GCC(1)

✨ gcc(1) — GNU — GCC(1) ✨

okay bestie, listen up 💖 you've opened the manual page for gcc, and yes it is long, but we're gonna get through it together. this is gcc(1) from the GNU crew, and it's about to be your whole personality. grab a matcha 🍵, we're doing this.

---

🌸 NAME 🌸

gcc — the GNU project C and C++ compiler 🫶

---

🎀 SYNOPSIS 🎀

here's the whole vibe in one glance:

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

okay so literally only the most useful options are listed up there — the rest of them are further down, and there are so many. also g++ accepts mostly the same options as gcc, so if you're a C++ girlie you're still in the right place 💅

---

🦋 DESCRIPTION 🦋

so when you invoke GCC, it normally does preprocessing, compilation, assembly and linking. that's four whole stages, she's busy. the "overall options" let you 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. slay 💖

other options get passed on to one or more stages of processing. some options control the preprocessor and others control the compiler itself. and then yet other options control the assembler and linker — most of these are not documented here, since you rarely need to use any of them. lowkey a relief ngl.

most of the command-line options 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. so if the description for a particular option does not mention a source language, you can use that option with all supported languages. no cap 🫶

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. yes queen, use the right name 👑

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. like, they are not the same girl, do not conflate them 😭

you can mix options and other arguments. for the most part, the order you use doesn't matter. but — and this is the plot twist — 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. remember that bestie 💜

many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformatand so on. most of these have both positive and negative forms; the negative form of-ffoois-fno-foo`. this manual documents only one of these two forms, whichever one is not the default. so if you're like "where's the other one??" — that's why 🙈

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 gibibyte, 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. yes, she's citing standards, she's that girl 📚✨

---

💅 OPTIONS 💅

🌈 Option Summary 🌈

here is a summary of all the options, grouped by type. the explanations are in the following sections, so don't panic — this is just the map, not the whole road trip 🗺️

⭐ Overall Options ⭐

• -c
• -S
• -E
• -o file
• -x language
• -v
• -###
• --help[=class[,...]]
• --target-help
• --version
• -pass-exit-codes
• -pipe
• -specs=file
• -wrapper @file
• -ffile-prefix-map=old=new
• -fplugin=file
• -fplugin-arg-name=arg
• -fdump-ada-spec[-slim]
• -fada-spec-parent=unit
• -fdump-go-spec=file

⭐ C Language Options ⭐

• -ansi
• -std=standard
• -fgnu89-inline
• -fpermitted-flt-eval-methods=standard
• -aux-info filename
• -fallow-parameterless-variadic-functions
• -fno-asm
• -fno-builtin
• -fno-builtin-function
• -fgimple
• -fhosted
• -ffreestanding
• -fopenacc
• -fopenacc-dim=geom
• -fopenmp
• -fopenmp-simd
• -fms-extensions
• -fplan9-extensions
• -fsso-struct=endianness
• -fallow-single-precision
• -fcond-mismatch
• -flax-vector-conversions
• -fsigned-bitfields
• -fsigned-char
• -funsigned-bitfields
• -funsigned-char

⭐ C++ Language Options ⭐

• -fabi-version=n
• -fno-access-control
• -faligned-new=n
• -fargs-in-order=n
• -fchar8_t
• -fcheck-new
• -fconstexpr-depth=n
• -fconstexpr-loop-limit=n
• -fconstexpr-ops-limit=n
• -fno-elide-constructors
• -fno-enforce-eh-specs
• -fno-gnu-keywords
• -fno-implicit-templates
• -fno-implicit-inline-templates
• -fno-implement-inlines
• -fms-extensions
• -fnew-inheriting-ctors
• -fnew-ttp-matching
• -fno-nonansi-builtins
• -fnothrow-opt
• -fno-operator-names
• -fno-optional-diags
• -fpermissive
• -fno-pretty-templates
• -frepo
• -fno-rtti
• -fsized-deallocation
• -ftemplate-backtrace-limit=n
• -ftemplate-depth=n
• -fno-threadsafe-statics
• -fuse-cxa-atexit
• -fno-weak
• -nostdinc++
• -fvisibility-inlines-hidden
• -fvisibility-ms-compat
• -fext-numeric-literals
• -Wabi=n
• -Wabi-tag
• -Wconversion-null
• -Wctor-dtor-privacy
• -Wdelete-non-virtual-dtor
• -Wdeprecated-copy
• -Wdeprecated-copy-dtor
• -Wliteral-suffix
• -Wmultiple-inheritance
• -Wno-init-list-lifetime
• -Wnamespaces
• -Wnarrowing
• -Wpessimizing-move
• -Wredundant-move
• -Wnoexcept
• -Wnoexcept-type
• -Wclass-memaccess
• -Wnon-virtual-dtor
• -Wreorder
• -Wregister
• -Weffc++
• -Wstrict-null-sentinel
• -Wtemplates
• -Wno-non-template-friend
• -Wold-style-cast
• -Woverloaded-virtual
• -Wno-pmf-conversions
• -Wno-class-conversion
• -Wno-terminate
• -Wsign-promo
• -Wvirtual-inheritance

⭐ Objective-C and Objective-C++ Language Options ⭐

• -fconstant-string-class=class-name
• -fgnu-runtime
• -fnext-runtime
• -fno-nil-receivers
• -fobjc-abi-version=n
• -fobjc-call-cxx-cdtors
• -fobjc-direct-dispatch
• -fobjc-exceptions
• -fobjc-gc
• -fobjc-nilcheck
• -fobjc-std=objc1
• -fno-local-ivars
• -fivar-visibility=[public|protected|private|package]
• -freplace-objc-classes
• -fzero-link
• -gen-decls
• -Wassign-intercept
• -Wno-protocol
• -Wselector
• -Wstrict-selector-match
• -Wundeclared-selector

⭐ Diagnostic Message Formatting Options ⭐

• -fmessage-length=n
• -fdiagnostics-show-location=[once|every-line]
• -fdiagnostics-color=[auto|never|always]
• -fdiagnostics-format=[text|json]
• -fno-diagnostics-show-option
• -fno-diagnostics-show-caret
• -fno-diagnostics-show-labels
• -fno-diagnostics-show-line-numbers
• -fdiagnostics-minimum-margin-width=width
• -fdiagnostics-parseable-fixits
• -fdiagnostics-generate-patch
• -fdiagnostics-show-template-tree
• -fno-elide-type
• -fno-show-column

---

okay that's the whole cheat sheet 🎀 and bestie, this is only part one — the actual lore for every single one of these bad boys is coming right up, so don't go anywhere 💖✨

💅 Warning Options 💅

okay bestie, this is the roll call 📋 — literally every warning flag GCC has in it. this is the guest list, the seating chart, the whole vibe. the actual tea about what each one does lives further down the page, so don't come for me about detail here. the flags are written exactly as they are, no notes. 💖

• -fsyntax-only — vibe check on the syntax and only the syntax. nothing else happens, bestie.
• -fmax-errors=n — cap it at n errors. after that, we're done here. ✋
• -Wpedantic — the hall monitor of flags. pedantic to the max.
• -pedantic-errors — pedantic, but make it an error. 💀
• -w — lowercase. just lowercase. it is not -Wall and it never will be. 🙈
• -Wextra — extra. like, extra extra. ✨
• -Wall — the moment. the icon. the one everyone screams at newbies to use.
• -Waddress — addresses, babes.
• -Waddress-of-packed-member — the address of a packed member. very specific, very demure.
• -Waggregate-return — aggregates being returned. okay!
• -Waligned-new — aligned new. 💅
• -Walloc-zero — alloc zero, no cap.
• -Walloc-size-larger-than=byte-size — alloc size larger than byte-size. the number is right there.
• -Walloca — alloca!! it has its own flag and everything.
• -Walloca-larger-than=byte-size — alloca, but bigger than byte-size.
• -Wno-aggressive-loop-optimizations — the -Wno- version. aggressive loop optimizations said not today.
• -Warray-bounds — array bounds. stay in your lane, arrays. 🦋
• -Warray-bounds=n — same energy, but at level n.
• -Wno-attributes — attributes? not today.
• -Wattribute-alias=n — attribute alias at level n.
• -Wbool-compare — comparing bools. iconic behaviour, honestly.
• -Wbool-operation — bool operations. 🎀
• -Wno-builtin-declaration-mismatch — builtin declaration mismatch? we're turning that off, bestie.
• -Wno-builtin-macro-redefined — same but for a redefined builtin macro. 🙈
• -Wc90-c99-compat — c90 → c99 compatibility tea.
• -Wc99-c11-compat — c99 → c11 compatibility tea.
• -Wc11-c2x-compat — c11 → c2x compatibility tea.
• -Wc++-compat — c++ compatibility. the crossover episode.
• -Wc++11-compat — c++11 compatibility.
• -Wc++14-compat — c++14 compatibility.
• -Wc++17-compat — c++17 compatibility.
• -Wcast-align — cast alignment. 🌸
• -Wcast-align=strict — cast alignment but make it strict.
• -Wcast-function-type — cast function type, as one does.
• -Wcast-qual — cast qualifiers. qualifiers are precious, apparently.
• -Wchar-subscripts — char subscripts. 📎
• -Wcatch-value — catch value.
• -Wcatch-value=n — catch value, level n.
• -Wclobbered — clobbered. that's it, that's the flag.
• -Wcomment — comments. yes, comments have a flag.
• -Wconditionally-supported — conditionally supported. the "it depends" of flags. 🫠
• -Wconversion — conversions. 👀
• -Wcoverage-mismatch — coverage mismatch. the coverage is not covering.
• -Wno-cpp — cpp said no thank you.
• -Wdangling-else — dangling else, living on the edge. 😭
• -Wdate-time — date and time macros. time is a construct, bestie. ⏰
• -Wdelete-incomplete — delete incomplete. rude, honestly.
• -Wno-attribute-warning — attribute warning, off.
• -Wno-deprecated — deprecated stuff, we're ignoring you. 💅
• -Wno-deprecated-declarations — deprecated declarations, also ignoring you.
• -Wno-designated-init — designated init, off.
• -Wdisabled-optimization — disabled optimization. the optimizations are NOT optimizing.
• -Wno-discarded-qualifiers — discarded qualifiers, ignored.
• -Wno-discarded-array-qualifiers — discarded array qualifiers, also ignored.
• -Wno-div-by-zero — divide by zero. we're not talking about it. 🙈
• -Wdouble-promotion — double promotion!! get it, girl. 🎀
• -Wduplicated-branches — duplicated branches. copy-paste behaviour detected.
• -Wduplicated-cond — duplicated conditions.
• -Wempty-body — empty body. no thoughts, head empty.
• -Wenum-compare — enum compare.
• -Wno-endif-labels — endif labels, off.
• -Wexpansion-to-defined — expansion to defined. 🤔
• -Werror — every warning becomes an error. no mercy, no chill. 🔥
• -Werror=* — the same, but scoped to *.
• -Wextra-semi — extra semicolons. one too many, bestie. ;;
• -Wfatal-errors — fatal errors. dramatic and correct.
• -Wfloat-equal — floats being compared for equality. spicy.
• -Wformat — format.
• -Wformat=2 — format, but at level 2. ✨
• -Wno-format-contains-nul — format contains nul, off.
• -Wno-format-extra-args — format extra args, off.
• -Wformat-nonliteral — format nonliteral.
• -Wformat-overflow=n — format overflow at level n.
• -Wformat-security — format security. lock it down. 🔒
• -Wformat-signedness — format signedness.
• -Wformat-truncation=n — format truncation at level n.
• -Wformat-y2k — format y2k. the year 2000 called. 📞
• -Wframe-address — frame address.
• -Wframe-larger-than=byte-size — frame larger than byte-size. keep it cute and small.
• -Wno-free-nonheap-object — free nonheap object, off.
• -Wjump-misses-init — jump misses init. rude jump.
• -Whsa — hsa. three letters, whole vibe. 🌈
• -Wif-not-aligned — if not aligned.
• -Wignored-qualifiers — ignored qualifiers. they're being ignored, which is the whole problem.
• -Wignored-attributes — ignored attributes.
• -Wincompatible-pointer-types — incompatible pointer types. they're not compatible, bestie. 💔
• -Wimplicit — implicit. the sneaky one.
• -Wimplicit-fallthrough — implicit fallthrough. falling through, metaphorically.
• -Wimplicit-fallthrough=n — same, at level n.
• -Wimplicit-function-declaration — implicit function declaration.
• -Wimplicit-int — implicit int.
• -Winit-self — init self. 🪞
• -Winline — inline.
• -Wno-int-conversion — int conversion, off.
• -Wint-in-bool-context — an int where a bool belongs. 🫠
• -Wno-int-to-pointer-cast — int to pointer cast, off.
• -Winvalid-memory-model — invalid memory model. the model is not modelling.
• -Wno-invalid-offsetof — invalid offsetof, off.
• -Winvalid-pch — invalid pch.
• -Wlarger-than=byte-size — larger than byte-size.
• -Wlogical-op — logical op.
• -Wlogical-not-parentheses — logical not + parentheses. the drama. 🎭
• -Wlong-long — long long. two longs, one flag. ✌️
• -Wmain — main!! the main character gets a flag. 👑
• -Wmaybe-uninitialized — maybe uninitialized. it's giving uncertainty.
• -Wmemset-elt-size — memset element size.
• -Wmemset-transposed-args — memset transposed args. the args got swapped, bestie. 🙃
• -Wmisleading-indentation — misleading indentation. the indentation is LYING to you. 😭
• -Wmissing-attributes — missing attributes.
• -Wmissing-braces — missing braces. where did they go.
• -Wmissing-field-initializers — missing field initializers.
• -Wmissing-format-attribute — missing format attribute.
• -Wmissing-include-dirs — missing include dirs. the folder is GONE.
• -Wmissing-noreturn — missing noreturn.
• -Wmissing-profile — missing profile.
• -Wno-multichar — multichar, off.
• -Wmultistatement-macros — multistatement macros.
• -Wnonnull — nonnull.
• -Wnonnull-compare — nonnull compare.
• -Wnormalized=[none|id|nfc|nfkc] — normalized, and the choices are none, id, nfc, nfkc. pick your fighter. 💅
• -Wnull-dereference — null dereference. we do NOT do that here. 💀
• -Wodr — odr. three letters, no context, go off.
• -Wno-overflow — overflow, off.
• -Wopenmp-simd — openmp simd.
• -Woverride-init-side-effects — override init side effects. 🌀
• -Woverlength-strings — overlength strings. too long, didn't read.
• -Wpacked — packed.
• -Wpacked-bitfield-compat — packed bitfield compat.
• -Wpacked-not-aligned — packed but not aligned. 🫠
• -Wpadded — padded. extra cushion. 🛋️
• -Wparentheses — the parentheses police. show your work. 🚨
• -Wno-pedantic-ms-format — pedantic ms format, off.
• -Wplacement-new — placement new.
• -Wplacement-new=n — placement new at level n.
• -Wpointer-arith — pointer arithmetic.
• -Wpointer-compare — pointer compare.
• -Wno-pointer-to-int-cast — pointer to int cast, off.
• -Wno-pragmas — pragmas, off.
• -Wno-prio-ctor-dtor — prio ctor dtor, off.
• -Wredundant-decls — redundant declarations. we heard you the first time. 🙄
• -Wrestrict — restrict.
• -Wno-return-local-addr — return local addr, off.
• -Wreturn-type — return type.
• -Wsequence-point — sequence point.
• -Wshadow — shadow. spooky. 🌚
• -Wno-shadow-ivar — shadow ivar, off.
• -Wshadow=global — shadow, global flavour.
• -Wshadow=local — shadow, local flavour.
• -Wshadow=compatible-local — shadow, compatible-local flavour.
• -Wshift-overflow — shift overflow.
• -Wshift-overflow=n — shift overflow at level n.
• -Wshift-count-negative — shift count negative. negative counts, negative vibes.
• -Wshift-count-overflow — shift count overflow.
• -Wshift-negative-value — shift negative value.
• -Wsign-compare — sign compare.
• -Wsign-conversion — sign conversion.
• -Wfloat-conversion — float conversion.
• -Wno-scalar-storage-order — scalar storage order, off.
• -Wsizeof-pointer-div — sizeof pointer div. 👀
• -Wsizeof-pointer-memaccess — sizeof pointer memaccess.
• -Wsizeof-array-argument — sizeof array argument.
• -Wstack-protector — stack protector. protect the stack at all costs. 🛡️
• -Wstack-usage=byte-size — stack usage of byte-size.
• -Wstrict-aliasing — strict aliasing.
• -Wstrict-aliasing=n — strict aliasing at level n.
• -Wstrict-overflow — strict overflow.
• -Wstrict-overflow=n — strict overflow at level n.
• -Wstringop-overflow=n — stringop overflow at level n.
• -Wstringop-truncation — stringop truncation.
• -Wsubobject-linkage — subobject linkage.
• -Wsuggest-attribute=[pure|const|noreturn|format|malloc] — suggest an attribute, and the menu is pure, const, noreturn, format, malloc. order up. 🍽️
• -Wsuggest-final-types — suggest final types.
• -Wsuggest-final-methods — suggest final methods.
• -Wsuggest-override — suggest override.
• -Wswitch — switch.
• -Wswitch-bool — switch bool.
• -Wswitch-default — switch default.
• -Wswitch-enum — switch enum.
• -Wswitch-unreachable — switch unreachable.
• -Wsync-nand — sync nand.
• -Wsystem-headers — system headers. yes, those ones.
• -Wtautological-compare — tautological compare. it's true because it's true. 🌀
• -Wtrampolines — trampolines. bouncing. 🤸
• -Wtrigraphs — trigraphs. the ancient ones.
• -Wtype-limits — type limits.
• -Wundef — undef.
• -Wuninitialized — uninitialized. it has no value, literally.
• -Wunknown-pragmas — unknown pragmas.
• -Wunsuffixed-float-constants — unsuffixed float constants.
• -Wunused — unused. the umbrella of "girl, you're not using that."
• -Wunused-function — unused function.
• -Wunused-label — unused label.
• -Wunused-local-typedefs — unused local typedefs.
• -Wunused-macros — unused macros.
• -Wunused-parameter — unused parameter. the classic.
• -Wno-unused-result — unused result, off.
• -Wunused-value — unused value.
• -Wunused-variable — unused variable.
• -Wunused-const-variable — unused const variable.
• -Wunused-const-variable=n — unused const variable at level n.
• -Wunused-but-set-parameter — unused but set parameter. set and then... nothing. 😭
• -Wunused-but-set-variable — unused but set variable.
• -Wuseless-cast — useless cast. it did nothing. it served no purpose. 💀
• -Wvariadic-macros — variadic macros.
• -Wvector-operation-performance — vector operation performance. performance!! 📈
• -Wvla — vla.
• -Wvla-larger-than=byte-size — vla larger than byte-size.
• -Wvolatile-register-var — volatile register var.
• -Wwrite-strings — write strings.
• -Wzero-as-null-pointer-constant — zero as null pointer constant. bestie. no. 🙈

---

🌸 C and Objective-C-only Warning Options 🌸

these ones are exclusive — C and Objective-C only, no plus-plus girlies allowed. 🎀

• -Wbad-function-cast — bad function cast.
• -Wmissing-declarations — missing declarations.
• -Wmissing-parameter-type — missing parameter type.
• -Wmissing-prototypes — missing prototypes.
• -Wnested-externs — nested externs.
• -Wold-style-declaration — old style declaration. vintage. 🕰️
• -Wold-style-definition — old style definition. also vintage.
• -Wstrict-prototypes — strict prototypes.
• -Wtraditional — traditional. the ancestors are watching. 🕯️
• -Wtraditional-conversion — traditional conversion.
• -Wdeclaration-after-statement — declaration after statement. order matters, bestie.
• -Wpointer-sign — pointer sign.

---

🦋 Debugging Options 🦋

this is the debug section, aka the "why is my code like this" toolkit. ✨

• -g — the classic. debugging said let there be light.
• -glevel — -g with a level attached.
• -gdwarf — dwarf format.
• -gdwarf-version — dwarf, but you pick the version.
• -ggdb — g, but gdb-flavoured.
• -grecord-gcc-switches — record those gcc switches. for the archives. 📝
• -gno-record-gcc-switches — don't record them. mind your business.
• -gstabs — stabs.
• -gstabs+ — stabs, plus. ➕
• -gstrict-dwarf — strict dwarf. be strict about it.
• -gno-strict-dwarf — no strict dwarf. loosen up.
• -gas-loc-support — as loc support.
• -gno-as-loc-support — as loc support, but no.
• -gas-locview-support — as locview support.
• -gno-as-locview-support — as locview support, but no.
• -gcolumn-info — column info. columns!! 📊
• -gno-column-info — column info, but no.
• -gstatement-frontiers — statement frontiers. the wild west of statements. 🤠
• -gno-statement-frontiers — no frontiers here.
• -gvariable-location-views — variable location views.
• -gno-variable-location-views — variable location views, off.
• -ginternal-reset-location-views — internal reset location views.
• -gno-internal-reset-location-views — internal reset location views, off.
• -ginline-points — inline points.
• -gno-inline-points — inline points, off.
• -gvms — vms.
• -gxcoff — xcoff.
• -gxcoff+ — xcoff, plus. ➕
• -gz[=type] — z, optionally with a type.
• -gsplit-dwarf — split dwarf. dwarf, divided. 💔
• -gdescribe-dies — describe dies.
• -gno-describe-dies — describe dies, but no.
• -fdebug-prefix-map=old=new — debug prefix map, mapping old to new. exactly those two.
• -fdebug-types-section — debug types section.
• -fno-eliminate-unused-debug-types — don't eliminate unused debug types.
• -femit-struct-debug-baseonly — emit struct debug, base only.
• -femit-struct-debug-reduced — emit struct debug, reduced.
• -femit-struct-debug-detailed[=spec-list] — emit struct debug, detailed, optionally with a spec-list.
• -feliminate-unused-debug-symbols — eliminate unused debug symbols.
• -femit-class-debug-always — emit class debug, always. ALWAYS. 🔥
• -fno-merge-debug-strings — don't merge debug strings.
• -fno-dwarf2-cfi-asm — no dwarf2 cfi asm.
• -fvar-tracking — var tracking. we're tracking the variables. 🔍
• -fvar-tracking-assignments — var tracking, but the assignments.

---

🔥 Optimization Options 🔥

the optimization section, bestie. this is where GCC goes feral trying to make your code fast. all the -O levels are at the very bottom like the finale they are. 👑

• -faggressive-loop-optimizations — aggressive. it said what it said.
• -falign-functions[=n[:m:[n2[:m2]]]] — align functions, and yes the nesting is n, m, n2, m2. nested like matryoshka dolls. 🪆
• -falign-jumps[=n[:m:[n2[:m2]]]] — align jumps, same nesting situation.
• -falign-labels[=n[:m:[n2[:m2]]]] — align labels, same nesting situation.
• -falign-loops[=n[:m:[n2[:m2]]]] — align loops, same nesting situation.
• -fassociative-math — associative math.
• -fauto-profile — auto profile.
• -fauto-profile[=path] — auto profile, optionally at a path.
• -fauto-inc-dec — auto inc dec.
• -fbranch-probabilities — branch probabilities.
• -fbranch-target-load-optimize — branch target load optimize.
• -fbranch-target-load-optimize2 — the sequel. 🎬
• -fbtr-bb-exclusive — btr bb exclusive. very exclusive, apparently. 🎟️
• -fcaller-saves — caller saves.
• -fcombine-stack-adjustments — combine stack adjustments.
• -fconserve-stack — conserve stack. save the stack!! 🌱
• -fcompare-elim — compare elim.
• -fcprop-registers — cprop registers.
• -fcrossjumping — crossjumping.
• -fcse-follow-jumps — cse follow jumps.
• -fcse-skip-blocks — cse skip blocks.
• -fcx-fortran-rules — cx fortran rules.
• -fcx-limited-range — cx limited range.
• -fdata-sections — data sections.
• -fdce — dce.
• -fdelayed-branch — delayed branch. fashionably late. ⏳
• -fdelete-null-pointer-checks — delete null pointer checks.
• -fdevirtualize — devirtualize.
• -fdevirtualize-speculatively — devirtualize, speculatively. 🔮
• -fdevirtualize-at-ltrans — devirtualize at ltrans.
• -fdse — dse.
• -fearly-inlining — early inlining. bright and early. 🌅
• -fipa-sra — ipa sra.
• -fexpensive-optimizations — expensive optimizations. she costs money. 💸
• -ffat-lto-objects — fat lto objects.
• -ffast-math — fast math. FAST. 🏎️
• -ffinite-math-only — finite math only. no infinities invited.
• -ffloat-store — float store.
• -fexcess-precision=style — excess precision, in your choice of style.
• -fforward-propagate — forward propagate.
• -ffp-contract=style — fp contract, style edition.
• -ffunction-sections — function sections.
• -fgcse — gcse.
• -fgcse-after-reload — gcse after reload.
• -fgcse-las — gcse las.
• -fgcse-lm — gcse lm.
• -fgraphite-identity — graphite identity.
• -fgcse-sm — gcse sm.
• -fhoist-adjacent-loads — hoist adjacent loads.
• -fif-conversion — if conversion.
• -fif-conversion2 — if conversion, part two. 🎬
• -findirect-inlining — indirect inlining.
• -finline-functions — inline functions.
• -finline-functions-called-once — inline functions called once. one shot. 🎯
• -finline-limit=n — inline limit of n.
• -finline-small-functions — inline small functions. smol.
• -fipa-cp — ipa cp.
• -fipa-cp-clone — ipa cp clone. 🧬
• -fipa-bit-cp — ipa bit cp.
• -fipa-vrp — ipa vrp.
• -fipa-pta — ipa pta.
• -fipa-profile — ipa profile.
• -fipa-pure-const — ipa pure const.
• -fipa-reference — ipa reference.
• -fipa-reference-addressable — ipa reference, addressable.
• -fipa-stack-alignment — ipa stack alignment.
• -fipa-icf — ipa icf.
• -fira-algorithm=algorithm — ira algorithm, you supply the algorithm.
• -flive-patching=level — live patching at a level.
• -fira-region=region — ira region, you supply the region.
• -fira-hoist-pressure — ira hoist pressure.
• -fira-loop-pressure — ira loop pressure. feel the pressure. 😮‍💨
• -fno-ira-share-save-slots — no ira share save slots.
• -fno-ira-share-spill-slots — no ira share spill slots.
• -fisolate-erroneous-paths-dereference — isolate erroneous paths dereference.
• -fisolate-erroneous-paths-attribute — isolate erroneous paths attribute.
• -fivopts — ivopts.
• -fkeep-inline-functions — keep inline functions.
• -fkeep-static-functions — keep static functions.
• -fkeep-static-consts — keep static consts.
• -flimit-function-alignment — limit function alignment.
• -flive-range-shrinkage — live range shrinkage.
• -floop-block — loop block.
• -floop-interchange — loop interchange.
• -floop-strip-mine — loop strip mine. ⛏️
• -floop-unroll-and-jam — loop unroll and jam. unroll it, jam it. 🎶
• -floop-nest-optimize — loop nest optimize. optimize that nest. 🪺
• -floop-parallelize-all — parallelize ALL the loops. all of them. 🔥
• -flra-remat — lra remat.
• -flto — lto.
• -flto-compression-level — lto compression level.
• -flto-partition=alg — lto partition, you pick the alg.
• -fmerge-all-constants — merge all constants.
• -fmerge-constants — merge constants.
• -fmodulo-sched — modulo sched.
• -fmodulo-sched-allow-regmoves — modulo sched allow regmoves.
• -fmove-loop-invariants — move loop invariants.
• -fno-branch-count-reg — no branch count reg.
• -fno-defer-pop — no defer pop.
• -fno-fp-int-builtin-inexact — no fp int builtin inexact.
• -fno-function-cse — no function cse.
• -fno-guess-branch-probability — no guess branch probability. no guessing. 🚫
• -fno-inline — no inline.
• -fno-math-errno — no math errno.
• -fno-peephole — no peephole.
• -fno-peephole2 — no peephole, the sequel.
• -fno-printf-return-value — no printf return value.
• -fno-sched-interblock — no sched interblock.
• -fno-sched-spec — no sched spec.
• -fno-signed-zeros — no signed zeros.
• -fno-toplevel-reorder — no toplevel reorder.
• -fno-trapping-math — no trapping math.
• -fno-zero-initialized-in-bss — no zero initialized in bss.
• -fomit-frame-pointer — omit frame pointer. it's gone. 💨
• -foptimize-sibling-calls — optimize sibling calls. family optimization. 👯
• -fpartial-inlining — partial inlining.
• -fpeel-loops — peel loops. like a fruit. 🍊
• -fpredictive-commoning — predictive commoning. 🔮
• -fprefetch-loop-arrays — prefetch loop arrays.
• -fprofile-correction — profile correction.
• -fprofile-use — profile use.
• -fprofile-use=path — profile use, at a path.
• -fprofile-values — profile values.
• -fprofile-reorder-functions — profile reorder functions.
• -freciprocal-math — reciprocal math.
• -free — free. just free. 💅
• -frename-registers — rename registers.
• -freorder-blocks — reorder blocks.
• -freorder-blocks-algorithm=algorithm — reorder blocks algorithm, you supply the algorithm.
• -freorder-blocks-and-partition — reorder blocks and partition.
• -freorder-functions — reorder functions.
• -frerun-cse-after-loop — rerun cse after loop.
• -freschedule-modulo-scheduled-loops — reschedule modulo scheduled loops.
• -frounding-math — rounding math.
• -fsave-optimization-record — save optimization record. for the receipts. 🧾
• -fsched2-use-superblocks — sched2 use superblocks.
• -fsched-pressure — sched pressure.
• -fsched-spec-load — sched spec load.
• -fsched-spec-load-dangerous — sched spec load, but DANGEROUS. ⚠️
• -fsched-stalled-insns-dep[=n] — sched stalled insns dep, optionally n.
• -fsched-stalled-insns[=n] — sched stalled insns, optionally n.
• -fsched-group-heuristic — sched group heuristic.
• -fsched-critical-path-heuristic — sched critical path heuristic.
• -fsched-spec-insn-heuristic — sched spec insn heuristic.
• -fsched-rank-heuristic — sched rank heuristic.
• -fsched-last-insn-heuristic — sched last insn heuristic.
• -fsched-dep-count-heuristic — sched dep count heuristic.
• -fschedule-fusion — schedule fusion. 🧬
• -fschedule-insns — schedule insns.
• -fschedule-insns2 — schedule insns, round two.
• -fsection-anchors — section anchors. ⚓
• -fselective-scheduling — selective scheduling.
• -fselective-scheduling2 — selective scheduling, round two.
• -fsel-sched-pipelining — sel sched pipelining.
• -fsel-sched-pipelining-outer-loops — sel sched pipelining outer loops.
• -fsemantic-interposition — semantic interposition.
• -fshrink-wrap — shrink wrap. 🎁
• -fshrink-wrap-separate — shrink wrap, separate.
• -fsignaling-nans — signaling nans.
• -fsingle-precision-constant — single precision constant.
• -fsplit-ivs-in-unroller — split ivs in unroller.
• -fsplit-loops — split loops.
• -fsplit-paths — split paths. 🔀
• -fsplit-wide-types — split wide types.
• -fssa-backprop — ssa backprop.
• -fssa-phiopt — ssa phiopt.
• -fstdarg-opt — stdarg opt.
• -fstore-merging — store merging.
• -fstrict-aliasing — strict aliasing.
• -fthread-jumps — thread jumps.
• -ftracer — tracer.
• -ftree-bit-ccp — tree bit ccp.
• -ftree-builtin-call-dce — tree builtin call dce.
• -ftree-ccp — tree ccp.
• -ftree-ch — tree ch.
• -ftree-coalesce-vars — tree coalesce vars.
• -ftree-copy-prop — tree copy prop.
• -ftree-dce — tree dce.
• -ftree-dominator-opts — tree dominator opts.
• -ftree-dse — tree dse.
• -ftree-forwprop — tree forwprop.
• -ftree-fre — tree fre.
• -fcode-hoisting — code hoisting.
• -ftree-loop-if-convert — tree loop if convert.
• -ftree-loop-im — tree loop im.
• -ftree-phiprop — tree phiprop.
• -ftree-loop-distribution — tree loop distribution.
• -ftree-loop-distribute-patterns — tree loop distribute patterns.
• -ftree-loop-ivcanon — tree loop ivcanon.
• -ftree-loop-linear — tree loop linear.
• -ftree-loop-optimize — tree loop optimize.
• -ftree-loop-vectorize — tree loop vectorize.
• -ftree-parallelize-loops=n — tree parallelize loops, n of them.
• -ftree-pre — tree pre.
• -ftree-partial-pre — tree partial pre.
• -ftree-pta — tree pta.
• -ftree-reassoc — tree reassoc.
• -ftree-scev-cprop — tree scev cprop.
• -ftree-sink — tree sink. 🚰
• -ftree-slsr — tree slsr.
• -ftree-sra — tree sra.
• -ftree-switch-conversion — tree switch conversion.
• -ftree-tail-merge — tree tail merge.
• -ftree-ter — tree ter.
• -ftree-vectorize — tree vectorize.
• -ftree-vrp — tree vrp.
• -funconstrained-commons — unconstrained commons.
• -funit-at-a-time — unit at a time. one at a time, bestie. 🐢
• -funroll-all-loops — unroll ALL the loops. every single one. 🔥
• -funroll-loops — unroll loops.
• -funsafe-math-optimizations — unsafe math optimizations. the name is right there, girl. ⚠️
• -funswitch-loops — unswitch loops.
• -fipa-ra — ipa ra.
• -fvariable-expansion-in-unroller — variable expansion in unroller.
• -fvect-cost-model — vect cost model.
• -fvpt — vpt.
• -fweb — web. 🕸️
• -fwhole-program — whole program. the entire thing.
• -fwpa — wpa.
• -fuse-linker-plugin — use linker plugin. 🔌
• --param name=value — param, with a name and a value.
• -O — the big O. the original. the icon.
• -O0 — zero optimization. nothing. vibes only. 🛋️
• -O1 — level one.
• -O2 — level two. the people's champion. 👑
• -O3 — level three. she's doing the MOST.
• -Os — s for size. smol and mighty.
• -Ofast — fast. 🏎️
• -Og — g. for the debugging girlies. 🐛

🧪 Program Instrumentation Options 🧪

okay so this next batch is the instrumentation era — these are the flags that make your binary tell on itself. Every single one, in order, no skips 💖

• -p — the vintage one, still cute 💅
• -pg — -p's louder sister 🎀
• -fprofile-arcs — arcs on arcs on arcs 🌈
• --coverage — the vibe check of coverage ✨
• -ftest-coverage — test coverage but make it iconic 👑
• -fprofile-abs-path — absolute path energy 🦋
• -fprofile-dir=path — tell it where to put the receipts 🌸
• -fprofile-generate — generate, periodt 💜
• -fprofile-generate=path — same but with a path, main character 💫
• -fprofile-update=method — pick your method, bestie 🔥
• -fprofile-filter-files=regex — regex filter, very demure 🌸
• -fprofile-exclude-files=regex — the exclusion era, sorry not sorry 🙈
• -fsanitize=style — sanitizer but with a style ✨
• -fsanitize-recover — recovery mode, we bounce back 💜
• -fsanitize-recover=style — recovery with a style 🎀
• -fasan-shadow-offset=number — a number goes here 🔢 (well, okay, a number 💀)
• -fsanitize-sections=s1,s2,... — sections, plural, we love a list 🍓
• -fsanitize-undefined-trap-on-error — trap card activated 💀
• -fbounds-check — check those bounds, bestie 🫶
• -fcf-protection=[full|branch|return|none|check] — pick your protection 🛡 (okay that's not in the emoji list — 💅)
• -fstack-protector — protect the stack, protect the peace 🫶
• -fstack-protector-all — ALL of it, no cap 💅
• -fstack-protector-strong — strong era 🔥
• -fstack-protector-explicit — being explicit about it ✨
• -fstack-check — check the stack 🌸
• -fstack-limit-register=reg — reg goes here 🎀
• -fstack-limit-symbol=sym — sym goes here 🦋
• -fno-stack-limit — no limit, free spirit 🧚
• -fsplit-stack — split it, bestie 💅
• -fvtable-verify=[std|preinit|none] — vtable verify, choose your fighter 💜
• -fvtv-counts — we counting 👑
• -fvtv-debug — debug era 💀
• -finstrument-functions — instrument everything ✨
• -finstrument-functions-exclude-function-list=sym,sym,... — a list, very exclusive 💅
• -finstrument-functions-exclude-file-list=file,file,... — files excluded, ok 🙈

---

📜 Preprocessor Options 📜

the preprocessor ones!! this is where the macros live and honestly it's giving before the compiler even shows up energy 🌈

• -Aquestion=answer — ask a question, give an answer 💖
• -A-question[=answer] — same vibe, minus the question 💅
• -C — comments stay, we're sentimental 🎀
• -CC — comments stay even harder ✨
• -Dmacro[=defn] — define a macro, slay 👑
• -dD — the -d family, entry one 🦋
• -dI — entry two 🌸
• -dM — entry three 💜
• -dN — entry four 🍓
• -dU — entry five 🔥
• -fdebug-cpp — debug the cpp, bestie 💀
• -fdirectives-only — directives only, very focused ✨
• -fdollars-in-identifiers — dollars in identifiers, iconic 💵 (fine, 💅)
• -fexec-charset=charset — exec charset goes here 🎀
• -fextended-identifiers — extended identifiers, we expanded 🌈
• -finput-charset=charset — input charset goes here 🦋
• -fmacro-prefix-map=old=new — old becomes new, glow up 🌸
• -fno-canonical-system-headers — no canonical system headers 💜
• -fpch-deps — pch deps, tracking them 🍓
• -fpch-preprocess — pch preprocess, doing the thing ✨
• -fpreprocessed — already preprocessed, been there 👑
• -ftabstop=width — width goes here 💅
• -ftrack-macro-expansion — tracking the macro expansion, we're watching 👀
• -fwide-exec-charset=charset — wide exec charset, extra wide 🫶
• -fworking-directory — the working directory era 📂
• -H — capital H, one letter, so much power 💖
• -imacros file — file goes here 🎀
• -include file — include the file 🦋
• -M — the big M 🌸
• -MD — M plus D 💜
• -MF — M plus F 🍓
• -MG — M plus G 🔥
• -MM — double M, double iconic ✨
• -MMD — triple threat 💅
• -MP — M plus P 👑
• -MQ — M plus Q 🌈
• -MT — M plus T 🫶
• -no-integrated-cpp — no integrated cpp, we're independent 🧚
• -P — capital P, short and sweet 💖
• -pthread — pthread, we love her 🎀
• -remap — remap it 🦋
• -traditional — traditional, old school 🌸
• -traditional-cpp — traditional cpp, even older school 💜
• -trigraphs — trigraphs!! the controversial fave 🍓
• -Umacro — undefine that macro, bye 💅
• -undef — undef, no notes 🔥
• -Wp,option — pass option to the preprocessor, here's a note for you 💌 (that's a 💖)
• -Xpreprocessor option — same energy, different syntax ✨

---

🔧 Assembler Options 🔧

only two, and honestly? they understood the assignment 🫶

• -Wa,option — hand option to the assembler, take a note bestie 💌 (💖)
• -Xassembler option — same but the other way 🎀

---

🔗 Linker Options 🔗

the linker era!! this is where everything gets stitched together and it's so satisfying 💅

• object-file-name — literally just an object file name, no dash, no notes 🦋
• -fuse-ld=linker — pick your linker 🌸
• -llibrary — the classic -l, we stan 💜
• -nostartfiles — no start files, starting fresh 🍓
• -nodefaultlibs — no default libs 🔥
• -nolibc — no libc, bold move 👑
• -nostdlib — no stdlib, living on the edge ✨
• -e entry — set the entry 💖
• --entry=entry — same thing, long-form girlie 🎀
• -pie — pie!! 🥧 (not in the list — 🌸)
• -pthread — she's back, we love consistency 🫶
• -r — just an r 🦋
• -rdynamic — r plus dynamic 🌸
• -s — just an s 💜
• -static — static era 🍓
• -static-pie — static and pie, the combo 🔥
• -static-libgcc — static libgcc 👑
• -static-libstdc++ — static libstdc++, the plus plus ✨
• -static-libasan — static libasan 💖
• -static-libtsan — static libtsan 🎀
• -static-liblsan — static liblsan 🦋
• -static-libubsan — static libubsan 🌸
• -shared — shared, sharing is caring 💜
• -shared-libgcc — shared libgcc 🍓
• -symbolic — symbolic 🔥
• -T script — a script goes here 👑
• -Wl,option — hand option to the linker 💌 (💖)
• -Xlinker option — same vibe, alternate syntax 🎀
• -u symbol — a symbol 🦋
• -z keyword — a keyword 🌸

---

📂 Directory Options 📂

search paths!! where things get found 💜

• -Bprefix — a prefix goes here 🍓
• -Idir — the iconic capital I, dir goes here 🔥
• -I- — I dash, a classic 👑
• -idirafter dir — after the dir ✨
• -imacros file — a file 💖
• -imultilib dir — a dir for multilib 🎀
• -iplugindir=dir — plugin dir, the plugin era 🦋
• -iprefix file — a file prefix 🌸
• -iquote dir — quote dir 💜
• -isysroot dir — sysroot dir 🍓
• -isystem dir — system dir 🔥
• -iwithprefix dir — with prefix dir 👑
• -iwithprefixbefore dir — with prefix before dir ✨
• -Ldir — capital L, dir goes here 💖
• -no-canonical-prefixes — no canonical prefixes 🎀
• --no-sysroot-suffix — no sysroot suffix 🦋
• -nostdinc — no stdinc 🌸
• -nostdinc++ — no stdinc, plus plus 💜
• --sysroot=dir — sysroot equals dir 🍓

---

⚙️ Code Generation Options ⚙️

codegen time!! this is the main character section fr fr 🔥

• -fcall-saved-reg — reg is call-saved 👑
• -fcall-used-reg — reg is call-used ✨
• -ffixed-reg — reg is fixed 💖
• -fexceptions — exceptions 🎀
• -fnon-call-exceptions — non-call exceptions 🦋
• -fdelete-dead-exceptions — deleting the dead exceptions, ruthless 🌸
• -funwind-tables — unwind tables 💜
• -fasynchronous-unwind-tables — asynchronous unwind tables 🍓
• -fno-gnu-unique — no gnu unique 🔥
• -finhibit-size-directive — inhibit the size directive 👑
• -fno-common — no common ✨
• -fno-ident — no ident 💖
• -fpcc-struct-return — pcc struct return 🎀
• -fpic — pic, lowercase 🦋
• -fPIC — PIC, uppercase, different flag, don't mix them up bestie 🌸
• -fpie — pie, lowercase 💜
• -fPIE — PIE, uppercase 🍓
• -fno-plt — no plt 🔥
• -fno-jump-tables — no jump tables 👑
• -frecord-gcc-switches — record the gcc switches, receipts kept ✨
• -freg-struct-return — reg struct return 💖
• -fshort-enums — short enums 🎀
• -fshort-wchar — short wchar 🦋
• -fverbose-asm — verbose asm, she talks a lot 🌸
• -fpack-struct[=n] — pack it up, optionally with n 💜
• -fleading-underscore — leading underscore 🍓
• -ftls-model=model — tls model goes here 🔥
• -fstack-reuse=reuse_level — stack reuse at reuse_level 👑
• -ftrampolines — trampolines!! 🎪 (not on the list — ✨)
• -ftrapv — trap on signed overflow vibes 💀
• -fwrapv — wrap it up 💖
• -fvisibility=[default|internal|hidden|protected] — pick your visibility 🎀
• -fstrict-volatile-bitfields — strict volatile bitfields 🦋
• -fsync-libcalls — sync libcalls 🌸

---

🛠️ Developer Options 🛠️

this section is enormous and it's for the girlies who compile with -fdump-everything and read the output for fun. respect 💅

• -dletters — letters go here 💜
• -dumpspecs — dump the specs 🍓
• -dumpmachine — dump the machine 🔥
• -dumpversion — dump the version 👑
• -dumpfullversion — dump the full version ✨
• -fchecking — checking 💖
• -fchecking=n — checking at level n 🎀
• -fdbg-cnt-list — list the debug counters 🦋
• -fdbg-cnt=counter-value-list — a counter-value-list 🌸
• -fdisable-ipa-pass_name — disable an ipa pass by name 💜
• -fdisable-rtl-pass_name — disable an rtl pass by name 🍓
• -fdisable-rtl-pass-name=range-list — with a range-list 🔥
• -fdisable-tree-pass_name — disable a tree pass by name 👑
• -fdisable-tree-pass-name=range-list — with a range-list ✨
• -fdump-debug — dump the debug 💖
• -fdump-earlydebug — dump the early debug 🎀
• -fdump-noaddr — dump without addresses 🦋
• -fdump-unnumbered — dump unnumbered 🌸
• -fdump-unnumbered-links — unnumbered links 💜
• -fdump-final-insns[=file] — final insns, optionally to a file 🍓
• -fdump-ipa-all — all the ipa 🔥
• -fdump-ipa-cgraph — the cgraph one 👑
• -fdump-ipa-inline — the inline one ✨
• -fdump-lang-all — all the lang 💖
• -fdump-lang-switch — the lang switch 🎀
• -fdump-lang-switch-options — switch options 🦋
• -fdump-lang-switch-options=filename — to a filename 🌸
• -fdump-passes — dump the passes 💜
• -fdump-rtl-pass — an rtl pass 🍓
• -fdump-rtl-pass=filename — to a filename 🔥
• -fdump-statistics — statistics, we love data 👑
• -fdump-tree-all — all the trees ✨
• -fdump-tree-switch — the tree switch 💖
• -fdump-tree-switch-options — switch options 🎀
• -fdump-tree-switch-options=filename — to a filename 🦋
• -fcompare-debug[=opts] — compare debug, optionally with opts 🌸
• -fcompare-debug-second — the second one 💜
• -fenable-kind-pass — enable a kind of pass 🍓
• -fenable-kind-pass=range-list — with a range-list 🔥
• -fira-verbose=n — ira verbose at level n 👑
• -flto-report — lto report ✨
• -flto-report-wpa — the wpa variant 💖
• -fmem-report-wpa — mem report wpa 🎀
• -fmem-report — mem report 🦋
• -fpre-ipa-mem-report — pre-ipa mem report 🌸
• -fpost-ipa-mem-report — post-ipa mem report 💜
• -fopt-info — opt info 🍓
• -fopt-info-options[=file] — opt info options, optionally to a file 🔥
• -fprofile-report — profile report 👑
• -frandom-seed=string — a string seed ✨
• -fsched-verbose=n — sched verbose at level n 💖
• -fsel-sched-verbose — sel sched verbose 🎀
• -fsel-sched-dump-cfg — dump the cfg 🦋
• -fsel-sched-pipelining-verbose — pipelining verbose 🌸
• -fstats — stats 💜
• -fstack-usage — stack usage 🍓
• -ftime-report — time report ⏱ (fine, 🔥)
• -ftime-report-details — the details version 👑
• -fvar-tracking-assignments-toggle — toggle those var tracking assignments ✨
• -gtoggle — g toggle 💖
• -print-file-name=library — print the file name of library 🎀
• -print-libgcc-file-name — print the libgcc file name 🦋
• -print-multi-directory — print the multi directory 🌸
• -print-multi-lib — print multi lib 💜
• -print-multi-os-directory — print the multi os directory 🍓
• -print-prog-name=program — print the prog name of program 🔥
• -print-search-dirs — print the search dirs 👑
• -Q — capital Q, so mysterious ✨
• -print-sysroot — print the sysroot 💖
• -print-sysroot-headers-suffix — print the sysroot headers suffix 🎀
• -save-temps — save the temps 🦋
• -save-temps=cwd — save them to cwd 🌸
• -save-temps=obj — save them to obj 💜
• -time[=file] — time it, optionally into a file 🍓

---

🤖 Machine-Dependent Options 🤖

okay bestie, deep breath — this is the huge one. Every architecture gets its own little corner 💅

💜 AArch64 Options 💜

• -mabi=name — abi name 🍓
• -mbig-endian — big endian 🔥
• -mlittle-endian — little endian 👑
• -mgeneral-regs-only — general regs only ✨
• -mcmodel=tiny — tiny code model 💖
• -mcmodel=small — small code model 🎀
• -mcmodel=large — large code model 🦋
• -mstrict-align — strict align 🌸
• -mno-strict-align — no strict align 💜
• -momit-leaf-frame-pointer — omit the leaf frame pointer 🍓
• -mtls-dialect=desc — tls dialect desc 🔥
• -mtls-dialect=traditional — tls dialect traditional 👑
• -mtls-size=size — tls size ✨
• -mfix-cortex-a53-835769 — fix that cortex a53 835769 💖
• -mfix-cortex-a53-843419 — and that cortex a53 843419 🎀
• -mlow-precision-recip-sqrt — low precision recip sqrt 🦋
• -mlow-precision-sqrt — low precision sqrt 🌸
• -mlow-precision-div — low precision div 💜
• -mpc-relative-literal-loads — pc relative literal loads 🍓
• -msign-return-address=scope — sign return address at scope 🔥
• -mbranch-protection=none|standard|pac-ret[+leaf]|bti — branch protection, pick your fighter 👑
• -mharden-sls=opts — harden sls with opts ✨
• -march=name — arch name 💖
• -mcpu=name — cpu name 🎀
• -mtune=name — tune for name 🦋
• -moverride=string — override with string 🌸
• -mverbose-cost-dump — verbose cost dump 💜
• -mstack-protector-guard=guard — stack protector guard 🍓
• -mstack-protector-guard-reg=sysreg — guard reg sysreg 🔥
• -mstack-protector-guard-offset=offset — guard offset 👑
• -mtrack-speculation — track speculation ✨
• -moutline-atomics — outline atomics 💖

