrecip-precision,-mno-recip-precision— assume (or don't assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI mandates. Selecting-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precision. The double-precision square root estimate instructions aren't generated by default on low-precision machines, since they don't provide an estimate that converges after three steps. Facts.- *
-mveclibabi=type* — specifies the ABI type to use for vectorizing intrinsics with an external library. The only type supported right now ismass, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics via external libraries. GCC currently emits calls toacosd2,acosf4,acoshd2,acoshf4,asind2,asinf4,asinhd2,asinhf4,atan2d2,atan2f4,atand2,atanf4,atanhd2,atanhf4,cbrtd2,cbrtf4,cosd2,cosf4,coshd2,coshf4,erfcd2,erfcf4,erfd2,erff4,exp2d2,exp2f4,expd2,expf4,expm1d2,expm1f4,hypotd2,hypotf4,lgammad2,lgammaf4,log10d2,log10f4,log1pd2,log1pf4,log2d2,log2f4,logd2,logf4,powd2,powf4,sind2,sinf4,sinhd2,sinhf4,sqrtd2,sqrtf4,tand2,tanf4,tanhd2, andtanhf4when generating code for power7. Both-ftree-vectorizeand-funsafe-math-optimizationsmust also be enabled, and the MASS libraries must be specified at link time. Got all that? 😭 -mfriz,-mno-friz— generate (or don't generate) thefrizinstruction when-funsafe-math-optimizationsis used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Little caveat: thefrizinstruction does not return the same value if the floating-point number is too large to fit in an integer. So like, watch out for that. 🌸-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— turns on (or off) code that loads up the static chain register (r11) when you call through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor. That descriptor gives the function address, the TOC value to load into registerr2, and the static chain value to load into registerr11.-mpointers-to-nested-functionsis on by default. 💅
⚠️ If you use -mno-pointers-to-nested-functions, you literally cannot call through pointers to nested functions, or through pointers to functions compiled in other languages that use the static chain. Like, at all.
-msave-toc-indirect,-mno-save-toc-indirect— generates (or doesn't) code to save the TOC value in the reserved stack location in the function prologue, when the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved right before the call through the pointer instead.-mno-save-toc-indirectis the default, bestie.-mcompat-align-parm,-mno-compat-align-parm— generates (or doesn't) code that passes structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. 🫶
So here's the lore: older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That's fixed in more recent versions of GCC. This option lets you generate code that's compatible with functions compiled by those older GCC versions. Ngl, it's giving legacy support.
-mno-compat-align-parm is the default.
-mstack-protector-guard=guard,-mstack-protector-guard-reg=*reg*,-mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default with GNU libc version 2.4 or later). ✨
With that tls choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which register to use as the base register for reading the canary, and what offset from that base register to read it from. The defaults for those are whatever the relevant ABI says. Periodt.
-mpcrel,-mno-pcrel— generates (or doesn't) pc-relative addressing. Heads up: the-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. No gatekeeping, just facts. 💖-mprefixed,-mno-prefixed— generates (or doesn't) addressing modes using prefixed load and store instructions. The-mprefixedoption requires that the option-mcpu=power10(or later) is enabled.-mmma,-mno-mma— generates (or doesn't) the MMA instructions. The-mmaoption requires that the option-mcpu=power10(or later) is enabled.-mrop-protect,-mno-rop-protect— generates (or doesn't) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). Lowkey a trade-off, but okay.-mprivileged,-mno-privileged— generates (or doesn't) code that will run in privileged state. 👑-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— generates (or doesn't) unaligned vsx loads and stores for inline expansion ofmemcpyandmemmove.--param rs6000-vect-unroll-limit=— the vectorizer checks in with target info to figure out whether unrolling the main vectorized loop would actually be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll the main loop. The default value isfour. 🔥
🌸 RX Options 🌸
These command-line options are defined for RX targets, bestie:
-m64bit-doubles,-m32bit-doubles— makes thedoubledata type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is-m32bit-doubles. Note that RX floating-point hardware only works on 32-bit values, which is literally why the default is-m32bit-doubles. 😭-fpu,-nofpu— enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point values, though, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.
⚠️ Note: if the-fpuoption is enabled then-funsafe-math-optimizationsis also enabled automatically. This is because the RX FPU instructions are themselves unsafe. Like, not me trusting them. 💀
-mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default isRX600.
The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction.
The RX200 series has no hardware floating-point unit, so -nofpu is enabled by default when this type is selected.
-mbig-endian-data,-mlittle-endian-data— store data (but not code!) in the big-endian format. The default is-mlittle-endian-data, i.e. to store data in the little-endian format. 🦋-msmall-data-limit=N — specifies the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited and it is up to the programmer to ensure that the area does not overflow. Also, when the small data area is used, one of the RX's registers (usuallyr13) is reserved for use pointing to this area, so it is no longer available for use by the compiler. That could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register. 🙈
Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, since they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note that this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. ✨
-msim,-mno-sim— use the simulator runtime. The default is to use the libgloss board-specific runtime.-mas100-syntax,-mno-as100-syntax— when generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.-mmax-constant-size=N — specifies the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction. Even though the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. So in some circumstances it can be beneficial to restrict the size of constants used in instructions. Constants that are too big get placed into a constant pool instead and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed. 💅
-mrelax— enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.-mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between0and4. A value of1means registerr13is reserved for the exclusive use of fast interrupt handlers. A value of2reservesr13andr12. A value of3reservesr13,r12andr11, and a value of4reservesr13throughr10. A value of0, the default, does not reserve any registers. 🎀-msave-acc-in-interrupts— specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, since that makes the interrupt handlers faster. ⭐-mpid,-mno-pid— enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This lets the location of constant data be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. 🧚
Note: using this feature reserves a register, usually r13, for the constant data base address. That can result in slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends on whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid, and finally -msmall-data-limit. So yes, it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line. 🤯
By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it's compiling a file. The default is to issue a warning for each extra fast interrupt handler found, since the RX only supports one such interrupt. 💜-mallow-string-insns,-mno-allow-string-insns— turns the string-manipulation instructions on or off:SMOVF,SCMPU,SMOVB,SMOVU,SUNTIL,SWHILE, plus theRMPAinstruction too 🎀
These babies may prefetch data, and that is not safe if you're poking at an I/O register 😭 (see section 12.2.7 of the RX62N Group User's Manual if you want the receipts).
The default is to allow them, and here's the tea: GCC literally cannot reliably detect every single circumstance where a string instruction might end up touching an I/O register, so it can't turn them off automatically for you 🙈. It's all on the programmer — if your program accesses I/O space, you use -mno-allow-string-insns. No cap.
When the instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__; otherwise it defines __RX_DISALLOW_STRING_INSNS__ ✨
-mjsr,-mno-jsr— use only (or not only)JSRinstructions to access functions 💅. You reach for this when your code size exceeds the range ofBSRinstructions. And lowkey important:-mno-jsrdoes not mean "don't useJSR" — it means any type of branch may be used. Different vibes entirely 🦋
Note, bestie, this one matters: the generic GCC command-line option-ffixed-reghas special significance to the RX port when used with theinterruptfunction attribute. That attribute marks a function intended to process fast interrupts. GCC makes sure it only uses the registersr10,r11,r12and/orr13— and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options ⭐
🦋 S/390 and zSeries Options 🦋
These are the -m options defined for the S/390 and zSeries architecture. Buckle up, there's a whole crew of them 💜
-mhard-float,-msoft-float— use (or don't use) the hardware floating-point instructions and registers for floating-point operations 🌸. With-msoft-float, functions inlibgcc.ado the floating-point work; with-mhard-float, the compiler generates IEEE floating-point instructions.-mhard-floatis the default.-mhard-dfp,-mno-hard-dfp— use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations ✨. With-mno-hard-dfp, functions inlibgcc.ahandle decimal-floating-point ops; with-mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for-march=z9-ecor higher.-mlong-double-64,-mlong-double-128— these switches control the size of thelong doubletype 🎀. A size of 64 bits makeslong doubleequivalent todouble. That's the default.-mbackchain,-mno-backchain— store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame 🦋. A backchain may be needed so debugging tools that don't understand DWARF call frame information can still do their thing. When-mno-packed-stackis in effect, the backchain pointer sits at the bottom of the stack frame; when-mpacked-stackis in effect, the backchain goes into the topmost word of the 96/160 byte register save area 👑
In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, using the backchain for debugging usually requires that the whole binary is built with -mbackchain 😭. 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 maintain the backchain.
-mpacked-stack,-mno-packed-stack— use (or don't use) the packed stack layout 💅. With-mno-packed-stack, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. With-mpacked-stack, register save slots are densely packed at the top of the register save area, and unused space gets reused for other purposes — more efficient use of the stack space you've got ✨. But when-mbackchainis 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 isn't 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 — and that 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 don't generate) code using thebrasinstruction to do subroutine calls 🌈. This only works reliably if the total executable size does not exceed 64k ⚠️. The default is to use thebasrinstruction instead, which does not have this limitation.-m64,-m31—-m31generates code compliant to the GNU/Linux for S/390 ABI;-m64generates code compliant to the GNU/Linux for zSeries ABI 🦋. This lets GCC in particular generate 64-bit instructions. For the s390 targets, the default is-m31, while the s390x targets default to-m64👑-mzarch,-mesa—-mzarchgenerates code using the instructions available on z/Architecture;-mesagenerates code using the instructions available on ESA/390 🎀. Note that-mesais not possible with-m64🙈. When generating code compliant to the GNU/Linux for S/390 ABI, the default is-mesa; when generating code compliant to the GNU/Linux for zSeries ABI, the default is-mzarch.-mhtm,-mno-htm—-mhtmenables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions ✨.-mhtmis enabled by default when using-march=zEC12.-mvx,-mno-vx—-mvxgenerates code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation 💜. Heads up: this option changes the ABI for some vector type values with regard to alignment and calling conventions 😳. In case vector type values are being used in an ABI-relevant context, a GAS.gnu_attributecommand will be added to mark the resulting binary with the ABI used.-mvxis enabled by default when using-march=z13.-mzvector,-mno-zvector—-mzvectorenables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation 🌸. This option adds support forvectorto be used as a keyword to define vector type variables and arguments.vectoris only available when GNU extensions are enabled — it will not be expanded when requesting strict standard compliance, e.g. with-std=c99🫶. In addition to the GCC low-level builtins,-mzvectorenables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To make use of these builtins, the header filevecintrin.hneeds to be included.-mzvectoris disabled by default.-mmvcle,-mno-mvcle— generate (or don't generate) code using themvcleinstruction to perform block moves 🎀. When-mno-mvcleis specified, use amvcloop instead. This is the default unless optimizing for size.-mdebug,-mno-debug— print (or don't print) additional debug information when compiling 👑. The default is to not print debug information.-march=cpu-type— generate code that runs on cpu-type, which is the name of a system representing a certain processor type 💅. Possible values for cpu-type arez900/arch5,z990/arch6,z9-109,z9-ec/arch7,z10/arch8,z196/arch9,zEC12,z13/arch11,z14/arch12,z15/arch13,z16/arch14,z17/arch15, andnative.
The default is -march=z900.
Specifying native as cpu-type can be used to select the best architecture option for the host processor. Note that -march=native has no effect if GCC does not recognize the processor 🌈
-mtune=cpu-type— tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions 🦋. The list of cpu-type values is the same as for-march. The default is the value used for-march.-mtpf-trace,-mno-tpf-trace— generate code that adds (or 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.-mtpf-trace-skip,-mno-tpf-trace-skip— generate code that changes (or does not change) the default branch targets enabled by-mtpf-traceto point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS ✨. This option is off by default, even when compiling for the TPF OS and specifying-mtpf-trace.-mfused-madd,-mno-fused-madd— generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions 🎀. These instructions are generated by default if hardware floating point is used.- *
-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size 💜. Because this is a compile-time check, it doesn't need to be a real problem when the program runs. It's intended to identify functions that most probably cause a stack overflow. Super useful in an environment with limited stack size, e.g. the linux kernel 👑 -mwarn-dynamicstack— emit a warning if the function callsallocaor uses dynamically-sized arrays 😭. This is generally a bad idea with a limited stack size ⚠️-mstack-guard=stack-guard💅-mstack-size=stack-size🫶
If you hand these over, the S/390 back end slips extra instructions into the function prologue that trip a trap whenever the stack size is stack-guard bytes above stack-size (and yes bestie, remember that the stack on S/390 grows downward — that detail matters, ok?).
Skip -mstack-guard entirely and it just picks the smallest power of 2 that's larger than the frame size of the compiled function. Iconic of it, honestly.
These two are literally here to help you debug stack overflow problems. The extra emitted code costs only a little overhead, so it can also chill in production-like systems without any greater performance degradation. ✨
Rules though, no cap:
- the values you give have to be exact powers of 2
stack-sizehas to be greater thanstack-guardwithout exceeding64k- to stay efficient, the extra code assumes the stack starts at an address aligned to the value you gave for
stack-size -mstack-guardcan only be used in conjunction withstack-size— it does not fly solo, periodt
-mhotpatch=pre-halfwords,post-halfwords🔥
Turn the hotpatch option on and a "hot-patching" function prologue gets generated for all functions in the compilation unit. The function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000). She really said "biggest NOP energy". 💀
If both arguments are zero, hotpatching is disabled.
You can override this for individual functions with the "hotpatch" attribute. 🎀
✨ SH Options ✨
These -m options are defined for the SH implementations, bestie — there's a lot of them, so buckle up:
-m1— generate code for the SH1.-m2— generate code for the SH2.-m2e— generate code for the SH2e.-m2a-nofpu— generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used. (So it exists, it just sits there, unbothered. 💅)-m2a-single-only— generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.-m2a-single— generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.-m2a— generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.-m3— generate code for the SH3.-m3e— generate code for the SH3e.-m4-nofpu— generate code for the SH4 without a floating-point unit.-m4-single-only— generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.-m4-single— generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.-m4— generate code for the SH4.-m4-100— generate code for SH4-100.-m4-100-nofpu— generate code for SH4-100 in such a way that the floating-point unit is not used.-m4-100-single— generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.-m4-100-single-only— generate code for SH4-100 in such a way that no double-precision floating-point operations are used.-m4-200— generate code for SH4-200.-m4-200-nofpu— generate code for SH4-200 in such a way that the floating-point unit is not used.-m4-200-single— generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.-m4-200-single-only— generate code for SH4-200 in such a way that no double-precision floating-point operations are used.-m4-300— generate code for SH4-300.-m4-300-nofpu— generate code for SH4-300 in such a way that the floating-point unit is not used.-m4-300-single— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-300-single-only— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-340— generate code for SH4-340 (no MMU, no FPU). Bare bones. Minimalist. She's giving stripped-down queen. 👑-m4-500— generate code for SH4-500 (no FPU). Passes-isa=sh4-nofputo the assembler.-m4a-nofpu— generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.-m4a-single-only— generate code for the SH4a, in such a way that no double-precision floating-point operations are used.-m4a-single— generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.-m4a— generate code for the SH4a.-m4al— same as-m4a-nofpu, except that it implicitly passes-dspto the assembler. And ngl, 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. ⚠️ Heads up bestie, this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with-mdalign.-mrelax— shorten some address references at link time, when possible; uses the linker option-relax.-mbigtable— use 32-bit offsets inswitchtables. The default is to use 16-bit offsets.-mbitops— enable the use of bit manipulation instructions on SH2A.-mfmovd— enable the use of the instructionfmovd. Check-mdalignfor alignment constraints. 👀-mrenesas— comply with the calling conventions defined by Renesas.-mno-renesas— comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain. (Yes, she's the default. Main character energy by decree. ✨)-mnomacsave— mark theMACregister as call-clobbered, even if-mrenesasis given.-mieee-mno-ieee— control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either-mieeeor-mno-ieee. 🦋-minline-ic_invalidate— inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if-musermodeis in effect and the selected code generation option (e.g.-m4) does not allow the use of theicbiinstruction. If the selected code generation option does not allow the use of theicbiinstruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. 💀 And that's not just a "requires privileged mode at run time" situation — it also fails if the cache line had been mapped via the TLB and has become unmapped. So like… careful.-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. You've been warned, bestie.-matomic-model=model— sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see__atomic Builtins. The following models and parameters are supported:none— disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is notsh*-*-linux*.soft-gusa— generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target issh*-*-linux*and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructionsmovli.landmovco.lto create more efficient code, unlessstrictis specified. 🔥soft-tcb— generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, thegbr-offset=parameter has to be specified as well. 👀soft-imask— generate software atomic sequences that temporarily disable interrupts by settingSR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target issh*-*-linux*and SH1* or SH2*.hard-llcs— generate hardware atomic sequences using themovli.landmovco.linstructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model. 💅gbr-offset=— this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when thesoft-tcbmodel has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range0-1020. 🔢strict— this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only. No mixing, no blending, one model only, periodt. 🎀
-mtas— makes the compiler spit out thetas.bopcode for__atomic_test_and_set. Heads up though bestie ⚠️ depending on your particular hardware and software configuration this can tank overall performance, because of the operand cache line flushes that thetas.binstruction implies. And on multi-core SH4A processors thetas.binstruction must be used with caution, since it can result in data corruption for certain cache configurations. Like, actual data corruption. Not cute. 😭-mprefergot— when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. that's the whole vibe 💅-musermode,-mno-usermode— don't allow (allow) the compiler to generate privileged mode code. specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code wouldn't work in user mode.-musermodeis the default when the target issh*-*-linux*. and if the target is SH1* or SH2*,-musermodehas no effect, because there's no user mode to speak of 🫠-multcost=number — set the cost to assume for a multiply insn. yeah, just a number, go off ✨-mdiv=strategy — sets the division strategy used for integer division operations. strategy can be one of these bad boys:call-div1— calls a library function that uses the single-step division instructiondiv1to do the operation. division by zero calculates an unspecified result and does not trap. this is the default except for SH4, SH2A and SHcompact.call-fp— calls a library function that performs the operation in double precision floating point. division by zero causes a floating-point exception 😬. this is the default for SHcompact with FPU. specifying this for targets that don't have a double precision FPU defaults tocall-div1.call-table— calls a library function that uses a lookup table for small divisors and thediv1instruction with case distinction for larger divisors. division by zero calculates an unspecified result and does not trap. this is the default for SH4. specifying this for targets that don't have dynamic shift instructions defaults tocall-div1.
and when a division strategy hasn't been specified, the default strategy gets picked based on the current target. for SH2A the default strategy is to use the divs and divu instructions instead of library function calls 👑
-maccumulate-outgoing-args— reserve space once for outgoing arguments in the function prologue rather than around each call. generally beneficial for performance and size. also needed for unwinding, to avoid changing the stack frame around conditional code. efficient queen 🎀-mdivsi3_libfunc=name — sets the name of the library function used for 32-bit signed division to name. this only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option weren't present. no cap 🫶-mfixed-range=register-range — generate code treating the given register range as fixed registers. a fixed register is one that the register allocator cannot use. super useful when compiling kernel code. a register range is specified as two registers separated by a dash. multiple register ranges can be specified, separated by a comma.-mbranch-cost=num — assume num to be the cost for a branch instruction. higher numbers make the compiler try to generate more branch-free code if it can. if you don't specify it, the value is selected depending on the processor type that's being compiled for.-mzdcbranch,-mno-zdcbranch— assume (do not assume) that zero displacement conditional branch instructionsbtandbfare fast. with-mzdcbranchspecified, the compiler prefers zero displacement branch code sequences. this is enabled by default when generating code for SH4 and SH4A. it can be explicitly disabled by specifying-mno-zdcbranch✨-mcbranch-force-delay-slot— forces the usage of delay slots for conditional branches, which stuffs the delay slot with anopif a suitable instruction can't be found. by default this option is disabled. it can be enabled to work around hardware bugs as found in the original SH7055. bug workaround era 💀-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. these instructions are generated by default if hardware floating point is used. the machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. glow-up migration, we love to see it 💅-mfsca,-mno-fsca— allow or disallow the compiler to emit thefscainstruction for sine and cosine approximations. the option-mfscamust be used in combination with-funsafe-math-optimizations. it's enabled by default when generating code for SH4A. using-mno-fscadisables sine and cosine approximations even if-funsafe-math-optimizationsis in effect.-mfsrra,-mno-fsrra— allow or disallow the compiler to emit thefsrrainstruction for reciprocal square root approximations. the option-mfsrramust be used in combination with-funsafe-math-optimizationsand-ffinite-math-only. it's enabled by default when generating code for SH4A. using-mno-fsrradisables reciprocal square root approximations even if-funsafe-math-optimizationsand-ffinite-math-onlyare in effect. two flags, bestie, don't forget 😤-mpretend-cmove— prefer zero-displacement conditional branches for conditional move instruction patterns. this can result in faster code on the SH4 processor. slay ⚡-mfdpic— generate code using the FDPIC ABI.