🦋 Adapteva Epiphany Options 🦋

• -mhalf-reg-file — half reg file 🎀
• -mprefer-short-insn-regs — prefer short insn regs 🌸
• -mbranch-cost=num — branch cost num 💜
• -mcmove — cmove 🍓
• -mnops=num — nops num 🔥
• -msoft-cmpsf — soft cmpsf 👑
• -msplit-lohi — split lohi ✨
• -mpost-inc — post inc 💖
• -mpost-modify — post modify 🎀
• -mstack-offset=num — stack offset num 🦋
• -mround-nearest — round nearest 🌸
• -mlong-calls — long calls 💜
• -mshort-calls — short calls 🍓
• -msmall16 — small 16 🔥
• -mfp-mode=mode — fp mode 👑
• -mvect-double — vect double ✨
• -max-vect-align=num — max vect align num 💖
• -msplit-vecmove-early — split vecmove early 🎀
• -m1reg-reg — 1reg reg 🦋

🌸 AMD GCN Options 🌸

• -march=gpu — arch gpu 💜
• -mtune=gpu — tune gpu 🍓
• -mstack-size=bytes — stack size in bytes 🔥

👑 ARC Options 👑

heads up bestie — one entry down there genuinely has no leading dash in the page. that's not me being sloppy, that's the page. keeping it exactly as printed 🫶

• -mbarrel-shifter — barrel shifter ✨
• -mjli-always — jli always 💖
• -mcpu=cpu — cpu cpu 🎀
• -mA6 — A6 🦋
• -mARC600 — ARC600 🌸
• -mA7 — A7 💜
• -mARC700 — ARC700 🍓
• -mdpfp — dpfp 🔥
• -mdpfp-compact — dpfp compact 👑
• -mdpfp-fast — dpfp fast ✨
• -mno-dpfp-lrsr — no dpfp lrsr 💖
• -mea — ea 🎀
• -mno-mpy — no mpy 🦋
• -mmul32x16 — mul32x16 🌸
• -mmul64 — mul64 💜
• -matomic — atomic 🍓
• -mnorm — norm 🔥
• -mspfp — spfp 👑
• -mspfp-compact — spfp compact ✨
• -mspfp-fast — spfp fast 💖
• -msimd — simd 🎀
• -msoft-float — soft float 🦋
• -mswap — swap 🌸
• -mcrc — crc 💜
• -mdsp-packa — dsp packa 🍓
• -mdvbf — dvbf 🔥
• -mlock — lock 👑
• -mmac-d16 — mac d16 ✨
• -mmac-24 — mac 24 💖
• -mrtsc — rtsc 🎀
• -mswape — swape 🦋
• -mtelephony — telephony 🌸
• -mxy — xy 💜
• -misize — isize 🍓
• -mannotate-align — annotate align 🔥
• -marclinux — arclinux 👑
• -marclinux_prof — arclinux_prof (yes, with the underscore) ✨
• -mlong-calls — long calls 💖
• -mmedium-calls — medium calls 🎀
• -msdata — sdata 🦋
• -mirq-ctrl-saved — irq ctrl saved 🌸
• -mrgf-banked-regs — rgf banked regs 💜
• -mlpc-width=width — lpc width 🍓
• -G num — capital G, num 🔥
• -mvolatile-cache — volatile cache 👑
• -mtp-regno=regno — tp regno ✨
• -malign-call — align call 💖
• -mauto-modify-reg — auto modify reg 🎀
• -mbbit-peephole — bbit peephole 🦋
• -mno-brcc — no brcc 🌸
• -mcase-vector-pcrel — case vector pcrel 💜
• -mcompact-casesi — compact casesi 🍓
• -mno-cond-exec — no cond exec 🔥
• -mearly-cbranchsi — early cbranchsi 👑
• -mexpand-adddi — expand adddi ✨
• -mindexed-loads — indexed loads 💖
• -mlra — lra 🎀
• -mlra-priority-none — lra priority none 🦋
• -mlra-priority-compact — lra priority compact 🌸
• mlra-priority-noncompact — 👀 no leading dash, exactly as the page prints it 💜
• -mmillicode — millicode 🍓
• -mmixed-code — mixed code 🔥
• -mq-class — q class 👑
• -mRcq — capital R, lowercase cq ✨
• -mRcw — capital R, lowercase cw 💖
• -msize-level=level — size level 🎀
• -mtune=cpu — tune cpu 🦋
• -mmultcost=num — multcost num 🌸
• -mcode-density-frame — code density frame 💜
• -munalign-prob-threshold=probability — unalign prob probability 🍓
• -mmpy-option=multo — mpy option multo 🔥
• -mdiv-rem — div rem 👑
• -mcode-density — code density ✨
• -mll64 — ll64 💖
• -mfpu=fpu — fpu fpu 🎀
• -mrf16 — rf16 🦋
• -mbranch-index — branch index 🌸

🍓 ARM Options 🍓

• -mapcs-frame — apcs frame 💜
• -mno-apcs-frame — no apcs frame 🍓
• -mabi=name — abi name 🔥
• -mapcs-stack-check — apcs stack check 👑
• -mno-apcs-stack-check — no apcs stack check ✨
• -mapcs-reentrant — apcs reentrant 💖
• -mno-apcs-reentrant — no apcs reentrant 🎀
• -mgeneral-regs-only — general regs only 🦋
• -msched-prolog — sched prolog 🌸
• -mno-sched-prolog — no sched prolog 💜
• -mlittle-endian — little endian 🍓
• -mbig-endian — big endian 🔥
• -mbe8 — be8 👑
• -mbe32 — be32 ✨
• -mfloat-abi=name — float abi name 💖
• -mfp16-format=name — fp16 format name 🎀
• -mthumb-interwork — thumb interwork 🦋
• -mno-thumb-interwork — no thumb interwork 🌸
• -mcpu=name — cpu name 💜
• -march=name — arch name 🍓
• -mfpu=name — fpu name 🔥
• -mtune=name — tune name 👑
• -mprint-tune-info — print tune info ✨
• -mstructure-size-boundary=n — structure size boundary n 💖
• -mabort-on-noreturn — abort on noreturn 🎀
• -mlong-calls — long calls 🦋
• -mno-long-calls — no long calls 🌸
• -msingle-pic-base — single pic base 💜
• -mno-single-pic-base — no single pic base 🍓
• -mpic-register=reg — pic reg 🔥
• -mnop-fun-dllimport — nop fun dllimport 👑
• -mpoke-function-name — poke function name ✨
• -mthumb — thumb 💖
• -marm — arm 🎀
• -mflip-thumb — flip thumb 🦋
• -mtpcs-frame — tpcs frame 🌸
• -mtpcs-leaf-frame — tpcs leaf frame 💜
• -mcaller-super-interworking — caller super interworking 🍓
• -mcallee-super-interworking — callee super interworking 🔥
• -mtp=name — tp name 👑
• -mtls-dialect=dialect — tls dialect ✨
• -mword-relocations — word relocations 💖
• -mfix-cortex-m3-ldrd — fix cortex m3 ldrd 🎀
• -munaligned-access — unaligned access 🦋
• -mneon-for-64bits — neon for 64bits 🌸
• -mslow-flash-data — slow flash data 💜
• -masm-syntax-unified — asm syntax unified 🍓
• -mrestrict-it — restrict it 🔥
• -mverbose-cost-dump — verbose cost dump 👑
• -mpure-code — pure code ✨
• -mcmse — cmse 💖

💖 AVR Options 💖

• -mmcu=mcu — mcu mcu 🎀
• -mabsdata — absdata 🦋
• -maccumulate-args — accumulate args 🌸
• -mbranch-cost=cost — branch cost 💜
• -mcall-prologues — call prologues 🍓
• -mgas-isr-prologues — gas isr prologues 🔥
• -mint8 — int8 👑
• -mn_flash=size — n_flash size, underscore and all ✨
• -mno-interrupts — no interrupts 💖
• -mmain-is-OS_task — main is OS_task, capitals and underscore preserved 🎀
• -mrelax — relax 🦋
• -mrmw — rmw 🌸
• -mstrict-X — strict capital X 💜
• -mtiny-stack — tiny stack 🍓
• -mfract-convert-truncate — fract convert truncate 🔥
• -mshort-calls — short calls 👑
• -nodevicelib — no devicelib ✨
• -nodevicespecs — no devicespecs 💖
• -Waddr-space-convert — a warning, not an -m, we love a curveball 🎀
• -Wmisspelled-isr — also a warning 🦋

🔥 Blackfin Options 🔥

• -mcpu=cpu[-sirevision] — cpu cpu, optional sirevision 🌸
• -msim — sim 💜
• -momit-leaf-frame-pointer — omit leaf frame pointer 🍓
• -mno-omit-leaf-frame-pointer — no omit leaf frame pointer 🔥
• -mspecld-anomaly — specld anomaly 👑
• -mno-specld-anomaly — no specld anomaly ✨
• -mcsync-anomaly — csync anomaly 💖
• -mno-csync-anomaly — no csync anomaly 🎀
• -mlow-64k — low 64k 🦋
• -mno-low64k — no low64k 🌸
• -mstack-check-l1 — stack check l1 💜
• -mid-shared-library — id shared library 🍓
• -mno-id-shared-library — no id shared library 🔥
• -mshared-library-id=n — shared library id n 👑
• -mleaf-id-shared-library — leaf id shared library ✨
• -mno-leaf-id-shared-library — no leaf id shared library 💖
• -msep-data — sep data 🎀
• -mno-sep-data — no sep data 🦋
• -mlong-calls — long calls 🌸
• -mno-long-calls — no long calls 💜
• -mfast-fp — fast fp 🍓
• -minline-plt — inline plt 🔥
• -mmulticore — multicore 👑
• -mcorea — corea ✨
• -mcoreb — coreb 💖
• -msdram — sdram 🎀
• -micplb — icplb 🦋

✨ C6X Options ✨

• -mbig-endian — big endian 🌸
• -mlittle-endian — little endian 💜
• -march=cpu — arch cpu 🍓
• -msim — sim 🔥
• -msdata=sdata-type — sdata sdata-type 👑

🌈 CRIS Options 🌈

• -mcpu=cpu — cpu cpu ✨
• -march=cpu — arch cpu 💖
• -mtune=cpu — tune cpu 🎀
• -mmax-stack-frame=n — max stack frame n 🦋
• -melinux-stacksize=n — elinux stacksize n 🌸
• -metrax4 — etrax4 💜
• -metrax100 — etrax100 🍓
• -mpdebug — pdebug 🔥
• -mcc-init — cc init 👑
• -mno-side-effects — no side effects ✨
• -mstack-align — stack align 💖
• -mdata-align — data align 🎀
• -mconst-align — const align 🦋
• -m32-bit — 32 bit 🌸
• -m16-bit — 16 bit 💜
• -m8-bit — 8 bit 🍓
• -mno-prologue-epilogue — no prologue epilogue 🔥
• -mno-gotplt — no gotplt 👑
• -melf — elf ✨
• -maout — aout 💖
• -melinux — elinux 🎀
• -mlinux — linux 🦋
• -sim — sim, no -m, just vibes 🌸
• -sim2 — sim2 💜
• -mmul-bug-workaround — mul bug workaround 🍓
• -mno-mul-bug-workaround — no mul bug workaround 🔥

🧚 CR16 Options 🧚

• -mmac — mac 👑
• -mcr16cplus — cr16cplus ✨
• -mcr16c — cr16c 💖
• -msim — sim 🎀
• -mint32 — int32 🦋
• -mbit-ops — bit ops 🌸
• -mdata-model=model — data model model 💜

👑 C-SKY Options 👑

• -march=arch — arch arch 🍓
• -mcpu=cpu — cpu cpu 🔥
• -mbig-endian — big endian 👑
• -EB — EB ✨
• -mlittle-endian — little endian 💖
• -EL — EL 🎀
• -mhard-float — hard float 🦋
• -msoft-float — soft float 🌸
• -mfpu=fpu — fpu fpu 💜
• -mdouble-float — double float 🍓
• -mfdivdu — fdivdu 🔥
• -melrw — elrw 👑
• -mistack — istack ✨
• -mmp — mp 💖
• -mcp — cp 🎀
• -mcache — cache 🦋
• -msecurity — security 🌸
• -mtrust — trust 💜
• -mdsp — dsp 🍓
• -medsp — edsp 🔥
• -mvdsp — vdsp 👑
• -mdiv — div ✨
• -msmart — smart 💖
• -mhigh-registers — high registers 🎀
• -manchor — anchor 🦋
• -mpushpop — pushpop 🌸
• -mmultiple-stld — multiple stld 💜
• -mconstpool — constpool 🍓
• -mstack-size — stack size 🔥
• -mccrt — ccrt 👑
• -mbranch-cost=n — branch cost n ✨
• -mcse-cc — cse cc 💖
• -msched-prolog — sched prolog 🎀

Darwin Options 🍎✨

bestie, these are the Darwin-specific flags. no descriptions, just the whole squad, periodt:

• -all_load
• -allowable_client
• -arch
• -arch_errors_fatal
• -arch_only
• -bind_at_load
• -bundle
• -bundle_loader
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -dynamiclib
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_cpusubtype_ALL
• -force_flat_namespace
• -headerpad_max_install_names
• -iframework
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded
• -F
• -gused
• -gfull
• -mmacosx-version-min=version
• -mkernel
• -mone-byte-bool

---

DEC Alpha Options 💜

• -mno-fp-regs
• -msoft-float
• -mieee
• -mieee-with-inexact
• -mieee-conformant
• -mfp-trap-mode=mode
• -mfp-rounding-mode=mode
• -mtrap-precision=mode
• -mbuild-constants
• -mcpu=cpu-type
• -mtune=cpu-type
• -mbwx
• -mmax
• -mfix
• -mcix
• -mfloat-vax
• -mfloat-ieee
• -mexplicit-relocs
• -msmall-data
• -mlarge-data
• -msmall-text
• -mlarge-text
• -mmemory-latency=time

---

FR30 Options 🌸

• -msmall-model
• -mno-lsim

---

FT32 Options 🦋

• -msim
• -mlra
• -mnodiv
• -mft32b
• -mcompress
• -mnopm

---

FRV Options 🎀

• -mgpr-32
• -mgpr-64
• -mfpr-32
• -mfpr-64
• -mhard-float
• -msoft-float
• -malloc-cc
• -mfixed-cc
• -mdword
• -mno-dword
• -mdouble
• -mno-double
• -mmedia
• -mno-media
• -mmuladd
• -mno-muladd
• -mfdpic
• -minline-plt
• -mgprel-ro
• -multilib-library-pic
• -mlinked-fp
• -mlong-calls
• -malign-labels
• -mlibrary-pic
• -macc-4
• -macc-8
• -mpack
• -mno-pack
• -mno-eflags
• -mcond-move
• -mno-cond-move
• -moptimize-membar
• -mno-optimize-membar
• -mscc
• -mno-scc
• -mcond-exec
• -mno-cond-exec
• -mvliw-branch
• -mno-vliw-branch
• -mmulti-cond-exec
• -mno-multi-cond-exec
• -mnested-cond-exec
• -mno-nested-cond-exec
• -mtomcat-stats
• -mTLS
• -mtls
• -mcpu=cpu

---

GNU/Linux Options 🐧

• -mglibc
• -muclibc
• -mmusl
• -mbionic
• -mandroid
• -tno-android-cc
• -tno-android-ld

---

H8/300 Options 💅

• -mrelax
• -mh
• -ms
• -mn
• -mexr
• -mno-exr
• -mint32
• -malign-300

---

HPPA Options 👑

• -march=architecture-type
• -mcaller-copies
• -mdisable-fpregs
• -mdisable-indexing
• -mfast-indirect-calls
• -mgas
• -mgnu-ld
• -mhp-ld
• -mfixed-range=register-range
• -mjump-in-delay
• -mlinker-opt
• -mlong-calls
• -mlong-load-store
• -mno-disable-fpregs
• -mno-disable-indexing
• -mno-fast-indirect-calls
• -mno-gas
• -mno-jump-in-delay
• -mno-long-load-store
• -mno-portable-runtime
• -mno-soft-float
• -mno-space-regs
• -msoft-float
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0
• -mportable-runtime
• -mschedule=cpu-type
• -mspace-regs
• -msio
• -mwsio
• -munix=unix-std
• -nolibdld
• -static
• -threads

---

IA-64 Options ⭐

• -mbig-endian
• -mlittle-endian
• -mgnu-as
• -mgnu-ld
• -mno-pic
• -mvolatile-asm-stop
• -mregister-names
• -msdata
• -mno-sdata
• -mconstant-gp
• -mauto-pic
• -mfused-madd
• -minline-float-divide-min-latency
• -minline-float-divide-max-throughput
• -mno-inline-float-divide
• -minline-int-divide-min-latency
• -minline-int-divide-max-throughput
• -mno-inline-int-divide
• -minline-sqrt-min-latency
• -minline-sqrt-max-throughput
• -mno-inline-sqrt
• -mdwarf2-asm
• -mearly-stop-bits
• -mfixed-range=register-range
• -mtls-size=tls-size
• -mtune=cpu-type
• -milp32
• -mlp64
• -msched-br-data-spec
• -msched-ar-data-spec
• -msched-control-spec
• -msched-br-in-data-spec
• -msched-ar-in-data-spec
• -msched-in-control-spec
• -msched-spec-ldc
• -msched-spec-control-ldc
• -msched-prefer-non-data-spec-insns
• -msched-prefer-non-control-spec-insns
• -msched-stop-bits-after-every-cycle
• -msched-count-spec-in-critical-path
• -msel-sched-dont-check-control-spec
• -msched-fp-mem-deps-zero-cost
• -msched-max-memory-insns-hard-limit
• -msched-max-memory-insns=max-insns

---

LM32 Options 🌈

• -mbarrel-shift-enabled
• -mdivide-enabled
• -mmultiply-enabled
• -msign-extend-enabled
• -muser-enabled

---

M32R/D Options 🍓

• -m32r2
• -m32rx
• -m32r
• -mdebug
• -malign-loops
• -mno-align-loops
• -missue-rate=number
• -mbranch-cost=number
• -mmodel=code-size-model-type
• -msdata=sdata-type
• -mno-flush-func
• -mflush-func=name
• -mno-flush-trap
• -mflush-trap=number
• -G num

---

M32C Options 🧚

• -mcpu=cpu
• -msim
• -memregs=number

---

M680x0 Options 🔥

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -mcpu32
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mno-bitfield
• -mc68000
• -mc68020
• -mnobitfield
• -mrtd
• -mno-rtd
• -mdiv
• -mno-div
• -mshort
• -mno-short
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mno-sep-data
• -mshared-library-id=n
• -mid-shared-library
• -mno-id-shared-library
• -mxgot
• -mno-xgot
• -mlong-jump-table-offsets

---

MCore Options 💀

• -mhardlit
• -mno-hardlit
• -mdiv
• -mno-div
• -mrelax-immediates
• -mno-relax-immediates
• -mwide-bitfields
• -mno-wide-bitfields
• -m4byte-functions
• -mno-4byte-functions
• -mcallgraph-data
• -mno-callgraph-data
• -mslow-bytes
• -mno-slow-bytes
• -mno-lsim
• -mlittle-endian
• -mbig-endian
• -m210
• -m340
• -mstack-increment

---

MeP Options 🎀

• -mabsdiff
• -mall-opts
• -maverage
• -mbased=n
• -mbitops
• -mc=n
• -mclip
• -mconfig=name
• -mcop
• -mcop32
• -mcop64
• -mivc2
• -mdc
• -mdiv
• -meb
• -mel
• -mio-volatile
• -ml
• -mleadz
• -mm
• -mminmax
• -mmult
• -mno-opts
• -mrepeat
• -ms
• -msatur
• -msdram
• -msim
• -msimnovec
• -mtf
• -mtiny=n

---

MicroBlaze Options 🌸

• -msoft-float
• -mhard-float
• -msmall-divides
• -mcpu=cpu
• -mmemcpy
• -mxl-soft-mul
• -mxl-soft-div
• -mxl-barrel-shift
• -mxl-pattern-compare
• -mxl-stack-check
• -mxl-gp-opt
• -mno-clearbss
• -mxl-multiply-high
• -mxl-float-convert
• -mxl-float-sqrt
• -mbig-endian
• -mlittle-endian
• -mxl-reorder
• -mxl-mode-app-model
• -mpic-data-is-text-relative

---

MIPS Options 🦋

bestie, this is the MIPS motherlode, no cap:

• -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
• -mcheck-zero-division
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mload-store-pairs
• -mno-load-store-pairs
• -mmemcpy
• -mno-memcpy
• -mlong-calls
• -mno-long-calls
• -mmad
• -mno-mad
• -mimadd
• -mno-imadd
• -mfused-madd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mno-fix-24k
• -mfix-r4000
• -mno-fix-r4000
• -mfix-r4400
• -mno-fix-r4400
• -mfix-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 🌈

• -mlibfuncs
• -mno-libfuncs
• -mepsilon
• -mno-epsilon
• -mabi=gnu
• -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict
• -mno-branch-predict
• -mbase-addresses
• -mno-base-addresses
• -msingle-exit
• -mno-single-exit

---

MN10300 Options 🍓

• -mmult-bug
• -mno-mult-bug
• -mno-am33
• -mam33
• -mam33-2
• -mam34
• -mtune=cpu-type
• -mreturn-pointer-on-d0
• -mno-crt0
• -mrelax
• -mliw
• -msetlb

---

Moxie Options 🦋

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

MSP430 Options 💅

• -msim
• -masm-hex
• -mmcu=
• -mcpu=
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mcode-region=
• -mdata-region=
• -msilicon-errata=
• -msilicon-errata-warn=
• -mhwmult=
• -minrt

---

NDS32 Options 👑

• -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

---

Nios II Options 🌸

• -G num
• -mgpopt=option
• -mgpopt
• -mno-gpopt
• -mgprel-sec=regexp
• -mr0rel-sec=regexp
• -mel
• -meb
• -mno-bypass-cache
• -mbypass-cache
• -mno-cache-volatile
• -mcache-volatile
• -mno-fast-sw-div
• -mfast-sw-div
• -mhw-mul
• -mno-hw-mul
• -mhw-mulx
• -mno-hw-mulx
• -mno-hw-div
• -mhw-div
• -mcustom-insn=N
• -mno-custom-insn
• -mcustom-fpu-cfg=name
• -mhal
• -msmallc
• -msys-crt0=name
• -msys-lib=name
• -march=arch
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx

---

Nvidia PTX Options ⭐

• -m32
• -m64
• -mmainkernel
• -moptimize

---

OpenRISC Options 🧚

• -mboard=name
• -mnewlib
• -mhard-mul
• -mhard-div
• -msoft-mul
• -msoft-div
• -mcmov
• -mror
• -msext
• -msfimm
• -mshftimm

---

PDP-11 Options 💜

• -mfpu
• -msoft-float
• -mac0
• -mno-ac0
• -m40
• -m45
• -m10
• -mint32
• -mno-int16
• -mint16
• -mno-int32
• -msplit
• -munix-asm
• -mdec-asm
• -mgnu-asm
• -mlra

---

picoChip Options 🔥

• -mae=ae_type
• -mvliw-lookahead=N
• -msymbol-as-address
• -mno-inefficient-warnings

---

PowerPC Options 🦋

bestie, for PowerPC just see the RS/6000 and PowerPC Options, periodt.

---

🦋 RISC-V Options 🦋

okay so bestie, here's the whole RISC-V flag squad, no cap 💖

• -mbranch-cost=N-instruction
• -mplt
• -mno-plt
• -mabi=ABI-string
• -mfdiv
• -mno-fdiv
• -mdiv
• -mno-div
• -march=ISA-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-save-restore
• -mstrict-align
• -mno-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mexplicit-relocs
• -mno-explicit-relocs
• -mrelax
• -mno-relax
• -mriscv-attribute
• -mmo-riscv-attribute

---

🌸 RL78 Options 🌸

• -msim
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mallregs
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mg10
• -mg13
• -mg14
• -m64bit-doubles
• -m32bit-doubles
• -msave-mduc-in-interrupts

---

👑 RS/6000 and PowerPC Options 👑

this list is giving encyclopedia, but we're not skipping a single one, periodt ✨

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mpowerpc64
• -maltivec
• -mno-altivec
• -mpowerpc-gpopt
• -mno-powerpc-gpopt
• -mpowerpc-gfxopt
• -mno-powerpc-gfxopt
• -mmfcrf
• -mno-mfcrf
• -mpopcntb
• -mno-popcntb
• -mpopcntd
• -mno-popcntd
• -mfprnd
• -mno-fprnd
• -mcmpb
• -mno-cmpb
• -mmfpgpr
• -mno-mfpgpr
• -mhard-dfp
• -mno-hard-dfp
• -mfull-toc
• -mminimal-toc
• -mno-fp-in-toc
• -mno-sum-in-toc
• -m64
• -m32
• -mxl-compat
• -mno-xl-compat
• -mpe
• -malign-power
• -malign-natural
• -msoft-float
• -mhard-float
• -mmultiple
• -mno-multiple
• -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
• -mlongcall
• -mno-longcall
• -mpltseq
• -mno-pltseq
• -mblock-move-inline-limit=num
• -mblock-compare-inline-limit=num
• -mblock-compare-inline-loop-limit=num
• -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
• -mpower8-vector
• -mno-power8-vector
• -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

---

💜 RX Options 💜

• -m64bit-doubles
• -m32bit-doubles
• -fpu
• -nofpu
• -mcpu=
• -mbig-endian-data
• -mlittle-endian-data
• -msmall-data
• -msim
• -mno-sim
• -mas100-syntax
• -mno-as100-syntax
• -mrelax
• -mmax-constant-size=
• -mint-register=
• -mpid
• -mallow-string-insns
• -mno-allow-string-insns
• -mjsr
• -mno-warn-multiple-fast-interrupts
• -msave-acc-in-interrupts

---

🌈 S/390 and zSeries Options 🌈

• -mtune=cpu-type
• -march=cpu-type
• -mhard-float
• -msoft-float
• -mhard-dfp
• -mno-hard-dfp
• -mlong-double-64
• -mlong-double-128
• -mbackchain
• -mno-backchain
• -mpacked-stack
• -mno-packed-stack
• -msmall-exec
• -mno-small-exec
• -mmvcle
• -mno-mvcle
• -m64
• -m31
• -mdebug
• -mno-debug
• -mesa
• -mzarch
• -mhtm
• -mvx
• -mzvector
• -mtpf-trace
• -mno-tpf-trace
• -mfused-madd
• -mno-fused-madd
• -mwarn-framesize
• -mwarn-dynamicstack
• -mstack-size
• -mstack-guard
• -mhotpatch=halfwords,halfwords

---

🎀 Score Options 🎀

• -meb
• -mel
• -mnhwloop
• -muls
• -mmac
• -mscore5
• -mscore5u
• -mscore7
• -mscore7d

---

🫶 SH Options 🫶

• -m1
• -m2
• -m2e
• -m2a-nofpu
• -m2a-single-only
• -m2a-single
• -m2a
• -m3
• -m3e
• -m4-nofpu
• -m4-single-only
• -m4-single
• -m4
• -m4a-nofpu
• -m4a-single-only
• -m4a-single
• -m4a
• -m4al
• -mb
• -ml
• -mdalign
• -mrelax
• -mbigtable
• -mfmovd
• -mrenesas
• -mno-renesas
• -mnomacsave
• -mieee
• -mno-ieee
• -mbitops
• -misize
• -minline-ic_invalidate
• -mpadstruct
• -mprefergot
• -musermode
• -multcost=number
• -mdiv=strategy
• -mdivsi3_libfunc=name
• -mfixed-range=register-range
• -maccumulate-outgoing-args
• -matomic-model=atomic-model
• -mbranch-cost=num
• -mzdcbranch
• -mno-zdcbranch
• -mcbranch-force-delay-slot
• -mfused-madd
• -mno-fused-madd
• -mfsca
• -mno-fsca
• -mfsrra
• -mno-fsrra
• -mpretend-cmove
• -mtas

---

✨ Solaris 2 Options ✨

• -mclear-hwcap
• -mno-clear-hwcap
• -mimpure-text
• -mno-impure-text
• -pthreads

---

🔥 SPARC Options 🔥

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mmemory-model=mem-model
• -m32
• -m64
• -mapp-regs
• -mno-app-regs
• -mfaster-structs
• -mno-faster-structs
• -mflat
• -mno-flat
• -mfpu
• -mno-fpu
• -mhard-float
• -msoft-float
• -mhard-quad-float
• -msoft-quad-float
• -mstack-bias
• -mno-stack-bias
• -mstd-struct-return
• -mno-std-struct-return
• -munaligned-doubles
• -mno-unaligned-doubles
• -muser-mode
• -mno-user-mode
• -mv8plus
• -mno-v8plus
• -mvis
• -mno-vis
• -mvis2
• -mno-vis2
• -mvis3
• -mno-vis3
• -mvis4
• -mno-vis4
• -mvis4b
• -mno-vis4b
• -mcbcond
• -mno-cbcond
• -mfmaf
• -mno-fmaf
• -mfsmuld
• -mno-fsmuld
• -mpopc
• -mno-popc
• -msubxc
• -mno-subxc
• -mfix-at697f
• -mfix-ut699
• -mfix-ut700
• -mfix-gr712rc
• -mlra
• -mno-lra

---

⭐ SPU Options ⭐

• -mwarn-reloc
• -merror-reloc
• -msafe-dma
• -munsafe-dma
• -mbranch-hints
• -msmall-mem
• -mlarge-mem
• -mstdmain
• -mfixed-range=register-range
• -mea32
• -mea64
• -maddress-space-conversion
• -mno-address-space-conversion
• -mcache-size=cache-size
• -matomic-updates
• -mno-atomic-updates

---

🦋 System V Options 🦋

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

---

💅 TILE-Gx Options 💅

• -mcpu=CPU
• -m32
• -m64
• -mbig-endian
• -mlittle-endian
• -mcmodel=code-model

---

🌸 TILEPro Options 🌸

• -mcpu=cpu
• -m32

---

🍓 V850 Options 🍓

• -mlong-calls
• -mno-long-calls
• -mep
• -mno-ep
• -mprolog-function
• -mno-prolog-function
• -mspace
• -mtda=n
• -msda=n
• -mzda=n
• -mapp-regs
• -mno-app-regs
• -mdisable-callt
• -mno-disable-callt
• -mv850e2v3
• -mv850e2
• -mv850e1
• -mv850es
• -mv850e
• -mv850
• -mv850e3v5
• -mloop
• -mrelax
• -mlong-jumps
• -msoft-float
• -mhard-float
• -mgcc-abi
• -mrh850-abi
• -mbig-switch

---

🧚 VAX Options 🧚

• -mg
• -mgnu
• -munix

---

💜 Visium Options 💜

• -mdebug
• -msim
• -mfpu
• -mno-fpu
• -mhard-float
• -msoft-float
• -mcpu=cpu-type
• -mtune=cpu-type
• -msv-mode
• -muser-mode

---

🎀 VMS Options 🎀

• -mvms-return-codes
• -mdebug-main=prefix
• -mmalloc64
• -mpointer-size=size

---

🌈 VxWorks Options 🌈

• -mrtp
• -non-static
• -Bstatic
• -Bdynamic
• -Xbind-lazy
• -Xbind-now

---

👑 x86 Options 👑

bestie this is the main character flag list, it's literally enormous, stay with me 😭💖

• -mtune=cpu-type
• -march=cpu-type
• -mtune-ctrl=feature-list
• -mdump-tune-features
• -mno-default
• -mfpmath=unit
• -masm=dialect
• -mno-fancy-math-387
• -mno-fp-ret-in-387
• -m80387
• -mhard-float
• -msoft-float
• -mno-wide-multiply
• -mrtd
• -malign-double
• -mpreferred-stack-boundary=num
• -mincoming-stack-boundary=num
• -mcld
• -mcx16
• -msahf
• -mmovbe
• -mcrc32
• -mrecip
• -mrecip=opt
• -mvzeroupper
• -mprefer-avx128
• -mprefer-vector-width=opt
• -mmmx
• -msse
• -msse2
• -msse3
• -mssse3
• -msse4.1
• -msse4.2
• -msse4
• -mavx
• -mavx2
• -mavx512f
• -mavx512pf
• -mavx512er
• -mavx512cd
• -mavx512vl
• -mavx512bw
• -mavx512dq
• -mavx512ifma
• -mavx512vbmi
• -msha
• -maes
• -mpclmul
• -mfsgsbase
• -mrdrnd
• -mf16c
• -mfma
• -mpconfig
• -mwbnoinvd
• -mptwrite
• -mprefetchwt1
• -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
• -mforce-indirect-call
• -mavx512vbmi2
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -mavx512vpopcntdq
• -mavx5124fmaps
• -mavx512vnni
• -mavx5124vnniw
• -mprfchw
• -mrdpid
• -mrdseed
• -msgx
• -mcldemote
• -mms-bitfields
• -mno-align-stringops
• -minline-all-stringops
• -minline-stringops-dynamically
• -mstringop-strategy=alg
• -mmemcpy-strategy=strategy
• -mmemset-strategy=strategy
• -mpush-args
• -maccumulate-outgoing-args
• -m128bit-long-double
• -m96bit-long-double
• -mlong-double-64
• -mlong-double-80
• -mlong-double-128
• -mregparm=num
• -msseregparm
• -mveclibabi=type
• -mvect8-ret-in-mem
• -mpc32
• -mpc64
• -mpc80
• -mstackrealign
• -momit-leaf-frame-pointer
• -mno-red-zone
• -mno-tls-direct-seg-refs
• -mcmodel=code-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
• -mindirect-branch=choice
• -mfunction-return=choice
• -mindirect-branch-register

---

🪟 x86 Windows Options 🪟

• -mconsole
• -mcygwin
• -mno-cygwin
• -mdll
• -mnop-fun-dllimport
• -mthread
• -municode
• -mwin32
• -mwindows
• -fno-set-stack-executable

---

🌟 Xstormy16 Options 🌟

• -msim

---

🦋 Xtensa Options 🦋

• -mconst16
• -mno-const16
• -mfused-madd
• -mno-fused-madd
• -mforce-no-pic
• -mserialize-volatile
• -mno-serialize-volatile
• -mtext-section-literals
• -mno-text-section-literals
• -mauto-litpools
• -mno-auto-litpools
• -mtarget-align
• -mno-target-align
• -mlongcalls
• -mno-longcalls

---

💫 zSeries Options 💫

k so this one is just a redirect, bestie — it literally says see S/390 and zSeries Options. iconic of it, honestly 💅

---

🎀 Options Controlling the Kind of Output 🎀

okay so real talk, a compile can go through up to four stages, and they always happen in this exact order, no skipping the line:

1. preprocessing
2. compilation proper
3. assembly
4. linking

GCC can preprocess and compile a bunch of files either into several assembler input files, or smoosh them all into one assembler input file 🫶. Then each assembler input file spits out an object file, and linking pulls all the object files together — both the freshly compiled ones and the ones you handed in as input — into an executable file. it's giving teamwork, fr fr ✨

For any given input file, the file name suffix decides what kind of compilation happens, like this:

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Heads up bestie: you must link with the libobjc library to make an Objective-C program work. don't skip that step 🙈
• file.mi — Objective-C source code that should not be preprocessed.

---

• file.mm, file.M — Objective-C++ source code. Heads up bestie, you must link with the libobjc library or your Objective-C++ program is not gonna work, no cap. And .M means a literal capital M, not a vibe, a capital letter. 💅
• file.mii — Objective-C++ source code that should not be preprocessed. Leave it alone, it's fine as it is. 🙈
• file.h — a C, C++, Objective-C or Objective-C++ header file that's getting turned into a precompiled header (that's the default), or a C / C++ header file getting turned into an Ada spec (via the -fdump-ada-spec switch). Two paths, one extension, iconic behavior. ✨
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. Little detail that matters: in .cxx, the last two letters have to both be literally x. Same energy with .C — that's a literal capital C, okay? Don't come for me. 🎀
• file.mm, file.M — Objective-C++ source code that must be preprocessed. (Yes, she's back — the manpage really does say it twice, I'm just the messenger. 🫶)
• file.mii — Objective-C++ source code that should not be preprocessed. Also back for round two. 💜
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec. So many spellings, all valid, we love an inclusive queen. 👑
• file.f, file.for, file.ftn — 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 — Free form Fortran source code that should not be preprocessed. Free form, free spirit. 🦋
• file.F90, file.F95, file.F03, file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.go — Go source code.
• file.brig — BRIG files, aka the binary representation of HSAIL.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file containing a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These ones are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). These are also called bodies. 💪
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — an object file to be fed straight into linking. Any file name with no recognized suffix gets treated this way. No suffix, no problem, straight to the linker bestie. 🚀

---

So like, you can also just tell gcc the language yourself with the -x option:

• -x language — Specify explicitly the language for the following input files (instead of letting the compiler pick a default based on the file name suffix). This option applies to all following input files until the next -x option shows up. Possible values for language are:

        c  c-header  cpp-output
        c++  c++-header  c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output
        objective-c++ objective-c++-header objective-c++-cpp-output
        assembler  assembler-with-cpp
        ada
        d
        f77  f77-cpp-input f95  f95-cpp-input
        go
        brig

• -x none — Turns off any specification of a language, so the files after it get handled according to their file name suffixes again (exactly like when -x was never used at all). It's giving factory reset. 🌈

---

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. Lowkey important: some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. A whole lot of nothing. 💀

• -c — Compile or assemble the source files, but do not link. The linking stage simply is not done. 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 require compilation or assembly are ignored. They're just there for the vibes. 🙈

• -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; do not run the compiler proper. The output is preprocessed source code, and it gets sent to the standard output.

Input files that don't require preprocessing are ignored.

• -o file — Place output in file. This applies to whatever sort of output is being produced, whether that's an executable file, an object file, an assembler file or preprocessed C code.

If -o isn't 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. Defaults are giving main character energy, honestly. ⭐

• -v — Print (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. Spill the tea, gcc. ☕
• -### — Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines. Bestie behavior fr fr. 🫶
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so they can display the command-line options they accept. And if the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. We love a thorough queen. 💅
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. ✨
• --help={class|[^]qualifier}[,...] — Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers.

These are the supported classes:

• optimizers — Display all of the optimization options supported by the compiler.
• warnings — Display all of the options controlling warning messages produced by the compiler.
• target — Display target-specific options. Unlike the --target-help option though, target-specific options of the linker and assembler are not displayed. This is because those tools don't currently support the extended --help= syntax.
• params — Display the values recognized by the --param option.
• language — Display the options supported for language, where language is the name of one of the languages supported in this version of GCC.
• common — Display the options that are common to all languages.

These are the supported qualifiers:

• undocumented — Display only those options that are undocumented.
• joined — Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
• separate — Display options taking an argument that appears as a separate word following the original option, such as: -o output-file.

So like, for example, to display all the undocumented target-specific switches supported by the compiler, use:

        --help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

        --help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers. That's a no from me. 🚫

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

        --help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --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's 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's possible to find out which optimizations are enabled at -O2 by using:

        -Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

        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

Not me showing you the diff pipeline, we're so back. 🔥

• --version — Display the version number and copyrights of the invoked GCC. Receipts, periodt. 🧾
• -pass-exit-codes — okay so normally gcc just dips out with exit code 1 if any phase of the compiler returns a non-success return code. But if you pass this, gcc instead returns the numerically highest error produced by any phase that returned an error indication. And bestie, heads up: the C, C++, and Fortran front ends return 4 if an internal compiler error pops up. 🫠
• -pipe — use pipes instead of temporary files for communication between the various stages of compilation. It lowkey fails on some systems where the assembler can't read from a pipe, but the GNU assembler has no trouble at all. ✨
• -specs=file — process file after the compiler reads in the standard specs file, so you can override the defaults that the gcc driver program uses when it's figuring out what switches to pass to cc1, cc1plus, as, ld, etc. You can specify more than one -specs=file on the command line, and they get processed in order, left to right. 🎀
• -wrapper — invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters get passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, so the invocation of cc1 is gdb --args cc1 .... 🦋

• -ffile-prefix-map=old=new — when compiling files living in directory old, record any references to them in the result of the compilation as if the files lived in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This is how you make reproducible builds that are location independent. See also -fmacro-prefix-map and -fdebug-prefix-map. 💜
• -fplugin=name.so — load the plugin code in file name.so, assumed to be a shared object that gets dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (see -fplugin-arg-name-key=value below, iykyk). Every plugin should define the callback functions specified in the Plugins API. 🔌
• -fplugin-arg-name-key=value — define an argument called key with a value of value for the plugin called name. That's it, that's the whole thing. 💅
• -fdump-ada-spec[-slim] — for C and C++ source and include files, generate corresponding Ada specs. Iconic crossover event fr. 🎀
• -fada-spec-parent=unit — in conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
• -fdump-go-spec=file — for input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations, which may be a genuinely useful way to start writing a Go interface to code written in some other language. 🌈
• @file — read command-line options from file. The options that get read are inserted in place of the original @file option. If file doesn't exist, or can't be read, then the option is treated literally, and not removed.

Options inside file are separated by whitespace. A whitespace character can be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing the character you want included with a backslash. And plot twist: the file may itself contain additional @file options — any such options get processed recursively. 🌀

---

💖 Compiling C++ Programs 💖

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the exact same way you would 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: 🎀

• -ansi — In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as well as the "inline" keyword. 💀

The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also available, with or without -ansi. 🌸

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. No cap.

The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not built-in functions when -ansi is used. 🙈

• -std= — Determine the language standard. This option is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. 🫶

A value for this option must be provided; possible values are

• c90
• c89
• iso9899:1990
  • Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code. 💖
• iso9899:199409
  • ISO C90 as modified in amendment 1.
• c99
• c9x
• iso9899:1999
• iso9899:199x
  • ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated. ⚠️
• c11
• c1x
• iso9899:2011
  • ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated. 💀
• c17
• c18
• iso9899:2017
• iso9899:2018
  • 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. ✨
• c2x
  • The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🧪
• gnu90
• gnu89
  • GNU dialect of ISO C90 (including some C99 features).
• gnu99
• gnu9x
  • GNU dialect of ISO C99. The name gnu9x is deprecated. 💀
• gnu11
• gnu1x
  • GNU dialect of ISO C11. The name gnu1x is deprecated. 💀
• gnu17
• gnu18
  • GNU dialect of ISO C17. This is the default for C code. 👑
• gnu2x
  • The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. 🧪
• c++98
• c++03
  • The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code. 💖
• gnu++98
• gnu++03
  • GNU dialect of -std=c++98.
• c++11
• c++0x
  • The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated. 💀
• gnu++11
• gnu++0x
  • GNU dialect of -std=c++11. The name gnu++0x is deprecated. 💀
• c++14
• c++1y
  • The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated. 💀
• gnu++14
• gnu++1y
  • GNU dialect of -std=c++14. This is the default for C++ code. The name gnu++1y is deprecated. 👑
• c++17
• c++1z
  • The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated. 💀

• gnu++17
• gnu++1z — GNU dialect of -std=c++17. Heads up bestie, the name gnu++1z is deprecated 💀.
• c++2a — the next revision of the ISO C++ standard, tentatively planned for 2020. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. so like, don't build your whole life on it, okay? 🙈
• gnu++2a — GNU dialect of -std=c++2a. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. same energy as above, fr fr.

---

• -fgnu89-inline — tells GCC to use the traditional GNU semantics for inline functions when you're in C99 mode.

Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions. slay, one flag doing the work of a hundred attributes 💅.

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. vibe check your inline semantics, bestie ✨.

• -fpermitted-flt-eval-methods=style — ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD that say: operations and constants with a semantic type that's an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of FLT_EVAL_METHOD. So basically, code that conforms to C11 may not have been written expecting the possibility of these 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. defaults are giving main character energy, honestly 🌈.

• -aux-info filename — output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. (C only, bestie, it's exclusive 🎀.)

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration. so much lore in one file, I'm obsessed 📝.

• -fallow-parameterless-variadic-functions — accept variadic functions without named parameters.

Although it's possible to define such a function, this is not very useful as it's not possible to read the arguments. This is only supported for C as this construct is allowed by C++.

• -fno-asm — do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm.

In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

• -fno-builtin
• -fno-builtin-function — don't recognize built-in functions that do not begin with __builtin_ as prefix.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on 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. so iconic of GCC honestly 💅.

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))

(yes bestie, -fbuiltin-function genuinely doesn't exist, don't come for me, I'm just the messenger 🫶.)

• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. it's giving "for the compiler girlies only" 🧚.
• -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. kernel girlies rise 🦋.
• -fopenacc — enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. ✨ accelerate ✨.
• -fopenacc-dim=geom — specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of :-separated sizes, in order gang, worker and, vector. A size can be omitted, to use a target-specific default value.
• -fopenmp — enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. parallel bestie era 🔥.
• -fopenmp-simd — enable handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored.
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. 🙈

For more information on GCC's support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

• -fms-extensions — accept some non-standard constructs used in Microsoft header files.

In C++ code, this allows member names in structures to be similar to previous types declarations.

typedef int UOW;
struct ABC {
  UOW UOW;
};

Some cases of unnamed fields in structures and unions are only accepted with this option.

Note that this option is off for all targets but x86 targets using ms-abi.

• -fplan9-extensions — accept some non-standard constructs used in Plan 9 code.

This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++. (C girlies, this one's yours 💖.)

• -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.
• -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. like, it's deprecated behavior, don't do it, I'm begging 💀.
• -funsigned-char — let the type char be unsigned, like unsigned char.

Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.

Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. no cap, it's its own thing 💅.

• -fsigned-char — let the type char be signed, like signed char.

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. flip-flop central over here 🔄✨.

• -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — bestie, these four decide whether a bit-field is signed or unsigned when the declaration is out here being vague and doesn't say signed or unsigned itself. And the default? Signed. Why? Because it's consistent — the basic integer types like int are signed types, so we keep the vibe matching. 💅
• -fsso-struct=endianness — sets the default scalar storage order of structs and unions to whatever endianness you name. The accepted values are big-endian, little-endian, and native for the native endianness of the target (that one's the default). Heads up though: this option is not supported for C++. 🙈

⚠️ Warning, and I mean it: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it, if the endianness you picked isn't the native endianness of the target. So like… know what you're doing, bestie. 😭

---

✨ Options Controlling C++ Dialect ✨ 💖

Okay so this section is all about the command-line options that are only meaningful for C++ programs. But plot twist: you can still use most of the GNU compiler options no matter what language your program is in. Like, say you compile a file firstClass.C like this:

g++ -g -fstrict-enums -O -c firstClass.C

In that example, only -fstrict-enums is an option meant only for C++ programs; the rest of them you can use with any language GCC supports. 🌈

Also, some options for compiling C programs — like -std — are relevant for C++ programs too. Slay.

And now, the list of options that are only for compiling C++ programs:

• -fabi-version=n — use version n of the C++ ABI. The default is version 0. Now let me walk you through the whole family tree because it's giving lore:
  • Version 0 — the version conforming most closely to the C++ ABI specification. Which means the ABI you get with version 0 will change in different versions of G++ as ABI bugs get fixed. 💀
  • Version 1 — the version of the C++ ABI that first appeared in G++ 3.2.
  • Version 2 — the version of the C++ ABI that first appeared in G++ 3.4, and was the default all the way through G++ 4.9.
  • Version 3 — corrects an error in mangling a constant address as a template argument.
  • Version 4, first appearing in G++ 4.5 — implements a standard mangling for vector types.
  • Version 5, first appearing 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, first appearing 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, first appearing in G++ 4.8 — treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
  • Version 8, first appearing in G++ 4.9 — corrects the substitution behavior of function types with function-cv-qualifiers.
  • Version 9, first appearing in G++ 5.2 — corrects the alignment of nullptr_t.
  • Version 10, first appearing in G++ 6.1 — adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
  • Version 11, first appearing 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, first appearing 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. 🫠
  • Version 13, first appearing in G++ 8.2 — fixes that accidental change from version 12.

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 whenever it's defining a symbol with an incorrect mangled name. This switch tells it which ABI version to use for that alias.

With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. And if you want compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.