🌞 Solaris 2 Options 🌞
these -m options are supported on Solaris 2, bestie:
-mclear-hwcap—-mclear-hwcaptells the compiler to remove the hardware capabilities generated by the Solaris assembler. this is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them. literally "check at runtime" energy 🕵️-mimpure-text—-mimpure-text, used in addition to-shared, tells the compiler to not pass-z textto the linker when linking a shared object. using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. however, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes 😭. instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. do that instead, fr fr 🫶
these switches are supported in addition to the above on Solaris 2:
-pthreads— this is a synonym for-pthread. that's it. that's the whole thing 💅
🔥 SPARC Options 🔥
these -m options are supported on the SPARC:
-mno-app-regs,-mapp-regs— specify-mapp-regsto generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. this is the default.
to be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. you should compile libraries and system software with this option 👑
-mflat,-mno-flat— with-mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. this model is compatible with the regular register window model. the local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.
with -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). this is the normal operating mode, aka main character energy ✨
-mfpu,-mhard-float— generate output containing floating-point instructions. this is the default 💖-mno-fpu,-msoft-float— generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC targets. normally the facilities of the machine's usual C compiler get 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 targetssparc-*-aoutandsparclite-*-*do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it's only useful if you compile all of a program with this option. in particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. all-or-nothing situation, bestie 😤
-mhard-quad-float— generate output containing quad-word (long double) floating-point instructions.-msoft-quad-float— generate output containing library calls for quad-word (long double) floating-point instructions. the functions called are those specified in the SPARC ABI. this is the default.
as of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. they all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. because of the trap handler overhead, this is much slower than calling the ABI library routines 💀. thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. this is the default.
with -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they're 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's not the default because it results in a performance loss, especially for floating-point code 😬
-muser-mode,-mno-user-mode— do not generate code that can only run in supervisor mode. this is relevant only for thecasainstruction emitted for the LEON3 processor. this is the default 🎀-mfaster-structs,-mno-faster-structs— with-mfaster-structs, the compiler just assumes structs are 8-byte aligned ✨ which is so convenient because it can then use pairs oflddandstdinstructions for copies in struct assignment instead of literally twice as manyldandstpairs. slay for speed. BUT 🚨 — that changed alignment directly violates the SPARC ABI. so it's only meant for targets where the dev is fully aware their resulting code isn't in line with the ABI rules. iykyk, proceed with caution 💅-mstd-struct-return,-mno-std-struct-return— with-mstd-struct-return, the compiler drops checking code into functions that return structs or unions, so it can detect size mismatches between the two sides of a function call, as per the 32-bit ABI 🫶
the default is -mno-std-struct-return. and heads up — this option has no effect in 64-bit mode 💀
-mcpu=cpu_type— sets the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. supported values for cpu_type arev7,cypress,v8,supersparc,hypersparc,leon,leon3,leon3v7,leon5,sparclite,f930,f934,sparclite86x,sparclet,tsc701,v9,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8👑
native Solaris and GNU/Linux toolchains also support the value native, which picks the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor 🙈
default instruction scheduling parameters get used for values that select an architecture and not an implementation. those are v7, v8, sparclite, sparclet, v9.
okay so here's the whole match-up of each supported architecture and their supported implementations, bestie:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclite—f930,f934,sparclite86xsparclet—tsc701v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
by default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. with -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. that one's also appropriate for the older SPARCStation 1, 2, IPX etc. 🎀
with -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. the only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7 🔥 with -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
with -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. this adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7 ✨ with -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. with -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU 💖
with -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. this adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7. with -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip 🦋
with -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. this adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. with -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. with -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. with -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. with -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. with -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. with -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. with -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. with -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips 🌈
-mtune=cpu_type— sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that-mcpu=cpu_typedoes 🫶
the same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are the ones that select a particular CPU implementation. those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. with native Solaris and GNU/Linux toolchains, native can also be used ✨
-mv8plus,-mno-v8plus— with-mv8plus, GCC generates code for the SPARC-V8+ ABI. the difference from the V8 ABI is that the global and out registers are considered 64 bits wide 👑 this is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.-mvis,-mno-vis— with-mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. the default is-mno-vis💅-mvis2,-mno-vis2— with-mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis2when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. setting-mvis2also sets-mvis🎀-mvis3,-mno-vis3— with-mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis3when targeting a cpu that supports such instructions, such as niagara-3 and later. setting-mvis3also sets-mvis2and-mvis🌸-mvis3b,-mno-vis3b— with-mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011 ✨ the default is-mvis3bwhen targeting a cpu that supports such instructions, such as niagara-7 and later. setting-mvis3balso sets-mvis3,-mvis2and-mvis.-mvis4,-mno-vis4— with-mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis4when targeting a cpu that supports such instructions, such as niagara-7 and later. setting-mvis4also sets-mvis3b,-mvis3,-mvis2and-mvis💖-mvis4b,-mno-vis4b— with-mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017 🔥 the default is-mvis4bwhen targeting a cpu that supports such instructions, such as m8 and later. setting-mvis4balso sets-mvis4,-mvis3b,-mvis3,-mvis2and-mvis.-mcbcond,-mno-cbcond— with-mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. the default is-mcbcondwhen targeting a CPU that supports such instructions, such as Niagara-4 and later 🦋-mfmaf,-mno-fmaf— with-mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. the default is-mfmafwhen targeting a CPU that supports such instructions, such as Niagara-3 and later ⭐-mfsmuld,-mno-fsmuld— with-mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. the default is-mfsmuldwhen targeting a CPU supporting the architecture versions V8 or V9 with FPU except-mcpu=leon🙈-mpopc,-mno-popc— with-mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. the default is-mpopcwhen targeting a CPU that supports such an instruction, such as Niagara-2 and later 💜-msubxc,-mno-subxc— with-msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. the default is-msubxcwhen targeting a CPU that supports such an instruction, such as Niagara-7 and later ✨-mfix-at697f— enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor) 🫶-mfix-ut699— enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.-mfix-ut700— enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.-mfix-gr712rc— enables the documented workaround for the back-to-back store errata of the GR712RC processor 💅
these -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments, bestie 👑
-m32,-m64— generate code for a 32-bit or 64-bit environment. the 32-bit environment setsint,longand pointer to 32 bits. the 64-bit environment setsintto 32 bits andlongand pointer to 64 bits 🌈-mcmodel=which— set the code model to one of:medlow— the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. programs can be statically or dynamically linked 🎀medmid— the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.medany— the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment 🌸
embmedany— the Medium/Anywhere code model for embedded systems, and bestie the name is literally longer than the vibe 💀 okay so: 64-bit addresses, and the text and data segments must each be less than 2GB in size, both of them starting anywhere in memory (that gets decided at link time). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported. Periodt.-mmemory-model=mem-model— sets the memory model in force on the processor to one of these babies:default— the default memory model for the processor and operating system, no notes ✨rmo— Relaxed Memory Orderpso— Partial Store Ordertso— Total Store Ordersc— Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field. iykyk 🫶
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes that the stack pointer, and the 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. That's it, that's the whole vibe 💅
🌈 Options for System V 🌈
These additional options are available on System V Release 4 for compatibility with other compilers on those systems, so like… legacy besties, we still love you:
-G— creates a shared object. It is recommended that-symbolicor-sharedbe used instead (so like, it works, but the cool kids use the other ones 👀).-Qy— identifies the versions of each tool used by the compiler, in a.identassembler directive in the output. Little receipts, love that for us 🧾-Qn— refrains from adding.identdirectives to the output file (this is the default). No receipts, no paper trail ✨-YP,dirs— searches the directories dirs, and no others, for libraries specified with-l. Exclusive much? 💅-Ym,dir— looks in the directory dir to find the M4 preprocessor. The assembler uses this option.
🦋 V850 Options 🦋
These -m options are defined for V850 implementations:
-mlong-calls,-mno-long-calls— treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. Long distance relationship core 😭-mno-ep,-mep— do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times, to copy that pointer into theepregister and use the shortersldandsstinstructions. The-mepoption is on by default if you optimize ⭐-mno-prolog-function,-mprolog-function— do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The-mprolog-functionoption is on by default if you optimize.-mspace— try to make the code as small as possible. At present, this just turns on the-mepand-mprolog-functionoptions. Tiny code, tiny everything 🎀-mtda=n— put static or global variables whose size is n bytes or less into the tiny data area that registereppoints to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references). It's giving pocket-sized.-msda=n— put static or global variables whose size is n bytes or less into the small data area that registergppoints to. The small data area can hold up to 64 kilobytes.-mzda=n— put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.-mv850— specify that the target processor is the V850.-mv850e3v5— specify that the target processor is the V850E3V5. The preprocessor constant__v850e3v5__is defined if this option is used.-mv850e2v4— specify that the target processor is the V850E3V5. This is an alias for the-mv850e3v5option. (yes bestie, that's what it says, don't come for me 🙈)-mv850e2v3— specify that the target processor is the V850E2V3. The preprocessor constant__v850e2v3__is defined if this option is used.-mv850e2— specify that the target processor is the V850E2. The preprocessor constant__v850e2__is defined if this option is used.-mv850e1— specify that the target processor is the V850E1. The preprocessor constants__v850e1__and__v850e__are defined if this option is used.-mv850es— specify that the target processor is the V850ES. This is an alias for the-mv850e1option.-mv850e— specify that the target processor is the V850E. The preprocessor constant__v850e__is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined. Slay, the compiler picks for you 🫶
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. Iconic, immovable, unbothered 👑
-mdisable-callt,-mno-disable-callt— this option suppresses generation of theCALLTinstruction for thev850e,v850e1,v850e2,v850e2v3andv850e3v5flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined.
-mrelax,-mno-relax— pass on (or do not pass on) the-mrelaxcommand-line option to the assembler.-mlong-jumps,-mno-long-jumps— disable (or re-enable) the generation of PC-relative jump instructions. Jump, bestie, jump 🦘-msoft-float,-mhard-float— disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol__FPU_OK__is defined, otherwise the symbol__NO_FPU__is defined.-mloop— enables the use of the e3v5LOOPinstruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental. Lowkey living on the edge, highkey not recommended yet 💀-mrh850-abi,-mghs— enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:- Integer sized structures and unions are returned via a memory pointer rather than a register.
- Large structures and unions (more than 8 bytes in size) are passed by value.
- Functions are aligned to 16-bit boundaries.
- The
-m8byte-aligncommand-line option is supported. - The
-mdisable-calltcommand-line option is enabled by default. The-mno-disable-calltcommand-line option is not supported.
When this version of the ABI is enabled the C preprocessor symbol __V850_RH850_ABI__ is defined.
-mgcc-abi— enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:- Integer sized structures and unions are returned in register
r10. - Large structures and unions (more than 8 bytes in size) are passed by reference.
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- The
-m8byte-aligncommand-line option is not supported. - The
-mdisable-calltcommand-line option is supported but not enabled by default.
- Integer sized structures and unions are returned in register
When this version of the ABI is enabled the C preprocessor symbol __V850_GCC_ABI__ is defined.
-m8byte-align,-mno-8byte-align— enables support fordoubleandlong longtypes to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4 bytes. When-m8byte-alignis in effect the C preprocessor symbol__V850_8BYTE_ALIGN__is defined.-mbig-switch— generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table. Fix-it energy, not a lifestyle 🩹-mapp-regs— this option causesr2andr5to be used in the code generated by the compiler. This setting is the default. ⭐-mno-app-regs— this option causesr2andr5to be treated as fixed registers.
🍓 VAX Options 🍓
These -m options are defined for the VAX:
-munix— do not output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX cannot handle across long ranges.-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used.-md,-md-float— use the D_floating data format for double-precision floating-point numbers instead of G_floating.-mg,-mg-float— use the G_floating data format for double-precision floating-point numbers instead of D_floating.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. Not me testing the experimental branch in prod 🙈
🧚 Visium Options 🧚
-mdebug— a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the librarieslibc.aandlibdebug.ato be linked. The program should be run on the target under the control of the GDB remote debugging stub.-msim— a program which performs file I/O and is destined to run on the simulator should be linked with this option. This causes librarieslibc.aandlibsim.ato be linked.-mfpu,-mhard-float— generate code containing floating-point instructions. This is the default. ✨-mno-fpu,-msoft-float— generate code containing library calls for floating-point.
⚠️ Bestie listen to me:-msoft-floatchanges the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work. Half-applying this is giving chaos, and not the cute kind 💀
-mcpu=cpu_type— set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type aremcm,gr5andgr6.
mcm is a synonym of gr5 present for backward compatibility. Nostalgia, we stan 🫶
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions. Block moves, main character energy, go off 👑
-mtune=cpu_type— sets the instruction scheduling parameters for machine typecpu_type, but it does not set the instruction set or register set that-mcpu=cpu_typewould. tuning only, bestie 🎛️-msv-mode— generates code for supervisor mode, where there are no restrictions on access to general registers. this one is the default 👑-muser-mode— generates code for user mode, where access to some general registers is forbidden: on the GR5, registersr24tor31cannot be accessed in this mode; on the GR6, only registersr29tor31are affected 🙈
🖥️ VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes— return VMS condition codes frommain. the default is to return POSIX-style condition (e.g. error) codes 💅-mdebug-main=prefix— flag the first routine whose name starts withprefixas the main routine for the debugger 🐛-mmalloc64— default to 64-bit memory allocation routines 🧠-mpointer-size=size— set the default size of pointers. possible options forsizeare32orshortfor 32 bit pointers,64orlongfor 64 bit pointers, andnofor supporting only 32 bit pointers. that last option disablespragma pointer_size✨
🌀 VxWorks Options
The options in this section are defined for all VxWorks targets. options specific to the target hardware are listed with the other options for that target. periodt 🫶
-mrtp— GCC can generate code for both VxWorks kernels and real time processes (RTPs). this option switches from the former to the latter. it also defines the preprocessor macro__RTP__🔥-msmp— select SMP runtimes for linking. not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice ⚠️-non-static— link an RTP executable against shared libraries rather than static libraries. the options-staticand-sharedcan also be used for RTPs;-staticis the default 🧚-Bstatic,-Bdynamic— these options are passed down to the linker. they are defined for compatibility with Diab 🌸-Xbind-lazy— enable lazy binding of function calls. this option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab 💜-Xbind-now— disable lazy binding of function calls. this option is the default and is defined for compatibility with Diab ⭐
💻 x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type— generate instructions for the machine typecpu-type. in contrast to-mtune=cpu-type, which merely tunes the generated code for the specifiedcpu-type,-march=cpu-typeallows GCC to generate code that may not run at all on processors other than the one indicated. specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise 👀
The choices for cpu-type are:
native— this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. using-march=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines). using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set 🖥️x86-64— a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support ✨x86-64-v2,x86-64-v3,x86-64-v4— these choices forcpu-typeselect the corresponding micro-architecture level from the x86-64 psABI. on ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level. since thesecpu-typevalues do not have a corresponding-mtunesetting, using-marchwith these values enables generic tuning. specific tuning can be enabled using the-mtune=other-cpu-typeoption with an appropriateother-cpu-typevalue 💅i386— original Intel i386 CPU 👑i486— Intel i486 CPU. (no scheduling is implemented for this chip.) 💀i586,pentium— Intel Pentium CPU with no MMX support 🌸lakemont— Intel Lakemont MCU, based on Intel Pentium CPU 🦋pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support 💖pentiumpro— Intel Pentium Pro CPU with no MMX support 🫶i686— when used with-march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. when used with-mtune, it has the same meaning asgeneric⭐pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support 🎀pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support 💜pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. used by Centrino notebooks 🧚pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support 🔥prescott— improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support 🌈nocona— improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support 🦋core2— Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support 🌸nehalem,corei7— Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support 👑westmere— Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support 💖sandybridge,corei7-avx— Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support ✨ivybridge,core-avx-i— Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support 🦋haswell,core-avx2— Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support 🔥broadwell— Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support 🌈skylake— Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support 💜skylake-avx512— Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support 👑cascadelake— Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support 🔥cannonlake— Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support 🌸cooperlake— Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support 🦋icelake-client— Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support 💖icelake-server— Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support 👑tigerlake— Intel Tiger Lake CPU with 64-bit extensions, and the full flex is:MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECTandKEYLOCKERinstruction set support. she's stacked 💅rocketlake— Intel Rocket Lake CPU with 64-bit extensions, rockingMMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPIDandAVX512VPOPCNTDQinstruction set support.alderlakeraptorlakemeteorlakegracemont— all four of these names point at the same vibe: Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with 64-bit extensions, servingMOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKLandAVX-VNNIinstruction set support. iconic group-chat energy fr fr 🫶arrowlake— Intel Arrow Lake CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERTandCMPCCXADDinstruction set support.arrowlake-slunarlake— both of these are the Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3andSM4instruction set support. yes bestie, SHA512 AND SM3 AND SM4, we ate 🌸pantherlake— Intel Panther Lake CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4andPREFETCHIinstruction set support.sapphirerapidsemeraldrapids— Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, and the list is honestly main-character:MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16andAVX512BF16instruction set support. 😭 it's giving server rack royaltygraniterapids— Intel Granite Rapids CPU with 64-bit extensions:MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16andPREFETCHIinstruction set support.graniterapids-d— Intel Granite Rapids D CPU with 64-bit extensions:MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHIandAMX-COMPLEXinstruction set support. (and yes, it's writtenAVX512FP16here, no dash — literally not me changing it, that's just how she's listed 👀)diamondrapids— Intel Diamond Rapids CPU with 64-bit extensions, and this list is so long it's basically a flex:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI, AMX-COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3, SM4, AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, AMX-TF32, AMX-TRANSPOSE, MOVRS, AMX-MOVRSandUSER_MSRinstruction set support. 🔥 ok she ate and left no crumbsbonnellatom— Intel Bonnell CPU with 64-bit extensions, keeping it cute and small:MOVBE, MMX, SSE, SSE2, SSE3andSSSE3instruction set support. tiny but mighty 🎀silvermontslm— Intel Silvermont CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHWandRDRNDinstruction set support.goldmont— Intel Goldmont CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPTandFSGSBASEinstruction set support.goldmont-plus— Intel Goldmont Plus CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPIDandSGXinstruction set support.tremont— Intel Tremont CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTEandWAITPKGinstruction set support.sierraforest— Intel Sierra Forest CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMDandUINTRinstruction set support. e-cores that serve looks 💅grandridge— Intel Grand Ridge CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMDandUINTRinstruction set support.clearwaterforest— Intel Clearwater Forest CPU with 64-bit extensions:MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSRandPREFETCHIinstruction set support. (note she's missingKL/WIDEKLfrom the earlier ones — that's literally just how she's listed, no vibe check needed 🦋)