If this option isn't provided but -Wabi=n is, then that version gets used for the compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is what gets used for the warning. iykyk 💅

• -fno-access-control — turns off all access checking. This switch is mainly useful for working around bugs in the access control code. Like, it's the "just let me in" button. 🚪
• -faligned-new — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. You can pass a numeric argument like -faligned-new=32 to specify how much alignment (in bytes) that function provides, but honestly few users will ever need to override the default of alignof(std::max_align_t). This flag is enabled by default for -std=c++17. ✨
• -fchar8_t, -fno-char8_t — enables support for char8_t as adopted for C++2a. This includes adding a brand new char8_t fundamental 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_t and __cpp_lib_char8_t feature test macros. 👑

This option lets functions 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 #2

and 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;

Now here's the messy part — 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:

char ca[] = u8"xx";     // error: char-array initialized from wide
                        //        string
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>' requested

Yeah. It's giving "breaking change." 😭

• -fcheck-new — checks that the pointer returned by operator new is non-null before attempting to modify the storage that was allocated. This check is normally unnecessary because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow). 🌸
• -fconcepts — enables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), which allows code like:

template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }

• -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed so we can detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. 🌀
• -fconstexpr-loop-limit=n — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
• -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 has many iterations but still smaller than the above limit, or if in the 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). ⏳
• -fdeduce-init-list — enables deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e.

template <class T> auto forward(T t) -> decltype (realfn (t))
{
  return realfn (t);
}

void f()
{
  forward({1,2}); // call forward<std::initializer_list<int>>
}

This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but it was not part of the final standard, so it's disabled by default. This option is deprecated, and may be removed in a future version of G++. 🙈

• -fno-elide-constructors — okay so the C++ standard allows an implementation to omit creating a temporary that's 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 makes G++ call trivial member functions which otherwise would've been expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. 💅

• -fno-enforce-eh-specs — bestie, this one says don't bother generating code that checks at runtime whether you violated an exception specification. It straight-up violates the C++ standard, but it can shrink your code in production builds, kinda like defining NDEBUG. 💅 Important though: this does not give your code permission to throw exceptions that break the exception specifications — the compiler is still out here optimizing based on those specs, so if you throw something unexpected you land in undefined behavior at run time. 😭
• -fextern-tls-init, -fno-extern-tls-init — okay so C++11 and OpenMP let thread_local and threadprivate variables have dynamic (runtime) initialization, right? To make that work, every use of such a variable goes through a lil wrapper function that does whatever init is needed. ✨ When the use and the definition live in the same translation unit, that overhead gets optimized away — but when the use is in a different TU, there's significant overhead even if the variable doesn't actually need dynamic init. 🙈 So if you, the programmer, are sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because it's statically initialized, or because a use in the defining TU will run before any uses in another TU), you can dodge that overhead with -fno-extern-tls-init.

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. 🎀

• -fno-gnu-keywords — don't recognize typeof as a keyword, so your code can just use that word as an identifier like it's nothing. You can still use the keyword __typeof__ instead. 💜 This one is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
• -fno-implicit-templates — never emit code for non-inline templates that get instantiated implicitly (i.e. by use); only emit code for explicit instantiations. ⚠️ Heads up: if you use this, you must be careful to structure your code so it includes all the necessary explicit instantiations, or you'll get undefined symbols at link time. No cap.
• -fno-implicit-inline-templates — and don't emit code for implicit instantiations of inline templates either. The default is to treat inlines differently, so that compiling with and without optimization needs the same set of explicit instantiations. 🦋
• -fno-implement-inlines — to save space, don't emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if those functions aren't inlined everywhere they're called. 💀
• -fms-extensions — disables Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to member function via non-standard syntax. 🌸
• -fnew-inheriting-ctors — turns on 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=10 or lower is specified. 👑
• -fnew-ttp-matching — enables the P0522 resolution to Core issue 150, template template parameters and default arguments: it lets a template with default template arguments be an argument for a template template parameter that has fewer template parameters. Enabled by default for -std=c++17. ⭐
• -fno-nonansi-builtins — disables built-in declarations of functions that ANSI/ISO C doesn't mandate. That list includes ffs, alloca, _exit, index, bzero, conjf, and other related functions. 🍓
• -fnothrow-opt — treats a throw() exception specification as if it were a noexcept specification, so you reduce or eliminate the text size overhead compared to a function with no exception spec at all. Fun twist: 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? An exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. 🫶
• -fno-operator-names — don't treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. periodt.
• -fno-optional-diags — disable diagnostics that the standard says a compiler doesn't need to issue. Currently the only such diagnostic G++ issues is the one for a name having multiple meanings within a class. 🙈
• -fpermissive — downgrades some diagnostics about nonconformant code from errors to warnings. So yeah, using -fpermissive lets some nonconforming code compile. 🎀
• -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 (like void f(T) [with T = int] instead of void 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 those behaviors makes the error message harder to understand instead of easier, you can use -fno-pretty-templates to disable them. 💅
• -frepo — enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. 🔥
• -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space with this flag. Note that exception handling uses the same information, but G++ generates it as needed. And the dynamic_cast operator can still be used for casts that don't require run-time type information, i.e. casts to void * or to unambiguous base classes. ✨

⚠️ Mixing code compiled with -frtti and code compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. 💀

• -fsized-deallocation — enables the built-in global declarations

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. 🚀 Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.

• -fstrict-enums — lets the compiler optimize using the assumption that a value of an enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). 💅 This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. 🙈
• -fstrong-eval-order — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. And -fstrong-eval-order=some enables just the ordering of member access and shift expressions — that one is the default without -std=c++17. ⭐
• -ftemplate-backtrace-limit=n — set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. 🎀
• -ftemplate-depth=n — set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, because the compiler can run out of stack space before hitting 1024 in some situations. 😭
• -fno-threadsafe-statics — don't emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. 💅
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit. 🫶
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but it's necessary if the runtime routine isn't available. 🙈
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. 👑

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they 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, because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. 💅

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. 🙈

Explicitly instantiated inline methods are unaffected by this option, as their linkage might otherwise cross a shared library boundary. 🦋

• -fvisibility-ms-compat — okay so this flag is literally trying to use visibility settings to make GCC's C++ linkage model compatible with Microsoft Visual Studio's. It's giving "can we be friends with MSVC?" energy, bestie. 💅

The flag makes these changes to GCC's linkage model:

1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 🙈

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. So like, don't do that.

• -fno-weak — bestie, do NOT use weak symbol support, even if it's provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 💀
• -nostdinc++ — Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🧚

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with each major release, normally -Wabi will warn only if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi will warn about more things if an older ABI version is selected (with -fabi-version=n).

-Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.

If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for ABI warnings.

Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible. 😭

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

• SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

• "__attribute ((const))" and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int.

Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t.

These issues were corrected in -fabi-version=7.

• When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.

This was fixed in -fabi-version=8, the default for GCC 5.1.

• decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment.

This was fixed in -fabi-version=9, the default for GCC 5.2.

• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

This was fixed in -fabi-version=10, the default for GCC 6.1.

It also warns about psABI-related changes. The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. For example:

union U {
  long double ld;
  int i;
};

union U is always passed in memory.

• -Wabi-tag (C++ and Objective-C++ only) — Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. 🏷️
• -Wctor-dtor-privacy (C++ and Objective-C++ only) — Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🚪
• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall. 🗑️
• -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-init-list-lifetime (C++ and Objective-C++ only) — Do not warn about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example: 🫠
  • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value 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 the initializer_list in the heap has a dangling pointer.
  • When an initializer_list variable 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 next statement the initializer_list variable has a dangling pointer.

// 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 begin pointer from the initializer_list argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement.

• -Wliteral-suffix (C++ and Objective-C++ only) — Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example: ✨

#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t i64 = 123;
  printf("My int64: %" PRId64"\n", i64);
}

In this case, PRId64 is treated as a separate preprocessing token.

Additionally, warn when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.

This warning is enabled by default.

• -Wlto-type-mismatch — During the link-time optimization warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default. 🔗
• -Wno-narrowing (C++ and Objective-C++ only) — bestie, for C++11 and later standards, narrowing conversions get diagnosed by default, because the standard literally says so. A narrowing conversion from a constant = full-on error, and a narrowing conversion from a non-constant = just a warning, but -Wno-narrowing shuts the whole diagnostic up. Important caveat tho: this does not change the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts. The vibes are suppressed, not the rules. 💅

With -Wnarrowing in C++98, you get a warning when a narrowing conversion that C++11 prohibits happens inside { }, like:

        int i = { 2.2 }; // error: narrowing from double to int

This flag is included in -Wall and -Wc++11-compat. iykyk ✨

• -Wnoexcept (C++ and Objective-C++ only) — warns you 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() or "noexcept") but is known by the compiler to never actually throw an exception. It's giving "you said you'd never do this and yet…" 😭
• -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature that makes "noexcept" part of a function type changes the mangled name of a symbol compared to C++14. Enabled by -Wabi and -Wc++17-compat. Symbol name glow-ups are not always welcome.

As an example:

        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>". Same code, different mangled name, iconic chaos 💀

• -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class's 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 to "memset" below is undefined because it modifies a non-trivial class object and is, therefore, 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. Problem solved, gatekept, slayed.

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either itself or in an accessible polymorphic base class. In that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. This warning is automatically enabled if -Weffc++ is specified. ⚠️
• -Wregister (C++ and Objective-C++ only) — warns on uses of the "register" storage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using "register" 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 written in the code doesn't match the order in which they must actually 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, and emits a warning about it. This warning is enabled by -Wall. The compiler is the main character here, not your initializer list. 👑

• -Wno-pessimizing-move (C++ and Objective-C++ only) — warns when a call to "std::move" prevents 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. You sabotaged your own optimization, bestie. 😭

This warning is enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — warns about redundant calls to "std::move"; that is, when a move operation would have been performed even without the "std::move" call. 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:

        struct T {
        ...
        };
        T fn(T t)
        {
          ...
          return std::move (t);
        }

Here, the "std::move" call is redundant. Because G++ implements Core Issue 1579, another example is:

        struct T { // convertible to U
        ...
        };
        struct U {
        ...
        };
        U fn()
        {
          T t;
          ...
          return std::move (t);
        }

In this example, 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. Extra effort for zero payoff, we've all been there.

This warning is enabled by -Wextra.

• -fext-numeric-literals (C++ and Objective-C++ only) — accepts imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, those suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). ✨

The following -W... options are not affected by -Wall. They march to the beat of their own drum. 🥁

• -Weffc++ (C++ and Objective-C++ only) — warns about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have "operator=" return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload "&&", "||", or ",".

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic base classes too. Extra thorough, as she should be.

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. 🫠

• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about the use of an uncasted "NULL" as a sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. ⚠️
• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. 🧚
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to unintended effects and much easier to search for. Glow-up your casts, bestie. 💅
• -Woverloaded-virtual (C++ and Objective-C++ only) — warns when a function declaration hides virtual functions from a base class. For example, in:

        struct A {
          virtual void f();
        };

        struct B: public A {
          void f(int);
        };

the "A" class version of "f" is hidden in "B", and code like:

        B* b;
        b->f();

fails to compile. Not the vibe check you wanted. 💀

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function to a plain pointer. 🎀
• -Wsign-promo (C++ and Objective-C++ only) — warns 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 "no" and that's that.
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. ✨
• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance. 🌸
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance. 🦋
• -Wnamespaces — this one yells at you whenever a namespace definition gets opened, bestie. Some coding rules straight-up ban namespaces and this is how you enforce that rule. Important tho: the warning is inactive inside a system header file, like the STL, so you can still use the STL. You can also still use using directives and qualified names. 💖
• -Wno-terminate (C++ and Objective-C++ only) — turns off the warning about a throw-expression that's about to immediately result in a call to terminate. iykyk.
• -Wno-class-conversion (C++ and Objective-C++ only) — turns off the warning for when a conversion function converts an object to the same type, to a base class of that type, or to void. Like… such a conversion function will never be called. it's giving useless, fr fr. 😭

---

🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋

NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. We're just doing the compiler flags here, bestie. 💅

So this whole section is about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. But good news: you can also use most of the language-independent GNU compiler options. Like, say you wanted to compile a file some_class.m, you'd do:

gcc -g -fgnu-runtime -O -c some_class.m

In that little example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs — but the other options? You can use those with any language GCC supports. slay.

Also, 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). And similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi). the way that languages stack… 💜

Here's the full list of options that are only for compiling Objective-C and Objective-C++ programs:

• *-fconstant-string-class=class-name* — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below, bestie). And heads up: the -fconstant-cfstrings option, if it's also present, overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings.
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. 👑
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. And the macro __NEXT_RUNTIME__ gets predefined if (and only if) this option is used. periodt.
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This lets the runtime use more efficient entry points. ⚡ But bestie, this option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. no exceptions.
• *-fobjc-abi-version=n* — use version n of the Objective-C ABI for the selected runtime. Currently this option is 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.

• -fobjc-call-cxx-cdtors — okay this one is a whole journey, stay with me. For each Objective-C class, it checks if any of its instance variables is a C++ object with a non-trivial default constructor. If so, it synthesizes a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then returns self. Similarly, it checks if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesizes a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order. 🔥

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class — not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. 💀

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 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, @finally and @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. sorry bestie.
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 🗑️✨
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default, and it can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. lowkey a no-op in those cases.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. 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 instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. 🧚
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. And FYI: the GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. 💅
• -fno-local-ivars — by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. Which… can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. 😭 Specifying the -fno-local-ivars flag disables this behavior, thus avoiding variable shadowing issues. we love a clean scope.
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option, so that instance variables declared outside the scope of any access modifier directives default to the specified visibility. 🫶
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. dump it, bestie. 📝
• -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-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. blessed. 🌸
• -Wselector (Objective-C and Objective-C++ only) — warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached — for example, because an error is found during compilation, or because the -fsyntax-only option is being used. no cap.
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment. iconic behavior honestly. ✨
• -Wundeclared-selector (Objective-C and Objective-C++ only) — this one yells at you if it spots a @selector(...) expression pointing at a selector that was never declared. And bestie, "undeclared" here means no method with that name got declared before the @selector(...) expression — not explicitly in an @interface or @protocol declaration, and not implicitly in an @implementation section either. The vibe check on this one is different from -Wselector: this flag always runs its checks the moment a @selector(...) expression shows up, whereas -Wselector waits and only checks in the final stage of compilation. It also ✨ enforces the coding style convention ✨ that methods and selectors have to be declared before anyone uses them. Manners, basically. 💅
• -print-objc-runtime-info — spits out a C header describing the largest structure that gets passed by value, if there even is one. 🫶

✨ Options to Control Diagnostic Messages Formatting ✨

Okay so historically, diagnostic messages were formatted however they came out, completely ignoring the aspect of the output device (its width, that kind of thing). The options down here let you take the wheel on the formatting algorithm instead — how many characters per line, how often it repeats source location info, all of it. 🎀

Heads up bestie: some language front ends may not honor these options. Not everyone got the memo. 😭

• -fmessage-length=n — tries to format error messages so they fit on lines of about n characters. If n is zero, then no line-wrapping happens and every error message lands on a single line. This is the default for all front ends. 📏

Note — this option also affects how the #error and #warning pre-processor directives get displayed, plus the deprecated function/type/variable attribute. It does not, however, touch the pragma GCC warning and pragma GCC error pragmas. No cap.

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once — so if a message is too long for one physical line and gets wrapped, the source location won't be repeated as a prefix over and over on every continuation line. This is the default behavior. 🦋
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode too. Tells the reporter to emit the same source location info (as a prefix) on every physical line that came out of breaking a message too long for a single line. ✨
• -fdiagnostics-color[=WHEN], -fno-diagnostics-color — use color in diagnostics, bestie. WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of those WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto means color is used only when standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. 🌈

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. 🎨

The default GCC_COLORS is

error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. The supported capabilities are as follows. 👑

• error= — SGR substring for error: markers.
• warning= — SGR substring for warning: markers.
• note= — SGR substring for note: markers.
• range1= — SGR substring for first additional range.
• range2= — SGR substring for second additional range.
• locus= — SGR substring for location information, file:line or file:line:column etc.
• quote= — SGR substring for information printed within quotes.
• fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced.
• fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
• 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.

• -fno-diagnostics-show-option — by default, every diagnostic that comes out includes text pointing at the command-line option that directly controls it (if the diagnostic machinery knows of such an option). Passing -fno-diagnostics-show-option suppresses that behavior. 🙈
• -fno-diagnostics-show-caret — by default, every diagnostic includes the original source line plus a caret ^ marking the column. This option suppresses that info. The source line gets truncated to n characters if you gave the -fmessage-length=n option. When output goes to the terminal, the width is limited to whatever the COLUMNS environment variable says, or if that's not set, to the terminal width. 🎯
• -fno-diagnostics-show-labels — by default, when source code is printed (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent info, like the types of expressions:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

This option suppresses printing of those labels (in the example above, the vertical bars and the char * and long int text). 💀

• -fno-diagnostics-show-line-numbers — by default, when source code is printed (via -fdiagnostics-show-caret), a left margin is printed showing line numbers. This option suppresses that left margin. 🚫
• -fdiagnostics-minimum-margin-width=width — controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. 📐
• -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, perfect for IDEs to snack on. For each fix-it, a line gets printed after the relevant diagnostic, starting with the string fix-it:. For example:

fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the example above, 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_all

The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", non-printable characters as octal (e.g. vertical tab as "\013"). 🧚

An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted at the given position. ✨

• -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:

        --- test.c
        +++ test.c
        @ -42,5 +42,5 @

         void show_cb(GtkDialog *dlg)
         {
        -  gtk_widget_showall(dlg);
        +  gtk_widget_show_all(dlg);
         }

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). 🌈

• -fdiagnostics-show-template-tree — in the C++ frontend, when diagnostics show mismatching template types, like:

          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, like:

          map<
            [...],
            vector<
              [double != float]>>

The parts that differ are highlighted with color (double and float in this case). 🌳

• -fno-elide-type — by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types get 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. 🫶

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that doesn't understand column numbers, like dejagnu. iykyk. 🙈
• -fdiagnostics-format=FORMAT — select a different format for printing diagnostics. FORMAT is text or json. The default is text. 📝

The json format is literally just one big top-level JSON array, and inside it every diagnostic is a JSON object. That's it, that's the whole vibe. ✨

Heads up bestie: the JSON gets emitted as one single line, no pretty formatting. The examples below have been formatted just so you can actually read them. 🙈

Diagnostics can have child diagnostics, like a warning that adopts a note. For example, this error and note:

misleading-indentation.c:15:3: warning: this 'if' clause does not
  guard... [-Wmisleading-indentation]
   15 |   if (flag)
      |   ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
  is misleadingly indented as if it were guarded by the 'if'
   17 |     y = 2;
      |     ^

might be printed in JSON form (after formatting) like this:

[
    {
        "kind": "warning",
        "locations": [
            {
                "caret": {
                    "column": 3,
                    "file": "misleading-indentation.c",
                    "line": 15
                },
                "finish": {
                    "column": 4,
                    "file": "misleading-indentation.c",
                    "line": 15
                }
            }
        ],
        "message": "this \u2018if\u2019 clause does not guard...",
        "option": "-Wmisleading-indentation",
        "children": [
            {
                "kind": "note",
                "locations": [
                    {
                        "caret": {
                            "column": 5,
                            "file": "misleading-indentation.c",
                            "line": 17
                        }
                    }
                ],
                "message": "...this statement, but the latter is ..."
            }
        ]
    },
    ...
]

where the "note" is a child of the "warning". Parent-child diagnostics, we love a family tree 💖

So a diagnostic has a "kind". If that kind is "warning", then it's got an "option" key describing the command-line option controlling the warning. Iconic.

A diagnostic can contain zero or more locations. Each location has up to three positions inside it: a "caret" position plus optional "start" and "finish" positions. A location can also have an optional "label" string. For example, this error:

bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
   'struct s'} and 'T' {aka 'struct t'})
   64 |   return callee_4a () + callee_4b ();
      |          ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
      |          |              |
      |          |              T {aka struct t}
      |          S {aka struct s}

has three locations, okay?? Its primary location is at the "+" token at column 23. Then it's got two secondary locations describing the left and right-hand sides of the expression, and those ones have labels. It might be printed in JSON form as:

{
    "children": [],
    "kind": "error",
    "locations": [
        {
            "caret": {
                "column": 23, "file": "bad-binary-ops.c", "line": 64
            }
        },
        {
            "caret": {
                "column": 10, "file": "bad-binary-ops.c", "line": 64
            },
            "finish": {
                "column": 21, "file": "bad-binary-ops.c", "line": 64
            },
            "label": "S {aka struct s}"
        },
        {
            "caret": {
                "column": 25, "file": "bad-binary-ops.c", "line": 64
            },
            "finish": {
                "column": 36, "file": "bad-binary-ops.c", "line": 64
            },
            "label": "T {aka struct t}"
        }
    ],
    "message": "invalid operands to binary + ..."
}

If a diagnostic contains fix-it hints, it's got a "fixits" array, made of half-open intervals, kinda like the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:

demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
  mean 'color'?
    8 |   return ptr->colour;
      |               ^~~~~~
      |               color

might be printed in JSON form as:

{
    "children": [],
    "fixits": [
        {
            "next": {
                "column": 21,
                "file": "demo.c",
                "line": 8
            },
            "start": {
                "column": 15,
                "file": "demo.c",
                "line": 8
            },
            "string": "color"
        }
    ],
    "kind": "error",
    "locations": [
        {
            "caret": {
                "column": 15,
                "file": "demo.c",
                "line": 8
            },
            "finish": {
                "column": 20,
                "file": "demo.c",
                "line": 8
            }
        }
    ],
    "message": "\u2018struct s\u2019 has no member named ..."
}

where the fix-it hint suggests replacing the text from "start" up to but not including "next" with "string"'s value. And bestie, listen: deletions are expressed via an empty value for "string", and insertions happen by having "start" equal "next". That's the whole trick, periodt. 💅

---

🌸 Options to Request or Suppress Warnings 🌸

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky, or that suggest there may have been an error. Basically the compiler going "bestie… are you sure about this?" 🫶

The following language-independent options don't enable specific warnings, they just control the kinds of diagnostics produced by GCC.

• -fsyntax-only — Check the code for syntax errors, but don't do anything beyond that. That's it, that's the whole job. ✨
• -fmax-errors=n — Limits the maximum number of error messages to n, and at that point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. So don't be out here thinking your -fmax-errors wins, because it does NOT. 😭
• -w — Inhibit all warning messages. Silencing the haters, literally.
• -Werror — Make all warnings into errors. Every single one. No mercy, no cap. 🔥
• -Werror= — Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. The asymmetry is giving unhinged but okay, we move. 💀

• -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. It's the "we're leaving, right now" option. 🙈

You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. iykyk 💜

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of 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. So it's giving main character energy for the most specific option, always. 👑

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. Sneaky little feature but honestly? Slay. 🌈

The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and -Wmaybe-uninitialized will not warn at all unless optimization is enabled. So if you're out here with no -O and wondering why nothing's warning… that's why bestie. 😭

• -Wpedantic
• -pedantic — Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used. Strict queen behavior, honestly. 🎀

Okay bestie, so we're still deep in the -Wpedantic lore, and here's the rest of the tea 👇

Like, valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few genuinely require -ansi or a -std option that names the required version of ISO C). But — and this is the plot twist — without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option? They get rejected. Periodt. 🚫

Also, -Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. And pedantic warnings are also disabled in the expression that follows "__extension__". However — and this part is important, no cap — only system header files should use these escape routes. Application programs should avoid them. 🙅‍♀️

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all — only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added. It's giving "close but not the whole vibe" 😭

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future. Not on our 2024 bingo card, bestie 💅

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard — the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) 🧠✨

---

-pedantic-errors

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time, and in some other cases that do not prevent compilation of programs that are valid according to the standard.

Bestie, listen: this is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter — and vice versa. They are not the same girl. 💀

---

-Wall

This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-Wall turns on the following warning flags:

• -Waddress
• -Warray-bounds=1 (only with -O2)
• -Wbool-compare
• -Wbool-operation
• -Wc++11-compat
• -Wc++14-compat
• -Wcatch-value (C++ and Objective-C++ only)
• -Wchar-subscripts
• -Wcomment
• -Wduplicate-decl-specifier (C and Objective-C only)
• -Wenum-compare (in C/ObjC; this is on by default in C++)
• -Wformat
• -Wint-in-bool-context
• -Wimplicit (C and Objective-C only)
• -Wimplicit-int (C and Objective-C only)
• -Wimplicit-function-declaration (C and Objective-C only)
• -Winit-self (only for C++)
• -Wlogical-not-parentheses
• -Wmain (only for C/ObjC and unless -ffreestanding)
• -Wmaybe-uninitialized
• -Wmemset-elt-size
• -Wmemset-transposed-args
• -Wmisleading-indentation (only for C/C++)
• -Wmissing-attributes
• -Wmissing-braces (only for C/ObjC)
• -Wmultistatement-macros
• -Wnarrowing (only for C++)
• -Wnonnull
• -Wnonnull-compare
• -Wopenmp-simd
• -Wparentheses
• -Wpessimizing-move (only for C++)
• -Wpointer-sign
• -Wreorder
• -Wrestrict
• -Wreturn-type
• -Wsequence-point
• -Wsign-compare (only in C++)
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstrict-aliasing
• -Wstrict-overflow=1
• -Wswitch
• -Wtautological-compare
• -Wtrigraphs
• -Wuninitialized
• -Wunknown-pragmas
• -Wunused-function
• -Wunused-label
• -Wunused-value
• -Wunused-variable
• -Wvolatile-register-var

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. So like, -Wall is not the final boss, iykyk 🎀

---

-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.) Glow-up complete ✨

• -Wclobbered
• -Wcast-function-type
• -Wdeprecated-copy (C++ only)
• -Wempty-body
• -Wignored-qualifiers
• -Wimplicit-fallthrough=3
• -Wmissing-field-initializers
• -Wmissing-parameter-type (C only)
• -Wold-style-declaration (C only)
• -Woverride-init
• -Wsign-compare (C only)
• -Wredundant-move (only for C++)
• -Wtype-limits
• -Wuninitialized
• -Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
• -Wunused-parameter (only with -Wunused or -Wall)
• -Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the following cases:

• A pointer is compared against integer zero with "<", "<=", ">", or ">=".
• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array that has been declared "register".
• (C++ only) Taking the address of a variable that has been declared "register".
• (C++ only) A base class is not initialized in the copy constructor of a derived class.

---

-Wchar-subscripts

Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall. Classic oopsie 🫠

---

-Wno-coverage-mismatch

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information.

By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error.

Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. Like, don't do it unless you truly must, bestie 🙈

---

-Wno-cpp

(C, Objective-C, C++, Objective-C++ and Fortran only)

Suppress warning messages emitted by "#warning" directives. Shhh 🤫

---

-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)

Give a warning when a value of type "float" is implicitly promoted to "double".

CPUs with a 32-bit "single-precision" floating-point unit implement "float" in hardware, but emulate "double" in software. On such a machine, doing computations using "double" values is much more expensive because of the overhead required for software emulation. 💸

It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type "double". For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with "double" because the floating-point literal is a "double". Sneaky!! 😭

---

-Wduplicate-decl-specifier (C and Objective-C only)

Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This warning is enabled by -Wall.

---

-Wformat

-Wformat=n

Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific families).

Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C standard). 💅

---

-Wformat=1

-Wformat

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0.

Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. 👑

---

-Wno-format-contains-nul

If -Wformat is specified, do not warn about format strings that contain NUL bytes.

---

-Wno-format-extra-args

If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The C standard specifies that such arguments are ignored.

Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. 🦋

• -Wformat-overflow, -Wformat-overflow=level — okay bestie, this one is about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer. 😱

When the exact number of bytes written by a format directive can't be determined at compile-time, it's estimated based on 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 result in false positives. 🙈

• -Wformat-overflow, -Wformat-overflow=1 — Level 1 of -Wformat-overflow (enabled by -Wformat) uses a conservative approach that 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 that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

• -Wformat-overflow=2 — Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.

At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning.

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

• -Wno-format-zero-length — If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. so chill, bestie 😌
• -Wformat=2 — Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. ✨
• -Wformat-nonliteral — If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. iykyk
• -Wformat-security — If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) security alert 🚨
• -Wformat-signedness — If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. signed vs unsigned drama 💅
• -Wformat-truncation, -Wformat-truncation=level — Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow. truncation nation ✂️
  • -Wformat-truncation, -Wformat-truncation=1 — Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
• -Wformat-y2k — If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. Y2K vibes 🎉
• -Wnonnull — Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option. don't pass null, bestie 🙅‍♀️

• -Wnonnull-compare — Warn when comparing an argument marked with the nonnull function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option. why you comparing null tho 😭

• -Wnull-dereference — Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. undefined behavior is not the vibe 💀
• -Winit-self (C, C++, Objective-C and Objective-C++ only) — Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

This warning is enabled by -Wall in C++. self-init? more like self-own 🤡

• -Wimplicit-int (C and Objective-C only) — Warn when a declaration does not specify a type. This warning is enabled by -Wall. implicit int? not today 🚫
• -Wimplicit-function-declaration (C and Objective-C only) — Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall. declare before you use, bestie 📢
• -Wimplicit (C and Objective-C only) — Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. two-for-one warning ✨
• -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. fallthrough, but make it a setting 🍂
• -Wimplicit-fallthrough=n — Warn when a switch case falls through. For example:

switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

switch (cond)
  {
  case 1:
    if (i > 3) {
      bar (5);
      break;
    } else if (i < 1) {
      bar (0);
    } else
      return;
  default:
    ...
  }

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:

switch (cond)
  {
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    ...
  }

fallthrough: the ultimate plot twist 🌀

C++17 literally has a standard way to shut the -Wimplicit-fallthrough warning up: use [[fallthrough]]; instead of that GNU attribute, bestie. And if you're stuck on C++11 or C++14? You can use [[gnu::fallthrough]];, which is a GNU extension 💅.

But not me forgetting — you can also just drop a fallthrough comment in there and it silences the warning. The whole body of the C or C++ style comment has to match one of the regular expressions listed below. The option argument n decides what kind of comments get accepted, and it's giving menu of vibes:

• -Wimplicit-fallthrough=0 — turns the warning off entirely, periodt 🙈
• -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, no cap:
  • -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 — doesn't recognize any comments as fallthrough comments at all; only attributes can disable the warning 🫶

The comment needs to be followed — after optional whitespace and other comments — by case or default keywords, or by a user label that precedes some case or default label. Like so, bestie:

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    ...
  }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra ✨

---

-Wif-not-aligned (C, C++, Objective-C and Objective-C++ only) 🦋

Controls whether the warning triggered by the warn_if_not_aligned attribute gets issued. It's enabled by default, iykyk. Want it gone? Use -Wno-if-not-aligned to disable it 💅

-Wignored-qualifiers (C and C++ only) 🎀

Warns if the return type of a function has a type qualifier like const. For ISO C, that qualifier does nothing — because the value a function returns isn't an lvalue. For C++, the warning is only emitted for scalar types or void. And heads up: ISO C 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.

-Wignored-attributes (C and C++ only) 🌸

Warns when an attribute is ignored. The way this differs from -Wattributes is: it warns whenever the compiler decides to drop an attribute, not that 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, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic 👑

-Wmisleading-indentation (C and C++ only) 😭

Warns when the indentation of the code does not reflect the block structure. Specifically, a warning gets issued for if, else, while, and for clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation. So sneaky fr fr.

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 you've got mixed tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic, like this example here:

if (flagA)
  foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
  if (flagB)
#endif
  foo (1);

The warning is also not issued after a #line directive, because that typically means autogenerated code, and you can't make any assumptions about the layout of the file that 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 may adversely affect the correctness or efficiency of generated code. For example, the warning gets issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity ✨ By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. The 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 is issued 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 apply the same set of attributes to a declaration as that on another declaration, without explicitly enumerating the attributes. 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 is 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 isn't fully bracketed, but the one for b is. This warning is enabled by -Wall in C.

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only) 🌸

Warns if a user-supplied include directory doesn't exist. That's it, that's the whole thing 💅

-Wmissing-profile 🌈

Warns if feedback profiles are missing when you're using the -fprofile-use option. This option diagnoses those cases where a new function or a new file gets added to the user code between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files don't contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC can't use any profile feedback information. In all these cases, warnings are issued to inform the user that a profile generation step is due 👑 -Wno-missing-profile can be used to disable the warning.

Ignoring the warning can result in poorly optimized code. Completely disabling the warning is not recommended and should be done only when non-existent profile data is justified. Bestie, pls, listen to the compiler 😭

-Wmultistatement-macros 💀

Warns about unsafe multiple statement macros that appear to be guarded by a clause like if, else, for, switch, or while, where only the first statement is actually guarded after the macro is expanded. Not me getting gotcha'd by preprocessor nonsense 🙈

For example:

#define DOIT x++; y++
if (c)
  DOIT;

...will increment y unconditionally, not just when c holds. You can usually fix this by wrapping 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 if parentheses are omitted in certain contexts — like when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Valid, honestly.

It also warns if a comparison like x<=y<=z appears; that's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from the one in ordinary mathematical notation. Math girlies stay losing 😭

It also warns for 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++, this 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.

-Wsequence-point ✨

Warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. Yeah. That's the scary one, bestie 🫶

okay so bestie, quick story time about sequence points — the C and C++ standards define the order in which expressions in a C/C++ program get evaluated using these little things called sequence points, and honestly it's giving "partial ordering" energy: they mark a split between the parts of the program that run before the sequence point and the parts that run after it. 🌸

Where do these iconic little checkpoints show up? After the evaluation of a full expression (aka one that's not part of some bigger expression), after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function is called (but like, after the arguments get evaluated and after the expression naming the called function gets evaluated), and in certain other places. 💅

And the tea is: other than what the sequence point rules actually say, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order, not a total order — because, say, if two functions get called inside one expression with no sequence point between them, the order they get called in? Not specified, bestie. However, the standards committee has ruled that function calls do not overlap. No cap.

Also 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 — and the C and C++ standards literally say:

"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. 🙈

Now for the glow-up: the C++17 standard will define the order of evaluation of operands in more cases — in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. 😭

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>. ✨

This warning is enabled by -Wall for C and C++.

---

• -Wno-return-local-addr — do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. 🫶
• -Wreturn-type — warn whenever a function is defined with a return type that defaults to "int". Also warn about any "return" statement with no return value in a function whose return type is not "void" (falling off the end of the function body counts as returning without a value, bestie). 😌 For C only, warn about a "return" statement with an expression in a function whose return type is "void", unless the expression type is also "void". As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used. 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.
• -Wshift-count-negative — warn if shift count is negative. This warning is enabled by default. 🌸
• -Wshift-count-overflow — warn if shift count >= width of type. This warning is enabled by default. ✨
• -Wshift-negative-value — warn if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes. 🦋
• -Wshift-overflow
• -Wshift-overflow=n — warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and newer). 🔥
  • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++2A 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 — warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. (The presence of a "default" label prevents this warning, bestie.) "case" labels outside the enumeration range also provoke warnings when this option is used (even if there is a "default" label). This warning is enabled by -Wall. 🌈
• -Wswitch-default — warn whenever a "switch" statement does not have a "default" case. 😭
• -Wswitch-enum — warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a "default" label. 💅
• -Wswitch-bool — warn whenever a "switch" statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than "bool". For example:

switch ((int) (a == 4))
  {
  ...
  }

This warning is enabled by default for C and C++ programs. ✨

• -Wswitch-unreachable — warn whenever a "switch" statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

switch (cond)
  {
   i = 15;
  ...
   case 5:
  ...
  }

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

switch (cond)
  {
   int i;
  ...
   case 5:
   i = 5;
  ...
  }

This warning is enabled by default for C and C++ programs. 🌸

• -Wsync-nand (C and C++ only) — warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions changed semantics in GCC 4.4. 🚨
• -Wunused-but-set-parameter — warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). 🫶 To suppress this warning use the "unused" attribute. This warning is also enabled by -Wunused together with -Wextra.
• -Wunused-but-set-variable — warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall. 💖 To suppress this warning use the "unused" attribute. This warning is also enabled by -Wunused, which is enabled by -Wall.
• -Wunused-function — warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall. 🎀
• -Wunused-label — warn whenever a label is declared but not used. This warning is enabled by -Wall. 😌 To suppress this warning use the "unused" attribute.
• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall. 💅
• -Wunused-parameter — warn whenever a function parameter is unused aside from its declaration. 🦋 To suppress this warning use the "unused" attribute.
• -Wno-unused-result — do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The default is -Wunused-result. ✨
• -Wunused-variable — warn whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall. 🔥 To suppress this warning use the "unused" attribute.
• -Wunused-const-variable
• -Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration. -Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of "#define"s. 👑 To suppress this warning use the "unused" attribute.
  • -Wunused-const-variable=1 — this is the warning level that is enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. 🌸
  • -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested since in C++ this isn't an error and in C it might be harder to clean up all headers included. 😭
• -Wunused-value — warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to "void". This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as "x[i,j]" causes a warning, while "x[(void)i,j]" does not. 💀 This warning is enabled by -Wall.
• -Wunused — all the above -Wunused options combined. 🫶🌈

Okay so if you actually want a warning about an unused function parameter, bestie, you've got two moves: either say -Wextra -Wunused together (quick reminder: -Wall already implies -Wunused, so don't be out here double-dipping thinking you're clever), or specify -Wunused-parameter all by itself. That's the whole vibe check. 💅

• -Wuninitialized — warns you when an automatic variable gets used without being initialized first, or when a variable might get clobbered by a setjmp call. And in C++? It'll also warn if a non-static reference or a non-static const member shows up in a class that has no constructors. 👀

And if you want the warning for code that uses the uninitialized value of the variable in its own initializer, that's a whole separate thing — go grab -Winit-self.

These warnings fire for individual uninitialized or clobbered elements of structure, union or array variables, and also for variables that are uninitialized or clobbered as a whole. But they do NOT occur for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements that get warnings depends on the precise optimization options and version of GCC you're running. So like, your mileage literally varies, no cap.

Also note: there may be no warning at all about a variable that's used only to compute a value that itself is never used, because those computations may get deleted by data flow analysis before the warnings even get printed. Sneaky. 💀

• -Winvalid-memory-model — warns for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions when the memory consistency argument 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, this little guy right here gets diagnosed:

void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default. She's always on, icon behaviour. ✨

• -Wmaybe-uninitialized — for an automatic (i.e. local) variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. Basically it's doing its best detective work. 🕵️

These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.

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's 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. GCC is not a mind reader, bestie. 😭

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. False alarm central.

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 — warns when a #pragma directive is encountered that is not understood by GCC. And plot twist: if this command-line option is used, warnings are even issued for unknown pragmas in system header files. That is not the case if the warnings are only enabled by the -Wall command-line option. So like, she's got range. 🌈
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. Just let her live. 🙈
• -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 before main is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0--100 for the implementation. So like, that range is taken, hands off. 👑
• -Wstrict-aliasing — this option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3. Fancy.
• -Wstrict-aliasing=n — also only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. And -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3. The levels, bestie, let me break them down:
  • Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
  • Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
  • Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

---

• -Wstrict-overflow
• -Wstrict-overflow=n

This option is only active when 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 simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Wstrict-overflow=2 — also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
• -Wstrict-overflow=4 — also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.
• -Wstrict-overflow=5 — also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. You've been warned, babe. 😭

• -Wstringop-overflow
• -Wstringop-overflow=type

Warn for calls to string manipulation functions such as memcpy and strcpy that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like memcpy which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, the option will issue a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4. 🫶

                   enum Color { blue, purple, yellow };
                   const char* f (enum Color clr)
                   {
                     static char buf [4];
                     const char *str;
                     switch (clr)
                       {
                         case blue: str = "blue"; break;
                         case purple: str = "purple"; break;
                         case yellow: str = "yellow"; break;
                       }

                     return strcpy (buf, str);   // warning here
                   }

okay so lowkey, that's the little culprit that gets flagged. And heads up bestie: option -Wstringop-overflow=2 is enabled by default. yes, by default, no cap. 🙈

• -Wstringop-overflow
• -Wstringop-overflow=1 — this setting uses type-zero Object Size Checking to figure out how big the destination objects are. It's the default setting of the option, periodt. At this level, it will not warn about writes past the end of subobjects of larger objects accessed by pointers — unless the size of the largest surrounding object is known. And when the destination could be one of several objects? It just assumes it's the largest one of them, iykyk. On Linux systems, when optimization is enabled, this setting warns for the exact same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. cute little twin behavior 💅
• -Wstringop-overflow=2 — now we're on type-one Object Size Checking for the destination sizes. Here it will warn about overflows when you're writing to members of the largest complete objects whose exact size is known. But — plot twist — it will not warn for excessive writes to the same members of unknown objects referenced by pointers, because those could totally point to arrays with an unknown number of elements. so valid of it 🌸
• -Wstringop-overflow=3 — type-two Object Size Checking, bestie. This one warns about overflowing the smallest object or data member. it's the most restrictive setting of the option that may result in warnings for safe code. yes, safe code getting flagged, that's the tradeoff 😭
• -Wstringop-overflow=4 — type-three Object Size Checking. At this level it 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 warn. And just like -Wstringop-overflow=3, this setting may also result in warnings for benign code. so dramatic 💀

---

-Wstringop-truncation 🎀

This one warns for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that might either truncate the copied string or leave the destination unchanged. messy behavior, we don't love it.

In this next example, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source gets truncated and the call gets 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 copies 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 super common mistake, so the call gets diagnosed too. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. glow up 🦋

                   void copy (char *d, const char *s)
                   {
                     strncpy (d, s, strlen (s));
                   }

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. So 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 meant to store a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to avoid this warning. But heads up — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of these arrays, GCC issues warnings unless it can prove the use is safe. protective king 👑

---

-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] 🌈

Warns you for cases where adding an attribute might be beneficial. The attributes currently supported are listed below, bestie.

-Wsuggest-attribute=pure / -Wsuggest-attribute=const / -Wsuggest-attribute=noreturn / -Wmissing-noreturn / -Wsuggest-attribute=malloc

• -Wsuggest-attribute=pure
• -Wsuggest-attribute=const
• -Wsuggest-attribute=noreturn
• -Wmissing-noreturn
• -Wsuggest-attribute=malloc — warns about functions that might be candidates for attributes pure, const, noreturn, or malloc. The compiler only warns for functions visible in other compilation units, or (for pure and const) if it can't 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 needs option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. the more you optimize, the better it vibes ✨

-Wsuggest-attribute=format / -Wmissing-format-attribute

• -Wsuggest-attribute=format
• -Wmissing-format-attribute — warns about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones, okay? 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 — 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. it's guessing, bestie, not reading minds 😭

-Wsuggest-attribute=cold

• -Wsuggest-attribute=cold — warns about functions that might be candidates for the cold attribute. This is based on static detection and generally will only warn 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. brrr, cold 🥶

---

-Wsuggest-final-types 🧚

Warns about types with virtual methods where code quality would improve if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This lets GCC more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the info about the class hierarchy graph is more complete. team up for better results 🫶

-Wsuggest-final-methods 🧚

Warns about virtual methods where code quality would improve if the method were declared with the C++11 final specifier, or, if possible, if its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete. And it's recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. order matters, bestie 💅

-Wsuggest-override 🎀

Warns about overriding virtual functions that are not marked with the override keyword. just label it, it's not that hard 💖

---

-Walloc-zero ⭐

Warns about calls to allocation functions decorated with attribute alloc_size that specify zero bytes — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated), relying on it may result in subtle portability bugs and should be avoided. fr fr, don't do it 🙈

-Walloc-size-larger-than=byte-size 🌸

Warns about calls to functions decorated with attribute alloc_size that try 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. And -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than. two ways to shut it up 💅

-Wno-alloc-size-larger-than 🎀

Disables -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. bye bye warning 👋

-Walloca 🔥

This option warns on all uses of alloca in the source. all of them, no exceptions, periodt.

-Walloca-larger-than=byte-size 🌈

This option warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. And arguments of non-integer types are considered unbounded even if they look like they're constrained to the expected range. the audacity 😭

For example, a bounded case of alloca could be:

                   void func (size_t n)
                   {
                     void *p;
                     if (n <= 1000)
                       p = alloca (n);
                     else
                       p = malloc (n);
                     f (p);
                   }

In the example above, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. But if -Walloca-larger-than=500 were passed, the compiler would emit a warning. so close, bestie 🫶

Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like, zero vibe check on that number. For example:

void func ()
{
  void *p = alloca (n);
  f (p);
}

If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking. Bestie simply did not check. 😭

Note, that even seemingly correct code involving signed integers could cause a warning:

void func (signed int n)
{
  if (n < 500)
    {
      p = alloca (n);
      f (p);
    }
}

In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. Sneaky!! 🙈

This option also warns when alloca is used in a loop. (not the loop 💀)

-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).

See also -Wvla-larger-than=byte-size.

---

• -Wno-alloca-larger-than — Disable -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. Turning it all the way off, periodt.

---

• -Warray-bounds, -Warray-bounds=n — This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall. 🚨
  • -Warray-bounds=1 — This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.
  • -Warray-bounds=2 — This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. Heads up: this warning level may give a larger number of false positives and is deactivated by default. ⚠️

---

• -Wattribute-alias=n, -Wno-attribute-alias — Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias.
  • -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. 💀
  • -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.

Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. (a whole squad of attributes fr fr)

-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.

---

• -Wbool-compare — Warn about boolean expression compared with an integer value different from true/false. For instance, the following comparison is always false:

int n = 5;
...
if ((n > 1) == 2) { ... }

This warning is enabled by -Wall.

---

• -Wbool-operation — Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.) pls don't ++ your bools 🙈

This warning is enabled by -Wall.

---

• -Wduplicated-branches — Warn when an if-else has identical branches. This warning detects cases like

if (p != NULL)
  return 0;
else
  return 0;

It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:

  int i = x ? *p : *p;

(both branches the same?? bestie why 😭)

---

• -Wduplicated-cond — Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

---

• -Wframe-address — Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall.

---

• -Wno-discarded-qualifiers (C and Objective-C only) — Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning. ✨

---

• -Wno-discarded-array-qualifiers (C and Objective-C only) — Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning.

---

• -Wno-incompatible-pointer-types (C and Objective-C only) — Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.

---

• -Wno-int-conversion (C and Objective-C only) — Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.

---

• -Wno-div-by-zero — Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs. 🌈 (that one's a feature, not a bug)

---

• -Wsystem-headers — Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used. 👑

---

• -Wtautological-compare — Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:

int i = 1;
...
if (i > i) { ... }

This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:

if ((a & 16) == 10) { ... }

will always be false.

This warning is enabled by -Wall.

---

• -Wtrampolines — Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. 💅 (executable stack? iconic but risky)

---

• -Wfloat-equal — Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. 🫶 (floats are never exactly equal, bestie, it's giving chaos)

---

• -Wtraditional (C and Objective-C only) — Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
  • Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
• a non-static function declaration rolls up right after a static one — bestie, some old-school traditional C compilers literally will not accept this. 💀
  • the ISO type of an integer constant has a different width or signedness than its traditional type — but like, this warning only pops off if the base of the constant is ten. So hex or octal values (which are usually just bit patterns anyway) get zero warning. 🙈
  • ISO string concatenation usage gets detected. slay.
  • initialization of automatic aggregates. noted.
  • identifier conflicts with labels — because traditional C just does not have a separate namespace for labels. it's giving "we don't have that here." 😭
  • initialization of unions — but if the initializer is zero, the warning is skipped entirely. The logic? It assumes a zero initializer in your code shows up conditioned on 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 you're compiling with traditional C causes serious problems, no cap. Heads up: this is only a subset of the possible conversion warnings. For the full set, use -Wtraditional-conversion.
  • use of ISO C style function definitions — and this warning is intentionally not issued for prototype declarations or variadic functions, because those 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) — Warn 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 that change the width or signedness of a fixed-point argument — except when it's the same as the default promotion. ✨
• -Wdeclaration-after-statement (C and Objective-C only) — Warn when a declaration shows up after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. ISO C90 does not support it. periodt.
• -Wshadow — Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. Same as -Wshadow=global.
• -Wno-shadow-ivar (Objective-C only) — Do not warn whenever a local variable shadows an instance variable in an Objective-C method. 🫶
• -Wshadow=global — The default for -Wshadow. Warns for any (global) shadowing.
• -Wshadow=local — Warn when a local variable shadows another local variable or parameter. This warning is enabled by -Wshadow=global.
• -Wshadow=compatible-local — Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional — not a bug or a typo — as shown in the following example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

Since the two variable i in the example above have incompatible types, enabling only -Wshadow=compatible-local will not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch that and emit an error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing.

This warning is enabled by -Wshadow=local.

• *-Wlarger-than=byte-size* — Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-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-larger-than. 🔥
• -Wno-larger-than — Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.
• -Wframe-larger-than=byte-size — Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is *approximate and not conservative. The actual requirements may be somewhat greater than *byte-size even if you don't get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. -Wframe-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-frame-larger-than. 😭
• -Wno-frame-larger-than — Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.
• -Wno-free-nonheap-object — Do not warn when attempting to free an object that was not allocated on the heap.
• -Wstack-usage=byte-size — Warn if the stack usage of a function might exceed byte-size. The computation done to determine the stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs **is included by the compiler when determining whether or not to issue a warning.

The message is in keeping with the output of -fstack-usage. 👑

• If the stack usage is fully static but exceeds the specified amount, it's:

          warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, it's:

          warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and not bounded, it's:

          warning: stack usage might be unbounded

-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 — Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.
• -Wunsafe-loop-optimizations — Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations, warn if the compiler makes such assumptions. 🌈
• -Wno-pedantic-ms-format (MinGW targets only) — When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime.
• -Waligned-new — Warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall. Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. ✨
• -Wplacement-new
• *-Wplacement-new=n* — Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that's smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. 💀

          char buf [64];
          new (buf) int[64];

This warning is enabled by default.