now we're sliding into the AMD section, bestie, and it's a whole different aesthetic:
k6— AMD K6 CPU withMMXinstruction set support.k6-2k6-3— improved versions of AMD K6 CPU withMMXand3DNow!instruction set support.athlonathlon-tbird— AMD Athlon CPU withMMX,3dNOW!, enhanced3DNow!andSSEprefetch instructions support. (yes,3dNOW!is spelled with a lowercase d there and3DNow!with a capital one elsewhere — copy her exactly, don't @ me 💀)athlon-4athlon-xpathlon-mp— improved AMD Athlon CPU withMMX,3DNow!, enhanced3DNow!and fullSSEinstruction set support.k8opteronathlon64athlon-fx— processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (this one supersetsMMX,SSE,SSE2,3DNow!, enhanced3DNow!and 64-bit instruction set extensions.) 👑 the OG 64-bit queenk8-sse3,opteron-sse3,athlon64-sse3— glow-up versions of the AMD K8 cores that added SSE3 instruction set support. 💫amdfam10,barcelona— CPUs built on AMD Family 10h cores with x86-64 instruction set support. (This supersetsMMX,SSE,SSE2,SSE3,SSE4A,3DNow!, enhanced3DNow!,ABMand 64-bit instruction set extensions.)bdver1— AMD Family 15h cores, x86-64 support, periodt. (SupersetsFMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)bdver2— AMD Family 15h core based CPUs with x86-64. (SupersetsBMI,TBM,F16C,FMA,FMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)bdver3— AMD Family 15h core based, x86-64, she ate. (SupersetsBMI,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)bdver4— AMD Family 15h core based, x86-64, the baddest of the bdver sisters 💅 (SupersetsBMI,BMI2,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,AVX2,XOP,LWP,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)znver1— AMD Family 17h core based CPUs, x86-64, the Zen era begins 🔥 (SupersetsBMI,BMI2,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT, and 64-bit instruction set extensions.)znver2— AMD Family 17h core based CPUs with x86-64. (SupersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD, and 64-bit instruction set extensions.)znver3— AMD Family 19h core based CPUs with x86-64 instruction set support. (SupersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES, and 64-bit instruction set extensions.)znver4— AMD Family 19h core based CPUs with x86-64. AVX-512 girlies rise 😭💜 (SupersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNIand 64-bit instruction set extensions.)znver5— AMD Family 1ah core based CPUs with x86-64 instruction set support. The newest one, main character energy fr fr 👑 (SupersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNI,AVXVNNI,MOVDIRI,MOVDIR64B,AVX512VP2INTERSECT,PREFETCHIand 64-bit instruction set extensions.)btver1— AMD Family 14h cores, x86-64 support. (SupersetsMMX,SSE,SSE2,SSE3,SSSE3,SSE4A,CX16,ABMand 64-bit instruction set extensions.)btver2— AMD Family 16h cores, x86-64 support. This one includesMOVBE,F16C,BMI,AVX,PCLMUL,AES,SSE4.2,SSE4.1,CX16,ABM,SSE4A,SSSE3,SSE3,SSE2,SSE,MMXand 64-bit instruction set extensions.winchip-c6— the IDT WinChip C6 CPU, dealt with the same way asi486, plus additionalMMXinstruction set support. Blast from the past ngl 🫶winchip2— IDT WinChip 2 CPU, same deal asi486, plus additionalMMXand3DNow!instruction set support.c3— VIA C3 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip — like, none, bestie.)c3-2— VIA C3-2 (Nehemiah/C5XL) CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.)c7— VIA C7 (Esther) CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.)samuel-2— VIA Eden Samuel 2 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip.)nehemiah— VIA Eden Nehemiah CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.)esther— VIA Eden Esther CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.)eden-x2— VIA Eden X2 CPU withx86-64,MMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.)eden-x4— VIA Eden X4 CPU withx86-64,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVXandAVX2instruction set support. (No scheduling is implemented for this chip.)nano— Generic VIA Nano CPU withx86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.)nano-1000— VIA Nano 1xxx CPU withx86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.)nano-2000— VIA Nano 2xxx CPU withx86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.)nano-3000— VIA Nano 3xxx CPU withx86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.)nano-x2— VIA Nano Dual Core CPU withx86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.)nano-x4— VIA Nano Quad Core CPU withx86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.)lujiazui— ZHAOXIN lujiazui CPU withx86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,FXSR,RDSEEDinstruction set support. Okay so heads up 🫢 — while the CPUs do supportAVXandF16C, these aren't enabled by-march=lujiazuifor performance reasons.yongfeng— ZHAOXIN yongfeng CPU withx86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,F16C,FXSR,RDSEED,AVX2,FMA,SHA,LZCNTinstruction set support.shijidadao— ZHAOXIN shijidadao CPU withx86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,F16C,FXSR,RDSEED,AVX2,FMA,SHA,LZCNTinstruction set support.geode— AMD Geode embedded processor withMMXand3DNow!instruction set support. Little retro icon 💾✨
-mtune=cpu-type— bestie, this one tunes everything applicable about the generated code tocpu-type, except for the ABI and the set of available instructions. So like… picking a specificcpu-typeschedules things appropriately for that particular chip, but the compiler does not generate any code that can't run on the default machine type unless you also use a-march=cpu-typeoption. Example time 🍓: if GCC is configured fori686-pc-linux-gnuthen-mtune=pentium4generates code that is tuned for Pentium 4 but still runs oni686machines. It's giving "dressed for the party but still safe to walk home".
The choices for cpu-type are the same as for -march. Plus, -mtune supports 2 extra choices for cpu-type:
generic— produces code optimized for the most commonIA32/AMD64/EM64Tprocessors. If you know the CPU on which your code will run, then you should use the corresponding-mtuneor-marchoption 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. So if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. Lowkey chaotic but that's the vibe.
Heads up: there is no-march=genericoption, because-marchindicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast,-mtuneindicates the processor (or, in this case, collection of processors) for which the code is optimized. Iykyk 💅
intel— produces code optimized for the most current Intel processors, which areHaswellandSilvermontfor this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding-mtuneor-marchoption instead of-mtune=intel. But if you want your application to perform better on bothHaswellandSilvermont, then you should use this option. Two birds, one slay 🦋
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.
💅 -mcpu=cpu-type
Okay so lowkey this is a deprecated synonym for -mtune. Like, it still works, but it's giving "old phone that still turns on." 🙈
First tho, a quick note that was just chillin' above: there is no -march=intel option, bestie, and here's why — -march tells you which instruction set the compiler is allowed to use, and there's literally no common instruction set that applies to all processors. Whereas -mtune is like... the processor (or, in this case, a whole collection of processors) that the code gets optimized for. Big difference, no cap. ✨
🌸 -mfpmath=unit
This one generates floating-point arithmetic for the unit you pick. The choices for unit are:
387— uses the standard 387 floating-point coprocessor that's on most chips and emulated everywhere else. Code compiled with this runs pretty much everywhere, iconic. The temporary results get computed in 80-bit precision instead of whatever precision the type actually specifies, so you get slightly different results compared to most other chips. Peep-ffloat-storefor the full tea. This is the default choice for non-Darwin x86-32 targets.sse— uses scalar floating-point instructions from the SSE instruction set. This one's supported by Pentium III and newer chips, and on the AMD side by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of SSE only does single-precision arithmetic, so double and extended-precision arithmetic still go through 387. A later version — only in Pentium 4 and AMD x86-64 chips — supports double-precision too. 🦋
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and actually make this option do anything. For the x86-64 compiler, those extensions are on by default, slay.
The resulting code should be considerably faster in the majority of cases and avoids the numerical instability problems of 387 code — but it may break some existing code that expects temporaries to be 80 bits. 💀
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.
sse,387sse+387both— tries to use both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE, the execution resources too. Use this with care bestie, it's still experimental — the GCC register allocator doesn't model separate functional units well, so performance is unstable. 😭
🎀 -masm=dialect
Outputs assembly instructions using the dialect you picked. It also affects which dialect gets used for basic asm and extended asm. Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. Periodt.
⭐ -mieee-fp / -mno-ieee-fp
Controls whether the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. Clean, simple, does what it says. 💜
🔥 -m80387 / -mhard-float
Generates output containing 80387 instructions for floating point.
🌈 -mno-80387 / -msoft-float
Generates output containing library calls for floating point.
⚠️ Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler get used, but that can't be done directly in cross-compilation. You have to make your own arrangements to provide suitable library functions for cross-compilation. Plan ahead, bestie. 🫶
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may still be emitted even if -msoft-float is used.
🧚 -mno-fp-ret-in-387
Don't 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's no FPU — the idea being that the operating system should emulate an FPU. The option -mno-fp-ret-in-387 makes such values come back in ordinary CPU registers instead. 🤷♀️
🍓 -mno-fancy-math-387
Some 387 emulators don't support the sin, cos and sqrt instructions for the 387. Specify this option to avoid generating those instructions. It gets overridden when -march indicates that the target CPU always has an FPU, so the instruction doesn't need emulation. Also, these instructions aren't generated unless you also use the -funsafe-math-optimizations switch.
👑 -malign-double / -mno-align-double
Controls whether GCC aligns double, long double and long long variables on a two-word boundary or a one-word boundary. Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory. Trade-offs, we love to see it. ✨
On x86-64, -malign-double is enabled by default.
⚠️ Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32, and are not binary compatible with structures in code compiled without that switch. Be so careful. 😭
💖 -m96bit-long-double / -m128bit-long-double
These switches control the size of the long double type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, that's just not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. 🎀
In the x86-64 compiler, -m128bit-long-double is the default choice since its ABI specifies that long double is aligned on a 16-byte boundary.
Notice that neither of these options enables 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, this changes the size of structures and arrays containinglong doublevariables, plus it modifies the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch.
🌸 -mlong-double-64 / -mlong-double-80 / -mlong-double-128
These switches control the size of the long double type. A size of 64 bits makes long double equivalent to the double type — this is the default for 32-bit Bionic C library. A size of 128 bits makes long double equivalent to the __float128 type — this is the default for 64-bit Bionic C library. ✨
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. Same warning, different switches, we stay consistent. 🫶
🦋 -malign-data=type
Controls how GCC aligns variables. Supported values for type are:
compat— uses increased alignment value compatible with GCC 4.8 and earlierabi— uses alignment value as specified by the psABIcacheline— uses increased alignment value to match the cache line size
compat is the default. Slay. 💅
🔥 -mlarge-data-threshold=threshold
When -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold get placed in large data sections. The default is 65535. Noted 📝
💀 -mrtd
Uses a different function-calling convention, where 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's no need to pop the arguments there. Efficiency era. ✨
You can specify that an individual function gets called with this calling sequence using 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 can't use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf) — otherwise incorrect code is generated for calls to those functions. 😭
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored, but not here bestie.)
🎀 -mregparm=num
Controls 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 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. That includes the system libraries and startup modules. Yeah, all of them. 👑😭
🌈 -msseregparm
Uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function 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. (Yes, same energy as the last warning — GCC is nothing if not consistent.)
🧚 -mvect8-ret-in-mem
Returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. Legit only for legacy vibes. 🙈
⭐ -mpc32 / -mpc64 / -mpc80
Sets 80387 floating-point precision to 32, 64 or 80 bits:
-mpc32— the significands of results of floating-point operations are rounded to 24 bits (single precision)-mpc64— rounds the significands of results of floating-point operations to 53 bits (double precision)-mpc80— rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default
When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. So like... set it yourself if you need it, bestie. 🫶💜
Okay so real talk: setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. 💨 But bestie, listen—some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 😭 So slay the speed but know the accuracy tea.
-mdaz-ftz— the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in theMXCSRregister are used to control floating-point calculations. SSE and AVX instructions—including scalar and vector instructions—could benefit from enabling the FTZ and DAZ flags when-mdaz-ftzis specified. 🫶 Don't set FTZ/DAZ flags when-mno-daz-ftzor-sharedis specified—but note that-mdaz-ftzwill set FTZ/DAZ flags even with-shared. 💀-mstackrealign— realign the stack at entry. On the x86, the-mstackrealignoption generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute"force_align_arg_pointer", applicable to individual functions. 🧚-mpreferred-stack-boundary=num— attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is4(16 bytes or 128 bits). ✨
⚠️ Warning: When generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with a misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87long doubleand__int128), leading to wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 😭💀
-mincoming-stack-boundary=num— assume the incoming stack is aligned to a 2 raised to num byte boundary. If-mincoming-stack-boundaryis not specified, the one specified by-mpreferred-stack-boundaryis used. 🫶
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. 🌸
To ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🎀
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. 👀
Okay bestie, buckle up—this next batch is a whole parade of instruction-set flags. Each one is its own bullet, and they all share the same big collective description below. 💅
-mmmx— enables this extension.-msse— enables this extension.-msse2— enables this extension.-msse3— enables this extension.-mssse3— enables this extension.-msse4— enables this extension.-msse4a— enables this extension.-msse4.1— enables this extension.-msse4.2— enables this extension.-mavx— enables this extension.-mavx2— enables this extension.-mavx512f— enables this extension.-mavx512cd— enables this extension.-mavx512vl— enables this extension.-mavx512bw— enables this extension.-mavx512dq— enables this extension.-mavx512ifma— enables this extension.-mavx512vbmi— enables this extension.-msha— enables this extension.-maes— enables this extension.-mpclmul— enables this extension.-mclflushopt— enables this extension.-mclwb— enables this extension.-mfsgsbase— enables this extension.-mptwrite— enables this extension.-mrdrnd— enables this extension.-mf16c— enables this extension.-mfma— enables this extension.-mpconfig— enables this extension.-mwbnoinvd— enables this extension.-mfma4— enables this extension.-mprfchw— enables this extension.-mrdpid— enables this extension.-mrdseed— enables this extension.-msgx— enables this extension.-mxop— enables this extension.-mlwp— enables this extension.-m3dnow— enables this extension.-m3dnowa— enables this extension.-mpopcnt— enables this extension.-mabm— enables this extension.-madx— enables this extension.-mbmi— enables this extension.-mbmi2— enables this extension.-mlzcnt— enables this extension.-mfxsr— enables this extension.-mxsave— enables this extension.-mxsaveopt— enables this extension.-mxsavec— enables this extension.-mxsaves— enables this extension.-mrtm— enables this extension.-mhle— enables this extension.-mtbm— enables this extension.-mmwaitx— enables this extension.-mclzero— enables this extension.-mpku— enables this extension.-mavx512vbmi2— enables this extension.-mavx512bf16— enables this extension.-mavx512fp16— enables this extension.-mgfni— enables this extension.-mvaes— enables this extension.-mwaitpkg— enables this extension.-mvpclmulqdq— enables this extension.-mavx512bitalg— enables this extension.-mmovdiri— enables this extension.-mmovdir64b— enables this extension.-menqcmd— enables this extension.-muintr— enables this extension.-mtsxldtrk— enables this extension.-mavx512vpopcntdq— enables this extension.-mavx512vp2intersect— enables this extension.-mavx512vnni— enables this extension.-mavxvnni— enables this extension.-mcldemote— enables this extension.-mserialize— enables this extension.-mamx-tile— enables this extension.-mamx-int8— enables this extension.-mamx-bf16— enables this extension.-mhreset— enables this extension.-mkl— enables this extension.-mwidekl— enables this extension.-mavxifma— enables this extension.-mavxvnniint8— enables this extension.-mavxneconvert— enables this extension.-mcmpccxadd— enables this extension.-mamx-fp16— enables this extension.-mprefetchi— enables this extension.-mraoint— enables this extension.-mamx-complex— enables this extension.-mavxvnniint16— enables this extension.-msm3— enables this extension.-msha512— enables this extension.-msm4— enables this extension.-mapxf— enables this extension.-musermsr— enables this extension.-mavx10.1— enables this extension.-mavx10.1-256— enables this extension.-mavx10.1-512— enables this extension.-mavx10.2— enables this extension.-mamx-avx512— enables this extension.-mamx-tf32— enables this extension.-mamx-transpose— enables this extension.-mamx-fp8— enables this extension.-mmovrs— enables this extension.-mamx-movrs— enables this extension.
✨ Collectively? These switches enable the use of instructions in the MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction sets. Each has a corresponding -mno- option to disable use of these instructions. No cap.
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. 📚
Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form of the option affects SSE4A support, controlled separately by -msse4a. 🎀
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse. 💫
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. 😌
These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. 🔍
-mdump-tune-features— this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in-mtune-ctrl=feature-list. 🗂️-mtune-ctrl=feature-list— this option is used to do fine grain control of x86 code generation features.feature-listis a comma separated list of feature names. See also-mdump-tune-features. When specified, the feature is turned on if it is not preceded with^, otherwise, it is turned off.-mtune-ctrl=feature-listis intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 💀-mno-default— this option instructs GCC to turn off all tunable features. See also-mtune-ctrl=feature-listand-mdump-tune-features. 🛑-mcld— this option instructs GCC to emit a"cld"instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the--enable-cldconfigure option. Generation of"cld"instructions can be suppressed with the-mno-cldcompiler option in this case. 🧵-mvzeroupper— this option instructs GCC to emit a"vzeroupper"instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary"zeroupper"intrinsics. 🌊-mprefer-avx128— this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. 🌀-mprefer-vector-width=opt— this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform. 📏-mpartial-vector-fp-math— this option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless-fno-trapping-mathis specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when-fno-trapping-mathis in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using-mdaz-ftzto set denormals-are-zero (DAZ) flag in the MXCSR register. 😤 This option is enabled by default.-mmove-max=bits— this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512. 📦-mstore-max=bits— okay so this one tells GCC to cap the biggest number of bits it can shove into memory efficiently at bits. Valid values are128,256and512. Slay.none— no extra limits on GCC other than whatever the selected platform already says. Free spirit energy.128— prefer 128-bit vector width for instructions.256— prefer 256-bit vector width for instructions.512— prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers— this one optimizes functions that have thenoreturnattribute or the_Noreturnspecifier by not saving the callee-saved registers used in the function in the function prologue (except theBPregister, bestie, BP stays). Heads up though: this can interfere with debugging the caller of thenoreturnfunction or literally any function further up in the call stack, which is why it's not enabled by default. It's giving "works but don't touch it if you like your debugger" 💀-mcx16— lets GCC generateCMPXCHG16Binstructions in 64-bit code so it can do compare-and-exchange ops on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures bigger than one machine word. The compiler uses this instruction for__syncBuiltins. But — and this is the plot twist — for__atomicBuiltins operating on 128-bit integers, a library call is always used. No cap.-msahf— enables generation ofSAHFinstructions in 64-bit code. Storytime: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step dropped in December 2005, were lowkey missing theLAHFandSAHFinstructions that AMD64 supports. Those two are load and store instructions (respectively) for certain status flags. In 64-bit mode, theSAHFinstruction is used to optimize thefmod,drem, andremainderbuilt-in functions; see Other Builtins for the details 🎀-mmovbe— enables use of themovbeinstruction to optimize byte swapping of four and eight byte entities. Little byte-flip icon 💅-mshstk— the-mshstkoption enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET).-mcrc32— enables the built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32siand__builtin_ia32_crc32dito generate thecrc32machine instruction.-mmwait— enables the built-in functions__builtin_ia32_monitorand__builtin_ia32_mwaitto generate themonitorandmwaitmachine instructions.-mrecip— enables use ofRCPSSandRSQRTSSinstructions (and their vectorized variantsRCPPSandRSQRTPS) with an extra Newton-Raphson step to bump precision, instead ofDIVSSandSQRTSS(and their vectorized variants) for single-precision floating-point arguments. These instructions only get generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Ngl, while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals0.99999994). Like, she's fast but she's approximate 😭
Fr fr note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the option combo above), and doesn't need -mrecip.