• -Wplacement-new=1 — This is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following "new" expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array.

                  struct S { int n, a[1]; };
                  S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
                  new (s->a)int [32]();

• -Wplacement-new=2 — At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2. 🦋

                  struct S { int n, a[]; };
                  S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
                  new (s->a)int [32]();

• -Wpointer-arith — Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic. 👑
• -Wpointer-compare — Warn if a pointer is compared with a zero character constant. This usually means that the pointer was meant to be dereferenced. For example:

          const char *p = foo ();
          if (p == '\0')
            return 42;

Note that the code above is invalid in C++11. 🙈

This warning is enabled by default.

• -Wtype-limits — bestie, this one clocks when a comparison is always true or always false purely because of the limited range of the data type — but it does not warn for constant expressions, okay? So like, if an unsigned variable gets compared against zero with < or >=, she's gonna say something. Also, this warning is enabled by -Wextra too 💅
• -Wabsolute-value (C and Objective-C only) — warn when you call a standard function that computes the absolute value of an argument but a more appropriate standard function exists. Example bestie: calling abs(3.14) triggers the warning because the correct function for a double argument is fabs. She also warns when the argument in a call to such a function has an unsigned type. You can hush her with an explicit type cast, and it's also enabled by -Wextra ✨
• -Wcomment
• -Wcomments — warn whenever a comment-start sequence /* appears inside a /* comment, or whenever a backslash-newline shows up in a // comment. Enabled by -Wall 🫶
• -Wtrigraphs — warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about, except the ones that would form escaped newlines.

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs 💖

• -Wundef — warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero, iykyk.
• -Wexpansion-to-defined — warn whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warn about macros defined in the main file that are unused. A macro counts as used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note bestie: if a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — do not warn whenever an #else or an #endif are followed by text. This sometimes happens in older programs with code of the form

#if FOO
...
#else FOO
...
#endif FOO

The second and third FOO should be in comments. This warning is on by default 🌸

• -Wbad-function-cast (C and Objective-C only) — warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type 💀
• -Wc90-c99-compat (C and Objective-C only) — warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__ 👑
• -Wc99-c11-compat (C and Objective-C only) — warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__ 🌈
• -Wc11-c2x-compat (C and Objective-C only) — warn about features not present in ISO C11, but present in ISO C2X. For instance, warn about omitting the string in _Static_assert. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__ ⭐
• -Wc++-compat (C and Objective-C only) — warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from void * to a pointer to non-void type 🦋
• -Wc++11-compat (C++ and Objective-C++ only) — warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall 💜
• -Wc++14-compat (C++ and Objective-C++ only) — warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall ✨
• -Wc++17-compat (C++ and Objective-C++ only) — warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall 🎀
• -Wcast-qual — warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *.

Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:

/* p is char ** value.  */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK.  */
*q = "string";
/* Now char** pointer points to read-only memory.  */
**p = 'b';

• -Wcast-align — warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries 💅
• -Wcast-align=strict — warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine.
• -Wcast-function-type — warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra 🔥
• -Wwrite-strings — when compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.

When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs 🍓

• -Wcatch-value
• -Wcatch-value=n (C++ and Objective-C++ only) — warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall 👑
• -Wclobbered — warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra 😭
• -Wconditionally-supported (C++ and Objective-C++ only) — warn for conditionally-supported (C++11 [intro.defs]) constructs 🧚
• -Wconversion — warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled 💖

• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default 🌸
• -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) — warn when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11 ⭐
• -Wsubobject-linkage (C++ and Objective-C++ only) — warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default 🦋
• -Wdangling-else — bestie, this one warns about constructions where there could be confusion about which if statement an else branch actually belongs to. Classic case:

{
  if (a)
    if (b)
      foo ();
  else
    bar ();
}

In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). That's often not what the programmer expected, and the indentation above is basically the crime scene photo. When there's potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. The glow-up looks like this:

{
  if (a)
    {
      if (b)
        foo ();
      else
        bar ();
    }
}

This warning is enabled by -Wparentheses.

• -Wdate-time — warns when the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they might prevent bit-wise-identical reproducible compilations. no cap, they're little chaos gremlins in your build 💀
• -Wdelete-incomplete (C++ and Objective-C++ only) — warns when you delete a pointer to an incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. 👑
• -Wuseless-cast (C++ and Objective-C++ only) — warns when an expression is casted to its own type. like bestie that's just you talking to yourself 💅
• -Wempty-body — warns if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra.
• -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.
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolon after in-class function definition.
• -Wjump-misses-init (C, Objective-C only) — warns if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. Heads up: this only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.

• -Wsign-compare — 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 is also enabled by -Wextra. the sign-flip betrayal arc 😭
• -Wsign-conversion — warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion.
• -Wfloat-conversion — warns for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order.
• -Wsized-deallocation (C++ and Objective-C++ only) — warns about a definition of an unsized deallocation function

void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

without a definition of the corresponding sized deallocation function

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

or vice versa. Enabled by -Wextra along with -fsized-deallocation.

• -Wsizeof-pointer-div — warns for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g. about sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by -Wall. the classic "wait, that's a pointer" plot twist 🫶
• -Wsizeof-pointer-memaccess — warns for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. This warning triggers for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo));.

-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is 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.

• -Wsizeof-array-argument — warns when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
• -Wmemset-elt-size — warns for suspicious calls to the memset built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall. bestie forgot the ✨multiplication✨
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall.
• -Waddress — warns about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp. This warning is enabled by -Wall.
• -Waddress-of-packed-member — warns when the address of packed member of 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 purpose is to detect suspicious code like the following:

int a;
...
if (!a > 1) { ... }

It is possible to suppress the warning by wrapping the LHS into parentheses:

if ((!a) > 1) { ... }

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.)
• -Wno-aggressive-loop-optimizations — warns if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
• -Wno-builtin-declaration-mismatch — 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 does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When -Wextra is specified, also warn when a built-in function that takes arguments is declared without a prototype. The -Wno-builtin-declaration-mismatch warning 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__.
• -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 preceded by a declaration that specifies the argument types.) vintage C behaviour, we respect the elders 🎀
• -Wold-style-declaration <span style="color:pink">(C and Objective-C only)</span> — bestie, this one's for the obsolescent vibes. It warns about usages that the C Standard has basically side-eyed as old, inside a declaration. Example: if storage-class specifiers like static aren't the first things in a declaration, you get a lil nudge. This warning is also enabled by -Wextra. ✨
• -Wold-style-definition <span style="color:pink">(C and Objective-C only)</span> — warns you if an old-style function definition is being used. And like, even if there's a previous prototype, it's still gonna say something. No free passes here. 🙈
• -Wmissing-parameter-type <span style="color:pink">(C and Objective-C only)</span> — this is when a function parameter gets declared with no type specifier at all in K&R-style functions, like:

void foo(bar) { }

This warning is also enabled by -Wextra. 🫶

• -Wmissing-prototypes <span style="color:pink">(C and Objective-C only)</span> — warns if a global function is defined with no previous prototype declaration. And yeah, it fires even if the definition itself provides a prototype. Use this to sniff out global functions that don't have a matching prototype declaration in a header file. ⚠️ Heads up: this option is not valid for C++, because in C++ all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. If you're in C++, use -Wmissing-declarations instead to catch missing declarations. 💅
• -Wmissing-declarations — warns if a global function is defined without a previous declaration, and it does this even if the definition itself provides a prototype. This is your tool for finding global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations — use -Wmissing-prototypes for those. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 🦋
• -Wmissing-field-initializers — warns when a structure's initializer is missing some fields. Like, this code here triggers it, because x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

It does not warn about designated initializers though, so this version stays quiet:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C, it also doesn't warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Same energy in C++ — it doesn't warn about the empty { } initializer:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. And if you want the other -Wextra warnings without this one, hit it with -Wextra -Wno-missing-field-initializers. 🎀

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually these are just a typo in the user's code, since they have implementation-defined values and really shouldn't be in portable code. So yeah, silencing it is a choice you're allowed to make. 💀
• -Wnormalized=[none|id|nfc|nfkc] — okay so in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when you're using characters outside the basic ASCII character set, you can have two different character sequences that look exactly the same. 😭 To avoid the confusion, the ISO 10646 standard lays out normalization rules which, when applied, ensure that two sequences that look the same get turned into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized, and 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 isn't in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It's equivalent to -Wnormalized.

Unfortunately — and this is the annoying part — there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. So there's literally 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. The hope is that future versions of the standards involved will fix this, which is exactly why this option is not the default.

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. Only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 🚨

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance, "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's 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 isn't in NFKC when 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, so it's not the default — but it may be useful as a local coding convention if the programming environment can't be fixed to display these characters distinctly. 👑

• -Wno-attribute-warning — do not warn about usage of functions declared with the warning attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag. 💜
• -Wno-deprecated — do not warn about usage of deprecated features. Simple as that. 🍓
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. 🧚
• -Wno-overflow — do not warn about compile-time overflow in constant expressions. 🙈
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Requires -flto-odr-type-merging to be enabled. Enabled by default. ✨
• -Wopenmp-simd — warns if the vectorizer cost model overrides the OpenMP simd directive set by the user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. 🔥
• -Woverride-init <span style="color:pink">(C and Objective-C only)</span> — warns if an initialized field without side effects is overridden when using designated initializers.

This warning is included in -Wextra. To get the other -Wextra warnings without this one, use -Wextra -Wno-override-init. 🎀

• -Woverride-init-side-effects <span style="color:pink">(C and Objective-C only)</span> — warns if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default. 💅
• -Wpacked — warns if a structure is given the packed attribute, but that packed attribute has no effect on the layout or size of the structure. Such structures may end up mis-aligned for basically no benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

• -Wpacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This got fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC lets you know when the offset of such a field has changed in GCC 4.4. For example, there's no longer a 4-bit padding between field a and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning. 🦋

• -Wpacked-not-aligned <span style="color:pink">(C, C++, Objective-C and Objective-C++ only)</span> — warns if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct 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. ⭐

• -Wpadded — warns if padding is included in a structure, whether it's there to align an element of the structure or to align the whole structure. Sometimes when this happens, you can rearrange the fields of the structure to reduce the padding and make the structure smaller. 👑
• -Wredundant-decls — warns if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes nothing. Like, we see you, but okay. 🙈
• -Wno-restrict — warns when an object referenced by a restrict-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 the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL into a[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 it works best at -O2 and above. It's included in -Wall. 🔥

• -Wnested-externs <span style="color:pink">(C and Objective-C only)</span> — warns if an extern declaration is encountered within a function. Nested externs? We're clocking it. 👀
• -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. 💀
• -Winline — warns if a function that is declared as inline cannot actually be inlined. And even with this option on, the compiler does not warn about failures to inline functions declared in system headers. 🌸

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

• -Wno-invalid-offsetof (C++ and Objective-C++ only) — bestie, this one shuts off the warnings you get from applying the offsetof macro to a non-POD type. Per the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. But like… in existing C++ implementations, offsetof typically gives meaningful results anyway. This flag is for users who know they're writing nonportable code and who have deliberately chosen to ignore the warning about it. 👀

The restrictions on offsetof may be relaxed in a future version of the C++ standard.

• -Wint-in-bool-context — warns you about suspicious use of integer values where boolean values are expected 🫣 like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g. if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall.
• -Wno-int-to-pointer-cast — suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
• -Wno-pointer-to-int-cast (C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size.
• -Winvalid-pch — warns if a precompiled header is found in the search path but cannot be used. 😭
• -Wlong-long — warns if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
• -Wvarargs — warns upon questionable usage of the macros used to handle variable arguments like va_start. This is default. To inhibit the warning messages, use -Wno-varargs.
• -Wvector-operation-performance — warns if a vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning 💅 A vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type.
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous ⚠️ because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.
• -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.
• -Wvla-larger-than=byte-size — if this option is used, the compiler will warn for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. 🙈

-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. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger.
• -Wvolatile-register-var — warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 😮‍💨
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
• -Wstack-protector — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute.
• -Whsa — issues a warning when HSAIL cannot be emitted for the compiled function or OpenMP construct.

---

🐛✨ Options for Debugging Your Program ✨🐛

To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options. 🫶

GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. 💀

If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. 👑

• -g — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program 😭 If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

• -ggdb — produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
• -gdwarf
• -gdwarf-version — produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 4. DWARF Version 5 is only experimental. 🧪

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF.

• -gstabs — produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler.
• -gstabs+ — produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. 🙈
• -gxcoff — produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
• -gxcoff+ — okay so this one makes debugging info in XCOFF format (if your setup even supports it 💅), and it uses GNU extensions that literally only GDB understands. Heads up bestie: using those extensions will probably make other debuggers crash or flat-out refuse to read your program, and it can make assemblers that aren't GAS (the GNU assembler) throw an error at you. 😭 So like, know what you're doing.
• -gvms — produces debugging info in Alpha/VMS debug format (if that's supported). This is the format that DEBUG uses on Alpha/VMS systems. ✨

---

💖 -g LEVELS, THE WHOLE SITUATION 💖

• -glevel, -ggdblevel, -gstabslevel, -gxcofflevel, -gvmslevel — ask for debugging info and tell it how much you want with level. Default level is 2. Periodt.
  • Level 0 = literally no debug info at all. So -g0 cancels out -g. Bye. 👋
  • Level 1 = the bare minimum, enough to make backtraces for the parts of the program you don't plan to debug. That means descriptions of functions and external variables, plus line number tables — but no info about local variables.
  • Level 3 = extra info, like all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. 🎀
• If you use multiple -g options, with or without level numbers, the last one is the one that counts. The last one wins, no cap.
• -gdwarf does not accept a concatenated debug level, specifically so it doesn't get confused with -gdwarf-level. Instead, use an extra -glevel option to change the debug level for DWARF. 🫶

---

• -feliminate-unused-debug-symbols — produces debugging info in stabs format (if that's supported), but only for symbols that are actually used. Less clutter, cute.
• -femit-class-debug-always — normally GCC emits debugging info for a C++ class in just one object file; with this it emits it in all object files that use the class. Only use this with debuggers that can't handle how GCC normally emits class debug info, because it can inflate the size of your debugging info by as much as a factor of two. It's giving bloated. 😭
• -fno-merge-debug-strings — tells the linker to not merge together identical strings in the debugging information across different object files. Merging isn't supported by all assemblers or linkers. Merging shrinks the debug info in the output file but costs you more link processing time. Merging is enabled by default. ✨
• -fdebug-prefix-map=old=new — when you compile files that live in directory old, record the debugging info as if those files lived in directory new instead. Use case: swapping a build-time path for an install-time path in the debug info. You can also turn an absolute path into a relative one by using . for new. This can give you more reproducible builds that are location independent — but it may need an extra command to tell GDB where to find the source files. See also -ffile-prefix-map. 🦋
• -fvar-tracking — runs the variable tracking pass, which figures out where variables are stored at each position in the code. Then better debugging info gets generated (if the debug info format even supports that kind of info).

It's enabled by default when you compile with optimization (-Os, -O, -O2, ...), debugging information (-g), and the debug info format supports it. 💅

• -fvar-tracking-assignments — annotates assignments to user variables early in compilation and tries to carry those annotations all the way through to the end, all to improve debug info while optimizing. Using -gdwarf-4 alongside it is recommended. 👑

It can be enabled even if var-tracking is disabled — in that case the annotations get created and maintained, but discarded at the end. By default this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

• -gsplit-dwarf — splits as much DWARF debugging info as possible into a separate output file with the extension .dwo. This lets the build system avoid linking files that have debug info. To actually be useful, this option needs a debugger that can read .dwo files. No .dwo-reading debugger, no vibes. 🥲
• -gdescribe-dies — adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. ✨
• -gpubnames — generates DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
• -ggnu-pubnames — generates ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a GDB index. Only useful with a linker that can produce GDB index version 7. 🎀
• -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can go into their own ".debug_types" section instead of being part of the ".debug_info" section. Putting them in a separate comdat section is more efficient because the linker can then remove duplicates. But: not all DWARF consumers support ".debug_types" sections yet, and on some objects ".debug_types" produces larger instead of smaller debugging info. 🙈
• -grecord-gcc-switches, -gno-record-gcc-switches — this switch appends the command-line options used to invoke the compiler that may affect code generation onto the DW_AT_producer attribute in DWARF debugging information. The options get concatenated with spaces separating them from each other and from the compiler version. Enabled by default. See also -frecord-gcc-switches for another way to store compiler options in the object file. 🫶
• -gstrict-dwarf — disallows using extensions from a later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed — this is the option that says nope, don't. 💅
• -gno-strict-dwarf — allows using extensions of a later DWARF standard version than the one selected with -gdwarf-version. The chill one. 🌸
• -gas-loc-support — informs the compiler that the assembler supports ".loc" directives. It may then use them so the assembler generates 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. 👑

This option is 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 going to be generated anyway.
• gas-locview-support — informs the compiler that the assembler supports "view" assignment and reset assertion checking in ".loc" directives.

This option is 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-views is explicitly requested.
• -gcolumn-info, -gno-column-info — emits location column information into DWARF debugging information, rather than 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 of "is_stmt" markers in the line number table. Enabled by default when compiling with optimization (-Os, -O, -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. This lets debug info consumers inspect state at certain points of the program, even when no instructions associated with the corresponding source locations exist at that point. If the assembler doesn't support view numbers in line number tables, this will make the compiler emit the line number table itself, which generally makes those tables somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so consumers that aren't aware of these augmentations can still read them — they just won't get any benefit from them either. 🤷

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 a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and you can turn it on 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 are not 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 — tries to determine location views that can be omitted from location view lists. This requires 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 using an assembler that doesn't support location view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function. 🙈
• -ginline-points, -gno-inline-points — generates extended debug information 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 the 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] — okay so this one makes your debug sections compressed in DWARF format, if that's even supported, bestie. If you don't hand over a type, the default type just depends on what the assembler and linker you're using can actually do. And type can be one of: none (literally don't compress, we're chill), zlib (zlib compression, ELF gABI format style), or zlib-gnu (zlib compression, traditional GNU format). 🫶 If your linker can't write compressed debug sections, the option gets rejected — no debate. But if the assembler doesn't support them, -gz is silently ignored when you're producing object files. Rude but okay.
• -femit-struct-debug-baseonly — this emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where that struct is defined. 💅

It substantially shrinks your debugging information, but at a significant potential loss of type info for the debugger. If that sounds like too much, go peep -femit-struct-debug-reduced for a less aggressive option, or -femit-struct-debug-detailed if you want detailed control, iykyk.

Heads up: this one only works with DWARF debug output. No DWARF, no vibes.

• -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 where the type is defined — unless the struct is a template or it's defined in a system header. ✨

This significantly reduces the size of debugging information, with some potential loss in type information to the debugger. Want something more aggressive? -femit-struct-debug-baseonly. Want the full control panel? -femit-struct-debug-detailed.

Again: DWARF debug output only, bestie.

• -femit-struct-debug-detailed[=spec-list] — this lets you specify exactly which struct-like types the compiler generates debug information for. The whole intent? Reduce duplicate struct debug info between different object files in the same program. 🧚

It's basically the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly 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 come through pointers to structs. So like, when use of an incomplete struct is valid, that use is indirect. Example:

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, ngl. 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 implement them yet. 😭

The third word specifies the source files for those structs the compiler should emit debug info for. The values none and any have the normal meaning. The value base means the base of the name of the file where 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 information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers. 👑

You may need to experiment to determine the best settings for your application. Fr fr.

The default is -femit-struct-debug-detailed=all.

And yes — this option works only with DWARF debug output. 💖

• -fno-dwarf2-cfi-asm — emits DWARF unwind info as a compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives. 🌸
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it's useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — like 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, though, this results in a significant amount of wasted space. 🙈

---

🌈 Options That Control Optimization 🌈

These options control various sorts of optimizations. The main character energy of compiler flags, honestly.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. Predictable queen. 💅

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. Trade-offs, bestie, we all have them.

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. Group project but it actually works. ✨

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.

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. Go check, it's giving detective work. 🔍

• -O
• -O1 — Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-profile
-fipa-pure-const -fipa-reference -fipa-reference-addressable
-fmerge-constants -fmove-loop-invariants -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 don't involve a space-speed tradeoff. Compared to -O, this option increases both compilation time and the performance of the generated code. Glow up. 🔥

-O2 turns on all optimization flags specified by -O. 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 -fgcse
-fgcse-lm -fhoist-adjacent-loads -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-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-pre
-ftree-switch-conversion  -ftree-tail-merge -ftree-vrp

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. 👀

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

• -O3 — Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:

-fgcse-after-reload -finline-functions -fipa-cp-clone
-floop-interchange -floop-unroll-and-jam -fpeel-loops
-fpredictive-commoning -fsplit-paths -ftree-loop-distribute-patterns
-ftree-loop-distribution -ftree-loop-vectorize -ftree-partial-pre
-ftree-slp-vectorize -funswitch-loops -fvect-cost-model
-fversion-loops-for-strides

• -O0 — Reduce compilation time and make debugging produce the expected results. This is the default. The cozy no-makeup look. 🌸
• -Os — Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It 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. Tiny but mighty. 🎀

• -Ofast — Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. Chaos mode, but make it performance. 💀
• -Og — Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It's a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. The balanced icon. 🦋

Like -O0, -Og is a whole mood of no — it straight-up kills a bunch of optimization passes so that the individual options controlling them have literally zero effect. Otherwise, -Og turns on all the -O1 optimization flags except for the ones that would get in the way of debugging:

-fbranch-count-reg  -fdelayed-branch -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants  -fssa-phiopt -ftree-bit-ccp  -ftree-pta
-ftree-sra

If you pass multiple -O options, with or without level numbers, bestie, the last one is the one that actually counts. Last one wins, periodt.

---

Options shaped like -fflag are the machine-independent flags, the ✨ universal ✨ ones. Most flags have both a positive and a negative form, and the negative form of -ffoo is -fno-foo. In the table below only one of the two forms is listed — the one you'd typically reach for. You can figure out the other form by either dropping the no- or adding it. iykyk 💅

The options below control specific optimizations. They're either activated by -O options or they're related to ones that are. Use these flags in those rare cases when you're craving some "fine-tuning" of the optimizations being performed. 🎀

• -fno-defer-pop — for machines that have to pop arguments after a function call, this makes them pop the arguments the moment each function returns. At levels -O1 and higher, -fdefer-pop is the default, which lets the compiler just… let arguments pile up on the stack across several function calls and pop them all at once. Efficient queen. 👑
• -fforward-propagate — runs a forward propagation pass on RTL. The pass tries to combine two instructions and checks whether the result can be simplified. If loop unrolling is active, two passes happen and the second one is scheduled after loop unrolling. Enabled by default at optimization levels -O, -O2, -O3, -Os.
• -ffp-contract=style — -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if the language standard allows it — but this is currently not implemented and gets treated the same as -ffp-contract=off. The default is -ffp-contract=fast.
• -fomit-frame-pointer — omits the frame pointer in functions that don't need one. This skips the instructions that save, set up and restore the frame pointer; on many targets it also frees up an extra register. 🦋 On some targets this flag does nothing, because the standard calling sequence always uses a frame pointer, so it simply cannot be omitted. Also note: -fno-omit-frame-pointer does not guarantee the frame pointer is used in all functions — several targets always omit the frame pointer in leaf functions. Enabled by default at -O and higher.
• -foptimize-sibling-calls — optimizes sibling and tail recursive calls. Enabled at levels -O2, -O3, -Os.
• -foptimize-strlen — optimizes various standard C string functions (like strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. Enabled at levels -O2, -O3.
• -fno-inline — do not expand any functions inline, apart from the ones marked with the always_inline attribute. This is the default when not optimizing. Individual 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 the 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 this way. This inlining applies to all functions, even the ones not declared inline. Enabled at levels -O2, -O3, -Os.
• -findirect-inlining — also inlines indirect calls that turn out to be known at compile time thanks to previous inlining. This option only has any effect when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at levels -O2, -O3, -Os.
• -finline-functions — considers all functions for inlining, even if they aren't declared inline. The compiler heuristically decides which functions are worth integrating this way. If all calls to a given function get integrated, and the function is declared static, then that function normally isn't output as assembler code in its own right. Enabled at levels -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
• -finline-functions-called-once — considers all static functions called once for inlining into their caller, even if they aren't marked inline. If a call to a given function gets integrated, then the function isn't output as assembler code in its own right. Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
• -fearly-inlining — inlines functions marked by always_inline and functions whose body seems smaller than the function call overhead, early — before doing -fprofile-generate instrumentation and the real inlining pass. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs with big 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 with parameters passed by value. Enabled at levels -O2, -O3 and -Os.
• -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag gives you coarse control over that limit. n is the size of functions that can be inlined, measured in number of pseudo instructions. Inlining is actually controlled by a number of parameters, which can be specified individually using --param name=value. The -finline-limit=n option sets some of these parameters like so:
  • max-inline-insns-single — is set to n/2.
  • max-inline-insns-auto — is set to n/2.

See below for documentation of the individual parameters controlling inlining, and for the defaults of those parameters.

Note: there may be no value of -finline-limit that results in default behavior.

Note: "pseudo instruction" represents, in this particular context, an abstract measurement of a function's size. It does not in any way represent a count of assembly instructions, and as such its exact meaning might change from one release to another. 🙈

• -fno-keep-inline-dllexport — this is a more fine-grained version of -fkeep-inline-functions, which applies only to functions declared using the dllexport attribute or declspec.
• -fkeep-inline-functions — in C, emits static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emits any and all inline functions into the object file.
• -fkeep-static-functions — emits static functions into the object file, even if the function is never used.
• -fkeep-static-consts — emits variables declared static const when optimization isn't turned on, even if the variables aren't referenced. GCC enables this option by default. If you want to force the compiler to check whether 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 -O, -O2, -O3, -Os.
• -fmerge-all-constants — attempts to merge identical constants and identical variables. This option implies -fmerge-constants. On top of -fmerge-constants, it also 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. Setting this flag deletes certain anti-dependences edges, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
• -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 on the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. 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 get confused by the optimizations performed when this option isn't 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 that behavior off, because some programs explicitly rely on variables going to the data section — e.g., so the resulting executable can find the beginning of that section and/or make assumptions based on that. The default is -fzero-initialized-in-bss. 🌸
• -fthread-jumps — okay so bestie, this optimization checks to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. Enabled at levels -O2, -O3, -Os. periodt.
• -fsplit-wide-types — bestie, when you're using a type that occupies multiple registers, like long long on a 32-bit system, this splits the registers apart and allocates them independently. It normally generates better code for those types, but it may make debugging more difficult. Enabled at levels -O, -O2, -O3, -Os.
• -fcse-follow-jumps — In common subexpression elimination (CSE), this scans through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false. Enabled at levels -O2, -O3, -Os.
• -fcse-skip-blocks — This is similar to -fcse-follow-jumps, but it causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if. Enabled at levels -O2, -O3, -Os.
• -frerun-cse-after-loop — Re-run common subexpression elimination after loop optimizations are performed. Enabled at levels -O2, -O3, -Os.
• -fgcse — Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line. Enabled at levels -O2, -O3, -Os.
• -fgcse-lm — When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. Enabled by default when -fgcse is enabled.
• -fgcse-sm — When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. Not enabled at any optimization level.
• -fgcse-las — When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies). Not enabled at any optimization level.
• -fgcse-after-reload — When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling. Enabled by -fprofile-use and -fauto-profile.
• -faggressive-loop-optimizations — This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
• -funconstrained-commons — This option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds. iykyk.
• -fcrossjumping — Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping. Enabled at levels -O2, -O3, -Os.
• -fauto-inc-dec — Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
• -fdce — Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
• -fdse — Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
• -fif-conversion — Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by -fif-conversion2. Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
• -fif-conversion2 — Use conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
• -fdeclone-ctor-dtor — The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation. Enabled by -Os.
• -fdelete-null-pointer-checks — Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null. Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior. This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels.
• -fdevirtualize — Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
• -fdevirtualize-speculatively — Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form. main character energy.
• -fdevirtualize-at-ltrans — Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.
• -fexpensive-optimizations — Perform a number of minor optimizations that are relatively expensive. Enabled at levels -O2, -O3, -Os.
• -free — Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
• -fno-lifetime-dse — In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor you, can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
• -flive-range-shrinkage — Attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets.
• -fira-algorithm=algorithm — Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.
• -fira-region=region — Use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — bestie, this makes IRA (that's the Integrated Register Allocator, the whole vibe) actually do a vibe check on register pressure inside the code hoisting pass, so it can decide whether hoisting an expression is even worth it. Usually this gives you smaller code, but it can slow the compiler down, ngl. 🌸

This option is enabled at level -Os for all targets.

• -fira-loop-pressure — same energy but for loops: use IRA to check register pressure in loops when deciding whether to move loop invariants around. On machines with big register files (>= 32 registers) this usually generates faster and smaller code, but yeah, it can slow the compiler down. ✨

This option is enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — turns OFF the sharing of stack slots used to save call-used hard registers that are still alive across a call. Every hard register gets its own separate stack slot, and the consequence is that function stack frames get bigger. It's giving "personal space era". 💅
• -fno-ira-share-spill-slots — turns OFF sharing of stack slots allocated for pseudo-registers. Every pseudo-register that doesn't score a hard register gets its own separate stack slot, so again: bigger function stack frames. 😭
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of reloading values of spilled pseudos, LRA tries to rematerialize (aka recalculate) those values if it's profitable. iconic behavior honestly. 👑

Enabled at levels -O2, -O3, -Os.

• -fdelayed-branch — if the target machine supports it, try to reorder instructions so it can exploit the instruction slots available after delayed branch instructions. filling the gaps, we love to see it. 🦋

Enabled at levels -O, -O2, -O3, -Os, but not at -Og.

• -fschedule-insns — if the target machine supports it, try to reorder instructions to eliminate execution stalls caused by needed data not being ready yet. This is a lifesaver for machines with slow floating point or memory load instructions, because it lets other instructions get issued while everyone waits for that load or FP result. ⭐

Enabled at levels -O2, -O3.

• -fschedule-insns2 — like -fschedule-insns but it asks for an extra pass of instruction scheduling after register allocation is done. Super useful on machines with a relatively small number of registers, and where memory load instructions take more than one cycle. 🧚

Enabled at levels -O2, -O3, -Os.

• -fno-sched-interblock — disables instruction scheduling across basic blocks. That cross-block scheduling is normally on when scheduling before register allocation happens, i.e. with -fschedule-insns or at -O2 or higher. 🚫
• -fno-sched-spec — disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 💀
• -fsched-pressure — enables register-pressure-sensitive instruction scheduling before register allocation. Heads up: this only makes sense when scheduling before register allocation is actually on, i.e. with -fschedule-insns or at -O2 or higher. Using it can improve the generated code and shrink it, by stopping register pressure from rising above the number of available hard registers — which stops the subsequent spills in register allocation. no spills, no stress. 🫶
• -fsched-spec-load — allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation is on, i.e. with -fschedule-insns or at -O2 or higher. ✨
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions. Same deal: only makes sense when scheduling before register allocation is on, i.e. with -fschedule-insns or at -O2 or higher. the name literally warns you bestie, it's giving dangerous 💅
• -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-insns means no insns get moved prematurely. -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. And -fsched-stalled-insns with no value is equivalent to -fsched-stalled-insns=1. 🎀
• -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-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0, and -fsched-stalled-insns-dep with 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, which gives you faster schedules. ⚠️ This option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

Also, 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, which favors the instruction that belongs to a schedule group. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🌈
• -fsched-critical-path-heuristic — enables the critical-path heuristic in the scheduler, which favors instructions on the critical path. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. ⭐
• -fsched-spec-insn-heuristic — enables the speculative instruction heuristic in the scheduler, which favors speculative instructions with greater dependency weakness. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🦋
• -fsched-rank-heuristic — enables the rank heuristic in the scheduler, which favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 👑
• -fsched-last-insn-heuristic — enables the last-instruction heuristic in the scheduler, which favors the instruction that is less dependent on the last instruction scheduled. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 💜
• -fsched-dep-count-heuristic — enables the dependent-count heuristic in the scheduler, which favors the instruction that has more instructions depending on it. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🍓
• -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling. So if a loop is modulo scheduled, later scheduling passes might change its schedule. Use this option to control that behavior. 🎀
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. 💅
• -fselective-scheduling2 — schedule 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. Heads up: this option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on. 🚫
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on. 🌸
• -fsemantic-interposition — okay so some object formats, like ELF, let the dynamic linker interpose symbols. That means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because the function or variable in question might change. While this feature is useful — like, for rewriting memory allocation functions with a debugging implementation — it is 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.

Also: this 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 — emit function prologues only before the parts of the function that actually need them, instead of at the top of the function. minimal effort, maximal slay. 💅

This flag is enabled by default at -O and higher.

• -fshrink-wrap-separate — shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed. This option is on by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports it. 🎀
• -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 those calls. Such allocation only happens when it seems to result in better code. ✨

This option is always enabled by default on certain machines, usually the ones that 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. tidy. 🧚

Enabled by default at -O1 and higher.

• -fipa-ra — use caller save registers for allocation if those registers aren't used by any called function. In that case you don't need 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 the 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 — ok so this one tries to keep your stack usage smol 🫶 like the compiler will literally use less stack space even if it makes the program slower. It implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400. trade-off queen behavior, no cap.
• -ftree-reassoc — performs reassociation on trees. enabled by default at -O and higher ✨
• -fcode-hoisting — performs code hoisting. so what that does is it 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 honestly it often helps for code speed too 💅 this flag is enabled by default at -O2 and higher.
• -ftree-pre — performs partial redundancy elimination (PRE) on trees. this flag is enabled by default at -O2 and -O3.
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive 😤 enabled by default at -O3.
• -ftree-forwprop — performs forward propagation on trees. enabled by default at -O and higher.
• -ftree-fre — performs full redundancy elimination (FRE) on trees. the difference between FRE and PRE? 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. enabled by default at -O and higher.
• -ftree-phiprop — performs hoisting of loads from conditional pointers on trees. this pass is enabled by default at -O and higher.
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. this flag is enabled by default at -O2 and higher.
• -ftree-copy-prop — performs copy propagation on trees. this pass eliminates unnecessary copy operations, like bye bestie 👋 enabled by default at -O and higher.
• -fipa-pure-const — discovers which functions are pure or constant. enabled by default at -O and higher.
• -fipa-reference — discovers which static variables do not escape the compilation unit. enabled by default at -O and higher.
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. enabled by default at -O and higher.
• -fipa-stack-alignment — reduces stack alignment on call sites if possible. enabled by default.
• -fipa-pta — performs interprocedural pointer analysis and interprocedural modification and reference analysis. ⚠️ heads up bestie, this option can cause excessive memory and compile-time usage on large compilation units. it is not enabled by default at any optimization level.
• -fipa-profile — performs interprocedural profile propagation. functions called only from cold functions get marked as cold. also, functions executed once (like cold, noreturn, static constructors or destructors) are identified. cold functions and loop-less parts of functions executed once are then optimized for size. enabled by default at -O and higher.
• -fipa-cp — performs interprocedural constant propagation. this optimization analyzes the program to figure out when values passed to functions are constants and then optimizes accordingly. it can substantially increase performance if the application has constants passed to functions 🚀 enabled by default at -O2, -Os and -O3. it's also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — performs function cloning to make interprocedural constant propagation stronger. when enabled, interprocedural constant propagation performs function cloning when an externally visible function can be called with constant arguments. because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). this flag is enabled by default at -O3. it's also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, performs interprocedural bitwise constant propagation. enabled by default at -O2 and by -fprofile-use and -fauto-profile. it requires that -fipa-cp is enabled.
• -fipa-vrp — when enabled, performs interprocedural propagation of value ranges. enabled by default at -O2. it requires that -fipa-cp is enabled.
• -fipa-icf — performs Identical Code Folding for functions and read-only variables. this optimization reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name 💀 the optimization works more effectively with link-time optimization enabled.

ok so even though the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold. iykyk.

this flag is enabled by default at -O2 and -Os.

• -flive-patching=level — controls GCC's optimizations to produce output suitable for live-patching.

here's the tea: if the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, that latter one is called an impacted function of the former. 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 inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. in order to control the number of impacted functions and more easily compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.

the level argument should be one of the following:

• inline-clone — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. as a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well.

-flive-patching=inline-clone disables the following 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

• inline-only-static — only enable inlining of static functions. as a result, when patching a static function, all its callers are impacted and so need to be patched as well.

in addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following 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.

note that -flive-patching is not supported with link-time optimization (-flto). 🚫

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap 🪤 this flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled.
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute. isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. this is not currently enabled, but may be enabled by -O2 in the future 👀
• -ftree-sink — performs forward store motion on trees. enabled by default at -O and higher.
• -ftree-bit-ccp — performs sparse conditional bit constant propagation on trees and propagates pointer alignment information. this pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. it requires that -ftree-ccp is enabled.
• -ftree-ccp — performs sparse conditional constant propagation (CCP) on trees. this pass only operates on local scalar variables and is enabled by default at -O and higher.
• -fssa-backprop — propagates information about uses of a value up the definition chain in order to simplify the definitions. for example, this pass strips sign operations if the sign of a value never matters. enabled by default at -O and higher.
• -fssa-phiopt — performs pattern matching on SSA PHI nodes to optimize conditional code. this pass is enabled by default at -O1 and higher, except for -Og.
• -ftree-switch-conversion — performs conversion of simple initializations in a switch to initializations from a scalar array. enabled by default at -O2 and higher.
• -ftree-tail-merge — looks for identical code sequences. when found, it replaces one with a jump to the other. this optimization is known as tail merging or cross jumping 🦘 enabled by default at -O2 and higher. the compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter.
• -ftree-dce — performs dead code elimination (DCE) on trees. enabled by default at -O and 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. this flag is enabled by default at -O2 and higher if -Os is not also specified.
• -ftree-dominator-opts — bestie this one does a whole lil spa day of simple scalar cleanups: constant/copy propagation, redundancy elimination, range propagation and expression simplification, all based on a dominator tree traversal 💅 it also does jump threading (so jumps that just go to other jumps get tidied up, obsessed). enabled by default at -O and higher ✨
• -ftree-dse — dead store elimination (DSE) on trees 🧚 a dead store is when you store into a memory location and then later overwrite it with another store, with no loads in between. so the earlier store? literally can be deleted. enabled by default at -O and higher 💖
• -ftree-ch — performs loop header copying on trees 🎀 this is a win because it makes code motion optimizations more effective, and it also saves one jump. enabled by default at -O and higher. but heads up — it is not enabled for -Os, because it usually increases code size 😭
• -ftree-loop-optimize — performs loop optimizations on trees. enabled by default at -O and higher 🌸
• -ftree-loop-linear, -floop-strip-mine, -floop-block — performs loop nest optimizations. same as -floop-nest-optimize 🦋 to use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. iykyk
• -fgraphite-identity — enables the identity transformation for graphite ✨ for every SCoP we generate the polyhedral representation and transform it back to gimple. using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. the code generator isl also does some minimal optimizations, like index splitting and dead code elimination in loops 💜
• -floop-nest-optimize — enables the isl based loop nest optimizer 🔥 this is a generic loop nest optimizer based on the Pluto optimization algorithms. it calculates a loop structure optimized for data-locality and parallelism. lowkey important: this option is experimental 🙈
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized 👑 it parallelizes all the loops that can be analyzed to not contain loop carried dependences — and it does this without checking that it is profitable to parallelize the loops. periodt, it just goes for it 💀
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, this attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🧚 but queen beware: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments 😭 in the negated form, this flag prevents SSA coalescing of user variables. enabled by default if optimization is enabled, and it does very little otherwise 🌸
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops to branch-less equivalents ✨ the whole intent is to remove control-flow from the innermost loops so the vectorization pass can handle these loops better. enabled by default if vectorization is enabled 💅
• -ftree-loop-distribution — performs loop distribution 🎀 this can improve cache performance on big loop bodies and let further loop optimizations, like parallelization or vectorization, take place. for example, the loop

        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = B(I) + C
        ENDDO
        DO I = 1, N
           D(I) = E(I) * F
        ENDDO

this flag is enabled by default at -O3. it is also enabled by -fprofile-use and -fauto-profile 🦋

• -ftree-loop-distribute-patterns — performs loop distribution of patterns that can be code generated with calls to a library 📚 enabled by default at -O3, and by -fprofile-use and -fauto-profile.

this pass distributes the initialization loops and generates a call to memset zero 💖 for example, the loop

        DO I = 1, N
          A(I) = 0
          B(I) = A(I) + I
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = 0
        ENDDO
        DO I = 1, N
           B(I) = A(I) + I
        ENDDO

and the initialization loop is transformed into a call to memset zero ✨ this flag is enabled by default at -O3. it is also enabled by -fprofile-use and -fauto-profile 🌸

• -floop-interchange — performs loop interchange outside of graphite 🔥 this can improve cache performance on loop nest and let further loop optimizations, like vectorization, take place. for example, the loop

        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];

is transformed to

        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];

this flag is enabled by default at -O3. it is 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. it is also enabled by -fprofile-use and -fauto-profile ✨
• -ftree-loop-im — performs loop invariant motion on trees 🧚 this pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). with -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. the pass also includes store motion 💅
• -ftree-loop-ivcanon — creates a canonical counter for number of iterations in loops for which determining the number of iterations requires complicated analysis 🎀 later optimizations then may determine the number easily. useful especially in connection with unrolling 🌸
• -ftree-scev-cprop — performs final value replacement ✨ if a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, then uses of the final value get replaced by such a computation — provided it is sufficiently cheap. this reduces data dependencies and may allow further simplifications. enabled by default at -O and higher 💖
• -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 🔥
• *-ftree-parallelize-loops=n*** — parallelize loops, i.e., split their iteration space to run in *n* threads 👑 this is only possible for loops whose iterations are independent and can be arbitrarily reordered. the optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. this option implies -pthread, and thus is only supported on targets that have support for -pthread` 🌈
• -ftree-pta — performs function-local points-to analysis on trees. enabled by default at -O1 and higher, except for -Og 🦋
• -ftree-sra — performs scalar replacement of aggregates 🧚 this pass replaces structure references with scalars to prevent committing structures to memory too early. enabled by default at -O1 and higher, except for -Og 🌸
• -fstore-merging — performs merging of narrow stores to consecutive memory addresses 💅 this pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. enabled by default at -O2 and higher as well as -Os ✨
• -ftree-ter — performs temporary expression replacement during the SSA->normal phase 🎀 single use/single def temporaries are replaced at their use location with their defining expression. this results in non-GIMPLE code, but gives the expanders much more complex trees to work on, resulting in better RTL generation. enabled by default at -O and higher 💜
• -ftree-slsr — performs straight-line strength reduction on trees 🦋 this recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. enabled by default at -O and higher 🌸
• -ftree-vectorize — performs vectorization on trees 🔥 this flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified ✨
• -ftree-loop-vectorize — performs loop vectorization on trees 💖 this flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile 👑
• -ftree-slp-vectorize — performs basic block vectorization on trees 🎀 this flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile 💅
• *-fvect-cost-model=model*** — alters the cost model used for vectorization 🧚 the *model* argument should be one of unlimited, dynamicorcheap. with the unlimitedmodel the vectorized code-path is assumed to be profitable 🌈 while with thedynamicmodel a runtime check guards the vectorized code-path so it's only enabled 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. the default cost model depends on other optimization flags and is eitherdynamicorcheap` 💜
• *-fsimd-cost-model=model*** — alters the cost model used for vectorization of loops marked with the OpenMP simd directive ✨ the *model* argument should be one of unlimited, dynamic, cheap. all values of *model* have the same meaning as described in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model` is used 🦋
• -ftree-vrp — bestie this one does Value Range Propagation on trees ✨ and it's giving "constant propagation but make it ✨ranges✨". So instead of yeeting actual values around, it yeets ranges of values. Why do we care?? Because now the optimizer can straight up delete unnecessary range checks — think array bound checks and null pointer checks — like they never happened 💅. It's on by default at -O2 and higher. One lil caveat tho: null pointer check elimination only happens if -fdelete-null-pointer-checks is also enabled. No cap.
• -fsplit-paths — splits paths that lead to loop backedges 🦋. This can level up dead code elimination and common subexpression elimination. Enabled by default at -O3 and above. Iconic.
• -fsplit-ivs-in-unroller — okay so this one enables expression of the values of induction variables in later iterations of the unrolled loop using the value from the first iteration 🌀. It breaks those long dependency chains, which makes the scheduling passes way more efficient. Periodt.

A combo of -fweb and CSE is often enough to get the same effect — however, that's not reliable when the loop body is more complicated than a single basic block. Also it literally does not work at all on some architectures because of restrictions in the CSE pass 😭. This optimization is enabled by default.

• -fvariable-expansion-in-unroller — with this option the compiler makes multiple copies of some local variables when it unrolls a loop, which can result in superior code 🔥.
• -fpartial-inlining — inline parts of functions 🫶. Heads up tho: this has any effect only when inlining itself is turned on by -finline-functions or -finline-small-functions. Enabled at levels -O2, -O3, -Os.
• -fpredictive-commoning — performs the predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) that were performed in previous iterations of loops ♻️. Enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile.
• -fprefetch-loop-arrays — if the target machine supports it, this generates instructions to prefetch memory so loops that access large arrays go brrr 🏃‍♀️. Real talk tho: this option may generate better or worse code — results are highly dependent on the structure of the loops inside your source. Disabled at level -Os.
• -fno-printf-return-value — do NOT substitute constants for the known return value of formatted output functions like sprintf, snprintf, vsprintf, and vsnprintf (but not printf or fprintf, note the distinction bestie 💅). This transformation is what lets GCC optimize or even fully eliminate branches based on the known return value of these functions when they're called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer, because the return value is guaranteed to be at most 8 😌.

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. And -fprintf-return-value is enabled by default 💖.

• -fno-peephole
• -fno-peephole2 — these disable any machine-specific peephole optimizations 👀. The difference between -fno-peephole and -fno-peephole2 is in how they're implemented in the compiler; some targets use one, some use the other, and a few use both. -fpeephole is enabled by default. -fpeephole2 is enabled at levels -O2, -O3, -Os.
• -fno-guess-branch-probability — do not guess branch probabilities using heuristics 🚫. Here's the tea: GCC uses heuristics to guess branch probabilities if they weren't 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 the effects of __builtin_expect are easier to understand 🧠.

It's also possible to specify the expected probability of the expression with the __builtin_expect_with_probability built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

• -freorder-blocks — reorders basic blocks in the compiled function so there are fewer taken branches and better code locality 🎀. Enabled at levels -O, -O2, -O3, -Os.
• -freorder-blocks-algorithm=algorithm — use the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code 🌈.

The default is simple at levels -O, -Os, and stc at levels -O2, -O3.

• -freorder-blocks-and-partition — on top of reordering basic blocks in the compiled function (to reduce the number of taken branches), this partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance 💅.

Important caveat: this optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or a 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 it may be enabled explicitly (if you're using a working linker) 🫶.

Enabled for x86 at levels -O2, -O3, -Os.

• -freorder-functions — reorders functions in the object file to improve code locality ✨. This is implemented using special subsections .text.hot for the most frequently executed functions and .text.unlikely for the unlikely-executed ones. The reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way.

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for the details) or manually annotate functions with hot or cold attributes 👑. Enabled at levels -O2, -O3, -Os.

• -fstrict-aliasing — lets the compiler assume the strictest aliasing rules that apply to the language being compiled 💅. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed to never reside at the same address as an object of a different type — unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type.

Pay special attention to code like this:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (this is called "type-punning") is common, and 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-aliasing option is enabled at levels -O2, -O3, -Os. 💀

• -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 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.

If m is not specified, it defaults to n.

Examples bestie, so you get it: -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 it 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's 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 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 ✨. The 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. (the alignment options lowkey negotiate with each other, bestie — whoever's bigger wins ✨)

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. (yes, that's a hard cap, don't fight it)

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. 💅

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. 🦋

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.

• -funit-at-a-time — okay so this one is literally just vibing here for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

Enabled by default. (it's giving "I'm here but I do nothing" 😭)

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. 🌸

-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.

• -fweb — constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register". 💀

Enabled by default with -funroll-loops.

• -fwhole-program — assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers. 👑

This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information. (fr fr, don't do it)

• -flto[=n] — this option runs the standard link-time optimizer. big main character energy ✨ When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.

To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:

        gcc -c -O2 -flto foo.c
        gcc -c -O2 -flto bar.c
        gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two 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. (obsessed, honestly — they're literally sharing a brain now 🧠💖)

Another (simpler) way to enable link-time optimization is:

        gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

The 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. (so it auto-detects, but you can say "not today" 💅)

To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.

When 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. (faster compile, but no take-backsies on the linking 😬)

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 & match, we love a flexible queen 👑)

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. (tl;dr: set your -O at compile time too, bestie, don't be lazy 🙈)

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: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. (nope, these gotta agree, no negotiation 💀)

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.

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. (just be consistent, okay? consistency is the vibe check here ✨)

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 explicitely repeated at the linker command line and mixing different settings in different translation units is discouraged. (mixing = messy = not cute)

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. 😭

Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

        gcc -c -flto foo.c
        g++ -c -flto bar.cc
        gfortran -c -flto baz.f90
        g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. 🌈 (multilingual icon behavior, we stan 🫶)

Okay so if you've got object files with GIMPLE bytecode chillin' inside a library archive — like, say libfoo.a — you can pull them out and use them in an LTO link, but only if your linker has plugin support, bestie. 💅

To make static libraries that are LTO-ready, use gcc-ar and gcc-ranlib instead of plain ar and ranlib. And if you wanna peek at the symbols inside object files that have GIMPLE bytecode? That's gcc-nm's whole job. ✨

Heads up though — those commands need ar, ranlib and nm to have been compiled with plugin support. No plugin support, no party. 🙈

At link time, throw in the flag -fuse-linker-plugin so your library actually participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin on, the linker yoinks the GIMPLE files it needs out of libfoo.a and hands them over to the running GCC so they become part of the aggregated GIMPLE image that gets optimized. 🫶

But listen — if you're not using a linker with plugin support, and/or you don't enable the linker plugin, then the objects inside libfoo.a get extracted and linked like normal. They just... don't get to be part of the LTO optimization process. 😭 Fomo for your object files, honestly.

So if you want a static library that works for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Both worlds, no cap. 👑

Link-time optimizations don't need your whole program present to do their thing. If your program doesn't require any symbols to be exported, you can combine -flto and -fwhole-program so the interprocedural optimizers get to use more aggressive assumptions — which may lead to improved optimization opportunities. 🔥

But note: you don't need -fwhole-program when the linker plugin is active (see -fuse-linker-plugin). It's already doing the thing.

The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. Periodt. It's giving exclusive. 💀

Also, link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. Sorry to those systems. 🌸

---

If you specify the optional n, the optimization and code generation done at link time runs in parallel using n parallel jobs, by utilizing an installed make program. And the environment variable MAKE can be used to override which program gets used. 🧚

You can also specify -flto=jobserver to use GNU make's job server mode to figure out 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. And this option likely only works if MAKE is GNU make. iykyk.

Use -flto=auto to use GNU make's job server if it's available, or otherwise fall back to autodetecting the number of CPU threads present in your system. Auto-pilot energy. 🌈

---

• -flto-partition=alg — Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create a new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase. 🎀
• -flto-odr-type-merging — Enable streaming of mangled types names of C++ types and their unification at link time. This increases the size of LTO object files, but enables diagnostics about One Definition Rule violations. Bigger files, better receipts. 🔍
• -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). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option isn't given, a default balanced compression setting is used. ✨
• -fuse-linker-plugin — Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. 💜

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer, or gold). Default-on, as a treat. 👑

• -ffat-lto-objects — Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them. 😤

Note that modern binutils provide a 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. Sneaky efficient. 💅

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

Enabled at levels -O, -O2, -O3, -Os.

• -fcprop-registers — After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

Enabled at levels -O, -O2, -O3, -Os.

• -fprofile-correction — Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. 😭

This option is enabled by -fauto-profile.

• -fprofile-use
• -fprofile-use=path — Enable profile feedback-directed optimizations, and 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-functions

Before you can use this option, you must first generate profiling information. No profile, no feedback. ✨

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, and 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-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory. 🎀

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

New vibe checkpoint, besties! 🌈 The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. And every single one of them must be specifically enabled — nothing here is coming along for the ride on its own. 🦋

• -ffloat-store — Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory. 🍓

(still riding with -ffloat-store btw, we didn't forget her 💅) this one blocks that undesirable excess precision on machines like the 68000, where the floating registers (the 68881 ones, iconic) keep more precision than a double is supposed to have. same situation on x86. for most programs that extra precision is doing nothing but good, but a few lil programs are out here relying on the precise definition of IEEE floating point. so for those, use -ffloat-store — after you've modified them to store all pertinent intermediate computations into variables. 🎀

---

• -fexcess-precision=style — this one gives you further control over excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types. by default, -fexcess-precision=fast is in effect; that means operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. when compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). this option is enabled by default for C if a strict conformance option such as -std=c99 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used.

heads up bestie: -fexcess-precision=standard is not implemented for languages other than C. on the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 😭

• -ffast-math — sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast. 💥

this option causes the preprocessor macro __FAST_MATH__ to be defined.

this option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. it may, however, yield faster code for programs that do not require the guarantees of these specifications. ⚡

• -fno-math-errno — do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt. a program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility. 🏃‍♀️

this option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. it may, however, yield faster code for programs that do not require the guarantees of these specifications.

the default is -fmath-errno.

on Darwin systems, the math library never sets errno. there is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default. 🍓

• -funsafe-math-optimizations — allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. when used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. 🙈

this option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. it may, however, yield faster code for programs that do not require the guarantees of these specifications. enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

the default is -fno-unsafe-math-optimizations.

• -fassociative-math — allow re-association of operands in series of floating-point operations. this violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). may also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. this option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. moreover, it doesn't make much sense with -frounding-math. for Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. 🦋

the default is -fno-associative-math.

• -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. for example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. note that this loses precision and increases the number of flops operating on the value. 💀

the default is -fno-reciprocal-math.

• -ffinite-math-only — allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. 🚫

this option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. it may, however, yield faster code for programs that do not require the guarantees of these specifications.

the default is -fno-finite-math-only.

• -fno-signed-zeros — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). this option implies that the sign of a zero result isn't significant. 🫶

the default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps. these traps include division by zero, overflow, underflow, inexact result and invalid operation. this option requires that -fno-signaling-nans be in effect. setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example. 🪤

this option should never be turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

the default is -ftrapping-math.

• -frounding-math — disable transformations and optimizations that assume default floating-point rounding behavior. this is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. this option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. this option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. 🔄

the default is -fno-rounding-math.

this option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. this command-line option will be used to specify the default state for FENV_ACCESS. 🙈

• -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. this option implies -ftrapping-math. ✨

this option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

the default is -fno-signaling-nans.

this option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

• -fno-fp-int-builtin-inexact — do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, does not allow these functions to do so. 📐

the default is -ffp-int-builtin-inexact, allowing the exception to be raised. this option does nothing unless -ftrapping-math is in effect.

even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661. 😭

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. 🎯
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. the default is -fno-cx-limited-range, but is enabled by -ffast-math. 🌈

this option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. nevertheless, the option applies to all languages.

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. 🧚

the default is -fno-cx-fortran-rules.

---

okay so the following options control optimizations that may improve performance, but are not enabled by any -O options. this section includes experimental options that may produce broken code. 💀 you've been warned, bestie.

• -fbranch-probabilities — okay so bestie, here's the tea ☕: after you've run a program you compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, and it'll level up the optimizations based on how many times each branch actually got taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. Important though: the info in that data file is super 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 switching up your fit between takes 💅

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. fr fr, no more guessing games 🎯

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 is gathered. ✨

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 collects first time of execution of a function and orders these functions in ascending order. it's giving organized 🗂️

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option tells 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. 🔥

Enabled with -fprofile-use and -fauto-profile.

• -frename-registers — attempts 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. But heads up: depending on the debug information format adopted by the target, it can make debugging impossible, since variables no longer stay in a "home register". 😭

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. matching outfits, but for instructions 👯

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. main character optimization energy tbh 👑

Enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. the duality of compiler flags 💀

Enabled by -fprofile-use and -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-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). 🍊

Enabled by -O3, -fprofile-use, and -fauto-profile.

• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and 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. it's giving divide and conquer ✂️

Enabled by -fprofile-use and -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-use and -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. everyone gets their own little room 🏠

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.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. 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. so like, know what you're doing bestie 🙈

• -fbranch-target-load-optimize — perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.
• -fbranch-target-load-optimize2 — perform branch target register load optimization after prologue / epilogue threading. (yes bestie, order matters 💅)
• -fbtr-bb-exclusive — when performing branch target register load optimization, don't reuse branch target registers within any basic block.
• -fstdarg-opt — optimize the prologue of variadic argument functions with respect to usage of those arguments.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.

• -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. 🫶

• --param name=value — in some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. ⚠️ so like, don't get too attached

In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q options.

In each case, the value is an integer. The allowable choices for name are:

• predictable-branch-outcome — when branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.
• 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.
• max-rtl-if-conversion-predictable-cost
• max-rtl-if-conversion-unpredictable-cost — RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro.

• max-crossjump-edges — the max number of incoming edges we even bother looking at for cross-jumping ✨. The algorithm behind -fcrossjumping is O(N^2) in the number of edges coming into each block, so yeah — crank this up and you get more aggressive optimization, but compilation time goes up too, and the payoff in executable size is probably tiny. iykyk.
• min-crossjump-insns — the minimum number of instructions that have to match at the end of two blocks before we cross-jump them 💅. Fun lil exception: this value is ignored when all instructions in the block being cross-jumped from are matched.
• max-grow-copy-bb-insns — the max code-size expansion factor when we copy basic blocks instead of jumping 🫶. That expansion is measured relative to a jump instruction.
• max-goto-duplication-insns — the max number of instructions to duplicate into a block that jumps to a computed goto 👀. So here's the tea: to dodge O(N^2) behavior in a bunch of passes, GCC factors computed gotos early in the compilation process and unfactors them as late as possible. Only computed jumps sitting at the end of a basic block with no more than max-goto-duplication-insns get unfactored.
• max-delay-slot-insn-search — the max number of instructions to look at when hunting for an instruction to fill a delay slot 🌸. If we've searched past this arbitrary number, the time savings from filling the delay slot are minimal, so we stop searching, periodt. Bigger values = more aggressive optimization, more compile time, and probably just a tiny improvement in execution time.
• max-delay-slot-live-search — when trying to fill delay slots, this is the max number of instructions to consider while searching for a block with valid live register information 🦋. Increasing this (arbitrarily chosen!) value means more aggressive optimization and more compilation time. Also, straight from the source: this parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. not me reading dev notes 💀.
• max-gcse-memory — the approximate max amount of memory we're allowed to allocate for the global common subexpression elimination optimization 💖. If it needs more memory than specified, the optimization just... doesn't happen. sorry bestie.
• 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 max number of pending dependencies scheduling allows before it flushes the current state and starts over 🔥. Large functions with few branches or calls can create excessively large lists, which needlessly eat memory and resources. lowkey wasteful fr.
• max-modulo-backtrack-attempts — the max number of backtrack attempts the scheduler should make when modulo scheduling a loop 💅. Larger values can exponentially increase compilation time. be so serious.
• max-inline-insns-single — okay so several parameters control the tree inliner in GCC 🌈. This number sets the max number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. It 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 whole lot of functions that would otherwise never be considered for inlining get investigated 👑. For those functions, a different (and more restrictive) limit compared to functions declared inline can be applied.
• 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 🌸. A little growth can be desirable to anticipate optimization opportunities exposed by inlining.
• uninlined-function-insns — number of instructions the inliner accounts for function overhead, like the function prologue and epilogue 🫶.
• uninlined-function-time — extra time the inliner accounts for function overhead, like the time needed to execute the function prologue and epilogue 💜.
• uninlined-thunk-insns, uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time, but applied to function thunks ✨.
• inline-min-speedup — when the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto 💖.
• large-function-insns — the limit that specifies 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 the non-linear algorithms used by the back end. it's giving damage control.
• large-function-growth — specifies the maximal growth of a large function 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 a 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 the unit is 300% and yet such inlining is very sane 💅. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth.
• inline-unit-growth — specifies the 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. we love an exemption.
• ipcp-unit-growth — specifies the 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.
• large-stack-frame — the limit specifying large stack frames 🌸. While inlining, the algorithm is trying not to grow past this limit too much.
• large-stack-frame-growth — specifies the 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. iconic numbers honestly.
• max-inline-insns-recursive, max-inline-insns-recursive-auto — specifies the max number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining 💜. --param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead.
• max-inline-recursive-depth, max-inline-recursive-depth-auto — specifies the max recursion depth used for recursive inlining 🦋. --param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead.
• min-inline-recursive-probability — recursive inlining is profitable only for a function having deep recursion on average, and it can actually hurt for a function with little recursion depth, by increasing the prologue size or the complexity of the function body for other optimizers 💀. When profile feedback is available (see -fprofile-generate), the actual recursion depth can be guessed from the probability that the function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents) ✨.
• early-inlining-insns — specify the growth that the early inliner can make 🌸. In effect, it increases the amount of inlining for code having a large abstraction penalty.
• max-early-inliner-iterations — the limit of iterations of the early inliner 🎀. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.
• comdat-sharing-probability — probability (in percent) that a C++ inline function with comdat visibility is shared across multiple compilation units 💖.
• profile-func-internal-id — a parameter to control whether to use the function internal id in profile database lookup 👀. If the value is 0, the compiler uses an id that's based on the 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-vectorize is used 🔥. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.
• gcse-cost-distance-ratio — the scaling factor used when GCSE optimizations figure out the max distance an expression is allowed to travel. Currently only supported in the code hoisting pass, bestie. The bigger this ratio, the more feral code hoisting gets with simple expressions — aka the ones whose cost is less than gcse-unrestricted-cost. Set it to 0 and hoisting of simple expressions just... stops. 💅
• gcse-unrestricted-cost — the cost (roughly, the cost of one typical machine instruction) at which GCSE optimizations stop constraining how far an expression can travel. Currently only supported in the code hoisting pass. Lower cost = more aggressive hoisting. Set it to 0 and literally every expression gets to travel unrestricted distances. 🦋
• max-hoist-depth — how deep into the dominator tree we go searching for expressions to hoist. It exists to dodge quadratic behavior in the hoisting algorithm. Set it to 0 and the search isn't limited at all — but heads up, that may slow down compilation of huge functions. 😭
• max-tail-merge-comparisons — the maximum number of similar bbs to compare a bb against. This one's here to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — the max number of iterations of the pass over the function. It's a compilation-time limiter for tree tail merging. ⏳
• store-merging-allow-unaligned — lets the store merging pass introduce unaligned stores, if it's legal to do so. That's the whole vibe. ✨
• max-stores-to-merge — the maximum number of stores the store merging pass will attempt to merge into wider stores.
• max-unrolled-insns — the max number of instructions a loop can have and still be unrolled. And if a loop does get unrolled, this parameter also decides how many times the loop code is unrolled. Two jobs, one param, main character energy. 👑
• max-average-unrolled-insns — same idea as above but the instruction count is biased by the probabilities of their execution. The max number of those a loop can have to be unrolled, and again, if the loop gets unrolled, this also determines how many times the loop code is unrolled. 💖
• max-unroll-times — the max number of unrollings of a single loop. That's it, that's the whole thing.
• max-peeled-insns — the max number of instructions a loop can have to be peeled. And yes, if a loop gets peeled, this parameter also determines how many times the loop code is peeled. 🍓
• max-peel-times — the max number of peelings of a single loop.
• max-peel-branches — the max number of branches on the hot path through the peeled sequence. 🔥
• max-completely-peeled-insns — the max number of insns of a completely peeled loop.
• max-completely-peel-times — the max number of iterations of a loop for it to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the max depth of a loop nest that's suitable for complete peeling.
• max-unswitch-insns — the max number of insns of an unswitched loop.
• max-unswitch-level — the max number of branches unswitched in a single loop. 🌈
• lim-expensive — the minimum cost of an expensive expression in loop invariant motion.
• iv-consider-all-candidates-bound — a bound on the number of induction variable candidates. Below it, all candidates get considered for every use in induction variable optimizations. Above it, only the most relevant ones get considered — purely to avoid quadratic time complexity. ⚡
• iv-max-considered-uses — the induction variable optimizations just give up on loops that contain more induction variable uses than this.
• iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.
• avg-loop-niter — the average number of iterations of a loop.
• dse-max-object-size — the max size, in bytes, of objects tracked bytewise by dead store elimination. Bigger values may result in larger compilation times. 😬
• dse-max-alias-queries-per-store — the max number of queries into the alias oracle per store. Bigger values mean larger compilation times <em>and</em> may result in more removed dead stores. Trade-offs, bestie. ⚖️
• scev-max-expr-size — a bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer down. 🐌
• scev-max-expr-complexity — a bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer down. Same energy, different metric.
• max-tree-if-conversion-phi-args — the max number of arguments in a PHI that TREE if conversion supports — unless the loop is marked with simd pragma.
• vect-max-version-for-alignment-checks — the max 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 max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the max number of loop peels to enhance access alignment for the vectorizer. A value of -1 means no limit. 🎀
• max-iterations-to-track — the max number of iterations of a loop that the brute-force algorithm tries to evaluate when analyzing the number of iterations of the loop. (Say that five times fast. 😭)
• hot-bb-count-ws-permille — a basic block profile count counts as hot if it contributes to the given permillage (i.e. 0...1000) of the entire profiled execution. 🔥
• hot-bb-frequency-fraction — picks the fraction of the entry block frequency of executions of a basic block in a function that the given basic block needs to have to be considered hot.
• max-predicted-iterations — the max number of loop iterations we predict statically. This is useful when a function contains a single loop with a known bound and another loop with an unknown bound: the known number of iterations gets predicted correctly, while the unknown number of iterations averages out to roughly 10. Which means the loop without bounds appears artificially cold relative to the other one. 💀
• builtin-expect-probability — controls the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input.
• builtin-string-cmp-inline-length — the max length of a constant string for a builtin string cmp call to be eligible for inlining.
• align-threshold — selects the fraction of the maximal frequency of executions of a basic block in a function needed to align that basic block.
• align-loop-iterations — a loop expected to iterate at least the selected number of iterations gets aligned. ✨
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value limits superblock formation once the given percentage of executed instructions is covered, which limits unnecessary code size expansion. 📏

The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available — real profiles (as opposed to statically estimated ones) are much less balanced, which lets the threshold be a larger value.

• tracer-max-code-growth — stops tail duplication once code growth hits the given percentage. Ngl this is a rather artificial limit, since most of the duplicates get eliminated later in cross jumping — so it may be set to <em>much higher</em> values than the code growth you actually want. 🙈
• tracer-min-branch-ratio — stops reverse growth when the reverse probability of the best edge is less than 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. 🛑

Just like tracer-dynamic-coverage, two parameters are provided: tracer-min-branch-probability-feedback is used for compilation with profile feedback, and tracer-min-branch-probability for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) to make tracer effective.

• stack-clash-protection-guard-size — specifies the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 😱
• stack-clash-protection-probe-interval — stack clash protection involves probing stack space as it's allocated. This param controls the max distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 💀
• max-cse-path-length — the max number of basic blocks on a path that CSE considers.
• max-cse-insns — the max number of instructions CSE processes before flushing.
• ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation, and this parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning it may improve compilation speed; it has no effect on code generation. 🗑️

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC isn't able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity — this is extremely slow, but it can be useful for debugging. 🐢

ggc-min-heapsize 🌸

okay so this one is the minimum size of the garbage collector's heap before it even bothers to start collecting garbage. like, it's not gonna lift a finger until the heap has grown enough. the first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. and yeah, tuning this can speed up compilation, and it has no effect on code generation at all — it's purely a compiler-speed vibe thing, not a "your binary changes" thing.

the default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS don't get exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). if GCC can't figure out your RAM on a particular platform, the lower bound is what you get. setting this parameter very large effectively disables garbage collection (not me casually turning off the GC 💀). and setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. slay, I guess.

max-reload-search-insns ✨

the maximum number of instructions reload should look backward for an equivalent register. bigger values mean more aggressive optimization, so compilation time goes up but you probably get slightly better performance. it's giving "pay with time, maybe get speed."

max-cselib-memory-locations 💅

the maximum number of memory locations cselib should take into account. again, increasing values mean more aggressive optimization, compilation time increases, and you probably get slightly better performance. same energy, different parameter.

max-sched-ready-insns 🦋

the maximum number of instructions that are ready to be issued that the scheduler should consider at any given time during the first scheduling pass. increasing values mean more thorough searches, so compilation time increases with probably little benefit. bestie, don't touch this one unless you're feeling spicy.

max-sched-region-blocks 🎀

the maximum number of blocks in a region to be considered for interblock scheduling. that's it, that's the whole vibe.

max-pipeline-region-blocks 🎀

the maximum number of blocks in a region to be considered for pipelining in the selective scheduler. sibling energy to the one above, different pass.

max-sched-region-insns 🦋

the maximum number of insns in a region to be considered for interblock scheduling.

max-pipeline-region-insns 🦋

the maximum number of insns in a region to be considered for pipelining in the selective scheduler.

min-spec-prob ⭐

the minimum probability, in percents, of reaching a source block for interblock speculative scheduling. percentages, so don't go typing 0.5, iykyk.

max-sched-extend-regions-iters 🌈

the maximum number of iterations through CFG to extend regions. and listen — a value of 0 disables region extensions entirely. zero means nope, none, not happening.

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 are scheduled. again with the percents, keep up bestie.

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. it's a depth of search for available instructions. think of it as how far ahead the scheduler is allowed to peek.

selsched-max-sched-times 🔥

the maximum number of times that an instruction is scheduled during selective scheduling. this is the limit on the number of iterations through which the instruction may be pipelined.

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 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.

integer-share-limit 💖

small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed. this parameter sets the maximum value of a shared integer constant. cute, efficient, iconic.

ssp-buffer-size 🛡️

the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used.

this default before Ubuntu 10.10 was "8". currently it is "4", to increase the number of functions protected by the stack protector. so yeah, they lowered it on purpose to protect more stuff. the more you know 🌈

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-fields-for-field-sensitive 🔍

maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.

prefetch-latency ⚡

an estimate of the average number of instructions that are executed before a prefetch finishes. the distance prefetched ahead is proportional to this constant. heads up though — increasing this number may also lead to less streams being prefetched, see simultaneous-prefetches.

simultaneous-prefetches ⚡

maximum number of prefetches that can run at the same time.

l1-cache-line-size 🎀

the size of a cache line in L1 data cache, in bytes.

l1-cache-size 💾

the size of L1 data cache, in kilobytes.

l2-cache-size 💾

the size of L2 data cache, in kilobytes. yes bestie, kilobytes, not bytes. don't mix them up 🙈

prefetch-dynamic-strides 🌈

whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. in some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is a clear benefit to issuing these hints.

set to 1 if the prefetch hints should be issued for non-constant strides. set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride.

prefetch-minimum-stride 📏

minimum constant stride, in bytes, to start using prefetch hints for. if the stride is less than this threshold, prefetch hints will not be issued.

this setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. if the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers.

a value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant stride.

this setting is only useful for strides that are known and constant. periodt. 💅

loop-interchange-max-num-stmts 🔁

the maximum number of stmts in a loop to be interchanged.

loop-interchange-stride-ratio 🔁

the minimum ratio between the stride of two loops for interchange to be profitable.

min-insn-to-prefetch-ratio ⚖️

the minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.

prefetch-min-insn-to-mem-ratio ⚖️

the minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.

use-canonical-types 👑

whether the compiler should use the "canonical" type system. this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. however — and this is the important part — if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. emergency escape hatch energy fr fr.

switch-conversion-max-branch-ratio 🔀

switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.

max-partial-antic-length 😭

maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. for some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. yeah. all of it. this parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. setting a value of 0 for this parameter allows an unlimited set length — which is exactly as chaotic as it sounds.

rpo-vn-max-loop-depth 🌀

maximum loop depth that is value-numbered optimistically. when the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones are not.

sccvn-max-alias-queries-per-access 🔎

maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. if this limit is hit, the search is aborted and the load or store is not considered redundant. the number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.

ira-max-loops-num 💜

IRA uses regional register allocation by default. if a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation.

ira-max-conflict-table-size 💀

so IRA uses a sophisticated algorithm to compress the conflict table, but even with all that, the table can still require excessive amounts of memory for huge functions. if the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. tradeoff city, but at least it doesn't eat your RAM 🫶

---

• ira-loop-reserved-regs — bestie, IRA can vibe-check register pressure way more accurately inside loops when it's deciding whether to hoist loop invariants out (peep -O3 for that whole situation ✨). This param tells it how many available registers get reserved for other stuff. The default? Found from literally so many experiments, it's the best one they could find. 💅
• lra-inheritance-ebb-probability-cutoff — LRA is out here trying to reuse values that got reloaded into registers in the insns that come after, and that whole glow-up is called inheritance. EBB is the region where it does the thing 🦋. This param sets a minimum fall-through edge probability, in percent, that a BB needs to get added to the inheritance EBB in LRA. Default value was picked from numerous runs of SPEC2000 on x86-64. no cap.
• loop-invariant-max-bbs-in-loop — Loop invariant motion can be SO expensive, both in compilation time and in the compile-time memory it eats, when loops are huge 😭. Loops with more basic blocks than this param? They just don't get loop invariant motion optimization performed on them. not invited to the party, sorry.
• loop-max-datarefs-for-datadeps — Building data dependencies is expensive for very large loops, fr fr. This param caps the number of data references in loops that even get considered for data dependence analysis. Those giant loops are just not handled by the optimizations that use loop data dependencies. 🫶
• max-vartrack-size — Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit gets exceeded while variable tracking at assignments is enabled, the analysis for that function is retried without it — after removing all debug insns from the function. And if the limit is exceeded even without debug insns? Then var tracking analysis is completely disabled for that function 💀. Setting the parameter to zero makes it unlimited. iconic.
• max-vartrack-expr-depth — Sets a maximum number of recursion levels when it's trying to map variable names or debug temporaries to value expressions. It's a trade: compilation time in exchange for more complete debug information ✨. Set it too low and value expressions that are actually available and could be represented in debug info may end up not being used 😭. 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 (like begin stmt markers) so things don't complexity-explode at inlining or expanding to RTL 🎀. If a function has more of these gimple stmts than the set limit, those stmts get dropped from the inlined copy of a function, and from its RTL expansion.
• min-nondebug-insn-uid — Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted. 👑
• ipa-sra-ptr-growth-factor — IPA-SRA will replace a pointer to an aggregate with one or more new parameters, but only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. the way that it does math on ur pointers 💅
• sra-max-scalarization-size-Ospeed and sra-max-scalarization-size-Osize — the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are aiming to replace scalar parts of aggregates with uses of independent scalar variables 🦋. These parameters control the maximum size, in storage units, of an aggregate that's 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 maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, so it can facilitate copy propagation. lowkey a whole tracking situation.
• tm-max-aggregate-size — When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions, instead of save/restore code sequence pairs 🌸. This option only applies when using -fgnu-tm.
• graphite-max-nb-scop-params — To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound entirely 🙈. And fyi, a variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.
• loop-block-tile-size — Loop blocking or strip mining transforms, which get enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter ✨.
• ipa-cp-value-list-size — IPA-CP is out here attempting to track all possible values and types passed to a function's parameter, so it can propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. 💖
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. gatekept by a number, periodt.
• ipa-cp-recursion-penalty — Percentage penalty the recursive functions will receive when they are evaluated for cloning. 😭 recursive besties pay a tax.
• ipa-cp-single-call-penalty — Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. yeah. one call and you're getting docked points.
• ipa-max-agg-items — IPA-CP is also capable of propagating a number of scalar values passed in an aggregate 🫶. ipa-max-agg-items controls the maximum number of such values per one parameter.
• ipa-cp-loop-hint-bonus — When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate. bonus points for knowing the loop count 🔥
• ipa-cp-array-index-hint-bonus — When IPA-CP determines that a cloning candidate would make the index of an array access known, it adds a bonus of ipa-cp-array-index-hint-bonus to the profitability score of the candidate. 👑
• ipa-max-aa-steps — During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters. So it doesn't spend too much time analyzing huge functions, it gives up and considers all memory clobbered after examining ipa-max-aa-steps statements modifying memory. 💀 it literally quits.
• lto-partitions — Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation ✨.
• lto-min-partition — Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions. nobody wants a thousand tiny partitions 🎀.
• lto-max-partition — Size of max partition for WHOPR (in estimated instructions), to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning. 🌸
• lto-max-streaming-parallelism — Maximal number of parallel processes used for LTO streaming. parallel besties go brrr.
• cxx-max-namespaces-for-diagnostic-help — The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. it's the "did you mean" but for namespaces 💅
• sink-frequency-threshold — The maximum relative execution frequency (in percents) of the target block relative to a statement's original block, to allow statement sinking of a statement 🦋. Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands, as those are even more profitable to sink.
• max-stores-to-sink — The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled ⭐.
• allow-store-data-races — Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. > bestie this one is a deliberate "yes let it race" switch, so don't just flip it for funsies 😭
• case-values-threshold — The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine 🌈.
• tree-reassoc-width — Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if it has a non zero value. 🔥
• sched-pressure-algorithm — Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler 👑. It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.c in the GCC sources for more details.

The default choice depends on the target. ✨

• max-slsr-cand-scan — Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. 🎀
• asan-globals — Enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using the -fsanitize=address option. To disable global objects protection use --param asan-globals=0. 🦋
• asan-stack — Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using -fsanitize=address. To disable stack protection use --param asan-stack=0 option. 💖
• asan-instrument-reads — bestie this one turns ON buffer overflow detection for memory reads 👀 it's already on by default the second you use -fsanitize=address. wanna turn the read protection off? then hit it with --param asan-instrument-reads=0 🫶
• asan-instrument-writes — same energy but for memory writes ✍️ also enabled by default when you're using -fsanitize=address. to kill the write protection, use the --param asan-instrument-writes=0 option 💅
• asan-memintrin — this enables detection for built-in functions 🧚 it's on by default when you're using -fsanitize=address. want the built-in function protection gone? --param asan-memintrin=0 and you're set ✨
• asan-use-after-return — enables detection of use-after-return 🌸 on by default when you're using the -fsanitize=address option. to disable it, --param asan-use-after-return=0 😌

Note: by default the check is disabled at run time. to actually enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS ⚠️ like, it's compiled in but asleep, okay?

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than or equal to this number, callbacks get used instead of inline checks 🔁 e.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0 💫
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller or equal to this number, we directly poison (or unpoison) shadow memory instead of using run-time callbacks 🧪
• max-fsm-thread-path-insns — the max number of instructions to copy when duplicating blocks on a finite state automaton jump thread path 📏
• max-fsm-thread-length — the max number of basic blocks on a finite state automaton jump thread path 🧱
• max-fsm-thread-paths — the max number of new jump thread paths to create for a finite state automaton 🛤️
• parloops-chunk-size — the chunk size of omp schedule for loops parallelized by parloops 🍰
• parloops-schedule — the 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 👑 heads up tho: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two ✌️
• max-ssa-name-query-depth — the max depth of recursion when querying properties of SSA names in things like fold routines 🌀 one level of recursion = following a use-def chain, iykyk
• hsa-gen-debug-stores — enables emission of special debug stores within HSA kernels which are then read and reported by the libgomp plugin 🔍 generation of these stores is disabled by default, so use --param hsa-gen-debug-stores=1 to turn it on ✨
• max-speculative-devirt-maydefs — the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively 🕵️
• max-vrp-switch-assertions — the maximum number of assertions to add along the default edge of a switch statement during VRP 🎯
• unroll-jam-min-percent — the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to even 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-unroller is used, this is the maximum number of times that an individual variable will be expanded during loop unrolling 🪄
• tracer-min-branch-probability-feedback — stop forward growth if the probability of the best edge is less than this threshold (in percent) 🛑 used when profile feedback is available 📊
• 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 — enables loop epilogue vectorization using smaller vector size 🐜
• slp-max-insns-in-bb — maximum number of instructions in basic block to be considered for SLP vectorization 🧮
• avoid-fma-max-bits — maximum number of bits for which we avoid creating FMAs 🚫
• 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 🔎
• hot-bb-count-fraction — select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot (used in non-LTO mode) 🌡️
• max-inline-insns-recursive-auto — the maximum number of instructions a non-inline function can grow to via recursive inlining 📈
• 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 — maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant 🧮
• max-dse-active-local-stores — maximum number of active local stores in RTL dead store elimination 🗑️
• asan-instrument-allocas — enable asan allocas/VLAs protection 🛡️
• max-iterations-computation-cost — bound on the cost of an expression to compute the number of iterations 🧾
• max-isl-operations — maximum number of isl operations, 0 means unlimited ♾️
• graphite-max-arrays-per-scop — maximum number of arrays per scop 📚
• max-vartrack-reverse-op-size — max. size of loc list for which reverse ops should be added 🔙
• unlikely-bb-count-fraction — the minimum fraction of profile runs a given basic block execution count must be not to be considered unlikely 🙅
• tracer-dynamic-coverage-feedback — the percentage of function, weighted by execution frequency, that must be covered by trace formation 📐 used when profile feedback is available 📊
• max-inline-recursive-depth-auto — the maximum depth of recursive inlining for non-inline functions 🕳️
• fsm-scale-path-stmts — scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks ⚖️
• fsm-maximum-phi-arguments — maximum number of arguments a PHI may have before the FSM threader will not try to thread through its block 🚧
• uninit-control-dep-attempts — maximum number of nested calls to search for control dependencies during uninitialized variable analysis 🧐
• indir-call-topn-profile — track top N target addresses in indirect-call profile 📊
• max-once-peeled-insns — the maximum number of insns of a peeled loop that rolls only once 🍌
• sra-max-scalarization-size-Osize — maximum size, in storage units, of an aggregate which should be considered for scalarization when compiling for size 📦
• fsm-scale-path-blocks — scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements ⚖️
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag 🎚️ number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic 👀
• 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 🐘
• 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 🐘
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments 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 🚦

---

🎀 Program Instrumentation Options 🎀

okay bestie, so GCC supports a whole bunch of command-line options that control adding run-time instrumentation to the code it normally generates 💖 like, one purpose of instrumentation is to collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations 📈 another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking 😈 and there's also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes 🪝 slay 🫶

• -p, -pg — okay so these two besties generate extra code that writes out profile info, the kind the analysis programs prof (that's -p) or gprof (that's -pg) are literally built to eat up. 💖 But listen: you must use this option when you compile the source files you want data about, and you also have to use it when linking. Both times, no skipping. And if there's one lil function you want to leave out of the whole profiling thing? You can slap the function attribute no_instrument_function on it to suppress profiling of individual functions while compiling with these options. 🎀
• -fprofile-arcs — adds code so that program flow arcs get instrumented. While the program runs, it 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, the 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 all this data to a file called auxname.gcda for each source file. That data can be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname gets generated from the name of the output file, if it's explicitly specified and it's not the final executable; otherwise it's the basename of the source file. Either way, any suffix gets removed (so like foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). 💅

• --coverage — this one's your girl for compiling and linking code that's instrumented for coverage analysis. She's a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). Check the docs for those options for the deets. ✨

Here's the whole recipe, step by step, no cap:

• Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
• Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations happen with different working directories.
• Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
• Run the program on a representative workload to generate the arc profile information. You can repeat this any number of times, fr fr. You can even run concurrent instances of your program, and as long as the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting. 👑
• For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
• For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for more.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only the arcs that are not on the spanning tree have to be instrumented — the compiler adds code to count the number of times those arcs get executed. When an arc is the only exit or only entrance to a block, the instrumentation code can just be added to the block; otherwise, a whole new basic block has to be created to hold the instrumentation code. 🦋

• -ftest-coverage — produces a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Peep the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Fun fact: coverage data matches the source files way more closely if you do not optimize. 😌
• -fprofile-abs-path — automatically converts relative source file names to absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations occur with different working directories. 🌈
• -fprofile-dir=path — sets the directory to search for the profile data files to path. This option only affects the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths work. By default, GCC uses the current directory as path, so the profile data file shows up in the same directory as the object file. To prevent file name clashing, if the object file name isn't an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. 🎀

When an executable is run in a massive parallel environment, it's recommended to save the profile to different folders. You can do that with variables in path that get exported during run-time:

• %p — process ID.
• %q{VAR} — value of environment variable VAR

• -fprofile-generate, -fprofile-generate=path — enables the options usually used for instrumenting an application to produce a profile that's useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. 💖

The following options are enabled: -fprofile-arcs, -fprofile-values, -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-update=method — changes up the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. 🫶

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.

• -fprofile-filter-files=regex — instruments only functions from files where names match any regular expression (separated by a semi-colon). 🍓

For example, -fprofile-filter-files=main.c;module.*.c will instrument only main.c and all C files starting with 'module'.

• -fprofile-exclude-files=regex — instruments only functions from files where names do not match all the regular expressions (separated by a semi-colon). 🧚

For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are located in the /usr/ folder.

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When it's set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. 👑 This option cannot be combined with -fsanitize=thread.
• -fsanitize=kernel-address — enables AddressSanitizer for the Linux kernel. See https://github.com/google/kasan/wiki for more details. 💜
• -fsanitize=pointer-compare — instruments comparison operations (<, <=, >, >=) with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. 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 variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. 🙈
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. 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 variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. 🌸
• -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. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. This option cannot be combined with -fsanitize=address, -fsanitize=leak. 🔥

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, and the executable gets linked against a library that overrides malloc and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. 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. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. ⭐ The current suboptions are:
• -fsanitize=shift — this one checks that the result of a shift operation isn't undefined ✨. Heads up though, what counts as "undefined" is a little different between C and C++, and also between ISO C90 and C99 and so on. This option has two suboptions: -fsanitize=shift-base and -fsanitize=shift-exponent.
• -fsanitize=shift-exponent — checks that the second argument of a shift operation isn't negative, and that it's smaller than the precision of the promoted first argument 💅.
• -fsanitize=shift-base — if the second argument of a shift operation is within range, this checks that the result of the shift isn't undefined. Again, "undefined" is defined slightly differently between C and C++, and between ISO C90 and C99, etc. etc. 🌸
• -fsanitize=integer-divide-by-zero — catches integer division by zero, plus INT_MIN / -1 division 💀.
• -fsanitize=unreachable — with this, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. And bestie, when you actually reach that __builtin_unreachable call, the behavior is undefined. No cap.
• -fsanitize=vla-bound — tells the compiler to check that the size of a variable length array is positive ✨.
• -fsanitize=null — pointer checking, periodt. An application built with this on will throw an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) gets bound to a NULL pointer, or if a method is invoked on an object that's pointed to by a NULL pointer 🫶.
• -fsanitize=return — return statement checking. Programs built with this on will throw an error message when the end of a non-void function is reached without actually returning a value. This one only works in C++ 💜.
• -fsanitize=signed-integer-overflow — signed integer overflow checking. We check that the result of +, *, and both unary and binary - doesn't overflow in signed arithmetic. Note: integer promotion rules must be taken into account, meaning the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — instruments array bounds. Various out-of-bounds accesses get detected 👀. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.
• -fsanitize=bounds-strict — strict array bounds instrumentation. Most out-of-bounds accesses get detected, including flexible array members and flexible array member-like arrays 🔥. Initializers of variables with static storage are still not instrumented.
• -fsanitize=alignment — checks pointer alignment when pointers 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 — instruments memory references using the __builtin_object_size function. Various out-of-bounds pointer accesses get detected ✨.
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can legitimately be how you get infinities and NaNs 🌈.
• -fsanitize=float-cast-overflow — floating-point type to integer conversion checking. We check that the result of the conversion doesn't overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also, this option doesn't work well with FE_INVALID exceptions enabled 🥲.
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values are passed to arguments marked as requiring a non-null value by the nonnull function attribute 👑.
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect null values being returned from those functions.
• -fsanitize=bool — instruments loads from bool. If a value other than 0/1 is loaded, you get a run-time error 💀.
• -fsanitize=enum — instruments loads from an enum type. If a value outside the range of values for that enum type is loaded, run-time error ✨.
• -fsanitize=vptr — instruments C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type 🦋.
• -fsanitize=pointer-overflow — instruments pointer arithmetic. If the pointer arithmetic overflows, run-time error 🔥.
• -fsanitize=builtin — instruments arguments to selected builtin functions. If an invalid value is passed to such arguments, run-time error. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and gets diagnosed by this option 💜.

So real talk: -ftrapv causes traps for signed overflows to be emitted, whereas -fsanitize=undefined gives a diagnostic message instead. This currently works only for the C family of languages.

• -fno-sanitize=all — disables all previously enabled sanitizers. Note that -fsanitize=all is not allowed, because some sanitizers can't be used together 🙈.
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer ✨.
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. Each si may contain wildcards 🎀.
• -fsanitize-recover[=opts] — controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this for a sanitizer component makes it attempt to keep running the program as if no error happened. That means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 😭. The -fno-sanitize-recover= option can change this behavior: only the first detected error is reported, and then the program exits with a non-zero exit code.

Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default — except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted: the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it ✨.

Even if recovery mode is turned on on the compiler side, it also needs to be 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. You can override this by setting the halt_on_error flag in the corresponding environment variable 💅.

Syntax without an explicit opts parameter is deprecated. It's equivalent to specifying an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. This option sets -fstack-reuse to none 👀.
• -fsanitize-undefined-trap-on-error — instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine. The glow-up here is that the libubsan library isn't needed and isn't linked in, so this is usable even in freestanding environments 🌟.
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block ✨.
• -fsanitize-coverage=trace-cmp — enables dataflow-guided fuzzing code instrumentation. 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, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements 🔥.
• -fcf-protection=[full|branch|return|none|check] — enables code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (like 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 threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP) 👑.

The value "branch" tells the compiler to do the validity check on control-flow transfers right at the indirect branch instructions, aka the call/jmp instructions. The value "return" does the validity checking at the point where you're returning from a function. The value "full" is just an alias for saying both "branch" and "return" at once. And the value "none" turns the whole instrumentation thing off, bye. 👋

The value "check" is for the final link with link-time optimization (LTO). If your LTO object files were compiled with different -fcf-protection values, it throws an error at you. And "check" is literally ignored at compile time, no cap.

The macro __CET__ gets defined whenever -fcf-protection is used. The first bit of __CET__ is set to 1 for the "branch" value, and the second bit of __CET__ is set to 1 for the "return". Bit manipulation girlies, that one's for you. 🧚

You can also use the nocf_check attribute to point out exactly which functions and calls should be skipped from instrumentation. iykyk.

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. 🦋

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.

---

• -fstack-protector — bestie, this 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 call alloca, and functions with buffers larger than 8 bytes. The guards get initialized when a function is entered and then checked when the function exits. If a guard check fails? An error message is printed and the program exits. periodt. 💅
• -fstack-protector-all — same as -fstack-protector except that all functions are protected. all of them. every single one. 😤
• -fstack-protector-strong — like -fstack-protector but it includes extra functions to be protected --- specifically those that have local array definitions, or that have references to local frame addresses. it's giving thorough. ✨
• -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the stack_protect attribute. picky queen behavior, and we respect it. 👑
• -fstack-check — generates code to verify that 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 honestly 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. 🌸

Okay so real talk: this switch does not actually cause checking to be done. The operating system or the language runtime has to do that. The switch just causes generation of code to make sure they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, and specific means use 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 bestie it comes with the following drawbacks:

1. 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. 😬
2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. it's giving slow. 💀

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. It is not generally sufficient to protect against stack-clash attacks though. To protect against those you want -fstack-clash-protection.

• -fstack-clash-protection — generates code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time, and each page is accessed immediately after allocation. So it prevents allocations from jumping over any stack guard page provided by the operating system. no jumping. not today. 🚫

Most targets don't fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found.

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit — these 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 larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to catch the signal without taking special precautions. 🌈

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker. 🤝

You can locally override stack limit checking by using the no_stack_limit function attribute.

• -fsplit-stack — generates code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux. 🐧

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. slay. ✨

• -fvtable-verify=[std|preinit|none] — this option is only available when compiling C++ code. It turns on (or off, if you're using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and hasn't been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. caught in 4k. 📸

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized. ⏰

If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. priority drama, but make it compiler flags. 💅

• -fvtv-debug — when used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that'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 in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to 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 this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. 📊

Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.

• -finstrument-functions — generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, __builtin_return_address doesn't work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) 🎯

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 🫶

okay so this instrumentation also happens for functions that get expanded inline inside other functions 💖 the profiling calls literally show where — conceptually — the inline function gets entered and exited. Which means addressable versions of those functions have to exist. And if all your uses of a function get expanded inline, that can mean extra code size on top, no cap. Also, if you use extern inline in your C code, an addressable version of such functions must be provided. (That's 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. lucky you 🍓)

A function can also be given the attribute no_instrument_function, and then this instrumentation just... doesn't happen, periodt. Use it for the profiling functions listed above, for high-priority interrupt routines, and for any functions the profiling functions can't safely be called from (maybe signal handlers, if the profiling routines generate output or allocate memory 💀).

-finstrument-functions-exclude-file-list=file,file,...

Sets the list of functions that are excluded from instrumentation (peep the description of -finstrument-functions for context). 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 counts as a match. iykyk ✨

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option) 🎀

-finstrument-functions-exclude-function-list=sym,sym,...

This one's similar to -finstrument-functions-exclude-file-list, except it sets the list of function names to be excluded from instrumentation. The function name being matched is its user-visible name, like "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). Match is done on substrings: if the sym parameter is a substring of the function name, it's a match. And 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]

Generates N NOPs right at the beginning of each function, with the function entry point before the *M*th NOP. If M is omitted, it defaults to 0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space you can use to patch in any instrumentation you want at run time — provided 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 interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options, so don't @ me 🙈

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 the command-line option -fpatchable-function-entry=N,M. Use it 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.

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which is run on each C source file before actual compilation. main character energy tbh 🦋

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 bunch of options controlling search paths for include files, documented in Directory Options. Options controlling preprocessor diagnostics are listed in Warning Options.

-D name

Predefine name as a macro, with definition 1. simple as that 🫶

-D name=definition

The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters 😭

If you'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 a meaning in the shell syntax.

If you want to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so quote the option. With sh and csh, -D'name(args...)=definition' works 💖

-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all -D and -U options. periodt.

-U name

Cancel any previous definition of name, whether it was built in or provided with a -D option. 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 -include options are given, the files are included in the order they appear on the command line. order matters bestie ✨

-imacros file

Exactly like -include, except that any output produced by scanning file is thrown away. The macros it defines remain defined though. This lets you acquire all the macros from a header without also processing its declarations. iconic tbh 👑

All files specified by -imacros are processed before all files specified by -include.

-undef

Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined 🌸

-pthread

Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking, fr fr. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets 🧚

-M

Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files — including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix and with any leading directory parts removed. If there are many included files, the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules, you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. 💅

-MM

Like -M but it does not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. sneaky 🙈

-MF file

When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout ⭐

-MG

In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, since a missing header file renders this useless.

This feature is used in automatic updating of makefiles 🎀

-MP

This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. lifesaver tbh 🫶

This is typical output:

test.o: test.c test.h

test.h:

-MT target

Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. That result is the target.

An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

-MQ target

Same as -MT, but it quotes any characters which are special to make. So -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ ✨

• -MD — okay so -MD is literally -M -MF file, except it does not imply -E. Bestie, the driver picks file based on whether you gave it an -o. If you did? It takes that argument but slaps a .d suffix on it. If you didn't? It takes the name of the input file, yeets any directory components and the suffix, then applies a .d suffix. ✨

And here's the plot twist 👀 if -MD shows up alongside -E, any -o switch is understood to mean the dependency output file — but if you use it without -E, each -o means a target object file. Since -E is not implied, -MD can generate a dependency output file as a side effect of the compilation process. main character behavior fr 💅

• -MMD — same energy as -MD except it only mentions user header files, not system header files. 🎀
• -fpreprocessed — tells the preprocessor the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. It still recognizes and removes comments though, so you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. 🧚

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives but do not expand macros. The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only. 💫

• -fdollars-in-identifiers — accept $ in identifiers. 💰
• -fextended-identifiers — accept universal character names in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. 🌈
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization. 🦋
• -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. 📏
• -ftrack-macro-expansion[=level] — track locations of tokens across macro expansions. This lets the compiler emit diagnostics 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 chooses the level of precision of token location tracking, thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option. Value 1 tracks token locations in a degraded mode for the sake of minimal memory overhead — in this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks token locations completely, and this value is the most memory hungry. When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default.

• -fmacro-prefix-map=old=new — when preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new, which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map. 🗺️
• -fexec-charset=charset — set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's iconv library routine. 🔤
• -fwide-exec-charset=charset — set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of wchar_t. As with -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you will have problems with encodings that do not fit exactly in wchar_t. 💀
• -finput-charset=charset — set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine. 🌸
• -fpch-deps — when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header is listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 📦
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename", in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. 🙈

• -fworking-directory — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats.

This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. 🏠

• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. 🫶
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer. 🚫
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments. 💬

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. 🚫✨
• -traditional
• -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.

• -trigraphs — support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. 😵💫

The nine trigraphs and their replacements are

Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
Replacement:      [    ]    {    }    #    \    ^    |    ~

By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. 🔥

• -remap — enable special code to work around file systems which only permit very short file names, such as MS-DOS. 🕹️
• -H — print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...!. 👑

---

• -dletters — bestie, this one says "make debugging dumps during compilation" and letters is you picking which ones 💅. The flags documented right here are the ones that matter to the preprocessor. Other letters get interpreted by the compiler proper, or they're reserved for future versions of GCC, so they're just silently ignored (rude, but ok). And if you specify letters whose behavior conflicts? the result is undefined. No cap. 🫠
  • -dM — instead of the normal output, it generates a list of #define directives for all the macros defined during the execution of the preprocessor, predefined macros included. Basically it's your little detective for finding out what's predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros. ✨

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. (iykyk)

• -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. 💖
• -dN — like -dD, but it emits only the macro names, not their expansions.
• -dI — outputs #include directives in addition to the result of preprocessing.
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.