Also note that GCC emits the above sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. So don't double up, bestie.
-mrecip=opt— controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, and any of them can be preceded by a!to invert it. The menu:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. It's giving "everything on the menu except the one thing you're allergic to" 🍓
-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type aresvmlfor the Intel short vector math library,aoclfor the math library (LibM) from AMD Optimizing CPU Libraries (AOCL) andacmlfor the end-of-life AMD core math library (to which AOCL-LibM is the successor). To use this option, both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. Rules are rules ✨
GCC currently emits calls to these for the corresponding function type when -mveclibabi=svml is used:
vmldExp2 vmldLn2 vmldLog102 vmldPow2 vmldTanh2 vmldTan2
vmldAtan2 vmldAtanh2 vmldCbrt2 vmldSinh2 vmldSin2 vmldAsinh2
vmldAsin2 vmldCosh2 vmldCos2 vmldAcosh2 vmldAcos2
vmlsExp4 vmlsLn4 vmlsLog104 vmlsPow4 vmlsTanh4 vmlsTan4
vmlsAtan4 vmlsAtanh4 vmlsCbrt4 vmlsSinh4 vmlsSin4 vmlsAsinh4
vmlsAsin4 vmlsCosh4 vmlsCos4 vmlsAcosh4 vmlsAcos4And these for the corresponding function type when -mveclibabi=aocl is used:
amd_vrs4_acosf amd_vrs16_acosf amd_vrd8_asin amd_vrs4_asinf
amd_vrs8_asinf amd_vrs16_asinf amd_vrd2_atan amd_vrd8_atan
amd_vrs4_atanf amd_vrs8_atanf amd_vrs16_atanf amd_vrd2_cos
amd_vrd4_cos amd_vrd8_cos amd_vrs4_cosf amd_vrs8_cosf
amd_vrs16_cosf amd_vrs4_coshf amd_vrs8_coshf amd_vrd2_erf
amd_vrd4_erf amd_vrd8_erf amd_vrs4_erff amd_vrs8_erff
amd_vrs16_erff amd_vrd2_exp amd_vrd4_exp amd_vrd8_exp
amd_vrs4_expf amd_vrs8_expf amd_vrs16_expf amd_vrd2_exp10
amd_vrs4_exp10f amd_vrd2_exp2 amd_vrd4_exp2 amd_vrd8_exp2
amd_vrs4_exp2f amd_vrs8_exp2f amd_vrs16_exp2f amd_vrs4_expm1f
amd_vrd2_log amd_vrd4_log amd_vrd8_log amd_vrs4_logf
amd_vrs8_logf amd_vrs16_logf amd_vrd2_log10 amd_vrs4_log10f
amd_vrs8_log10f amd_vrs16_log10f amd_vrd2_log1p amd_vrs4_log1pf
amd_vrd2_log2 amd_vrd4_log2 amd_vrd8_log2 amd_vrs4_log2f
amd_vrs8_log2f amd_vrs16_log2f amd_vrd2_pow amd_vrd4_pow
amd_vrd8_pow amd_vrs4_powf amd_vrs8_powf amd_vrs16_powf
amd_vrd2_sin amd_vrd4_sin amd_vrd8_sin amd_vrs4_sinf
amd_vrs8_sinf amd_vrs16_sinf amd_vrd2_tan amd_vrd4_tan
amd_vrd8_tan amd_vrs16_tanf amd_vrs4_tanhf amd_vrs8_tanhf
amd_vrs16_tanhfAnd these for the corresponding function type when -mveclibabi=acml is used:
__vrd2_sin __vrd2_cos __vrd2_exp __vrd2_log __vrd2_log2
__vrd2_log10 __vrs4_sinf __vrs4_cosf __vrs4_expf __vrs4_logf
__vrs4_log2f __vrs4_log10f __vrs4_powfLike, that's a lot of names but every single one of them is iconic in her own way 👑
-mabi=name— generate code for the specified calling convention. Permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributesms_abiandsysv_abi. Pick your fighter 💅-mforce-indirect-call— forces all calls to functions to be indirect. Useful when you're using Intel Processor Trace, since it generates more precise timing information for function calls. No direct calls allowed, we go around the block now 🌈-mmanual-endbr— inserts the ENDBR instruction at function entry only via thecf_checkfunction attribute. Useful when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry. You're the main character deciding where ENDBR goes.-mcet-switch— by default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets reachable via any indirect jumps. Safety girlies, this one's for you 🫶-mcall-ms2sysv-xlogues— because 64-bit ABIs are messy, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default the code for saving and restoring these registers is emitted inline, giving you fairly lengthy prologues and epilogues (iykyk 😭). Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores, reducing function size at the cost of a few extra instructions. Trade-off queen.-mtls-dialect=type— generate code to access thread-local storage using thegnuorgnu2conventions.gnuis the conservative default;gnu2is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. Bold choice, bestie, but check your platform first.-mpush-args-mno-push-args— use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. Sometimes the shorter way isn't the faster way, and that's growth ✨-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies-mno-push-args. Fast but thicc, no cap.-mthreads— supports thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the-mthreadsoption. When compiling,-mthreadsdefines-D_MT; when linking, it links in a special thread helper library-lmingwthrdwhich cleans up per-thread exception-handling data. All or nothing, diva 💜-mms-bitfields,-mno-ms-bitfields— enable/disable bit-field layout compatible with the native Microsoft Windows compiler 💅
okay so if packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.
This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determined by these rules:
- Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
- Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the
alignedattribute or thepackpragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0- Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
- If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.
- If a zero-length bit-field is inserted after a bit-field,
foo, and the alignment of the zero-length bit-field is greater than the member that follows it,bar,baris aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example,
t2has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. - Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;Here, t4 takes up 4 bytes.
- Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;Here, t5 takes up 2 bytes.
-mno-align-stringops— do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it 🎀-minline-all-stringops— by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fastmemcpyandmemsetfor short lengths. The option enables inline expansion ofstrlenfor all pointer alignments ✨-minline-stringops-dynamically— for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks 🦋-mstringop-strategy=alg— override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:rep_byte,rep_4byte,rep_8byte— expand using i386repprefix of the specified size.byte_loop,loop,unrolled_loop— expand into an inline loop.libcall— always use a library call.
-mmemcpy-strategy=strategy— override the internal decision heuristic to decide if__builtin_memcpyshould be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list ofalg:max_size:dest_aligntriplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be-1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is0for the first triplet andmax_size + 1of the preceding range 🔥-mmemset-strategy=strategy— the option is similar to-mmemcpy-strategy=except that it is to control__builtin_memsetexpansion 🌸-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option-fomit-leaf-frame-pointerremoves the frame pointer for leaf functions, which might make debugging harder 💅-mtls-direct-seg-refs,-mno-tls-direct-seg-refs— controls whether TLS variables may be accessed with offsets from the TLS segment register (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. For systems that use the GNU C Library, the default is on 🫶-msse2avx,-mno-sse2avx— specify that the assembler should encode SSE instructions with VEX prefix. The option-mavxturns this on by default ⭐-mfentry,-mno-fentry— if profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attributems_hook_prologueisn't possible at the moment for-mfentryand-pg😭-mrecord-mcount,-mno-record-mcount— if profiling is active (-pg), generate a__mcount_locsection that contains pointers to each profiling call. This is useful for automatically patching and out calls 👑-mnop-mcount,-mno-nop-mcount— if profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with-mrecord-mcount🌈-minstrument-return=type— instrument function exit in-pg -mfentryinstrumented functions with call to specified function. This only instruments true returns ending withret, but not sibling calls ending withjump. Valid type values arenoneto not instrument,callto generate a call to__return__, ornop5to generate a 5 byte nop 🎀-mrecord-return,-mno-record-return— generate a__return_locsection pointing to all return instrumentation code 💀-mfentry-name=name— set name of__fentry__symbol called at function entry for-pg -mfentryfunctions 🧚-mfentry-section=name— set name of section to record-mrecord-mcountcalls (default__mcount_loc) 💜-mskip-rax-setup,-mno-skip-rax-setup— when generating code for the x86-64 architecture with SSE extensions disabled,-mskip-rax-setupcan be used to skip setting up RAX register when there are no variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't have those issues, regardless the RAX register value.
-m8bit-idiv,-mno-8bit-idiv— on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of0to255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide 🍓-mavx256-split-unaligned-load,-mavx256-split-unaligned-store— split 32-byte AVX unaligned load and store 🔥-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol— generate stack protection code using canary at guard. Supported locations areglobalfor global canary ortlsfor per-thread canary in the TLS block (the default). This option has effect only when-fstack-protectoror-fstack-protector-allis specified 👑
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which segment register (%fs or %gs) to use as the base register for reading the canary, and what offset from that base register to use. The default for those is whatever the relevant ABI says. ✨
-mstack-protector-guard-symbol=symbol— overrides the offset with a symbol reference to a canary in the TLS block. 🎀-mgeneral-regs-only— generates code that uses only the general-purpose registers. This stops the compiler from using floating-point, vector, mask and bound registers. 💅-mrelax-cmpxchg-loop— when emitting a compare-and-swap loop for__syncBuiltins and__atomicBuiltins that don’t have a native instruction, this optimizes for the highly contended case by issuing an atomic load before the"CMPXCHG"instruction, and using the"PAUSE"instruction to save CPU power when restarting the loop. 🔥-mindirect-branch=choice— converts indirect call and jump with choice. The default iskeep, which keeps indirect call and jump unmodified.thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute"indirect_branch".
🚨 Note that-mcmodel=largeis incompatible with-mindirect-branch=thunkand-mindirect-branch=thunk-externsince the thunk function may not be reachable in the large code model. ✨ Note that-mindirect-branch=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check.
-mfunction-return=choice— converts function return with choice. The default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute"function_return".
🚨 Note that-mindirect-return=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check. 🚨 Note that-mcmodel=largeis incompatible with-mfunction-return=thunkand-mfunction-return=thunk-externsince the thunk function may not be reachable in the large code model.
-mindirect-branch-register— forces indirect call and jump via register. 💫-mharden-sls=choice— generates code to mitigate against straight line speculation (SLS) with choice. The default isnone, which disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables all SLS hardening. 🛡️-mindirect-branch-cs-prefix— adds CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so that the call and jmp instruction length is 6 bytes to allow them to be replaced withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run-time. ⚡-mapx-inline-asm-use-gpr32— for inline asm support with APX, by default the EGPR feature was disabled to prevent potential illegal instruction with EGPR occurs. To invoke egpr usage in inline asm, use new compiler option-mapx-inline-asm-use-gpr32and user should ensure the instruction supports EGPR. 🧚-mevex512,-mno-evex512— enables/disables 512-bit vector. It will be default on if AVX512F is enabled. 🌈
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. 👑
-m32— generates code for a 16-bit, 32-bit or 64-bit environment. The-m32option setsint,long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.-m64— setsintto 32 bits andlongand pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the-m64option also turns off the-fno-picand-mdynamic-no-picoptions.-mx32— setsint,long, and pointer types to 32 bits, and generates code for the x86-64 architecture.-m16— is the same as-m32, except for that it outputs the.code16gccassembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.-miamcu— generates code which conforms to Intel MCU psABI. It requires the-m32option to be turned on.-mno-red-zone— do not use a so-called “red zone” for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag-mno-red-zonedisables 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. Small symbols are also placed there. Symbols with sizes larger than-mlarge-data-thresholdare put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.-mcmodel=large— generate code for the large model. This model makes no assumptions about addresses and sizes of sections.-maddress-mode=long— generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.-maddress-mode=short— generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.-mneeded,-mno-needed— emitGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. ⭐-mno-direct-extern-access— without-fpicnor-fPIC, always use the GOT pointer to access external symbols. With-fpicor-fPIC, treat access to protected symbols as local symbols. The default is-mdirect-extern-access.
💀 Warning: shared libraries compiled with-mno-direct-extern-accessand executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable.
-munroll-only-small-loops— controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop with less than 4 insns by 1 time. Explicit-f[no-]unroll-[all-]loopswould disable this flag to avoid any unintended unrolling behavior that user does not want. 🌀-mlam=choice— LAM(linear-address masking) allows special bits in the pointer to be used for metadata. The default isnone. Withu48, pointer bits in positions 62:48 can be used for metadata; withu57, pointer bits in positions 62:57 can be used for metadata. 🎛️
x86 Windows Options 🪟
Xstormy16 Options 🌪️
These options are defined for Xstormy16:
-msim— choose startup files and linker script suitable for the simulator. 🧪
Xtensa Options 🎚️
These options are supported for Xtensa targets:
-mconst16,-mno-const16— enable or disable use of"CONST16"instructions for loading constant values. The"CONST16"instruction is currently not a standard option from Tensilica. When enabled,"CONST16"instructions are always used in place of the standard"L32R"instructions. The use of"CONST16"is enabled by default only if the"L32R"instruction is not available. 💖-mfused-madd,-mno-fused-madd— enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler’s ability to combine multiply and add/subtract operations. 🧮-mserialize-volatile,-mno-serialize-volatile— when this option is enabled, GCC inserts"MEMW"instructions before"volatile"memory references to guarantee sequential consistency. The default is-mserialize-volatile. Use-mno-serialize-volatileto omit the"MEMW"instructions. ⚡-mforce-no-pic— for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. 🐧-mtext-section-literals,-mno-text-section-literals— these options control the treatment of literal pools. The default is-mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With-mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. 📚-mauto-litpools— bestie, this turns on auto literal pools: the assembler intersperses literals in the text section. The compiler does not produce explicit".literal"directives and loads literals into registers withMOVIinstructions instead ofL32R, so the assembler can do relaxation and place literals as necessary. This option lets the assembler create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals.-mno-auto-litpools— the default vibe: literals go in a separate section in the output file unless-mtext-section-literalsis used. These options control the treatment of literal pools, and this is the default one. 🎀-mtarget-align— when this is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is-mtarget-align. These options do not affect the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by insertingNOPinstructions. ✨-mno-target-align— the off switch for that alignment behavior; the default is-mtarget-align, so this is the one you reach for when you want it off. SameLOOPcaveat applies: the assembler always aligns those bad boys. 💅-mlongcalls— when this is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a directCALLinstruction into anL32Rfollowed by aCALLXinstruction. The default is-mno-longcalls. This option should be used in programs where the call target can potentially be out of range. It is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. 👑-mno-longcalls— the default, no long-call translation. Direct calls stay direct unless you opt into-mlongcalls, and the same assembler-not-compiler note applies. 🫶-mabi=name— generate code for the specified ABI. Permissible values are:call0,windowed. Default ABI is chosen by the Xtensa core configuration.-mabi=call0— when this option is enabled, function parameters are passed in registersa2througha7, registersa12througha15are caller-saved, and registera15may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_CALL0_ABI__is defined. 🌸-mabi=windowed— when this option is enabled, function parameters are passed in registersa10througha15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registersa2througha7. Registera7may be used as a frame pointer. The register window is rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__is defined. 🦋-mextra-l32r-costs=n— specify an extra cost of instruction RAM/ROM access forL32Rinstructions, in clock cycles. This affects, when optimizing for speed, whether loading a constant from a literal pool usingL32Ror synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is0. ⭐-mstrict-align— avoid generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-alignfor cores that support both unaligned loads and stores in hardware and-mstrict-alignfor all other cores. No cap, alignment girlies stay winning. 💖-mno-strict-align— allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. This is the default for cores that support both unaligned loads and stores in hardware; all other cores default to-mstrict-align.
zSeries Options ✨
These are listed under
🌸 ENVIRONMENT 🌸
okay bestie, this section is literally the backstage pass tour of every environment variable that changes how GCC behaves 💅 Some of them work by handing GCC directories or prefixes to go hunting in when it's looking for files, and some of them just set the whole vibe of the compilation environment. Both count!
real talk though: you can also point GCC at places to search with options like-B,-Iand-L. Those take precedence over anything the environment variables say, and the environment variables in turn take precedence over what GCC's own configuration specifies. Order matters, bestie. 👑
🎀 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These four cuties control how GCC uses localization info, which is the whole thing that lets GCC vibe with different national conventions. GCC checks the locale categories LC_CTYPE and LC_MESSAGES — but only if it was configured to do so. These locale categories can be set to any value your installation supports. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8 (iconic, honestly 🇬🇧✨).
LC_CTYPE specifies character classification. GCC uses it to figure out the character boundaries in a string — which is genuinely necessary for some multibyte encodings that contain quote and escape characters which would otherwise get read as a string end or an escape. not me getting parsed wrong 😭
LC_MESSAGES specifies the language to use in diagnostic messages.
And if LC_ALL 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. periodt. 💀
🔧 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 that's going to be the input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. little relay race, we love to see it 🏃♀️✨
🔍 GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of that option for more details — bestie, go read the option, it's right there 👀
🧭 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 absolutely specify a prefix that ends with a slash if you wish. your call, main character 🫶
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.
If GCC can't find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. it's not lost, it's just checking other spots 💅
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix. (told you options win ✨)
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition — and this is the funky part — the prefix is used in an unusual way when finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. So, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And if a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. it's giving switcheroo 🦋
📚 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. fallback queen 👑
📖 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, always and forever. 💖
🌍 LANG
okay yes, LANG again — it has a second job bestie, she's versatile. This variable is used to pass locale information to the compiler. One way that info gets used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
C-JIS— recognize JIS characters.C-SJIS— recognize SJIS characters.C-EUCJP— recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters. no notes, it just does its thing 🧚
🩹 GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. Heads up: -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable.
fixits-v1— emit parseable fix-it hints, equivalent to-fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.fixits-v2— as"fixits-v1", but columns are expressed as display columns, as per-fdiagnostics-column-unit=display.
🧚 Preprocessor env vars, the sequel
Some additional environment variables affect the behavior of the preprocessor. buckle up, we're not done yet ✨
🗂️ CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. That special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon. iykyk 🪄
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each one 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. empty string carrying the whole team fr fr 🫶
🧾 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. (they're not invited to this party 🚫)
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. combo deal, we stan 🌈
☀️ SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted. nearly the same twin, different lore 💅
⏰ SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible. reproducible builds girlies rise 🔥
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII — identical to the output of "date +%s" on GNU/Linux and other systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last modification time of the source or package, and it should be set by the build process. please don't just make one up 🙈
💀 BUGS
For instructions on reporting bugs, see <https://gitlab.archlinux.org/archlinux/packaging/packages/or1k-elf-gcc/-/issues>. go tell them everything, they want to know 🫶
📎 FOOTNOTES
- On some systems,
gcc -sharedneeds to build supplementary stub code for constructors to work. On multi-libbed systems,gcc -sharedmust select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.-sharedsuppresses the addition of startup code to alter the floating-point environment as done with-ffast-math,-Ofastor-funsafe-math-optimizationson some targets. 😭 please pass the right flags bestie
🔗 SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb. the whole friend group, linked and ready 🌸
👑 AUTHOR
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. legends, every single one 💜
©️ COPYRIGHT
Copyright (c) 1988-2025 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
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.
okay that's literally the whole page bestie, we made it to the end together 🎀✨ this one's gcc-15.2.0, dated 2025-08-08, and it's the GCC(1) man page — go compile something iconic with it. mwah 💋🦋
📦 where this page comes from
- arch rolling: or1k-elf-gcc 15.2.0-1 · GPL-3.0-with-GCC-exception AND GFDL-1.3-or-later
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on : a language model's rewrite of the original, under its terms