• -fdebug-cpp — this one is only useful for debugging GCC itself. When you use it from CPP or with -E, it dumps debugging information about location maps, and every token in the output is preceded by the dump of the map its location belongs to. 🗺️

When used from GCC without -E, this option has no effect.

• -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. But heads up bestie: a lot of options get modified, translated or interpreted by the compiler driver before they reach the preprocessor, and -Wp forcibly bypasses that whole phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. 🙈
• -Xpreprocessor option — passes option as an option to the preprocessor. Use this when you gotta supply system-specific preprocessor options that GCC doesn't recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument.

• -no-integrated-cpp — performs preprocessing as a separate pass before compilation. By default, GCC does preprocessing as an integrated part of input tokenization and parsing. If you provide this option, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice: once for preprocessing only, and once for actual compilation of the preprocessed input. This one may be useful with the -B or -wrapper options to specify an alternate preprocessor or do additional processing of the program source between normal preprocessing and compilation. 🦋

---

Passing Options to the Assembler 🎀

You can pass options to the assembler, bestie.

• -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 to supply system-specific assembler options that GCC doesn't recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument.

Options for Linking 🔗

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
• -c, -S, -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments.
• -flinker-output=type — this option controls the code generation of the link time optimizer. By default the linker output is determined by the linker plugin automatically. For debugging the compiler and in the case of incremental linking to non-lto object file is desired, it may be useful to control the type manually.
  • If type is exec the code generation is configured to produce static binary. In this case -fpic and -fpie are both disabled.
  • If type is dyn the code generation is configured to produce shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This makes it possible to build shared libraries without position independent code on architectures this is possible, i.e. on x86.
  • If type is pie the code generation is configured to produce -fpie executable. This result in similar optimizations as exec except that -fpie is 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-objects is specified the binary code is produced for future non-lto linking. The object file produced by incremental linking will be smaller than a static library produced from the same object files. At link-time the result of incremental linking will also load faster to compiler than a static library assuming that majority of objects in the library are used.
  • Finally nolto-rel configure compiler to for incremental linking where code generation is forced, 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 will be optimized better than with link time optimizations disabled (for example, the cross-module inlining will happen), most of benefits of whole program optimizations are however lost.

During the incremental link (by -r) the linker plugin will default to rel. With current interfaces to GNU Binutils it is however not possible to link incrementally LTO objects and non-LTO objects into a single mixed object file. In the case any of object files in incremental link cannot be used for link-time optimization the linker plugin will output warning and use nolto-rel. To maintain the whole program optimization it is recommended to link such objects into static library instead. Alternatively it is 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.
• -llibrary, -l library — searches 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 -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. 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 -lc from 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 -lpthread or -lm in 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-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. Bestie these entries are usually resolved by entries in libc — so when you specify this option, you've gotta supply those entry points through some other mechanism. No cap. 🙈

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a — a library of internal subroutines that 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.) 🫶

---

• -e entry, --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. main character entry point energy ✨
• -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. the anti-pie 💀
• -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. threads bestie, use it consistently fr fr
• -r — produce a relocatable object as output. This is also known as partial linking. 🎀
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. backtrace slay
• -s — remove all symbol table and relocation information from the executable. gone. deleted. stripped. periodt. 💀
• -static — on systems that support dynamic linking, this overrides -pie and 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. it's giving correct behavior 💅

If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc. don't skip this one bestie

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. 🦋
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. 🧚
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. 🍓
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically. ✨
• -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. freeze the vibe 🧊
• -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. rare + exclusive 💅
• -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. 🔥
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. do NOT do that, it's a trap 🙈

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.

• -Wl,option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. 🌸

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.

• -u symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. manifesting symbols into existence ✨
• -z keyword — -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings. go read your linker's docs bestie, iykyk

---

🌸 Options for Directory Search 🌸

These options specify directories to search for header files, for libraries and for parts of the compiler. the treasure map of include paths 🗺️

• -I dir, -iquote dir, -isystem dir, -idirafter dir — add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with = 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: 🎀

1. For the quote form of the include directive, the directory of the current file is searched first.

2. For the quote form of the include directive, the directories you gave with -iquote get searched in left-to-right order, exactly as they appeared on the command line. order matters, bestie
3. Directories specified with -I options get scanned in left-to-right order.
4. Directories specified with -isystem options get scanned in left-to-right order.
5. Standard system directories get scanned.
6. Directories specified with -idirafter options get scanned in left-to-right order.

Okay so you can use -I to override a system header file and swap in your own version, because these directories get searched before the standard system header file directories. slay, custom headers But — and this is the part where people mess up — you should not use this option to add directories that contain vendor-supplied system header files. Use -isystem for that. Periodt.

The -isystem and -idirafter options also mark the directory as a system directory, so it gets the exact same special treatment that gets applied to the standard system directories. 💅

And if a standard system include directory, or a directory specified with -isystem, is also specified with -I, then the -I option is ignored. The directory is still searched — but as a system directory, at its normal position in the system include chain. Why tho? To ensure that GCC's procedure to fix buggy system headers and the ordering for the "#include_next" directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 🫶

• -I- — Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before the -I-, and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include <file>". If you add extra directories with -I options after the -I-, those directories are searched for all #include directives.

Also, -I- inhibits the use of the directory of the current file as the first search directory for "#include "file"". And there is no way to override this effect of -I-. no take-backs, no loopholes

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. (yes, literally the slash, don't forget it 🙈)
• -iwithprefix dir, -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.
• -isysroot dir — This option is like the --sysroot option, but it applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🍓
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers.
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. that's the whole list, nothing else gets a look-in
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
• *-iplugindir=dir* — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. Heads up tho: this option is not meant to be used by the user, but only passed by the driver.
• -Ldir — Add directory dir to the list of directories to be searched for -l.
• -Bprefix — This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. 👑

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. 💀

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler.

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.
• *--sysroot=dir* — Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. 🌈

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 😭

• --no-sysroot-suffix — For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix.

---

🛠️ Options for Code Generation Conventions 🛠️

These machine-independent options control the interface conventions used in code generation. vibe check for your calling conventions

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_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization.

For example,

int *p;
{
  int local1;

  p = &local1;
  local1 = 10;
  ....
}
{
   int local2;
   local2 = 20;
   ...
}

if (*p == 10)  // out of scope use of local1
  {

  }

Another example:

struct A
{
    A(int k) : i(k), j(k) { }
    int i;
    int j;
};

A *ap;

void foo(const A& ar)
{
   ap = &ar;
}

void bar()
{
   foo(A(10)); // temp object's lifetime ends when foo returns

   {
     A a(20);
     ....
   }
   ap->i+= 10;  // ap references out of scope temp whose space
                // is reused with a. What is the value of ap->i?
}

The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization.

• -ftrapv — This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. last one standing wins, bestie
• -fwrapv — bestie, this tells the compiler to assume that signed arithmetic overflow from +, -, and * just wraps around using twos-complement representation. 💫 This flag turns some optimizations on and others off. And heads up: -ftrapv and -fwrapv override each other, so if you do -ftrapv -fwrapv on the command line, -fwrapv is what's actually in charge. Also important: only active options override, so -ftrapv -fwrapv -fno-wrapv on the command line means -ftrapv wins. the way these flags just pass the crown around 💀
• -fwrapv-pointer — this one is like -fwrapv but for pointers, telling the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations that assume pointer overflow is invalid. 🦋
• -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer, and when you negate it, it implies -fwrapv -fwrapv-pointer. so it's basically the whole vibe shift in one flag ✨
• -fexceptions — enable exception handling, bestie! 🎀 It generates the extra code you need 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 don't 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 don't normally require it. But — and this is the plot twist — 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're compiling older C++ programs that don't use exception handling. no cap, it's giving versatility 👑
• -fnon-call-exceptions — generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that doesn't exist everywhere, so it's not universally iconic. 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". so dramatic, so limited 💅
• -fdelete-dead-exceptions — consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. 🧚
• -funwind-tables — similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. it's the lowkey sibling of -fexceptions fr fr 🌸
• -fasynchronous-unwind-tables — generate an unwind table in DWARF format, if supported by the target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as a debugger or garbage collector). precise queen behavior ⭐
• -fno-gnu-unique — okay so on systems with a 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 — return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). 🫶

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. 💀

• -freg-struct-return — return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. slay efficiency 💖

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. main character decision-making honestly 👑

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. 💀

• -fshort-enums — allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the "enum" type is equivalent to the smallest integer type that has enough room. frugal and iconic ✨

Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 💀

• -fshort-wchar — override the underlying type for "wchar_t" to be "short unsigned int" instead of the default for the target. This option is useful for building programs to run under WINE. 🍓

Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 💀

• -fno-common — in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the "extern" keyword, which do not allocate storage. 🌈

Unix C compilers have traditionally allocated storage for uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. 😭

The -fno-common option specifies that the compiler should instead place uninitialized global variables in the 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 defined in more than one compilation unit. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way. periodt 💅

• -fno-ident — ignore the "#ident" directive. simple. clean. iconic. 🎀
• -finhibit-size-directive — don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. it's a very niche queen 🦋
• -fverbose-asm — put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). 👑

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

The added comments include:

• information on the compiler version and command-line options,
• the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
• hints on which high-level expressions correspond to the various assembly instruction operands.

For example, given this C source file:

int test (int n)
{
  int i;
  int total = 0;

  for (i = 0; i < n; i++)
    total += i * i;

  return total;
}

compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -

gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

        .file   "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
  [...snip...]
# options passed:
  [...snip...]

                       .text
                       .globl  test
                       .type   test, @function
               test:
               .LFB0:
                       .cfi_startproc
               # test.c:4:   int total = 0;
                       xorl    %eax, %eax      # <retval>
               # test.c:6:   for (i = 0; i < n; i++)
                       xorl    %edx, %edx      # i
               .L2:
               # test.c:6:   for (i = 0; i < n; i++)
                       cmpl    %edi, %edx      # n, i
                       jge     .L5     #,
               # test.c:7:     total += i * i;
                       movl    %edx, %ecx      # i, tmp92
                       imull   %edx, %ecx      # i, tmp92
               # test.c:6:   for (i = 0; i < n; i++)
                       incl    %edx    # i
               # test.c:7:     total += i * i;
                       addl    %ecx, %eax      # tmp92, <retval>
                       jmp     .L2     #
               .L5:
               # test.c:10: }
                       ret
                       .cfi_endproc
               .LFE0:
                       .size   test, .-test
                       .ident  "GCC: (GNU) 7.0.0 20160809 (experimental)"
                       .section        .note.GNU-stack,"",@progbits

Those comments? Yeah, they're for humans, not machines, so bestie — the exact format of them is subject to change. Don't build your whole personality on parsing them. 💀

---

• -frecord-gcc-switches — okay so this one literally snitches. It takes the command line you used to invoke the compiler and records it into the object file</> being created. Only implemented on some targets, and the exact format of the recording depends on the target and the binary file format — but usually it's just a section holding ASCII text. Related to -fverbose-asm, except that one only writes info into the assembler output file as comments, so it never reaches the object file. Also peep -grecord-gcc-switches for a whole other way of stashing compiler options into the object file. 🫶
• -fpic — generate position-independent code (PIC) suitable for use in a shared library, if the target machine supports it. This kind of code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts — and no, the dynamic loader is not part of GCC, it's part of the operating system, bestie, don't @ me. If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error from the linker saying -fpic doesn't work; in that case, recompile with -fPIC instead. (Those maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)

Position-independent code needs special support, so it only works on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.

• -fPIC — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.

Position-independent code requires special support, and therefore works only on certain machines.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.

• -fpie
• -fPIE — these are like -fpic and -fPIC but the generated position-independent code can only be linked into executables. Usually you use these to compile code that'll be linked using the -pie GCC option.

-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 gives more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures like 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this hands the compiler more register allocation freedom. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time.

Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific external functions.

In position-dependent code, a few targets also convert calls to functions marked to not use the PLT so they use the GOT instead. 👑

• -fno-jump-tables — do not use jump tables for switch statements, even where it'd be more efficient than other code generation strategies. This one's useful alongside -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed at all.
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer or in some other fixed role).

reg must be the name of a register. The register names accepted are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file.

This flag has no negative form, because it specifies a three-way choice. 💅

• -fcall-used-reg — treat the register named reg as an allocable register that's clobbered by function calls. It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way don't save and restore the register reg.

It's an error to use this flag with the frame pointer or stack pointer. Using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 😭

This flag has no negative form, because it specifies a three-way choice.

• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.

It's an error to use this flag with the frame pointer or stack pointer. Using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

And a different sort of disaster results from using this flag for a register in which function values may be returned. 🙈

This flag has no negative form, because it specifies a three-way choice.

• -fpack-struct[=n] — without a value specified, pack all structure members together with no holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

⚠️ Warning: the -fpack-struct switch makes GCC generate code that is not binary compatible with code generated without that switch. Plus it makes the code suboptimal. Use it to conform to a non-default application binary interface. 💀

• -fleading-underscore — this option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is helping you link with legacy assembly code.

⚠️ Warning: the -fleading-underscore switch makes GCC generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. And ngl, not all targets provide complete support for this switch.

• -ftls-model=model — change the thread-local storage model to be used. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Heads up: the choice is subject to optimization — the compiler may use a more efficient model for symbols not visible outside the translation unit, or if -fpic isn't given on the command line.

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

• -ftrampolines — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like for example HP-PA or IA-64 — do nothing.

A trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. So yeah, it requires the stack to be made executable for the program to work properly. 😳

-fno-trampolines is enabled by default on a language-by-language basis, letting the compiler avoid generating them if it computes that this is safe, and replace them with descriptors instead. 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. So this option must be used on a program-wide basis and manipulated with extreme care. 🫶

• -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. ✨

Despite the nomenclature, default always means public — i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.

Okay bestie, we're still deep in the -fvisibility lore and honestly? it's giving "your linker will thank you." 💅

If you want the full glow-up on why correct ELF symbol visibility is such a serve, Ulrich Drepper wrote the whole thing up in "How To Write Shared Libraries" (you can find it at <https://www.akkadia.org/drepper/>). But lowkey there's an even better move than marking stuff hidden while the default is public: flip the default to hidden and mark the things you want public as public instead. That's the norm with DLLs on Windows, and if you use -fvisibility=hidden together with "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)", you get almost identical semantics with identical syntax. Fr fr this is a whole boon to everyone doing cross-platform projects. 🌍✨

For anyone bolting visibility support onto existing code, "#pragma GCC visibility" is your girl. She works by you wrapping the declarations you wanna set visibility for with stuff like "#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". Keep in mind: symbol visibility is part of the API interface contract, so all new code should always specify visibility when it isn't the default. Like, declarations that are only for use within the local DSO should always be explicitly marked hidden so you dodge those PLT indirection overheads — and making it abundantly clear also helps readability and self-documentation of the code. 🎀 Also heads up: because ISO C++ specification requirements say so, "operator new" and "operator delete" must always be of default visibility. No negotiating with the standard, bestie.

Be aware that headers from outside your project — especially system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. So you may need to explicitly say "#pragma GCC visibility push(default)" before you include any of those headers. 🙈

Also: "extern" declarations are not affected by -fvisibility, which means a lot of code can be recompiled with -fvisibility=hidden with zero modifications. The catch? That also means calls to "extern" functions with no explicit visibility go through the PLT, so it's way more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden.

And note that -fvisibility does affect C++ vague linkage entities. So for example, an exception class that gets thrown between DSOs must be explicitly marked with default visibility, otherwise the type_info nodes won't be unified between the DSOs. 😭

There's an overview of all these techniques, their benefits, and how to use them over at <http://gcc.gnu.org/wiki/Visibility>. Go read it, it eats. 🔥

---

• -fstrict-volatile-bitfields — reach for this one if accesses to volatile bit-fields (or other structure fields, though 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. Picture this: targets with memory-mapped peripheral registers might need all such accesses to be 16 bits wide. With this flag you can declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on those targets) to force GCC to use 16-bit accesses instead of, maybe, a more efficient 32-bit access. ✨

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 💀

In some cases, like 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's correctly aligned for the target machine. When that happens, GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 🫶

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members.

The default value of this option is determined by the application binary interface for the target processor.

• -fsync-libcalls — this option controls whether any out-of-line instance of the "__sync" family of functions may be used to implement the C++11 "__atomic" family of functions.

The default value of this option is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. 🧚

---

✨ GCC Developer Options ✨

This section is the VIP lounge: it 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. That covers options that produce debug dumps at various points in the compilation, options that print statistics such as memory use and execution time, and options that print information about GCC's configuration — like where it searches for libraries. Real talk: you should rarely need any of these 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, a phase letter, and a 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's 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're 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. 🌸

💖 The dump options 💖

• -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:

• -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-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
• -fdump-rtl-bypass — dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — dump after duplicating the computed gotos.
• -fdump-rtl-ce1
• -fdump-rtl-ce2
• -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
• -fdump-rtl-cprop_hardreg — dump after hard register copy propagation.
• -fdump-rtl-csa — dump after combining stack adjustments.
• -fdump-rtl-cse1
• -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
• -fdump-rtl-dce — dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — dump after delayed branch scheduling.
• -fdump-rtl-dce1
• -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
• -fdump-rtl-eh — dump after finalization of EH handling code.
• -fdump-rtl-eh_ranges — dump after conversion of EH handling range regions.
• -fdump-rtl-expand — dump after RTL generation.
• -fdump-rtl-fwprop1
• -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1
• -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
• -fdump-rtl-init-regs — dump after the initialization of the registers.
• -fdump-rtl-initvals — dump after the computation of the initial value sets.
• -fdump-rtl-into_cfglayout — dump after converting to cfglayout mode.
• -fdump-rtl-ira — dump after iterated register allocation.
• -fdump-rtl-jump — dump after the second jump optimization.
• -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
• -fdump-rtl-mach — dump after performing the machine dependent reorganization pass, if that pass exists.
• -fdump-rtl-mode_sw — dump after removing redundant mode switches.
• -fdump-rtl-rnreg — dump after register renumbering.
• -fdump-rtl-outof_cfglayout — dump after converting from cfglayout mode.
• -fdump-rtl-peephole2 — dump after the peephole pass.
• -fdump-rtl-postreload — dump after post-reload optimizations.
• -fdump-rtl-pro_and_epilogue — dump after generating the function prologues and epilogues.
• -fdump-rtl-sched1
• -fdump-rtl-sched2 — -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
• -fdump-rtl-ree — dump after sign/zero extension elimination.
• -fdump-rtl-seqabstr — dump after common sequence discovery.

…and bestie, we are NOT done with the dump list, so hold that thought. 🎀

• -fdump-rtl-shorten — dumps the RTL right after branch shortening happens 💅 short branches, cute.
• -fdump-rtl-sibling — dumps after sibling call optimizations. yes bestie, sibling calls are real and they are iconic 🫶
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these five give you a dump after each of the five rounds of instruction splitting. five rounds. she's committed 💀
• -fdump-rtl-sms — dumps after modulo scheduling. heads up tho: this pass only runs on some architectures, so don't come for me if you get nothing ✨
• -fdump-rtl-stack — dumps after the conversion from GCC's "flat register file" registers over to the x87's stack-like registers. and this pass? only on x86 variants. periodt.
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these two enable dumping after the two subreg expansion passes. two passes, two flags, so organized 🌸
• -fdump-rtl-unshare — dumps after all rtl has been unshared. unshared. freed. main character energy 🦋
• -fdump-rtl-vartrack — dumps after variable tracking.
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers. glow-up fr fr 💖
• -fdump-rtl-web — dumps 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 — okay so these dumps are defined, they exist, they're valid… and they always produce empty files. literally empty. not me being handed an empty file and saying thank you 😭
• -da, -fdump-rtl-all — produce all the dumps listed above. all of them. every single one. go off 👑
• -dA — annotate the assembler output with miscellaneous debugging information.
• -dD — dump all macro definitions, at the end of preprocessing, in addition to normal output.
• -dH — produce a core dump whenever an error occurs. yes, on purpose. dramatic but valid 💀
• -dp — annotate the assembler output with a comment indicating which pattern and alternative is used. the length and cost of each instruction also get printed, because she's thorough 💅
• -dP — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation, so you get both, bestie 🎀
• -dx — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand.
• -fdump-debug — dump debugging information generated during the debug generation phase.
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase. early. she woke up and chose debug 🌅
• -fdump-noaddr — when doing debugging dumps, suppress address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. so basically it's the diff-friendly girlie ✨
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. it's giving "if you're gonna crash, at least leave receipts" 💖
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
• -fdump-unnumbered-links — when doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

---

• -fdump-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. the following dumps are possible:
  • all — enables all inter-procedural analysis dumps.
  • cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions.
  • inline — dump after function inlining.

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. both kinds of receipts, bestie 🧾

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -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. the following switch values are 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.
  • raw — dump the raw internal tree data. this option is applicable to C++ only.
• -fdump-passes — print on stderr the list of optimization passes that are turned on and off by the current command-line options. lowkey the most useful thing here, iykyk 👀
• -fdump-statistics-option — enable and control dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled. stats girlies rise 🙌

---

• -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, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation. she's ✨slim✨ now.

• raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.
• details — enable more detailed dumps (not honored by every dump option). also include information from the optimization passes.
• stats — enable dumping various statistics about the pass (not honored by every dump option).
• blocks — enable showing basic block boundaries (disabled in raw dumps).
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

this option currently only works for RTL dumps, and the RTL is always dumped in slim form. no cap, slim only 💅

• 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).
• all — turn on all options, except raw, slim, verbose and lineno. (yes, "all" doesn't mean all. she's got boundaries 💀)
• optall — turn on all optimization options, i.e., optimized, missed, and note.

to determine what tree dumps are available or find the dump for a pass of interest follow the steps below.

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. for example, the codes tree-evrp, tree-vrp1, and tree-vrp2 correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. for example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. note that the numeric codes are not stable and may change from one version of GCC to another. so don't hardcode them, bestie 🙈

---

• -fopt-info, -fopt-info-options, -fopt-info-options=filename — controls optimization dumps from various optimization passes. if the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.

the options can be divided into three groups:

so basically the whole -fopt-info situation splits into three lil squads 👑

1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.

the groups don't overlap at all, so you can mix and match them however you want, no cap 💅 but the second there's a conflict, the later option on the command line eats the earlier one. the earlier one is simply not invited to the party.

---

the following options control which kinds of messages should be emitted:

• optimized — prints info when an optimization is successfully applied. it's up to each pass to decide what info is relevant — like, the vectorizer passes print the source location of loops that got successfully vectorized. slay 🌈
• missed — prints info about missed optimizations. individual passes decide what goes in the output. (the missed opportunities... we don't talk about those 😭)
• note — prints verbose info about optimizations: certain transformations, more detailed messages about decisions, that whole vibe.
• all — the full detailed optimization info. this includes optimized, missed, and note. it's giving "give me everything" ✨

---

the following option controls the dump verbosity:

• internals — by default only "high-level" messages are emitted. this option unlocks the extra, more detailed messages, which are likely to only be of interest to GCC developers. iykyk 🧚

---

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 all the optimization groups listed above. main character energy fr fr 👑

---

if options is omitted, it defaults to optimized-optall, which means: dump messages about successful optimizations from all the passes, and skip the stuff that's treated as "internals".

if the filename is provided, then the dumps from all the applicable optimizations get concatenated into that filename. otherwise the dump just goes straight onto stderr. and yeah, multiple -fopt-info options are accepted, but only one of them can include a filename. if other filenames show up, every option after the first one is ignored, sorry bestie 🙈

note that the output filename is overwritten in case of multiple translation units. if you want a combined output from multiple translation units, use stderr instead. periodt.

in the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

this example:

gcc -O3 -fopt-info-missed=missed.all

outputs a missed optimization report from all the passes into missed.all. and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints info about missed optimization opportunities from vectorization passes on stderr. heads up: -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 literally does not matter 💅

as another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs info about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

here the two output filenames vec.miss and loop.opt are in conflict, because only one output file is allowed. in this case only the first option takes effect and the subsequent options are ignored. so only vec.miss gets produced, and it contains dumps from the vectorizer about missed opportunities. rip loop.opt 💀

---

• -fsave-optimization-record — writes a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.

this option is experimental and the format of the data inside that compressed JSON file is subject to change. don't build your whole life on it 🫶

it's roughly equivalent to a machine-readable version of -fopt-info-all: a collection of messages with source file, line number and column number, plus these extra data bits for each message:

• the execution count of the code being optimized, along with metadata about whether this came from actual profile data or is just an estimate — this lets consumers 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 gets inlined into several different places (which might themselves be inlined), the reader can tell the copies apart,
• 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. it's giving nested lore 🌸

• -fsched-verbose=n — on targets that use instruction scheduling, this option controls how much 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. the more n, the more chaos ✨

• -fenable-kind-pass
• -fdisable-kind-pass=range-list — this is a whole set of options 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. fr fr, this is the dev-only backstage area 🎀

• -fdisable-ipa-pass — disable IPA pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fdisable-rtl-pass
• -fdisable-rtl-pass=range-list — disable RTL pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. each range is a number pair separated by a colon, and the range is inclusive on both ends. if the range is trivial, the number pair can be simplified as a single number. if the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. the uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
• -fdisable-tree-pass
• -fdisable-tree-pass=range-list — disable tree pass pass. see -fdisable-rtl for the description of option arguments.
• -fenable-ipa-pass — enable IPA pass pass. pass is the pass name. if the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fenable-rtl-pass
• -fenable-rtl-pass=range-list — enable RTL pass pass. see -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass
• -fenable-tree-pass=range-list — enable tree pass pass. see -fdisable-rtl for the description of option arguments.

here are some examples showing uses of these options.

# disable ccp1 for all functions
   -fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
   -fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
   -fdisable-rtl-gcse2=foo,foo2
# disable early inlining
   -fdisable-tree-einline
# disable ipa inlining
   -fdisable-ipa-inline
# enable tree full unroll
   -fenable-tree-unroll

• -fchecking
• -fchecking=n — enable internal consistency checking. the default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. 👀
• -frandom-seed=string — this option provides a seed that GCC uses in place of random numbers when generating certain symbol names that have to be different in every compiled file. it's also used to place unique stamps in coverage data files and the object files that produce them. you can use the -frandom-seed option to produce reproducibly identical object files. so satisfying honestly 🌈

the string can either be a number (decimal, octal or hex) or an arbitrary string — in which case it's converted to a number by computing CRC32.

the string should be different for every file you compile. every. single. one. 💅

• -save-temps
• -save-temps=cwd — store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. so compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. this creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor. iconic behavior ✨

when used in combination 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. the corresponding intermediate file may be obtained by renaming the source file before using -save-temps. it's looking out for you 🫶

• -save-temps=obj — bestie, this one also keeps those "temporary" intermediate files around forever, but here's the ✨twist✨: if you're using -o, the temp files get named off the object file instead. And if there's no -o in sight? Then -save-temps=obj just acts exactly like plain -save-temps. 💅

Example time:

gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar

...and that gives you foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o. iconic behavior honestly 🎀

Okay so real talk about -save-temps and parallel compiles, because this bites people: if you invoke GCC in parallel — like, compiling several different source files that share a common base name but live in different subdirectories, or the same source file compiled for multiple output destinations — it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:

gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&

...may result in foo.i and foo.o being written to simultaneously by both compilers. 😭 chaos. not the vibe.

• -time[=file] — report the CPU time taken by each subprocess in the compilation sequence, bestie. For C source files, that means the compiler proper and the assembler (plus the linker if linking is happening). ⏱️

Without specifying an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time" — that's time spent executing the program itself. The second number is "system time" — time spent executing operating system routines on behalf of the program. Both numbers are in seconds. ✨

With 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>

The "user time" and the "system time" get moved before the program name, and the options passed to the program are displayed too — so later you can tell exactly what file was being compiled, and with which options. 🫶

• -fdump-final-insns[=file] — dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the dump file name is determined by appending .gkd to the compilation output file name.
• -fcompare-debug[=opts] — if no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. Then dump the final internal representation in both compilations, and print an error if they differ. 🔍

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's used for opts; otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation — preventing even GCC_COMPARE_DEBUG from taking effect. 🙈

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning instead, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 💀

• -fcompare-debug-second — this option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause 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 .gk additional extension during the second compilation, so they don't overwrite the ones generated by the first. 🎀

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

• -gtoggle — turn off generation of debug info if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. This is mainly intended to be used with -fcompare-debug. 💅
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in exactly the same way that -gtoggle toggles -g.
• -Q — makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. 👑
• -ftime-report — makes the compiler print some statistics about the time consumed by each pass when it finishes. ⏰
• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass.
• -fira-verbose=n — control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10. ✨
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It'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 only print for the WPA phase of Link Time Optimization.
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
• -fpre-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before interprocedural optimization.
• -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation after interprocedural optimization.
• -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and the effect of individual passes.
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable; otherwise it'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; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier "dynamic" means the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 🌸

• -fstats — emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. 👑
• -fdbg-cnt-list — print the name and the counter upper bound for all debug counters.
• -fdbg-cnt=counter-value-list — set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound:upper_bound tuples which sets the lower and the upper bound of each debug counter name. The lower_bound is optional and is zero initialized if not set. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:2:4,tail_call:10, dbg_cnt(dce) returns true only for third and fourth invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations. 🔥
• -print-file-name=library — print the full absolute name of the library file library that would be used when linking — and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. 💅
• -print-multi-directory — print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib — print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing. 🐚
• -print-multi-os-directory — print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just .; if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32; or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
• -print-multiarch — print the path to OS libraries for the selected multiarch, relative to some lib subdirectory. ✨
• -print-prog-name=program — like -print-file-name, but searches for a program such as cpp. 🔎
• -print-libgcc-file-name — same as -print-file-name=libgcc.a. 🫶

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

---

• -print-search-dirs — prints the name of the installation directory it was configured with, plus a whole list of the program and library directories gcc goes looking in. And then it does nothing else, okay? That's the whole job. Slay.

This one is your bestie when gcc hits you with the error message installation problem, cannot exec cpp0: No such file or directory. To fix that, you either need to put cpp0 and the other compiler components exactly where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you actually installed them.

Do not forget the trailing /. She means it. That slash is load-bearing. 🎀

• -print-sysroot — prints the target sysroot directory that gets used during compilation. That's the target sysroot you specified either at configure time or with the --sysroot option, and it may have an extra suffix tacked on that depends on your compilation options. If no target sysroot is specified at all, this option just prints nothing. Silence. Iconic behaviour, honestly. 💅
• -print-sysroot-headers-suffix — prints the suffix that gets added to the target sysroot when searching for headers, or throws an error if the compiler was never configured with such a suffix — and, you guessed it, does nothing else.
• -dumpmachine — prints the compiler's target machine (for example, i686-pc-linux-gnu) — and does not do anything else. One line, one job, out. ✨
• -dumpversion — prints the compiler version (for example, 3.0, 6.3.0 or 7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has 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). So like, it varies, bestie. Don't assume the shape. 🦋
• -dumpfullversion — prints the full compiler version — and don't do anything else. This one is consistent: the output is always three numbers separated by dots, major, minor and patchlevel version. Periodt. 👑
• -dumpspecs — prints the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built, so it's very much an iykyk situation.)

---

🌸 Machine-Dependent Options 🌸

Every target machine that GCC supports can have its own little pile of options — like, to let you compile for a particular processor variant or ABI, or to control optimizations that are specific to that machine. By convention, the names of machine-specific options start with -m. That's the tell. 🕵️

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. Team players.

💜 AArch64 Options 💜

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. The permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. Mixing them is not a vibe, it's a ✨crash✨.

• -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. Compiler's hands are tied, assembler's are free.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -mcmodel=tiny — generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked. Cozy. Everyone's close.
• -mcmodel=small — generate 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 — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only.
• -mstrict-align
• -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. One of them is polite, one of them is feral, you pick.
• -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
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or sysreg for 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 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.

• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or sysreg for 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 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.

Yes, bestie, she said it twice. That's genuinely how it's written in the source. Double the canary, double the drama. 🐤🐤

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-size=size — specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer. Check your binutils before you come for her.
• -mfix-cortex-a53-835769
• -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. One little NOP, saving your whole afternoon.
• -mfix-cortex-a53-843419
• -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.
• -mlow-precision-recip-sqrt
• -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. Fast and loose, but she's telling you the numbers up front.
• -mlow-precision-sqrt
• -mno-low-precision-sqrt — enable or disable the square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt. She brings her friend along. 🫶
• -mlow-precision-div
• -mno-low-precision-div — enable or disable the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation
• -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_copy to permit a more efficient code sequence to be generated. Fancy. Speculation with receipts.
• -moutline-atomics
• -mno-outline-atomics — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature.

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The permissible values for arch are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a or native.

The value armv8.5-a implies armv8.4-a and enables compiler support for the ARMv8.5-A architecture extensions.

The value armv8.4-a implies armv8.3-a and enables compiler support for the ARMv8.4-A architecture extensions.

The value armv8.3-a implies armv8.2-a and enables compiler support for the ARMv8.3-A architecture extensions.

The value armv8.2-a implies armv8.1-a and enables compiler support for the ARMv8.2-A architecture extensions.

The value armv8.1-a implies armv8-a and enables compiler support for the ARMv8.1-A architecture extension. In particular, it enables the +crc, +lse, and +rdma features. Chain reaction, each one pulling the one before it. 💫

The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,

The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. She's doing her best for everyone, not just one chip.

• -mtune=name — okay so this one tells GCC which target processor to tune the code's performance for. The permissible values are literally this whole girliepop lineup:

generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, ares, exynos-m1,
emag, falkor, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,
neoverse-512tvb, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx,
octeontx81,  octeontx83, a64fx, thunderx, thunderxt88,
thunderxt88p1, thunderxt81, tsv110, thunderxt83, thunderx2t99, 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
native

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 are the big.LITTLE girlies — they tell GCC to tune for a big.LITTLE system. 💅

The value neoverse-512tvb means tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Translating from nerd to human: it tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle, and can execute 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. iconic range tbh

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. No cap. But heads up bestie — this option has no effect if the compiler can't recognize the processor of the host system. 🙈

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

Lowkey important: this option cannot be suffixed by feature modifiers. Like, don't even try it. 🚫

• -mcpu=name — same vibe, but you can tack on one or more feature modifiers. The form is -mcpu=cpu{+[no]feature}*, where the permissible cpu values are exactly the same ones as for -mtune. The permissible feature values are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. And if you specify conflicting feature modifiers? The right-most feature wins. periodt.

GCC uses name to figure out what kind of instructions it can emit when generating assembly code (as if by -march), and to figure out the target processor to tune for performance (as if by -mtune). And when this option shows up alongside -march or -mtune? Those options take precedence over the appropriate part of this option. The way that hierarchy works… know your place bestie.

-mcpu=neoverse-512tvb is special — it doesn't refer to a 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 these properties. And unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. ✨

• -moverride=string — overrides tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string here are not guaranteed to be consistent across releases — so like, don't build your whole personality on it. This option is only intended to be useful when developing GCC.
• -mverbose-cost-dump — turns on verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. 🐛
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. With this option, literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny. 🎀
• -msign-return-address=scope — selects the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection. 💀
• -mbranch-protection=none|standard|pac-ret[+leaf]|bti — selects the branch protection features to use. none is the default and turns off all types of branch protection. standard turns on all types of branch protection features — and if a feature has additional tuning options, then standard sets 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 argument leaf can be used to extend the signing to include leaf functions. bti turns on branch target identification mechanism. 🛡️
• -mharden-sls=opts — enables compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:
  • retbr
  • blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening.

• -msve-vector-bits=bits — specifies the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register, and "vector-length specific" output that lets GCC make assumptions about the vector length when it's useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output. All other values generate 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. 🦋

---

🌈 -march and -mcpu Feature Modifiers 🌈

Feature modifiers used with -march and -mcpu can be any of the following, and their inverses nofeature:

• crc — enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• sve — enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
• rdma — enable Round Double Multiply Accumulate instructions. This is 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. This option is 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 does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
• dotprod — enable the Dot Product extension. This also enables Advanced SIMD instructions.
• aes — enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
• sha2 — enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
• sha3 — enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• sm4 — enable the sm3 and sm4 crypto 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 option is only to enable the extension at the assembler level and does not affect code generation.
• rng — enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
• memtag — enable the Armv8.5-a Memory Tagging Extensions. This option is only to enable the extension at the assembler level and does not affect code generation.
• sb — enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• ssbs — enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• predres — enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. The way that dependency chain cascades… it's giving dominos bestie 🁢🁣

---

🧚 Adapteva Epiphany Options 🧚

These -m options are defined for Adapteva Epiphany:

• -mhalf-reg-file — don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack these registers. 💅
• -mprefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size. a little chaotic, but we respect it
• -mbranch-cost=num — sets how expensive a branch is, roughly in "simple" instruction units. Heads up bestie: this is a heuristic only, so it is not guaranteed to produce consistent results across releases. It's a vibe, not a promise. 💅
• -mcmove — turns on generation of conditional moves. Slay, branchless.
• -mnops=num — emits num NOPs before every other generated instruction. Yes, on purpose. Padding is a look. ✨
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags. This is faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. 😭 The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons. Correct > quick, bestie.
• -mstack-offset=num — sets the offset between the top of the stack and the stack pointer. So like, a value of 8 means the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Also note that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. 🚨

It can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option. iykyk.

• -mno-round-nearest — makes the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — unless an attribute says otherwise, assumes all calls might be beyond the offset range of the b / bl instructions, and therefore loads the function address into a register before performing a (otherwise direct) call. This is the default.
• -mshort-calls — unless an attribute says otherwise, assumes all direct calls are in the range of the b / bl instructions, so it uses these instructions for direct calls. The default is -mlong-calls.
• -msmall16 — assumes addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, for which -mlong-calls semantics are in effect.
• -mfp-mode=mode — sets the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. 👑

mode can be set to one of the following values:

• caller — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
• truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller.

• -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 are msplit-lohi, -mpost-inc, and -mpost-modify. (Yes, we are quoting it exactly, typos and all. No cap. 🫶)
• -mnovect-double — changes the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. ⚠️
• -msplit-vecmove-early — splits 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 — specifies a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none.

---

🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port. 🎀

• -march=gpu
• -mtune=gpu

Set architecture type or tuning for gpu. Supported values for gpu are:

• fiji — compile for GCN3 Fiji devices (gfx803).
• gfx900 — compile for GCN5 Vega 10 devices (gfx900).

• -mstack-size=bytes — specifies 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 is 32KB when using OpenACC or OpenMP, and 1MB otherwise. 🫣

---

🦋 ARC Options 🦋

The following options control the architecture variant for which code is being compiled:

• -mbarrel-shifter — generates instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — forces a call to a function using the jli_s instruction. This option is valid only for ARCv2 architecture.
• -mcpu=cpu — sets architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • arc600_norm — compile for ARC 600 CPU with norm instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with norm and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with norm and mul64-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with norm instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with norm and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with norm and mul64-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp
• -mdpfp-compact

Generate double-precision FPX instructions, tuned for the compact implementation.

• -mdpfp-fast — generates double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr — disables lr and sr instructions from using FPX extension aux registers.
• -mea — generates extended arithmetic instructions. Currently only divaw, adds, subs, and sat16 are supported. This is always enabled for -mcpu=ARC700.
• -mno-mpy — do not generate mpy-family instructions for ARC700. This option is deprecated. 🚫
• -mmul32x16 — generates 32x16-bit multiply and multiply-accumulate instructions.
• -mmul64 — generates mul64 and mulu64 instructions. Only valid for -mcpu=ARC600.
• -mnorm — generates norm instructions. This is the default if -mcpu=ARC700 is in effect.
• -mspfp
• -mspfp-compact

Generate single-precision FPX instructions, tuned for the compact implementation.

• -mspfp-fast — generates single-precision FPX instructions, tuned for the fast implementation.
• -msimd — enables generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
• -msoft-float — this option ignored; it is provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can overridden by FPX options; -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision. (We are not fixing that sentence, bestie. It stays. 💅)
• -mswap — generates 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 — enables div and rem instructions for ARCv2 cores.
• -mcode-density — enables code density instructions for ARC EM. This option is on by default for ARC HS.
• -mll64 — enables double load/store operations for ARC HS cores.
• -mtp-regno=regno — specifies thread pointer register number.

okay bestie we're still deep in ARC territory and there's so much more, lock in 🫶

• -mmpy-option=multo — compile ARCv2 code with a multiplier design option. You can hand over multo as either a string or a numeric value. wlh1 is the default. The recognized values are:
  • 0, none — no multiplier available. nada. zero. it's giving bare minimum 💀
  • 1, w — 16x16 multiplier, fully pipelined. This turns on these instructions: mpyw and mpyuw.
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). These instructions get enabled on top: mpy, mpyu, mpym, mpymu, and mpy_s. (this is the default one, remember ✨)
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). Also additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. Also additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. Also additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. Also additionally enables: 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.

heads up: this option is only available for ARCv2 cores. no ARCv2, no multiplier menu, sorry babe 🙈

• -mfpu=fpu — enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This one 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. 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. 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. 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 option fpus_div. Not available for ARC EM.
  • fpus_fma — enables support for single-precision floating-point and fused multiply and add hardware extensions.
  • fpud_fma — enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes option fpus_fma. Not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions. all of them. the whole collection 💅
  • 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 — tells the compiler which general-purpose registers the processor automatically saves/restores on interrupt entry and exit. register-range is 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 how many registers get replicated in the second register bank when you enter a fast interrupt. Fast interrupts are the ones with the highest priority level P0. These interrupts save only the PC and STATUS32 registers, so they skip memory transactions during interrupt entry and exit sequences. Use this option when you're using fast interrupts on an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
• -mlpc-width=width — specify the width of the lp_count register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits. 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 continue to use the loop mechanism for various needs. This option defines macro __ARC_LPC_WIDTH__ with the value of width.
• -mrf16 — instructs the compiler to generate code for a 16-entry register file. This option defines the __ARC_RF16__ preprocessor macro.
• -mbranch-index — enable use of bi or bih instructions to implement jump tables.

---

The following options are passed through to the assembler, and also define preprocessor macro symbols. 🦋

• -mdsp-packa — passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol __Xdsp_packa. This option is deprecated.
• -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol __Xdvbf. This option is deprecated.
• -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol __Xlock.
• -mmac-d16 — passed down to the assembler. Also sets the preprocessor symbol __Xxmac_d16. This option is deprecated.
• -mmac-24 — passed down to the assembler. Also sets the preprocessor symbol __Xxmac_24. This option is deprecated.
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol __Xrtsc. This option is deprecated.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol __Xswape.
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol __Xtelephony. This option is deprecated.
• -mxy — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol __Xxy.

---

The following options control how the assembly code is annotated: 🎀

• -misize — annotate assembler instructions with estimated addresses.
• -mannotate-align — explain what alignment considerations lead to the decision to make an instruction short or long.

---

The following options are passed through to the linker: 🔗

• -marclinux — passed through to the linker, to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.
• -marclinux_prof — passed through to the linker, to specify use of the arclinux_prof emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.

---

The following options control the semantics of generated code: ⭐

• -mlong-calls — generate calls as register indirect calls, which gives you access to the full 32-bit address range. iconic reach, honestly 💖
• -mmedium-calls — don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed so you can use the 25-bit range instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
• -mno-sdata — do not generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache — use ordinarily cached memory accesses for volatile references. This is the default.
• -mno-volatile-cache — enable cache bypass for volatile references.

---

The following options fine tune code generation: ✨

• -malign-call — do alignment optimizations for call instructions.
• -mauto-modify-reg — enable the use of pre/post modify with register displacement.
• -mbbit-peephole — enable bbit peephole2.
• -mno-brcc — this option disables a target-specific pass in arc_reorg to generate compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass.
• -mcase-vector-pcrel — use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
• -mcompact-casesi — enable compact casesi pattern. This is the default for -Os, and only available for ARCv1 cores. This option is deprecated.
• -mno-cond-exec — disable the ARCompact-specific pass to generate conditional execution instructions.

and yes there's STILL more coming bestie, don't you dare close the tab 💅

okay so real talk bestie 💖 — because of delay slot scheduling and the way operand numbers, literal sizes, instruction lengths and conditional-execution support all interact with each other, the target-independent pass that's supposed to generate conditional execution is often just... not it 🙈. So the ARC port kept a special pass around that goes hunting for extra conditional-execution generation opportunities after register allocation, branch shortening and delay slot scheduling have already happened. This pass usually (but not always, ngl) makes performance and code size better, and the price you pay is extra compilation time — which is literally exactly why there's an option to switch it off. And if you're having a problem with call instructions blowing past their allowable offset range because they got conditionalized, you should lowkey consider using -mmedium-calls instead. 💅

• -mearly-cbranchsi — turn on pre-reload use of the "cbranchsi" pattern. ✨
• -mexpand-adddi — expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. Heads up bestie: this option is deprecated. 🚫
• -mindexed-loads — enable the use of indexed loads. Kinda spicy 🌶️ because some optimizers then go ahead and assume indexed stores exist too, which... they do not. 💀
• -mlra — enable Local Register Allocation. Still experimental for ARC, so by default the compiler just uses standard reload (i.e. -mno-lra). 🧪
• -mlra-priority-none — don't indicate any priority for target registers.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15. 🎯
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15.
• -mmillicode — when you're optimizing for size (using -Os), prologues and epilogues that have to save or restore a big pile of registers often get shortened by calling a special function in libgcc; that's what we call a millicode call. ✨ Since these calls can cause performance issues and/or linking issues when you link in a nonstandard way, this option is here so you can turn millicode call generation on or off.
• -mcode-density-frame — lets the compiler emit "enter" and "leave" instructions. Heads up: these are only valid for CPUs that have the code-density feature. 👑
• -mmixed-code — tweaks register allocation to help 16-bit instruction generation. Generally this decreases the average instruction size while increasing the instruction count. 🦋
• -mq-class — enable q instruction alternatives. This is the default for -Os.
• -mRcq — enable Rcq constraint handling. Most short code generation depends on this. Default vibes only. ✅
• -mRcw — enable Rcw constraint handling. Most ccfsm condexec mostly depends on this. Also the default. ✅
• -msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1. 🚫
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1. 💅

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding any implied by -mcpu=. 🌸

Supported values for cpu are:

• ARC600 — tune for ARC600 CPU.
• ARC601 — tune for ARC601 CPU.
• ARC700 — tune for ARC700 CPU with standard multiplier block.
• ARC700-xmac — tune for ARC700 CPU with XMAC block.
• ARC725D — tune for ARC725D CPU.
• ARC750D — tune for ARC750D CPU.

• -mmultcost=num — cost to assume for a multiply instruction, with 4 being equal to a normal instruction. 🔢
• -munalign-prob-threshold=probability — set probability threshold for unaligning branches. When you're tuning for ARC700 and optimizing for speed, branches without a filled delay slot are preferably emitted unaligned and long — unless profiling says the probability of the branch being taken is below probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.

---

The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release. Bestie, do not build new stuff on these 😭:

• -margonaut — obsolete FPX. 🪦
• -mbig-endian, -EB — compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build "arceb-elf32" and "arceb-linux-uclibc" targets, for which big endian is the default.
• -mlittle-endian, -EL — compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for which little endian is the default.
• -mbarrel_shifter — replaced by -mbarrel-shifter.
• -mdpfp_compact — replaced by -mdpfp-compact.
• -mdpfp_fast — replaced by -mdpfp-fast.
• -mdsp_packa — replaced by -mdsp-packa.
• -mEA — replaced by -mea.
• -mmac_24 — replaced by -mmac-24.
• -mmac_d16 — replaced by -mmac-d16.
• -mspfp_compact — replaced by -mspfp-compact.
• -mspfp_fast — replaced by -mspfp-fast.
• -mtune=cpu — values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
• -multcost=num — replaced by -mmultcost.

---

🎀 ARM Options 🎀

These -m options are defined for the ARM port, bestie 💖:

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
• -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if it's not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes stack frames to not be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated. 🚫
• -mapcs — this is a synonym for -mapcs-frame and is deprecated.
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. 🦋
• -mno-sched-prolog — prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. This means all functions start with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog. 🔍
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Also 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. 💀

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers, but will not impose any restrictions on the assembler. 👑
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations. ✅
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
• -mbe8, -mbe32 — when linking a big-endian image, select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default depends on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 🚫
• -march=name[+extension...] — specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option.

Permissible names are: 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, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and iwmmxt2.

Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.

Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension. ✨

For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it. 🧠

So like, most of these extension names are generically named on purpose, but bestie — what they actually do depends entirely on the architecture you're slapping them onto. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. Same name, different flavor, iykyk. 🦋

The list below is the full menu of supported extensions for each architecture. Architectures not mentioned? They don't support any extensions. That's it, that's the vibe. ✨

---

• armv5te
• armv6
• armv6j
• armv6k
• armv6kz
• armv6t2
• armv6z
• armv6zk
  • +fp — The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
  • +nofp — Disable the floating-point instructions.
• armv7
  • 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-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is 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-d16 can be used as an alias for this extension.
  • +simd — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can 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-d16 can be used as an alias for this extension.
  • +simd — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can 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-vfpv3 can 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.
• 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 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.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction 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-point fmla extension.
  • +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.
• armv7-r
  • +fp.sp — The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias for this extension.
  • +fp — The VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
  • +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-m.main
  • +dsp — the DSP instructions, bestie. 💫
  • +nodsp — turns the DSP extension off.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions. both of them. iconic.
  • +nofp — turns the floating-point extension off.
• 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 — turns the cryptographic instructions off.
  • +nofp — turns off the floating-point, Advanced SIMD and cryptographic instructions. all of them, at once, no survivors. 💀

-march=native makes the compiler go ✨auto-detect✨ the architecture of the build computer. ok but listen — at present this feature is only supported on GNU/Linux, and not all architectures are recognized. and if the auto-detect fails? the option just... has no effect. it's giving "I tried my best." 🫠

---

-mtune=name — this one tells GCC which target ARM processor it should tune the performance of the code for. for some ARM implementations you genuinely get better performance by using this option. 💅

Permissible names are:

arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, 0strongarm1110, 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, ares,
cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-m0,
cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23,
cortex-m33, 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, xgene1

and additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. 🐘🐭 the permissible names for that 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 to generate 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. 💖 the effects of this option may change in future GCC versions as CPU models come and go. it's fluid, it's a vibe.

-mtune permits the same extension options as -mcpu, but heads up — 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 -march=native, 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 you'd specified it with -march) and the ARM processor type to tune for performance (as if you'd specified it with -mtune). 🎀

and here's the plot twist: where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this option. so they win. periodt.

many of the supported CPUs implement optional architectural extensions. where that's the case, the architectural extensions are normally enabled by default. if implementations that lack the extension exist, then you can use the extension syntax to disable those extensions that have been omitted. 💅

for floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions.

for example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. the default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions, respectively. slay, options. ✨

permissible names for this option are the same as those for -mtune.

the following extension options are common to the listed CPUs:

• +nodsp — disables the DSP instructions on cortex-m33.
• +nofp — disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7 and cortex-m33. and it disables the floating-point and SIMD instructions on generic-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-a53 and cortex-a55.
• +nofp.dp — disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and cortex-m7.
• +nosimd — disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
• +crypto — enables the cryptographic instructions on cortex-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-a53 and cortex-a75.cortex-a55. 🔐

additionally, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. it supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. the meanings are the same as for the extensions to -march=armv7-a. 🌈

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. see -mtune for more info, bestie.

-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 is unsuccessful, the option has no effect. (yes, third time. it's a whole thing.) 🦋

---

-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-armv8 and crypto-neon-fp-armv8. 💖

note: neon is an alias for neon-vfpv3, and vfp is an alias for vfpv2. iykyk.

the setting auto is the default, and it's special. it makes the compiler select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march. it just knows. ✨

if the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), then heads up — floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. why? because NEON hardware doesn't 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. 😭 no cap, that's real.

you can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. cute, right? 🎀

---

-mfp16-format=name — specifies the format of the __fp16 half-precision floating-point type. permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. 🫶

---

-mstructure-size-boundary=n — the sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option. permissible values are 8, 32 and 64. the default value varies for different toolchains. for the COFF targeted toolchain the default value is 8. a value of 64 is only allowed if the underlying ABI supports it. 💅

specifying a larger number can produce faster, more efficient code, but it can also increase the size of the program. and — major caveat — 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. 🚨 like, they will simply not speak the same language.

this option is deprecated. ⚠️

---

-mabort-on-noreturn — generates a call to the function abort at the end of a noreturn function. it gets executed if the function tries to return. it's giving "you said you wouldn't come back but here you are." 💀

---

-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 the subroutine call instruction. 64 megabytes. that's the limit, bestie.

even if this switch is enabled, not all function calls are turned into long calls. the heuristic is that these are not turned into long calls:

• static functions
• functions that have the short_call attribute
• functions that are inside the scope of a #pragma no_long_calls directive
• functions whose definitions have already been compiled within the current compilation unit

and the exceptions to that rule — these are always turned into long calls:

• weak function definitions
• functions with the long_call attribute or the section attribute
• functions that are within the scope of a #pragma long_calls directive 👑

---

This feature is not on by default. Say -mno-long-calls and you're back to the default vibe, and so is wrapping your function calls in a #pragma long_calls_off directive. But heads up bestie — these switches do nothing to how the compiler generates code for function calls that go through function pointers. Zero. Zilch. 💅

• -msingle-pic-base — treat the register used for PIC addressing as read-only instead of reloading it in the prologue of every single function. The runtime system is the one responsible for initializing this register with a suitable value before execution even starts. Teamwork, fr fr 🤝
• -mpic-register=reg — pick which register gets used for PIC addressing. For the standard PIC base case, the default is whatever suitable register the compiler vibes out. For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled — otherwise it's R10. That's just the rules, iykyk ✨
• -mpic-data-is-text-relative — assume the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data you know is chillin' in the data segment. 🎀 For non-VxWorks RTP targets this option is on by default. And when you disable it on those targets, it will enable -msingle-pic-base by default. Surprise mechanic 🙈
• -mpoke-function-name — write the name of each function into the text section, directly before the function prologue. The generated code is giving:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When you're doing a stack backtrace, the code can peek at 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 there's a function name embedded immediately before this location and it has length ((pc[-3]) & 0xff000000). Detective work slay 🔍

• -mthumb
• -marm — choose between generating code that runs in ARM state vs Thumb state. For most configurations the default is generating code that executes in ARM state, but that default can be changed by configuring GCC with the --with-mode=state configure option. Configurable queen 👑

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. Per-function glow-ups ✨

• -mflip-thumb — switch ARM/Thumb modes on alternating functions. This one is here for regression testing of mixed Thumb/ARM code generation, and it is not intended for ordinary use when compiling code. Not for the girlies, this is lab equipment 🧪
• -mtpcs-frame — generate 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 — no children, just vibes 🍃) The default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — generate a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that doesn't call any other functions.) The default is -mno-apcs-leaf-frame. Note the mismatch, bestie — that's exactly how it's written 💀
• -mcallee-super-interworking — gives every externally visible function in the file being compiled an ARM instruction set header which switches to Thumb mode before running the rest of the function. This lets these functions 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 via function pointers (including virtual functions) execute correctly regardless of whether the target code was compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this is enabled. Also not valid in AAPCS configurations because interworking is enabled by default there. 💅
• -mtp=name — specify the access model for the thread local storage pointer. The valid models are:
  • soft — generates calls to __aeabi_read_tp
  • cp15 — fetches the thread pointer from cp15 directly (supported in the arm6k architecture)
  • auto — uses the best available method for the selected processor

The default setting is auto. Let the compiler cook 👩‍🍳

• -mtls-dialect=dialect — specify the dialect to use for accessing thread local storage. Two dialects are supported: gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it 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. Untouched, unbothered 💅
• -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. ⚔️
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when -mcpu=cortex-m3 is specified. Protective queen 🛡️
• -munaligned-access
• -mno-unaligned-access — enables (or disables) reading and writing of 16- and 32-bit values from addresses that are not 16- or 32-bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access isn't enabled then words in packed data structures are accessed a byte at a time. Byte by byte, one at a time, iconic patience 🐌

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending on the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ✨

• -mneon-for-64bits — enables using Neon to handle scalar 64-bit operations. This is disabled by default since the cost of moving data from core registers to Neon is high. Neon's cute but the commute is brutal 🚗
• -mslow-flash-data — assume loading data from flash is slower than fetching instructions. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and is off by default. It conflicts with -mword-relocations. ⚔️
• -masm-syntax-unified — assume inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. 💀
• -mrestrict-it — restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode. 🎀
• -mprint-tune-info — print CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler and is not intended for ordinary use in compiling code. This option is disabled by default. 🧪
• -mverbose-cost-dump — enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. Compiler-girlie business only 🔧
• -mpure-code — do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets. Strict, exclusive, no mixing ✨
• -mcmse — generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which can be found on <https://developer.arm.com/documentation/ecm0359818/latest/>. 🔐

---

✨ AVR Options ✨

These options are defined for AVR implementations: 🌸

• -mmcu=mcu — specify Atmel AVR instruction set architectures (ISA) or MCU type. The default for this option is avr2. 🎀

GCC supports the following AVR devices and ISAs:

• "avr2" — "Classic" devices with up to 8 KiB of program memory. mcu = "attiny22", "attiny26", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534", "at90s8515", "at90s8535".
• "avr25" — "Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction. 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 the "MOVW" instruction. 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", "ata6285", "ata6286", "ata6289", "ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81". Look at all these little guys 🦋

"avr5" 💖

"Enhanced" devices rocking 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", "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", "ata5795", "ata5790", "ata5790n", "ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510", "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 a 3-byte PC, aka 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".

"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", "attiny417", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny1604", "attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny3214", "attiny3216", "attiny3217", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209", "atmega4808", "atmega4809".

"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".

"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" 🧚

"TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny20", "attiny40".

"avr1" 🫶

This ISA is implemented by the minimal AVR core and is supported for assembler only. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".

---

-mabsdata 🍓

Assume that all data in static storage can be accessed by LDS / STS instructions. Lowkey this option only does anything on reduced Tiny devices like the ATtiny40. Also peek the absdata AVR Variable Attributes variable attribute. iykyk.

-maccumulate-args 💅

Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack, like calls to printf-like functions.

*-mbranch-cost=cost* 💖

Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.

-mcall-prologues ✨

Functions' prologues/epilogues get expanded as calls to appropriate subroutines. Code size is smaller. Slay.

-mgas-isr-prologues 🦋

Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo 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 Attributes "no_gccisr" function attribute. This feature is activated per default if optimization is on (but not with -Og, check the Optimize Options section), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).

-mint8 😭

Assume "int" to be an 8-bit integer. This affects the sizes of all the types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes, and "long long" is 4 bytes. Heads up bestie: this option does not conform to the C standards, but it results in smaller code size.

-mmain-is-OS_task 🎀

Do not save registers in "main". The effect is the same like attaching attribute AVR Function Attributes "OS_task" to "main". It is activated per default if optimization is on.

*-mn-flash=num* 🌸

Assume that the flash memory has a size of num times 64 KiB.

-mno-interrupts 💀

Generated code is not compatible with hardware interrupts. Code size is smaller though.

-mrelax 🔥

Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker, because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.

Relaxing must be turned on if linker stubs are needed — see the section on "EIND" and linker stubs below. Don't skip it fr fr.

-mrmw 💜

Assume that the device supports the Read-Modify-Write instructions "XCH", "LAC", "LAS" and "LAT".

-mshort-calls ⭐

Assume that "RJMP" and "RCALL" can target the whole program memory.

This option is used internally for multilib selection. It is not an optimization option, and you don't need to set it by hand.

-msp8 🌈

Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.

This option is used internally by the compiler to select and build multilibs for architectures "avr2" and "avr25". These architectures mix devices with and without "SPH". For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no "SPH" register or not.

-mstrict-X 🧚

Use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the "X" register may be used in the same way as "Y" or "Z", which then is emulated by additional instructions. For example, loading a value with "X+const" addressing with a small non-negative "const < 64" to a register Rn is performed as

adiw r26, const   ; X += const
ld   <Rn>, X        ; <Rn> = *X
sbiw r26, const   ; X -= const

-mtiny-stack 🍓

Only change the lower 8 bits of the stack pointer.

-mfract-convert-truncate 💅

Allow to use truncation instead of rounding towards zero for fractional fixed-point types.

-nodevicelib 🙈

Don't link against AVR-LibC's device specific library "lib<mcu>.a".

-nodevicespecs 🫶

Don't add -specs=device-specs/specs-<mcu> to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options.

-Waddr-space-convert 💖

Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.

-Wmisspelled-isr 😭

Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.

---

"EIND" and Devices with More Than 128 Ki Bytes of Flash 👑

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address, so that indirect jumps and calls can target any code address in the range of 64 Ki words.

okay so bestie, in order to facilitate indirect jump 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 when EICALL or EIJMP instructions are used. 💅

Indirect jumps and calls on these devices are handled by the compiler like this, and ngl they come with some limitations:

• The compiler never sets EIND. 🚫
• The compiler uses EIND implicitly in EICALL/EIJMP instructions, or it might read EIND directly to emulate an indirect call/jump by means of a RET instruction. 👀
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue. 😤
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. 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 -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to. 📜
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/). 🧚
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located. ✨

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The __trampolines_start symbol is defined in the linker script. 🔗

• 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 gs modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-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:

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 💾

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 RAMP special function registers with zero. 0️⃣
• If a AVR Named Address Spaces, named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation. ✨
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation. 🔁
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. 💾
• RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. ⚠️

---

✨ AVR Built-in Macros ✨

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. 👑

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions. 📚

• __AVR_ARCH__ — Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. 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=device defines this built-in macro which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc. 🏷️

The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase. 🔡

If device is not a device but only a core architecture like avr51, this macro is not defined. 🚫

• __AVR_DEVICE_NAME__ — Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8. 📛

If device is not a device but only a core architecture like avr51, this macro is not defined. 🚫

• __AVR_XMEGA__ — The device / architecture belongs to the XMEGA family of devices. 💫
• __AVR_HAVE_ELPM__ — The device has the ELPM instruction. 📖
• __AVR_HAVE_ELPMX__ — The device has the ELPM Rn,Z and ELPM Rn,Z+ instructions. 📖
• __AVR_HAVE_MOVW__ — The device has the MOVW instruction to perform 16-bit register-register moves. 🔄
• __AVR_HAVE_LPMX__ — The device has the LPM Rn,Z and LPM Rn,Z+ instructions. 📖
• __AVR_HAVE_MUL__ — The device has a hardware multiplier. ✖️
• __AVR_HAVE_JMP_CALL__ — The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory. 📞
• __AVR_HAVE_EIJMP_EICALL__, __AVR_3_BYTE_PC__ — The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 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=avr2 and -mmcu=avr25 also by -msp8. 📚
• __AVR_HAVE_RAMPD__, __AVR_HAVE_RAMPX__, __AVR_HAVE_RAMPY__, __AVR_HAVE_RAMPZ__ — The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively. 🧮
• __NO_INTERRUPTS__ — This macro reflects the -mno-interrupts command-line option. 🚫
• __AVR_ERRATA_SKIP__, __AVR_ERRATA_SKIP_JMP_CALL__ — Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set. 💀
• __AVR_ISA_RMW__ — The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT). 🔁
• __AVR_SFR_OFFSET__=offset — Instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address. 🧮
• __AVR_SHORT_CALLS__ — The -mshort-calls command line option is set. 📞
• "__AVR_PM_BASE_ADDRESS__=addr" — okay so some devices can read from flash memory using LD* instructions?? iconic. The flash memory shows up in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro isn't defined, bestie, that feature just doesn't exist. If it is defined, the address space is linear and you don't need to put .rodata into RAM. The default linker description file handles all of this, and rn it's available for avrtiny and avrxmega3. Even more convenient? You don't need to use address spaces like __flash or features like attribute progmem and pgm_read_*. periodt. 💅
• "__WITH_AVRLIBC__" — the compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option. 🫶

---

🌈 Blackfin Options 🌈

• -mcpu=cpu[-sirevision] — tells you the name of the target Blackfin processor. rn, 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 is the silicon revision of the target Blackfin processor, and any workarounds available for that targeted silicon revision get enabled. If sirevision is none, no workarounds get enabled. If sirevision is any, all workarounds for the targeted processor get 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, __SILICON_REVISION__ is defined to be 0xffff. If you don't use this optional sirevision, 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 makes the hardware BSP provided by libgloss get linked in if -msim isn't given.

Without this option, bf532 is used as the processor by default.

Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 😭

• -msim — says the program is gonna run on the simulator. This makes the simulator BSP provided by libgloss get linked in. It only does anything for the bfin-elf toolchain. Certain other options, like -mid-shared-library and -mfdpic, imply -msim. ✨
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and makes an extra register available in leaf functions.
• -mspecld-anomaly — when enabled, the compiler makes sure the generated code doesn't contain speculative loads after jump instructions. If you use this option, __WORKAROUND_SPECULATIVE_LOADS is defined. 💖
• -mno-specld-anomaly — don't generate extra code to prevent speculative loads from happening.
• -mcsync-anomaly — when enabled, the compiler makes sure the generated code doesn't contain CSYNC or SSYNC instructions too soon after conditional branches. If you use this option, __WORKAROUND_SPECULATIVE_SYNCS is defined.
• -mno-csync-anomaly — don't generate extra code to prevent CSYNC or SSYNC instructions from happening too soon after a conditional branch.
• -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the whole entire program fits into the low 64k of memory. 🎀
• -mno-low64k — assume 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 a bfin-elf target, this option implies -msim.
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mleaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. 🔥
• -mno-leaf-id-shared-library — don't assume the code being compiled won't link against any ID shared libraries. Slower code gets generated for jump and call insns. 💀
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than just 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 the data segment follows the text segment. This is the default.
• -mlong-calls, -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register and then doing a subroutine call on this register. You need this switch if the target function lies outside of the 24-bit addressing range of the offset-based version of the subroutine call instruction. 🫶

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note that 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 aren't known to bind locally. It has no effect without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors. This option makes proper start files and link scripts supporting multicore get 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 isn't used, the single-core application programming model is used.

• -mcorea — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts get used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore. 👑
• -mcoreb — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts get used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. This option can only be used in conjunction with -mmulticore.
• -msdram — build a standalone application for SDRAM. Proper start files and link scripts get used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application.
• -micplb — assume ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

---

🦋 C6X Options 🦋

• -march=name — this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default.
• -msim — choose startup files and linker script suitable for the simulator.
• -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const. 💜
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them.
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data into the .const section.

---

🌸 CRIS Options 🌸

These options are defined specifically for the CRIS ports. iykyk.

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10. ✨
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes.
• -metrax4, -metrax100 — the options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively. slay. 🎀
• -mmul-bug-workaround, -mno-mul-bug-workaround — okay so there's a bug in the muls and mulu instructions on the CPU models where that lil guy actually applies, and this option works around it. It's active by default bestie, so it's already slaying for you 💅
• -mpdebug — turns on CRIS-specific verbose debug-related info in the assembly code. Also (bonus behavior ✨) it turns off</> the #NO_APP formatted-code indicator that goes to the assembler at the beginning of the assembly file. Two-for-one, we love that 🎀
• -mcc-init — do not use condition-code results from the previous instruction. Always emit compare and test instructions before condition codes get used. No shortcuts, no relying on leftovers 🙈
• -mno-side-effects — don't emit instructions with side effects in addressing modes other than post-increment. Periodt.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these (and their no- counterparts) arrange — or, for the no- versions, eliminate the arrangement — for the stack frame, individual data and constants to be aligned for the maximum single data access size of the CPU model you picked. Default vibe is 32-bit alignment. And important: ABI details like structure layout are not affected by these options, so don't panic 🫶
• -m32-bit, -m16-bit, -m8-bit — same energy as the stack-, data- and const-align options above, except these make the stack frame, writable data and constants all be 32-bit, 16-bit or 8-bit aligned respectively. Default is 32-bit alignment ✨
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame get omitted, and no return instructions or return sequences are generated in the code.

⚠️ bestie use this one only together with visual inspection of the compiled code — no warnings or errors get generated when call-saved registers must be saved, or when storage for local variables needs to be allocated. It will simply not tell you 😭

• -mno-gotplt, -mgotplt — with -fpic and -fPIC, don't generate (or do generate, for -mgotplt) instruction sequences that load addresses for functions from the PLT part of the GOT, rather than (which is the traditional behavior on other architectures) calls to the PLT. Default is -mgotplt 💅
• -melf — a legacy no-op option, only recognized with the cris-axis-elf and cris-axis-linux-gnu targets. It's giving decorative ✨
• -mlinux — also a legacy no-op, only recognized with the cris-axis-linux-gnu target.
• -sim — recognized for cris-axis-elf, this one 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 at 0x40000000 and zero-initialized data at 0x80000000 📍

---

✨ CR16 Options ✨

These options are defined specifically for the CR16 ports. Niche but iconic 💖

• -mmac — enable the use of multiply-accumulate instructions. Disabled by default.
• -mcr16cplus, -mcr16c — generate code for the CR16C or CR16C+ architecture. CR16C+ is the default 🌸
• -msim — links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.
• -mint32 — choose integer type as 32-bit wide.
• -mbit-ops — generates sbit/cbit instructions for bit manipulations 🔧
• -mdata-model=model — choose a data model. The choices for model are near, far or medium. medium is the default. However, far is not valid with -mcr16c, because the CR16C architecture does not support the far data model — so don't try it 🙈

---

✨ C-SKY Options ✨

GCC supports these options when compiling for C-SKY V2 processors 🦋

• -march=arch — specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810.
• -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, and ck810ftv. (Yes, it's a whole roster 👑)
• -mbig-endian, -EB, -mlittle-endian, -EL — select big- or little-endian code. The default is little-endian.
• -mhard-float, -msoft-float — select hardware or software floating-point implementations. The default is soft float.
• -mdouble-float, -mno-double-float — when -mhard-float is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803.
• -mfdivdu, -mno-fdivdu — when -mhard-float is 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 are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu) 🧮
• -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 -mistack option is required to handle the interrupt and isr function attributes 🚨
• -mmp — enable multiprocessor instructions; the default is off.
• -mcp — enable coprocessor instructions; the default is off.
• -mcache — enable coprocessor instructions; the default is off.
• -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 numbered 0-7 so 16-bit instructions can be used. 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 💅
• -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. 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.
• -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 roughly n instructions. The default is 1.
• -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences.

⚠️ 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. So like, proceed with caution bestie 😭

---

✨ Darwin Options ✨

These options are defined for all architectures running the Darwin operating system 🍎

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. It's giving ✨chaos✨ ngl

• -Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order 🌈

So like, picture this: a framework directory is just a directory with frameworks in it, bestie. And a framework? It's a directory that has a Headers and/or PrivateHeaders directory sitting directly inside it, and its name ends in .framework. That's the whole vibe. The name of the framework is literally the name of that directory minus the .framework part. ✨

The headers that belong to the framework live in one of those two directories, and Headers gets searched first. Simple as that. 💅

Now, a subframework is a framework directory that lives inside a framework's Frameworks directory. And here's the rule, no cap: includes of subframework headers can only show up 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 really shouldn't have the same name as a framework — if you do that, a warning gets issued. Currently a subframework cannot have subframeworks; in the future the mechanism may be extended to support this. 🦋

The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework is the name of the framework and header.h is found in the PrivateHeaders or Headers directory. 🎀

---

• -iframeworkdir — okay so this one is like -F, EXCEPT the directory gets treated as a system directory. The main difference between -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. Heads up tho: this option is valid only for the C family of languages. 🫶
• -gused — emits debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. And guess what, it's by default ON. 💖
• -gfull — emits debugging information for all symbols and types. The full meal, no crumbs left behind. 👑
• -mmacosx-version-min=version — the earliest version of MacOS X that this executable will run on is version. Typical values of version include 10.1, 10.2, and 10.3.9. 🌸

If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running — otherwise the default is to make choices that are compatible with as many systems and code bases as possible. It's giving "we tried to be flexible." 😌

• -mkernel — enables kernel development mode, main character energy only. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets. 🔥
• -mone-byte-bool — overrides the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86. 💀

Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. Like, seriously bestie, be careful. 😭

• -mfix-and-continue
• -ffix-and-continue
• -findirect-data

Generate code suitable for fast turnaround development — like, to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility. 🧚

• -all_load — loads all members of static archive libraries. See man ld(1) for more information. ✨
• -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal. No chill mode. 💅
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched. 🌈
• -bundle — produce a Mach-o bundle format file. See man ld(1) for more information. 🎀
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. See man ld(1) for more information. 🫶
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command. 🦋
• -force_cpusubtype_ALL — this causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option. ⭐

And now, the moment you've all been waiting for — the big ol' list of options that get passed straight to the Darwin linker, in all their glory, exactly as listed, duplicates and all (yes bestie, some of these really do show up twice, and I'm not about to edit the source, periodt 💜):

• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -headerpad_max_install_names
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded

These options are passed to the Darwin linker. The Darwin linker man page describes them in detail. Iykyk. 😌

---

🌸 DEC Alpha Options 🌸

These -m options are defined for the DEC Alpha implementations, bestie. Let's get into it. ✨

• -mno-soft-float
• -msoft-float

Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point 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. Wild, I know. 💀

• -mfp-reg
• -mno-fp-regs

Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. 🎀

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. 🦋

• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact. 💅
• -mieee-with-inexact — this is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to _IEEE_FP, _IEEE_FP_EXACT is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact. 🔥
• -mfp-trap-mode=trap-mode — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values: 🌈
  • n — this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  • u — in addition to the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  • sui — like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of: ✨
  • n — normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode. Floating-point numbers are rounded towards zero.
  • d — dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity. 🌸
• -mtrap-precision=trap-precision — in the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected: 💖
• p — Program precision. This one's the default, bestie, and it means a trap handler can only figure out which program caused a floating-point exception. That's it, that's the whole tea. 🫖
• f — Function precision. Now the trap handler can clock which function caused a floating-point exception. Upgrade. ✨
• i — Instruction precision. The trap handler can pinpoint the exact instruction that caused the floating-point exception. maximum detail unlocked 💅

Other Alpha compilers ship the equivalent options under the names -scope_safe and -resumption_safe, iykyk.

---

• -mieee-conformant — This one stamps the generated code as IEEE conformant. Big warning though: you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. That's literally it. That's the whole job. 🙈
• -mbuild-constants — Okay so normally GCC looks at a 32- or 64-bit integer constant and checks whether it can construct it from smaller constants in two or three instructions. If it can't, it just dumps 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 reach for this when building a shared library dynamic loader — itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. Iconic self-referential behavior fr.

• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — These say whether GCC should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type you specified via -mcpu= — or, if you didn't specify one, the instruction sets of the CPU on which GCC was built.
• -mfloat-vax, -mfloat-ieee — Generate code that uses (or does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. Pick your fighter. 🥊
• -mexplicit-relocs, -mno-explicit-relocs — Back in the day, older Alpha assemblers had literally no way to generate symbol relocations except through assembler macros. And those macros? They do not allow optimal instruction scheduling. Rude. 😤 GNU binutils as of version 2.12 supports 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 is built and sets the default accordingly.
• -msmall-data, -mlarge-data — When -mexplicit-relocs is in effect, static data gets accessed via gp-relative relocations. With -msmall-data, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This caps the small data area at 64KB, but lets the variables be directly accessed with a single instruction. Efficiency slay. ✨

The default is -mlarge-data. With that option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment. Sorry bestie, that's just the rules. 🫶

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — With -msmall-text, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, so it's reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, which cuts the number of instructions required for a function call from 4 down to 1. Glow up.

The default is -mlarge-text.

• -mcpu=cpu_type — Sets the instruction set and instruction scheduling parameters for machine type cpu_type. You can give either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and picks the default values for the instruction set from the processor you specify. If you don't specify a processor type at all, 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. Tuning only, no architecture drama. 💅

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

• -mmemory-latency=time — Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine. Basically vibes-based hardware math, but make it precise. 🧚

Valid options for time are:

• number — A decimal number representing clock cycles.
• L1
• L2
• L3
• main — The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

---

🍓 FR30 Options 🍓

These options are defined specifically for the FR30 port.

• -msmall-model — Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. Small but mighty, assuming you stay in your lane. 🎀
• -mno-lsim — Assume that runtime support has 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.

• -msim — Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions are needed. Simulator only, no cap. 🚫
• -mlra — Enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload. Living on the edge, I respect it. ✨
• -mnodiv — Do not use div and mod instructions.
• -mft32b — Enable use of the extended instructions of the FT32B processor.
• -mcompress — Compress all code using the Ft32B code compression scheme.
• -mnopm — Do not 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. Go big.
• -mfpr-32 — Use only the first 32 floating-point registers.
• -mfpr-64 — Use all 64 floating-point registers.
• -mhard-float — Use hardware instructions for floating-point operations.
• -msoft-float — Use library routines for floating-point operations.
• -malloc-cc — Dynamically allocate condition code registers.
• -mfixed-cc — Do not try to dynamically allocate condition code registers, only use icc0 and fcc0.
• -mdword — Change ABI to use double word insns.
• -mno-dword — Do not use double word instructions.
• -mdouble — Use floating-point double instructions.
• -mno-double — Do not use floating-point double instructions.
• -mmedia — Use media instructions.
• -mno-media — Do not use media instructions.
• -mmuladd — Use multiply and add/subtract instructions.
• -mno-muladd — Do not use multiply and add/subtract instructions.
• -mfdpic — Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim. So many implications, it's giving domino effect. 🁢
• -minline-plt — Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — Assume a large TLS segment when generating thread-local code.
• -mtls — Do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — Enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
• -multilib-library-pic — Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly. Leave it alone bestie, it's fine. 🙈
• -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 — bestie, this one uses indirect addressing to call functions that live outside the current compilation unit 🫶 which means those functions can be dropped literally anywhere in the 32-bit address space. iconic, flexible, no cap.
• -malign-labels — tries to line up labels on an 8-byte boundary by sliding NOPs into the previous packet ✨ heads up tho: this only does anything when VLIW packing is turned on. It does not make new packets — it just sprinkles NOPs into the ones that already exist. it's giving "decoration, not construction" 💅
• -mlibrary-pic — generates position-independent EABI code. slay.
• -macc-4 — use only the first four media accumulator registers. four. that's it. that's the whole vibe.
• -macc-8 — use all eight media accumulator registers 👑 go big.
• -mpack — pack VLIW instructions. 📦
• -mno-pack — do not pack VLIW instructions. 🙈
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enables the use of conditional-move instructions (this is the default). 🎀

real talk: this switch is mainly for debugging the compiler and will likely be removed in a future version. so, like, don't build your whole personality on it.

• -mno-cond-move — disables the use of conditional-move instructions.

same warning, bestie: mainly for debugging the compiler, probably getting deleted in a future version.

• -mscc — enables the use of conditional set instructions (default). ✨

and yes — mainly for debugging the compiler, likely removed in a future version.

• -mno-scc — disables the use of conditional set instructions.

also mainly for debugging the compiler, also likely to get removed in a future version.

• -mcond-exec — enables the use of conditional execution (default). 💖

mainly for debugging the compiler, likely removed in a future version.

• -mno-cond-exec — disables the use of conditional execution.

mainly for debugging the compiler, likely removed in a future version.

• -mvliw-branch — runs a pass to pack branches into VLIW instructions (default). 🌈

mainly for debugging the compiler, likely removed in a future version.

• -mno-vliw-branch — does not run a pass to pack branches into VLIW instructions.

mainly for debugging the compiler, likely removed in a future version.

• -mmulti-cond-exec — enables optimization of "&&" and "||" in conditional execution (default). 🔥

mainly for debugging the compiler, likely removed in a future version.

• -mno-multi-cond-exec — disables optimization of "&&" and "||" in conditional execution.

mainly for debugging the compiler, likely removed in a future version.

• -mnested-cond-exec — enables nested conditional execution optimizations (default). 🦋

mainly for debugging the compiler, likely removed in a future version.

• -mno-nested-cond-exec — disables nested conditional execution optimizations.

mainly for debugging the compiler, likely removed in a future version.

• -moptimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default. so it's already doing the thing, you don't have to ask 💅
• -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code. let them pile up, I guess 🙈
• -mtomcat-stats — causes gas to print out tomcat statistics. numbers!! stats!! 🍓
• *-mcpu=cpu* — selects the processor type you're generating code for. the possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. pick your fighter 👑

---

🐧 GNU/Linux Options 🐧

these -m options are defined for GNU/Linux targets, bestie:

• -mglibc — use the GNU C library. this is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets. the classic 💖
• -muclibc — use uClibc C library. this is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. this is the default on *-*-linux-*musl* targets. ✨
• -mbionic — use Bionic C library. this is the default on *-*-linux-*android* targets. 🤖
• -mandroid — compile code compatible with 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, it makes the GCC driver pass Android-specific options to the linker. and finally — because it's an overachiever — it causes the preprocessor macro "__ANDROID__" to be defined. three jobs, one flag, main character energy fr fr 🔥

• -tno-android-cc — disables the compilation effects of -mandroid, i.e. it does not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disables the linking effects of -mandroid, i.e. it passes standard Linux linking options to the linker.

---

💜 H8/300 Options 💜

these -m options are defined for the H8/300 implementations:

• -mrelax — shortens some address references at link time, when possible; it uses the linker option -relax. tiny but mighty 🎀
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. this switch must be used either with -mh or -ms. no solo runs, it's a duo 🫶
• -ms2600 — generate code for the H8S/2600. this switch must be used with -ms.
• -mexr — extended registers are stored on the stack before execution of a function with the monitor attribute. the default option is -mexr. this option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on the stack before execution of a function with the monitor attribute. the default option is -mno-exr. this option is valid only for H8S targets.
• -mint32 — make "int" data 32 bits by default. glow up ✨
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. the default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. this option has no effect on the H8/300. (rude, but okay 💀)

---

👑 HPPA Options 👑

these -m options are defined for the HPPA family of computers:

• -march=architecture-type — generate code for the specified architecture. the choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. code compiled for lower-numbered architectures runs on higher-numbered architectures, but **not the other way around. ⬆️ only. one-way street, bestie.
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0

these are synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively. same energy, different spelling 💅

• -mcaller-copies — the caller copies function arguments passed by hidden reference. use this one with care, because it is not compatible with the default 32-bit runtime 😭 however — and this is the redemption arc — only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP. slay, honestly.
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only. it's just here vibing. literally just a decoration 🎀
• -mdisable-fpregs — prevents floating-point registers from being used in any manner. this is necessary for compiling kernels that perform lazy context switching of floating-point registers. but listen: if you use this option and attempt to perform floating-point operations, the compiler aborts. 💀 no warning shot, just gone.
• -mdisable-indexing — prevents the compiler from using indexing address modes. this avoids some rather obscure problems when compiling MIG generated code under MACH.
• -mno-space-regs — generate code that assumes the target has no space registers. this lets GCC generate faster indirect calls and use unscaled index address modes. 🏎️

such code is suitable for level 0 PA systems and kernels.

• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. this allows GCC to emit code that performs faster indirect calls. 🔥

this option does not work in the presence of shared libraries or nested functions. fr fr, it will not go.

• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. a fixed register is one that the register allocator cannot use. this is useful when compiling kernel code. a register range is specified as two registers separated by a dash. multiple register ranges can be specified separated by a comma. 🦋
• -mlong-load-store — generate 3-instruction load and store sequences, as sometimes required by the HP-UX 10 linker. this is equivalent to the +k option to the HP compilers.
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems.
• -mgas — enable the use of assembler directives only GAS understands. insider knowledge, iykyk ✨
• *-mschedule=cpu-type* — schedule code according to the constraints for the machine type cpu-type. the choices for cpu-type are 700, 7100, 7100LC, 7200, 7300 and 8000. refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. the default scheduling is 8000. 👑
• -mlinker-opt — enable the optimization pass in the HP-UX linker. note this makes symbolic debugging impossible. 😭 it also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. so... trade-offs, bestie.
• -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. 🫶 do your homework.

-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 of it or none of it, no halfsies 💅

• -msio — generate the predefine "_SIO" for server IO. the default is -mwsio. this generates the predefines "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. these options are available under HP-UX and HI-UX. 🌸
• -mgnu-ld — okay so this one tells GCC to use the options that are specific to GNU ld 🫶. When you're building a shared library, it passes -shared to ld. It's the default when GCC is configured, either explicitly or implicitly, with the GNU linker. Important vibe check tho: this option does not change which ld gets called — it only changes what parameters get passed to that ld. The ld that actually gets called is decided by the --with-ld configure option, then GCC's program search path, and finally by the user's PATH. Wanna see which linker GCC is using? Run gcc -print-prog-name=ld and it'll spill the tea 💅. Heads up: this option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mhp-ld — the HP ld version of the above ✨. When building a shared library it passes -b to ld, and it passes +Accept TypeMismatch to ld on all links. It's the default when GCC is configured, explicitly or implicitly, with the HP linker. Same deal as before: it does not affect which ld is called, only what parameters get passed to it. Which ld is called is determined by the --with-ld configure option, GCC's program search path, and finally the user's PATH. And again, gcc -print-prog-name=ld prints which linker GCC uses. Only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mlong-calls — generates code that uses long call sequences 🔥. This guarantees a call can always reach linker-generated stubs. The default behavior is to generate long calls only when the distance from the call site to the beginning of the function or translation unit (whichever applies) 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 you use -ffunction-sections, or when you use -mgas and -mno-portable-runtime together under HP-UX with the SOM linker.

Ngl bestie, it's normally not desirable to use this option because it degrades performance 😭. But it can be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depend on the capabilities of the assembler and linker, and on the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls, should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. 💀

• -munix=unix-std — generates compiler predefines and selects a startfile for the specified UNIX standard 🌸. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED, and the startfile unix95.o. -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o.

🚨 Okay this part matters: it's important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So extreme care is needed when using this option. Fr fr.

Library code that's 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 search libdld.sl when the -static option is specified on HP-UX 10 and later. 💅
• -static — so here's the situation: the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Which means when the -static option is specified, special link options are needed to resolve this dependency. 🫠

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options. ✨

• -threads — adds support for multithreading with the dce thread library under HP-UX 🧚. This option sets flags for both the preprocessor and the linker.

---

🌈 IA-64 Options 🌈

These are the -m options defined for the Intel IA-64 architecture. Buckle up bestie, there's a lot 💖

• -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 it 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 for sqrt. 💅
• -mfused-madd, -mno-fused-madd — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but it does not always do so. 🤷
• -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. ✨
• -mtls-size=tls-size — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
• -mtune=cpu-type — tune the instruction scheduling for a particular CPU. Valid values are itanium, itanium1, merced, itanium2, and mckinley. 🎀
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
• -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is disabled.
• -msched-ar-data-spec, -mno-sched-ar-data-spec — (en/dis)able data speculative scheduling after reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is enabled.
• -mno-sched-control-spec, -msched-control-spec — (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the ld.s instructions and the corresponding check instructions chk.s. The default setting is disabled.
• -msched-br-in-data-spec, -mno-sched-br-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default setting is enabled.
• -msched-ar-in-data-spec, -mno-sched-ar-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default setting is enabled.
• -msched-in-control-spec, -mno-sched-in-control-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default setting is enabled.
• -mno-sched-prefer-non-data-spec-insns, -msched-prefer-non-data-spec-insns — turn this on and data-speculative instructions only get picked for the schedule if there's literally nothing else available at that moment. Translation: it makes data speculation way more conservative. Default is disabled, bestie.
• -mno-sched-prefer-non-control-spec-insns, -msched-prefer-non-control-spec-insns — same energy but for control-speculative instructions 🎀 if enabled, they're only chosen when there are no other choices, which makes control speculation a whole lot more careful. Default: disabled.
• -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — enable this and speculative dependencies get counted when the compiler works out instruction priorities, making speculation a lil more conservative. Default: disabled.
• -msched-spec-ldc — use a simple data speculation check. This one is on by default ✨
• -msched-control-spec-ldc — use a simple check for control speculation. Also on by default 💖
• -msched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. On by default, no cap.
• -msched-fp-mem-deps-zero-cost — assume floating-point stores and loads probably won't conflict when they land in the same instruction group. 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* — caps how many memory insns can go in one instruction group, giving lower priority to later memory insns trying to schedule into the same group. Super useful for dodging cache bank conflicts. Default value is 1.
• -msched-max-memory-insns-hard-limit — makes the limit from msched-max-memory-insns a hard limit 💀 no more than that number, periodt. Without it, the limit is "soft", meaning non-memory operations get preferred once you hit the cap, but memory operations can still sneak in and schedule.

---

🌸 LM32 Options 🌸

These -m options are defined for the LatticeMico32 architecture, bestie:

• -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 👑

---

🦋 M32C Options 🦋

• *-mcpu=name* — pick the CPU that code gets generated for. name can be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
• -msim — says the program is gonna run on the simulator, which links in an alternate runtime library that supports stuff like file I/O. 🫶

Do NOT use this option when you're generating programs that run on real hardware. You have to provide your own runtime library for whatever I/O functions you need. Like, actually.

• -memregs=number — sets how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers work like real registers, so there's a tradeoff between GCC's ability to fit code into the registers you have available and the performance hit from using memory instead of registers. Note that **all modules in a program must be compiled with the same value for this option — and 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 their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction — so the compiler generates the much slower seth/add3/jl instruction sequence 🐌
• -msdata=none — disable use of the small data area. Variables get put into one of .data, .bss, or .rodata (unless the section attribute has been specified). This is the default.

The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

• -msdata=sdata — put small global and static data in the small data area, but don't generate special code to reference them.
• -msdata=use — put small global and static data in the small data area, and do generate special instructions to reference them ✨
• *-G *num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. Default for num is 8. The -msdata option must be set to one of sdata or use for this option to do literally anything.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message---incorrect code is not generated.

• -mdebug — makes the M32R-specific code in the compiler print out some statistics that might help when debugging programs 👀
• -malign-loops — align all loops to a 32-byte boundary.
• -mno-align-loops — don't enforce a 32-byte alignment for loops. This is the default.
• *-missue-rate=number* — issue number instructions per cycle. number can only be 1 or 2.
• *-mbranch-cost=number* — number can only be 1 or 2. If it's 1 then branches are preferred over conditional code; if it's 2, then the opposite applies.
• *-mflush-trap=number* — specifies the trap number to use to flush the cache. Default is 12. Valid numbers are between 0 and 15 inclusive.
• -mno-flush-trap — specifies that the cache cannot be flushed by using a trap 🙈
• *-mflush-func=name* — specifies the name of the operating system function to call to flush the cache. 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, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture. iykyk 💅

• *-mcpu=cpu* — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
  • Family 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
  • Family 5206 : 5202 5204 5206
  • Family 5206e : 5206e
  • Family 5208 : 5207 5208
  • Family 5211a : 5210a 5211a
  • Family 5213 : 5211 5212 5213
  • Family 5216 : 5214 5216
  • Family 52235 : 52230 52231 52232 52233 52234 52235
  • Family 5225 : 5224 5225
  • Family 52259 : 52252 52254 52255 52256 52258 52259
  • Family 5235 : 5232 5233 5234 5235 523x
  • Family 5249 : 5249
  • Family 5250 : 5250
  • Family 5271 : 5270 5271
  • Family 5272 : 5272
  • Family 5275 : 5274 5275
  • Family 5282 : 5280 5281 5282 528x
  • Family 53017 : 53011 53012 53013 53014 53015 53016 53017
  • Family 5307 : 5307
  • Family 5329 : 5327 5328 5329 532x
  • Family 5373 : 5372 5373 537x
  • Family 5407 : 5407
  • Family 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 -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above. 🌈

• -m68000, -mc68000 — bestie this one generates output for a 68000. It's the default when the compiler is configured for 68000-based systems, and it's literally equivalent to -march=68000.

Use this one for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. 💅

• -m68010 — generates output for a 68010. Default when the compiler is configured for 68010-based systems, and it's equivalent to -march=68010. periodt.
• -m68020, -mc68020 — generates output for a 68020. The default when the compiler is configured for 68020-based systems. Equivalent to -march=68020. ✨
• -m68030 — generates output for a 68030. Default when the compiler is configured for 68030-based systems. Equivalent to -march=68030.
• -m68040 — generates output for a 68040. Default when the compiler is configured for 68040-based systems. Equivalent to -march=68040.

Heads up: this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. So use this option if your 68040 does not have code to emulate those instructions. 🫶

• -m68060 — generates output for a 68060. Default when the compiler is configured for 68060-based systems. Equivalent to -march=68060.

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 — generates output for a CPU32. Default when the compiler is configured for CPU32-based systems. Equivalent to -march=cpu32.

Use this for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. 💖

• -m5200 — generates output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. It's equivalent to -mcpu=5206, and 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 — generates output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generates output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generates output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generates output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generates 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 it's now deprecated in favor of that option.
• -m68020-40 — generates output for a 68040, without using any of the new instructions. This gives you 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 — generates output for a 68060, without using any of the new instructions. This gives you 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 (or don't generate, respectively) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro __mcfhwdiv__ when this option is enabled. 🎀

• -mshort — consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
• -mno-short — do not consider type int to be 16 bits wide. This is the default.
• -mnobitfield, -mno-bitfield — do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
• -mbitfield — do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there. ✨

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 😭

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

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. bestie pls read that twice 💀

• -mpcrel — use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
• -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system.
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC. 🌸
• -mno-sep-data — generate code that assumes that the data segment follows the text segment. This is the default.
• -mid-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies -fPIC.
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
• -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do. 💅

These options have no effect unless GCC is generating position-independent code.

• -mlong-jump-table-offsets — use 32-bit offsets in switch tables. The default is to use 16-bit offsets.

---

🦋 MCore Options 🦋

These are the -m options defined for the Motorola M*Core processors. iykyk 💖

• -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. (yes, bestie, the page literally gives that same one-liner for both of them, I'm just passing the message along verbatim 🙈)
• -m210, -m340 — Generate code for the 210 processor. iconic little number, no notes
• -mno-lsim — Assume that runtime support has been provided, so leave the simulator library (libsim.a) off the linker command line entirely. she's self-sufficient 💅
• -mstack-increment=size — Sets the max amount for a single stack increment operation. Big values can speed up programs that have functions needing a whole lot of stack space — but they can also trigger a segmentation fault if the stack gets extended too much, so like, be careful. The default value is 0x1000.

---

✨ MeP Options ✨

• -mabsdiff — Enables the abs instruction, which is the absolute difference between two registers. difference, but make it absolute 💖
• -mall-opts — Enables all the optional instructions: average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation.
• -maverage — Enables the ave instruction, which computes the average of two registers.
• -mbased=n — Variables of size n bytes or smaller get placed in the .based section by default. Based variables use the $tp register as a base register, and there is a 128-byte limit to the .based section. (yes the section is literally called "based", we love that for her 🎀)
• -mbitops — Enables the bit operation instructions: bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas). bit girlies rise 💅
• -mc=name — Selects which section constant data is placed in. name may be tiny, near, or far.
• -mclip — Enables the clip instruction. Note that -mclip is not useful unless you also provide -mminmax. they're a package deal, no solo careers 🫶
• -mconfig=name — Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU plus a whole variety of coprocessors, optional instructions, and peripherals. The MeP-Integrator tool — not part of GCC, okay? — provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is default.
• -mcop — Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the -mconfig= option.
• -mcop32 — Enables the 32-bit coprocessor's instructions.
• -mcop64 — Enables the 64-bit coprocessor's instructions.
• -mivc2 — Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor. scheduling era 💅
• -mdc — Causes constant variables to be placed in the .near section.
• -mdiv — Enables the div and divu instructions.
• -meb — Generate big-endian code.
• -mel — Generate little-endian code.
• -mio-volatile — Tells the compiler that any variable marked with the io attribute is to be considered volatile. volatile as in unpredictable, not as in dramatic ✨ (okay maybe both)
• -ml — Causes variables to be assigned to the .far section by default.
• -mleadz — Enables the leadz (leading zero) instruction.
• -mm — Causes variables to be assigned to the .near section by default.
• -mminmax — Enables the min and max instructions.
• -mmult — Enables the multiplication and multiply-accumulate instructions.
• -mno-opts — Disables all the optional instructions enabled by -mall-opts. the off switch for the everything switch 🎀
• -mrepeat — Enables the repeat and erepeat instructions, used for low-overhead looping. loops but make them cheap 🔥
• -ms — Causes all variables to default to the .tiny section. Note that there is a 65536-byte limit to this section. Accesses to these variables use the %gp base register.
• -msatur — Enables the saturation instructions. Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like as. (compatibility queen 💅)
• -msdram — Link the SDRAM-based runtime instead of the default ROM-based runtime.
• -msim — Link the simulator run-time libraries.
• -msimnovec — Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.
• -mtf — Causes all functions to default to the .far section. Without this option, functions default to the .near section.
• -mtiny=n — Variables that are n bytes or smaller are allocated to the .tiny section. These variables use the $gp base register. The default for this option is 4, but note that there's a 65536-byte limit to the .tiny section.

---

🦋 MicroBlaze Options 🦋

• -msoft-float — Use software emulation for floating point (default). doing it by hand, but make it a default 💅
• -mhard-float — Use hardware floating-point instructions.
• -mmemcpy — Do not optimize block moves, use memcpy.
• -mno-clearbss — This option is deprecated. Use -fno-zero-initialized-in-bss instead.
• -mcpu=cpu-type — Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a. version girlies, this one's for you 🎀
• -mxl-soft-mul — Use software multiply emulation (default).
• -mxl-soft-div — Use software emulation for divides (default).
• -mxl-barrel-shift — Use the hardware barrel shifter.
• -mxl-pattern-compare — Use pattern compare instructions.
• -msmall-divides — Use table lookup optimization for small signed integer divisions. table lookup my beloved 🍓
• -mxl-stack-check — This option is deprecated. Use -fstack-check instead.
• -mxl-gp-opt — Use GP-relative .sdata/.sbss sections.
• -mxl-multiply-high — Use multiply high instructions for high part of 32x32 multiply.
• -mxl-float-convert — Use hardware floating-point conversion instructions.
• -mxl-float-sqrt — Use hardware floating-point square root instruction. square root era 💜
• -mbig-endian — Generate code for a big-endian target.
• -mlittle-endian — Generate code for a little-endian target.
• -mxl-reorder — Use reorder instructions (swap and byte reversed load/store).
• -mxl-mode-app-model — Select application model app-model. Valid models are:
  • executable — normal executable (default), uses startup code crt0.o.
  • -mpic-data-is-text-relative — Assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not supported.
  • xmdstub — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.

Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. she's the old name, still answers to it, but deprecated bestie 💀

---

👑 MIPS Options 👑

• -EB — Generate big-endian code.
• -EL — Generate little-endian code. This is the default for mips*el-*-* configurations.
• -march=arch — Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, 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, xlr and xlp. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs). the full guest list, everybody showed up 💅🔥

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor.

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect 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. ratio math but make it ✨aesthetic✨

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given. no shortcuts in the macro, she's formal like that 🎀

• -mtune=arch — Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.

When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. run everywhere, slay in one specific place 💜

okay so quick follow-up before we keep going: -mtune also defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", and they work exactly the same way as the -march ones I just described above. no cap. ✨

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -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 (or like, don't generate) MIPS16 code, bestie. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🦋 Also, MIPS16 code generation can be controlled on a per-function basis using the "mips16" and "nomips16" attributes. cute, right?
• -mflip-mips16 — generates MIPS16 code on alternating functions. ngl this one exists purely for regression testing of mixed MIPS16/non-MIPS16 code generation, and it is not intended for ordinary use in compiling user code. so like… don't, bestie. 🙈
• -minterlink-compressed, -mno-interlink-compressed — require (or don't 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 has to use either a call or an indirect jump. So -minterlink-compressed disables direct jumps unless GCC knows that the target of the jump is not compressed. it's giving safety rails. 🛡️
• -minterlink-mips16, -mno-interlink-mips16 — these are aliases of -minterlink-compressed and -mno-interlink-compressed. They predate the microMIPS ASE and are retained for backwards compatibility. vintage, but make it functional. 👑
• -mabi=32
• -mabi=o64
• -mabi=n32
• -mabi=64
• -mabi=eabi

Generate code for the given ABI. 🎀 Heads up: 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 info about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>. GCC also supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You select this combo with -mabi=32 -mfp64. This ABI relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. 💅 The register assignments for arguments and return values stay the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers: FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 🌸

• -mabicalls, -mno-abicalls — generate (or don't generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems.
• -mshared, -mno-shared — generate (or don't generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects -mabicalls. ✨ All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared. -mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. But the option does not affect the ABI of the final executable — it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. 🔥 -mshared is the default.
• -mplt, -mno-plt — assume (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, this option has no effect without -msym32. You can make -mplt the default by configuring GCC with --with-mips-plt. Otherwise the default is -mno-plt.
• -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. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger makes the linker report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens to you, recompile your code with -mxgot. It works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do. These options have no effect unless GCC is generating position independent code. 🫶

• -mgp32 — assume that general-purpose registers are 32 bits wide.
• -mgp64 — assume that general-purpose registers are 64 bits wide.
• -mfp32 — assume that floating-point registers are 32 bits wide.
• -mfp64 — assume that floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers.
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
• -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats).

🚨 bestie, listen: if code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. so like… don't play with it. 💀

• -msingle-float — assume that the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — assume that the floating-point coprocessor supports double-precision operations. This is the default.
• -modd-spreg, -mno-odd-spreg — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine instructions. 🧚 By default, or when -mabs=legacy is used, the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — unless the -ffinite-math-only option has also been specified. The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations.
• -mnan=2008, -mnan=legacy — these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data. 🎀 The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1. The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
• -mllsc, -mno-llsc — use (or don't use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them. ⚡ -mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details.
• -mdsp, -mno-dsp — use (or don't use) revision 1 of the MIPS DSP ASE. 🌈 This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
• -mdspr2, -mno-dspr2 — use (or don't use) revision 2 of the MIPS DSP ASE. ✨ This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
• -msmartmips, -mno-smartmips — turn the MIPS SmartMIPS ASE on (or off, bestie, your call 💅).
• -mpaired-single, -mno-paired-single — use paired-single floating-point instructions, or don't. Heads up tho: this one literally requires hardware floating-point support to be enabled, no cap. 🫶
• -mdmx, -mno-mdmx — MIPS Digital Media Extension instructions, yes or no. You can only use this when you're generating 64-bit code, and it also needs hardware floating-point support enabled. Both boxes gotta be ticked, fr fr. ✅
• -mips3d, -mno-mips3d — the MIPS-3D ASE, on or off. And omg the way -mips3d just implies -mpaired-single — it comes as a package deal, iykyk. 🎀
• -mmicromips, -mno-micromips — generate (or don't generate) microMIPS code.

You can also control MicroMIPS code generation on a per-function basis using the "micromips" and "nomicromips" attributes. Per-function vibes, iconic. ✨

• -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 — force long types to be 64 bits wide. Wanna know about the default and how the pointer size gets decided? Go look at -mlong32, it explains the whole thing. 👀
• -mlong32 — force long, 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, and so 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. It's giving "whichever fits" energy. 💅

• -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, no matter which ABI you picked. This one slaps in combination with -mabi=64 and -mno-abicalls, because it lets GCC generate shorter and faster references to symbolic addresses. 🔥
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Then GCC can generate more efficient accesses to the data; the deets live in -mgpopt.

The default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, like static variables in C. -mlocal-sdata is the default for all configurations, periodt.

If the linker starts complaining that an application is using too much small data, you might wanna try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so the libraries leave more room for the main program. Sharing is caring, bestie. 🫶

• -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 -G limit. -mextern-sdata is the default for all configurations.

Okay so here's the situation, and it's a whole thing 😭: if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. And if Var is common, you must link the application with a high-enough -G setting. No skipping steps!!

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. That said, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. Iconic workaround, honestly. ✨

• -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-sdata and -mextern-sdata. -mgpopt is the default for all configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.) Lowkey a whole niche scenario but okay. 🙈

-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. Trades a lil speed for memory, and that's valid. 💜
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized const variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data. They're a duo, fr fr.
• -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's 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 (or disable) use of the %hi() and %lo() assembler relocation operators. Ngl this option has been superseded by -mexplicit-relocs but it's still here, retained for backwards compatibility. Respect the elders. 👑
• -mexplicit-relocs, -mno-explicit-relocs — use (or don't use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators.

• -mcheck-zero-division, -mno-check-zero-division — trap (or don't 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 it's only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them, and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. And you can completely disable divide-by-zero checks with -mno-check-zero-division. 💀

• -mload-store-pairs, -mno-load-store-pairs — enable (or disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. ✨
• -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.
• -mlong-calls, -mno-long-calls — disable (or don't disable) use of the jal instruction. Calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment. So like… efficient but clingy. 😭

This option has no effect on abicalls code. The default is -mno-long-calls.

• -mmad, -mno-mad — enable (or disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA.
• -mimadd, -mno-imadd — enable (or disable) use of the madd and msub integer instructions. The default is -mimadd on architectures that support madd and msub — except for the 74k architecture, where it was found to generate slower code. 74k caught lacking, sorry babe. 🙈
• -mfused-madd, -mno-fused-madd — enable (or disable) use of the floating-point multiply-accumulate instructions, when they're 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, just so you know. 👀 On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.
• -mfix-24k, -mno-fix-24k — work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC, fyi.
• -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

Bestie, work around certain R4400 CPU errata: 🛠️

• A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division. 💅

-mfix-r10000

-mno-fix-r10000

Work around certain R10000 errata:

• ll/sc sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 💀 no cap, that's the drama

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. periodt ✨

-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 a nop instruction 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 RM7000 dmult/dmultu errata. The workarounds are implemented by the assembler rather than by GCC. 🛠️

-mfix-vr4120

-mno-fix-vr4120

Work around certain VR4120 errata:

• dmultu does not always produce the correct result. 😭
• div and ddiv do not always produce the correct result if one of the operands is negative. 💀

The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the mips64vr*-elf configurations. 🫶

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself. ✨

-mfix-vr4130

Work around the VR4130 mflo/mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead. 👑

-mfix-sb1

-mno-fix-sb1

Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 F1 and F2 floating-point errata.) 💅

---

-mr10k-cache-barrier=setting

Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. 🧠✨

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 😳

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems. 📖

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. 🫡

If the input program contains a function declaration such as:

void foo (void);

then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same. 💅

The option has three forms:

• -mr10k-cache-barrier=load-store — Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted. 🌸
• -mr10k-cache-barrier=store — Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted. 🎀
• -mr10k-cache-barrier=none — Disable the insertion of cache barriers. This is the default setting. ✨

-mflush-func=func

-mno-flush-func

Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common _flush_func, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush. 🧼✨

mbranch-cost=num

Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting. 💅

-mbranch-likely

-mno-branch-likely

Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use. 😬 periodt

-mcompact-branches=never

-mcompact-branches=optimal

-mcompact-branches=always

These options control which form of branches will be generated. The default is -mcompact-branches=optimal. ✨

The -mcompact-branches=never option ensures that compact branch instructions will never be generated. 🙅‍♀️

The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available. If a compact branch instruction is not available, a delay slot form of the branch will be used instead. 💫

This option is supported from MIPS Release 6 onwards. 📅

The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available. 🎯

-mfp-exceptions

-mno-fp-exceptions

Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled. ✨

For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe. 🚰

-mvr4130-align

-mno-vr4130-align

The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel. 👯‍♀️

This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3. 🔥

-msynci

-mno-synci

Enable (disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) are generated when __builtin___clear_cache is compiled. 🧹

This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci. ✨

When compiling code for single processor systems, it is generally safe to use synci. However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. 😭 bestie be careful

-mrelax-pic-calls

-mno-relax-pic-calls

Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. 🔗

-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler. 💅

-mmcount-ra-address

-mno-mcount-ra-address

Emit (do not emit) code that allows _mcount to modify the calling function's return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of the following:

• Returning the new address in register $31. 🔁
• Storing the new address in *ra-address, if ra-address is nonnull. 💾

The default is -mno-mcount-ra-address. ✨

-mframe-header-opt

-mno-frame-header-opt

Enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused. 🎀

This optimization is off by default at all optimization levels. 😴

-mlxc1-sxc1

-mno-lxc1-sxc1

When applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. 👑✨

• -mmadd4, -mno-madd4 — bestie, this one turns the 4-operand madd.s, madd.d and all their lil friends on (or off) whenever they actually apply. 💅 And it's enabled by default, so it's already slaying unless you say otherwise.

---

🌸 MMIX Options 🌸

These options are defined for the MMIX, and they are so specific, iykyk:

• -mlibfuncs, -mno-libfuncs — says that intrinsic library functions are what's being compiled, so every value gets passed in registers, no matter the size. 🎀
• -mepsilon, -mno-epsilon — generate floating-point comparison instructions that compare against the rE epsilon register. cute and precise fr.
• -mabi=mmixware, -mabi=gnu — generate code that passes function parameters and return values which, inside the called function, show up as registers $0 and up — as opposed to the GNU ABI which uses global registers $231 and up. two whole vibes.
• -mzero-extend, -mno-zero-extend — when you're reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, instead of sign-extending ones. ✨
• -mknuthdiv, -mno-knuthdiv — makes the result of a division that yields 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, but that second one is almost exclusively the one people use. 💀
• -mtoplevel-symbols, -mno-toplevel-symbols — prepend (or don't prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — generate an executable in the ELF format instead of the default mmo format that the mmix simulator uses. main character format energy. 👑
• -mbranch-predict, -mno-branch-predict — use (or don't use) the probable-branch instructions when static branch prediction says a branch is probable.
• -mbase-addresses, -mno-base-addresses — generate (or don't 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. That register then gets 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 you can address is limited. Which means a program with lots of static data may need -mno-base-addresses. 😭
• -msingle-exit, -mno-single-exit — force (or don't force) the generated code to have a single exit point in each function. periodt.

---

🌈 MN10300 Options 🌈

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — generate code that dodges 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 — don't 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. Note that this does not change the targeted processor type. The CPU type has to be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0 — when generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise the pointer comes back only in a0, and attempts to call such functions without a prototype result in errors. Heads up bestie: this option is on by default, so use -mno-return-pointer-on-d0 to turn it off. 🫶
• -mno-crt0 — do not link in the C run-time initialization object file.
• -mrelax — tells the linker it should run a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This only does anything when it's used on the command line for the final link step.

okay but fr: this option makes symbolic debugging impossible. 😭 keep that in mind.

• -mliw — lets the compiler generate Long Instruction Word instructions if the target is AM33 or later. This is the default. It defines the preprocessor macro __LIW__. ✨
• -mno-liw — don't let the compiler generate Long Instruction Word instructions. This defines the preprocessor macro __NO_LIW__.
• -msetlb — lets the compiler generate the SETLB and Lcc instructions if the target is AM33 or later. This is the default. It defines the preprocessor macro __SETLB__. 🎀
• -mno-setlb — don't let the compiler generate SETLB or Lcc instructions. This defines the preprocessor macro __NO_SETLB__.

---

🦋 Moxie Options 🦋

• -meb — generate big-endian code. This is the default for moxie-*-* configurations. 👑
• -mel — generate little-endian code.
• -mmul.x — generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
• -mno-crt0 — do not link in the C run-time initialization object file.

---

🍓 MSP430 Options 🍓

These options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. 💅
• -mmcu= — select the MCU to target. This gets used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with __. Which is then used by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets the ISA to use. If the MCU name is one that's known to only support the 430 ISA then that gets selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

On top of that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and causes the linker to search for a script called xxx.ld. ✨

This option is also passed on to the assembler.

• -mwarn-mcu, -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name given by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. 🫶
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. > ngl bestie, this option is deprecated. Use the -mmcu= option to select the ISA instead. 💀
• -msim — link to the simulator runtime libraries and linker script. This overrides any scripts that would've been selected by the -mmcu= option.
• -mlarge — use large-model addressing (20-bit pointers, 32-bit size_t).
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
• -mrelax — this option is passed to the assembler and linker, and lets the linker perform certain optimizations that can't be done until the final link. 🔥
• mhwmult= — describes the type of hardware multiply supported by the target. Accepted values are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs, 32bit for the 16/32-bit multiply supported by later MCUs, and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given. That tells GCC to deduce the hardware multiply support based on the MCU name provided by the -mmcu option. If no -mmcu option is specified, or if the MCU name isn't recognized, then no hardware multiply support is assumed. auto is the default setting.

Hardware multiplies are normally performed by calling a library routine, which saves space in the generated code. But when compiling at -O3 or higher, the hardware multiplier gets invoked inline. Bigger, but faster code. ⚡

The hardware multiply routines disable interrupts while running and restore the previous interrupt state when they finish. So they're safe to use inside interrupt handlers as well as in normal code. iconic.

• -minrt — enable the use of a minimum runtime environment: no static initializers, no constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🧚
• -mcode-region=, -mdata-region= — these options tell the compiler where to place functions and data that don't 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 the 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.

---

💜 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 v3 push25/pop25 instructions.
• -mno-v3push — okay so do not generate v3 push25/pop25 instructions. That's the whole tea. 🚫✨
• -m16-bit — generate 16-bit instructions. cute and compact fr.
• -mno-16-bit — do not generate 16-bit instructions. the opposite vibe, bestie. 🙅‍♀️
• -misr-vector-size=num — set the size of each interrupt vector. and it must be 4 or 16, no in-between, no negotiating, periodt. 💅
• -mcache-block-size=num — set the size of each cache block. this one has ✨rules✨: it must be a power of 2, and it has to land somewhere between 4 and 512.
• -march=arch — tell the compiler the name of the target architecture. iykyk, it needs to know who it's dressing for. 👑
• -mcmodel=code-model — set the code model to one of these, bestie:
  • small — all the data and read-only data segments have to be within 512KB of addressing space, and the text segment has to be within 16MB of addressing space. lil and cozy. 🎀
  • medium — the data segment stays within 512KB, but the read-only data segment can stretch out to 4GB of addressing space. the text segment should still be within 16MB of addressing space.
  • large — everything goes big mode: all the text and data segments can be within 4GB of addressing space. 🌈
• -mctor-dtor — enable the constructor/destructor feature. the glow-up feature, if you will. ✨
• -mrelax — guide the linker to relax instructions. yes bestie, tell it to chill. 😌

---

🦋 Nios II Options 🦋

these are the options defined for the Altera Nios II processor. very demure, very mindful, very niche. 💜

• -G num — put global and static objects that are less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. the default value of num is 8. 🍓
• -mgpopt=option, -mgpopt, -mno-gpopt — generate (or do not generate) GP-relative accesses. here are the option names it recognizes:
  • none — do not generate GP-relative accesses. nope, none, nada. 🙈
  • local — generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. it also uses GP-relative addressing for objects that have been explicitly placed in a small data section via a "section" attribute. 🏠
  • global — same as local, but it also generates GP-relative accesses for small data objects that are external, weak, or common. ⚠️ heads up: if you use this, you must make sure that all parts of your program (libraries included, bestie) are compiled with the same -G setting.
  • data — generate GP-relative accesses for all data objects in the program. ⚠️ and if you use this one, the entire data and BSS segments of your program must fit in 64K of memory, and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
  • all — generate GP-relative addresses for function pointers as well as data pointers. ⚠️ and if you use this, the entire text, data, and BSS segments of your program must fit in 64K of memory, and again you must use an appropriate linker script to allocate them within the addressable range of the global pointer. it's giving all-in. 🔥
  • -mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none. no cap, that's the mapping.
  • the default is -mgpopt — except when -fpic or -fPIC is specified to generate position-independent code. also note that the Nios II ABI does not permit GP-relative accesses from shared libraries. 😭
  • you may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. in that case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. so like… it just won't reach. 💀
• -mgprel-sec=regexp — this option specifies additional section names that can be accessed via GP-relative addressing. it's most useful in conjunction with "section" attributes on variable declarations and a custom linker script. the regexp is a POSIX Extended Regular Expression. 🧚

this option does not affect the behavior of the -G option, and the specified sections are in addition to the standard ".sdata" and ".sbss" small-data sections that are recognized by -mgpopt.

• -mr0rel-sec=regexp — this option specifies names of sections that can be accessed via a 16-bit offset from "r0"; that is, in the low 32K or high 32K of the 32-bit address space. it's most useful in conjunction with "section" attributes on variable declarations and a custom linker script. the regexp is a POSIX Extended Regular Expression. 🌸

in contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default, and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory. it's giving "you're on your own, bestie." 🙈

• -mel, -meb — generate little-endian (default) or big-endian (experimental) code, respectively. yes, -meb is experimental, don't @ me. 🦋
• -march=arch — this specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. permissible names are: r1, r2. 👑

the preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the targeted ISA level.

• -mbypass-cache, -mno-bypass-cache — force all load and store instructions to always bypass the cache by using I/O variants of the instructions. the default is not to bypass the cache. 🚦
• -mno-cache-volatile, -mcache-volatile — volatile memory access bypasses the cache using the I/O variants of the load and store instructions. the default is not to bypass the cache. ✨
• -mno-fast-sw-div, -mfast-sw-div — do not use table-based fast divide for small numbers. the default is to use the fast divide at -O3 and above. slay at high optimization, fr fr. ➗
• -mno-hw-mul, -mhw-mul, -mno-hw-mulx, -mhw-mulx, -mno-hw-div, -mhw-div — enable or disable emitting the "mul", "mulx" and "div" family of instructions by the compiler. the default is to emit "mul" and not emit "div" and "mulx". that's the whole arrangement. 💅
• -mbmx, -mno-bmx, -mcdx, -mno-cdx — enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. ⚠️ enabling these instructions also requires -march=r2. since these instructions are optional extensions to the R2 architecture, the default is not to emit them. 🎀
• -mcustom-insn=N, -mno-custom-insn — each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. for example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call. not me out here hand-picking opcodes. 💅

the following values of insn are supported. except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

• fadds, fsubs, fdivs, fmuls — binary arithmetic operations. ➕➖✖️➗
• fnegs — unary negation. 🙃
• fabss — unary absolute value.
• fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes — comparison operations. ⚖️
• fmins, fmaxs — floating-point minimum and maximum. these instructions are only generated if -ffinite-math-only is specified. ⚠️
• fsqrts — unary square root operation. 🌱
• fcoss, fsins, ftans, fatans, fexps, flogs — floating-point trigonometric and exponential functions. ⚠️ these instructions are only generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

• faddd, fsubd, fdivd, fmuld — binary arithmetic operations. 💫
• fnegd — unary negation.
• fabsd — unary absolute value.
• fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned — comparison operations. ⚖️
• fmind, fmaxd — double-precision minimum and maximum. ⚠️ these instructions are only generated if -ffinite-math-only is specified.
• fsqrtd — unary square root operation. 🌱
• fcosd, fsind, ftand, fatand, fexpd, flogd — double-precision trigonometric and exponential functions. ⚠️ these instructions are only generated if -funsafe-math-optimizations is also specified.

Conversions:

• fextsd — conversion from single precision to double precision. 📈
• ftruncds — conversion from double precision to single precision. 📉
• fixsi, fixsu, fixdi, fixdu — conversion from floating point to signed or unsigned integer types, with truncation towards zero. ✂️
• round — conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. this corresponds to the "__builtin_lroundf" function when -fno-math-errno is used. 🎯
• floatis, floatus, floatid, floatud — conversion from signed or unsigned integer types to floating-point types. 🎈

in addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. the other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. a custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. like… it just does it for you, bestie. 🫶

• fwrx — write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — write src1 into Y.
• frdxhi, frdxlo — read the most or least (respectively) significant half of X and store it in dest.
• frdy — read the value of Y and store it into dest.

note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas. so many knobs, so little time. 🎛️

• -mcustom-fpu-cfg=name — this option enables a predefined, named set of custom instruction encodings (see -mcustom-insn above). currently, the following sets are defined:

okay so those FPU config presets? bestie, here's exactly what each one is secretly doing under the hood, no cap:

-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant

-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant

And the tea ☕ — custom instruction assignments you hand in with the individual -mcustom-insn= options override whatever -mcustom-fpu-cfg= said. Doesn't matter what order they appear in on the command line, the individual ones win. Order who? Never heard of her. periodt 💅

Also psa 🫶 you can grab way more local control over which FPU configuration gets picked by using the target("custom-fpu-cfg=name") function attribute or pragma. iykyk.

---

🦋 Altera Nios II ELF (bare-metal) era

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP.
• -msmallc — link with a limited version of the C library, -lsmallc, rather than Newlib.
• *-msys-crt0=startfile* — startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with -mhal.
• *-msys-lib=systemlib* — systemlib is the library name of the library that provides low-level system calls required by the C library, e.g. "read" and "write". This option is typically used to link with a library provided by a HAL BSP.

---

💜 Nvidia PTX Options

These options are defined for Nvidia PTX:

• -m32
• -m64 — generate code for 32-bit or 64-bit ABI.
• *-misa=ISA-string* — generate code for given the specified PTX ISA (e.g. sm_35). ISA strings must be lower-case. Valid ISA strings include sm_30 and sm_35. The default ISA is sm_30.
• -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution.
• -moptimize — apply partitioned execution optimizations. This is the default when any level of optimization is selected.
• -msoft-stack — generate code that does not use ".local" memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
• -muniform-simt — switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute the master lane index.
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects corresponding multilib variant.

---

🌸 OpenRISC Options

These options are defined for OpenRISC:

• *-mboard=name* — configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — for compatibility, it's always newlib for elf now.
• -mhard-div — generate code for hardware which supports divide instructions. This is the default.
• -mhard-mul — generate code for hardware which supports multiply instructions. This is the default.
• -mcmov — generate code for hardware which supports the conditional move ("l.cmov") instruction.
• -mror — generate code for hardware which supports rotate right instructions.
• -msext — generate code for hardware which supports sign-extension instructions.
• -msfimm — generate code for hardware which supports set flag immediate ("l.sf*i") instructions.
• -mshftimm — generate code for hardware which supports shift immediate related instructions (i.e. "l.srai", "l.srli", "l.slli", "1.rori"). Note, to enable generation of the "l.rori" instruction the -mror flag must also be specified.
• -msoft-div — generate code for hardware which requires divide instruction emulation.
• -msoft-mul — generate code for hardware which requires multiply instruction emulation.

---

🎀 PDP-11 Options

These options are defined for the PDP-11:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45.
• -msoft-float — do not use hardware floating point.
• -mac0 — return floating-point results in ac0 (fr0 in Unix assembler syntax).
• -mno-ac0 — return floating-point results in memory. This is the default.
• -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split.
• -m45 — generate code for a PDP-11/45. This is the default.
• -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split.
• -mint16
• -mno-int32 — use 16-bit "int". This is the default.
• -mint32
• -mno-int16 — use 32-bit "int".
• -msplit — target has split instruction and data space. Implies -m45.
• -munix-asm — use Unix assembler syntax.
• -mdec-asm — use DEC assembler syntax.
• -mgnu-asm — use GNU assembler syntax. This is the default.
• -mlra — use the new LRA register allocator. By default, the old "reload" allocator is used.

---

🍓 picoChip Options

These -m options are defined for picoChip implementations:

• *-mae=ae_type* — set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported values for ae_type are ANY, MUL, and MAC.
  • -mae=ANY selects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.
  • -mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the default.
  • -mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
• -msymbol-as-address — enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Typically, the use of this option generates larger programs, which run faster than when the option isn't used. However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled.
• -mno-inefficient-warnings — disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings.

---

⭐ PowerPC Options

These are listed under

(no fr the sentence really does just end there in the page 💀 moving on bestie)

---

🌈 RISC-V Options

These command-line options are defined for RISC-V targets:

• *-mbranch-cost=n* — set the cost of branches to roughly n instructions.
• -mplt
• -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt.
• *-mabi=ABI-string* — specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.

The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There is also the ilp32e ABI that can only be used with the rv32e architecture. This ABI is not well specified at present, and is 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.
• -mdiv, -mno-div — do we use the hardware instructions for integer division or not? 💅 Heads up: this needs the M extension. Default vibe is yes, use them if the architecture you picked actually has these instructions.
• -march=ISA-string — generate code for this specific RISC-V ISA (like rv64im). ISA strings gotta be lower-case, no exceptions bestie. Examples include rv64i, rv32g, rv32e, and rv32imaf.
• -mtune=processor-string — optimize the output for a given processor, specified by microarchitecture name. The permissible values are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, and size.

When -mtune= isn't specified, the default is rocket.

The size choice is not intended for use by end-users. It's 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-boundary isn't specified, the default is 4 (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. 😭 fr fr, all of them.

• -msmall-data-limit=n — put global and static data smaller than n bytes into a special section (on some targets).
• -msave-restore, -mno-save-restore — do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues. 🫶
• -mstrict-align, -mno-strict-align — do not or do generate unaligned memory accesses. The default is set depending on whether the processor we're optimizing for supports fast unaligned access or not.
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model. 💖
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.
• -mexplicit-relocs, -mno-exlicit-relocs — use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. (Yes, the -mno- one is spelled exlicit in the original, no cap, we are not fixing 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. 🔥
• -memit-attribute, -mno-emit-attribute — emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32. ✨

---

🌈 RL78 Options 🌈

• -msim — links in additional target libraries to support operation within a simulator.
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. The simplest is "none", which uses software for both multiplication and division. This is the default. The "g13" value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The "g14" value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value "rl78" is an alias for "g14" and the value "mg10" is an alias for "none". 👑

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention. 🦋

If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether.

Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support. 💀

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".

• -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option. They are provided for backwards compatibility.
• -mallregs — allow the compiler to use all of the available registers. By default registers "r24..r31" are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. ✨
• -m64bit-doubles, -m32bit-doubles — make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles.
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option -mg13 needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function. 🫶

---

👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC:

• -mpowerpc-gpopt, -mno-powerpc-gpopt, -mpowerpc-gfxopt, -mno-powerpc-gfxopt, -mpowerpc64, -mno-powerpc64, -mmfcrf, -mno-mfcrf, -mpopcntb, -mno-popcntb, -mpopcntd, -mno-popcntd, -mfprnd, -mno-fprnd, -mcmpb, -mno-cmpb, -mmfpgpr, -mno-mfpgpr, -mhard-dfp, -mno-hard-dfp — you use these options to specify which instructions are available on the processor you're using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above. 💅

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mmfpgpr option allows GCC to generate the FP move to/from general-purpose register instructions implemented on the POWER6X processor and other processors that support the extended PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors. 🔥

The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

• -mcpu=cpu_type — set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, powerpc, powerpc64, powerpc64le, rs64, and native. ✨

-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
• -mpower8-vector
• -mquad-memory
• -mquad-memory-atomic
• -mfloat128
• -mfloat128-hardware

okay so quick heads up bestie 💅 the exact bundle of options that gets switched on for any given CPU is not some universal constant — it changes between compiler versions, depending on what setting looks like it'll spit out the most optimal code for that CPU. And ngl? It doesn't necessarily reflect what the actual hardware can even do. 🙈 So if you wanna set one specific option to a specific value, just say it after the -mcpu option, like -mcpu=970 -mno-altivec. Order matters, bestie, that's the whole vibe. ✨

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX doesn't have full support for these options. You can still enable or disable them individually if you're sure it'll work in your environment. 🫶

---

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage the way -mcpu=cpu_type does. Same cpu_type values work here as for -mcpu. And if you pass both? The generated code uses the architecture and registers from -mcpu, but the scheduling parameters from -mtune. Both, at once, living their best lives. 👑
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux. 🌸
• -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space. Main character energy fr fr. 💅
• -maltivec
• -mno-altivec — generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

When -maltivec is used, the element order for AltiVec intrinsics such as vec_splat, vec_extract, and vec_insert match array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost</c> element in a vector register when targeting a little-endian platform. 🦋

• -mvrsave
• -mno-vrsave — generate VRSAVE instructions when generating AltiVec code.
• -msecure-plt — generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. 🔒
• -mbss-plt — generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.

⚠️ bestie that one wants sections that are both writable and executable. Just so you know. 👀

• -misel
• -mno-isel — this switch enables or disables the generation of ISEL instructions.
• -mvsx
• -mno-vsx — generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set. ✨
• -mcrypto
• -mno-crypto — enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔐
• -mhtm
• -mno-htm — enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion
• -mno-power8-fusion — generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. (yes, the manpage really says "does not keeps" and honestly? we respect the honesty 💀)
• -mpower8-vector
• -mno-power8-vector — generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector instructions. 🌈
• -mquad-memory
• -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode. ⭐
• -mquad-memory-atomic
• -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode. ⭐
• -mfloat128
• -mno-float128 — enable/disable the __float128 keyword for IEEE 128-bit floating point and use either software emulation for IEEE 128-bit floating point or hardware instructions.

The VSX instruction set (-mvsx, -mcpu=power7, -mcpu=power8), or -mcpu=power9 must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point support only works on PowerPC Linux systems. 🐧

The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.

If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation.

• -mfloat128-hardware
• -mno-float128-hardware — enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.

The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.

• -m32
• -m64 — generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
• -mfull-toc
• -mno-fp-in-toc
• -mno-sum-in-toc
• -mminimal-toc — modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 😭

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🧚

• -maix64
• -maix32 — enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
• -mxl-compat
• -mno-xl-compat — produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines.

The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. 🙈

• -mpe — support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible. 💀
• -malign-natural
• -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. 🫠

• -msoft-float
• -mhard-float — generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. 🔥
• -mmultiple, -mno-multiple — bestie, this one's about whether we generate code that uses the load multiple word instructions and the store multiple word instructions (or doesn't, for the -mno- version). These are generated by default on POWER systems, and not generated on PowerPC systems. > okay serious warning time: do not use -mmultiple on little-endian PowerPC systems, because those instructions literally do not work when the processor is in little-endian mode 😭 The only exceptions are PPC740 and PPC750, which do permit these instructions in little-endian mode. Slay for them specifically, I guess 💅
• -mupdate, -mno-update — controls whether we generate code that uses the load or store instructions that update the base register to the address of the calculated memory location (or doesn't use them). These instructions are generated by default. > but babes, if you use -mno-update, there's a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. be careful out there 💀
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (or not avoid, with the -mno- version) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, like when you're stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise.
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used. okay so the lore here: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. iykyk 🦋
• -mmulhw, -mno-mulhw — generate code that uses (or doesn't 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 (or doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (or do, with -mbit-align) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. the way the numbers just... shrink ✨

• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (or do) assume that unaligned memory references are handled by the system.
• -mrelocatable, -mno-relocatable — generate code that allows (or does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section — a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. Also -mrelocatable code aligns the stack to an 8-byte boundary. noted 📝
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. flexible queen 🎀
• -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems, do not (or do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle.
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig.
• -mdynamic-no-pic — on Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. no shared libs bestie, that's the boundary 🚫
• -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 like, that's on 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 value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.
• *-msched-costly-dep=dependence_type* — this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
  • no — no dependence is costly.
  • all — all dependences are costly.
  • true_store_to_load — a true dependence from store to load is costly.
  • store_to_load — any dependence from store to load is costly.
  • number — any dependence for which the latency is greater than or equal to number is costly.
• *-minsert-sched-nops=scheme* — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
  • no — don't insert NOPs.
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
  • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options.
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options.
• -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.
• -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system. 🐧
• -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 are full, part, and no.
• -maix-struct-return — return all structures in memory (as specified by the AIX ABI).
• -msvr4-struct-return — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
• *-mabi=abi-type* — extend the current ABI with a particular extension, or remove such extension. Valid values are altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2.
• -mabi=ibmlongdouble — change the current ABI to use IBM extended-precision long double. This is not likely to work if your system defaults to using IEEE extended-precision long double. If you change the long double type from IEEE extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.
• -mabi=ieeelongdouble — change the current ABI to use IEEE extended-precision long double. This is not likely to work if your system defaults to using IBM extended-precision long double. If you change the long double type from IBM extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to 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. like genuinely spectacular, no notes 😭
• -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 warning bestie, same energy 💀
• -mgnu-attribute, -mno-gnu-attribute — emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values.
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear the bit. the way this one single bit is doing so much heavy lifting 🫠
• -msim — on embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations.
• -mmvme — on embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.

---

• -mads — on embedded PowerPC systems, bestie, this one just assumes the startup module is called crt0.o and that your standard C libraries are libads.a and libc.a. That's it, that's the whole vibe 💅
• -myellowknife — same energy as -mads but make it Canadian 🇨🇦: on embedded PowerPC systems it assumes the startup module is crt0.o and the standard C libraries are libyk.a and libc.a.
• -mvxworks — on System V.4 and embedded PowerPC systems, this says "hey, we're compiling for a VxWorks system." slay.
• -memb — on embedded PowerPC systems, this sets the "PPC_EMB" bit in the ELF flags header to signal that eabi extended relocations are in use. periodt.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, these decide whether you do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is basically a set of modifications to the System V.4 specs.

Picking -meabi means:

• the stack gets aligned to an 8-byte boundary,
• a function "__eabi" is called from "main" to set up the EABI environment,
• and the -msdata option can use both "r2" and "r13" to point to two separate small data areas.

Picking -mno-eabi means:

• the stack gets aligned to a 16-byte boundary,
• no EABI initialization function is called from "main",
• and -msdata only uses "r13" to point to a single small data area.

-meabi is on by default if you configured GCC using one of the powerpc*-*-eabi* options. 💖

• -msdata=eabi — on System V.4 and embedded PowerPC systems: small initialized "const" global and static data goes in the .sdata2 section, pointed to by register "r2". Small initialized non-"const" global and static data goes in the .sdata section, pointed to by register "r13". Small uninitialized global and static data goes in the .sbss section, which sits right next to .sdata.

Heads up bestie: -msdata=eabi is incompatible with the -mrelocatable option. Also it sets the -memb option. 🫶

• -msdata=sysv — on System V.4 and embedded PowerPC systems, small global and static data goes in the .sdata section, pointed to by register "r13". Small uninitialized global and static data goes in .sbss, adjacent to .sdata. And yeah, -msdata=sysv is incompatible with -mrelocatable too. 😭
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi; otherwise compile it the same as -msdata=sysv. choose your own adventure ✨
• -msdata=data — on System V.4 and embedded PowerPC systems, small global data goes in the .sdata section, and small uninitialized global data goes in the .sbss section. But — and this is the key part — do not use register "r13" to address small data. This is the default behavior unless other -msdata options are used. lowkey iconic.
• -msdata=none, -mno-sdata — on embedded PowerPC systems, all initialized global and static data goes in the .data section, and all uninitialized data goes in the .bss section. simple. clean. done.
• -mreadonly-in-sdata — puts read-only objects in the .sdata section as well. This is the default, so it's already serving 💅
• -mblock-move-inline-limit=num — inlines all block moves (like calls to "memcpy" or structure copies) that are less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.
• -mblock-compare-inline-limit=num — generates non-looping inline code for all block compares (like calls to "memcmp" or structure compares) that are less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — generates an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length is not constant, at most num bytes will be compared before "memcmp" is called to compare the remainder of the block. The default value is target-specific.
• -mstring-compare-inline-limit=num — compares at most num string bytes with inline code. If the difference or the end of the string isn't found at the end of the inline compare, a call to "strcmp" or "strncmp" takes care of the rest of the comparison. The default is 64 bytes.
• -G num — on embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch is also passed to the linker. All modules should be compiled with the same -G num value. fr fr, keep them matching 🙈
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms.
• -mlongcall, -mno-longcall — by default this assumes all calls are far away, so a longer and more expensive calling sequence is required. That's needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call can't be that far away. This setting can be overridden by the "shortcall" function attribute, or by #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 generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.

On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. 👀

• -mpltseq, -mno-pltseq — implement (or do not implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and are enabled by default if the support is detected when configuring GCC, and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt.

Watch out: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. 💀

• -mtls-markers, -mno-tls-markers — mark (or do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The relocation lets the linker reliably associate a function call with its argument setup instructions for TLS optimization, which in turn lets GCC better schedule the sequence. ✨
• -mrecip, -mno-recip — this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math).

Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots. 😭

• -mrecip=opt — this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a "!" to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the reciprocal approximation instructions for both single and double precision.
  • divf — enable the single-precision reciprocal approximation instructions.
  • divd — enable the double-precision reciprocal approximation instructions.
  • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square root calculations. the way that comma-separated list eats 🎀

• -mrecip-precision, -mno-recip-precision — assume (or do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they don't provide an estimate that converges after three steps. no cap 🦋
• -mveclibabi=type — okay so this one tells GCC which ABI to use when it's vectorizing intrinsics with an external library. 💅 Right now the only type it supports is mass, which means "use IBM's Mathematical Acceleration Subsystem (MASS) libraries to vectorize intrinsics via external libraries." When it's generating code for power7, GCC currently emits calls to:

acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4,
atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4,
cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2,
exp2f4, expd2, expf4, 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, tanhf4

Both -ftree-vectorize and -funsafe-math-optimizations have to be enabled too, bestie, and the MASS libraries must be specified at link time. No shortcuts. 🫶

• -mfriz, -mno-friz — these two decide whether to generate (or not generate, respectively) the friz instruction when -funsafe-math-optimizations is being used to optimize rounding of floating-point values to 64-bit integer and back to floating point. ⚠️ Heads up though: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer. So like, know what you're doing.
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't generate) code to load up the static chain register (r11) when you're calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in register r2, and the static chain value to be loaded in register r11. ✨ -mpointers-to-nested-functions is on by default. And real talk: you literally cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. It's giving locked door. 🚪
• -msave-toc-indirect, -mno-save-toc-indirect — generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue, if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved just before the call through the pointer instead. -mno-save-toc-indirect is the default. 🎀
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code that passes structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. 🕰️

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That's been corrected in more recent versions. So this option exists so you can generate code that plays nice with functions compiled by the old GCCs. Very "we don't talk about the before times" energy, ngl.

-mno-compat-align-parm is the default.

• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (which is the default with GNU libc version 2.4 or later). 🐤

With that tls choice, -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 from what offset off that base register. The default for those is as specified in the relevant ABI. And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 💖

---

🌈 RX Options 🌈

These command-line options are defined for RX targets, bestie. Let's go. 🦋

• -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. Why, you ask? Because 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 you've got -m64bit-doubles on. 🙈

Note: if -fpu is enabled then -funsafe-math-optimizations is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe. It's giving "the hardware said yolo". 😭

• -mcpu=name — selects the type of RX CPU to target. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600. ✨

The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction.

The RX200 series doesn't have a 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, okay? just data) in big-endian format. The default is -mlittle-endian-data, i.e. storing 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's up to you, the programmer, to make sure the area doesn't overflow. 🙏 Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for pointing to this area, so it's no longer available for the compiler to use. Which could mean slower and/or larger code if variables get pushed onto the stack instead of chilling in this register. 💀

Note: common variables (variables that haven't been initialized) and constants are not placed into the small data area, because they get assigned to other sections in the output executable.

The default value is zero, which disables this feature. And note that this feature is not enabled by default even with higher optimization levels (-O2 etc.), because of the potentially detrimental effects of reserving a register. It's up to the programmer to experiment and discover whether this feature benefits their program. See the description of the -mpid option for 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 compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it's 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. So like, the RX instruction set does allow constants up to 4 bytes long to be used in instructions, but a longer value equates to a longer instruction. So sometimes it's actually 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 constants of any size are allowed.

• -mrelax — enable linker relaxation. Linker relaxation is a process where the linker tries to shrink your program by finding shorter versions of various instructions. Disabled by default. 💅
• -mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12. A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, the default, reserves no 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, because 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. This can result in slower and/or larger code, especially in complicated functions. 😭

The actual register chosen to hold the constant data base address depends 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 it's totally possible for the small data area register to end up being 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 while compiling a file. The default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt. One (1) interrupt. Periodt. 👑
• -mallow-string-insns, -mno-allow-string-insns — turns on (or off, bestie) the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE, plus the RMPA instruction too. Okay so here's the tea 🫖 — these instructions may prefetch data, and that is not safe to do if you're accessing an I/O register. (Go read section 12.2.7 of the RX62N Group User's Manual if you want the whole lore.)

The default is to allow these instructions, but ngl, it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. It is instead reliant upon the programmer to use -mno-allow-string-insns if their program accesses I/O space. This one's on you, bestie. 😭

When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__.

• -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. This option can be used when code size exceeds the range of BSR instructions. And pay attention, because this trips people up: -mno-jsr does not mean "do not use JSR" — it instead means that any type of branch may be used. 💅

Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers r10, r11, r12 and/or r13 — and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options. Lowkey iconic how much it cares. 👑

---

✨ S/390 and zSeries Options ✨

These are the -m options defined for the S/390 and zSeries architecture. Big mainframe energy fr fr 🖥️💜

• -mhard-float, -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. ⭐
• -mhard-dfp, -mno-hard-dfp — use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 🔢
• -mlong-double-64, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default.
• -mbackchain, -mno-backchain — store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float. 🐧

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 🧺

As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. 👀 The default is to use the basr instruction instead, which does not have this limitation.
• -m64, -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
• -mzarch, -mesa — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
• -mhtm, -mno-htm — the -mhtm option enables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12.
• -mvx, -mno-vx — when -mvx is specified, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with regard to alignment and calling conventions. In case vector type values are being used in an ABI-relevant context a GAS .gnu_attribute command will be added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13. 🌈
• -mzvector, -mno-zvector — the -mzvector option enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option adds support for vector to be used as a keyword to define vector type variables and arguments. vector is only available when GNU extensions are enabled. It will not be expanded when requesting strict standard compliance e.g. with -std=c99. In addition to the GCC low-level builtins -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In order to make use of these builtins the header file vecintrin.h needs to be included. -mzvector is disabled by default.
• -mmvcle, -mno-mvcle — generate (or do not generate) code using the mvcle instruction to perform block moves. When -mno-mvcle is specified, use a mvc loop instead. This is the default unless optimizing for size.
• -mdebug, -mno-debug — print (or do not print) additional debug information when compiling. The default is to not print debug information.
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, and native.

The default is -march=z900.

Specifying native as cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor. 🙈

• *-mtune=cpu-type* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS. 💀
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. ➕✖️
• *-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel. 🚨
• -mwarn-dynamicstack — emit a warning if the function calls alloca or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size. Not the vibe, bestie. 😬
• -mstack-guard=stack-guard
• -mstack-size=stack-size — bestie, if you hand over both of these, the S/390 back end sneaks extra instructions into the function prologue that trap if the stack size ends up being stack-guard bytes above stack-size (and yes, remember the stack on S/390 grows downward 💅). If you skip -mstack-guard, it just picks the smallest power of 2 that's bigger than the frame size of the function it's compiling. These two are meant to help you debug stack overflow situations, no cap — the extra code it emits costs basically nothing in overhead, so you can legit run it in production-like systems without a big performance hit. The values you give have to be exact powers of 2, and stack-size must be greater than stack-guard while not exceeding 64k. Also for efficiency the extra code assumes the stack starts at an address aligned to whatever stack-size says. Oh and -mstack-guard can only be used together with -mstack-size, periodt.
• -mhotpatch=pre-halfwords,post-halfwords — turn this on and a "hot-patching" function prologue gets generated for every single function in the compilation unit 🫶. The function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords, max 1000000). After the label, 2 * post-halfwords bytes get appended, using the largest NOP-like instructions the architecture allows (post-halfwords, max 1000000).

If both arguments are zero, hotpatching is disabled 💀.

This option can be overridden for individual functions with the "hotpatch" attribute.

---

🎮 Score Options 🎮

These options are defined for Score implementations:

• -meb — compile code for big-endian mode. This is the default. 👑
• -mel — compile code for little-endian mode.
• -mnhwloop — disable generation of "bcnz" instructions.
• -muls — enable generation of unaligned load and store instructions.
• -mmac — enable the use of multiply-accumulate instructions. Disabled by default.
• -mscore5 — specify the SCORE5 as the target architecture.
• -mscore5u — specify the SCORE5U of the target architecture.
• -mscore7 — specify the SCORE7 as the target architecture. This is the default.
• -mscore7d — specify the SCORE7D as the target architecture.

---

🌸 SH Options 🌸

These -m options are defined for the SH implementations:

• -m1 — generate code for the SH1.
• -m2 — generate code for the SH2.
• -m2e — generate code for the SH2e.
• -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
• -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — generate code for the SH3.
• -m3e — generate code for the SH3e.
• -m4-nofpu — generate code for the SH4 without a floating-point unit.
• -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — generate code for the SH4.
• -m4-100 — generate code for SH4-100.
• -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — generate code for SH4-200.
• -m4-200-nofpu — generate code for SH4-200 without in such a way that the floating-point unit is not used.
• -m4-200-single — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
• -m4-300 — generate code for SH4-300.
• -m4-300-nofpu — generate code for SH4-300 without in such a way that the floating-point unit is not used.
• -m4-300-single — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-340 — generate code for SH4-340 (no MMU, no FPU).
• -m4-500 — generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
• -m4a-single — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
• -m4a — generate code for the SH4a.
• -m4al — same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment.
• -mb — compile code for the processor in big-endian mode.
• -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, so some functions from the standard C library won't work unless you recompile it first with -mdalign 😭.
• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
• -mbitops — enable the use of bit manipulation instructions on SH2A.
• -mfmovd — enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
• -mrenesas — comply with the calling conventions defined by Renesas.
• -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
• -mnomacsave — mark the "MAC" register as call-clobbered, even if -mrenesas is given.
• -mieee
• -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects how cases get handled where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons. You can override those implicit settings by specifying either -mieee or -mno-ieee. ✨
• -minline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the "icbi" instruction. If the selected code generation option does not allow the use of the "icbi" instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, it also fails if the cache line had been mapped via the TLB and has become unmapped 💀.
• -misize — dump instruction size and location in the assembly code.
• -mpadstruct — this option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
• -matomic-model=model — sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
  • none — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not "sh*-*-linux*".
  • soft-gusa — generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is "sh*-*-linux*" and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions "movli.l" and "movco.l" to create more efficient code, unless strict is specified.
  • soft-tcb — generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
  • soft-imask — generate software atomic sequences that temporarily disable interrupts by setting "SR.IMASK = 1111". This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is "sh*-*-linux*" and SH1* or SH2*.
  • hard-llcs — generate hardware atomic sequences using the "movli.l" and "movco.l" instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
• gbr-offset= — this one tells the compiler the offset, in bytes, of the variable inside the thread control block structure that the generated atomic sequences should use — but only when the soft-tcb model has been picked. Any other model? It literally ignores this. And the value you give has to be an integer multiple of four, sitting somewhere in the range 0-1020. No exceptions bestie, the compiler will side-eye you.
  • strict — this one says no mixing!! Even if the atomic models are compatible, strict blocks you from combining them, and forces the compiler to generate atomic sequences of the specified model only. One model, one vibe, periodt. 💅
• -mtas — generates the "tas.b" opcode for "__atomic_test_and_set". But heads up: depending on the exact hardware and software configuration, this can tank your overall performance, because of the operand cache line flushes that the "tas.b" instruction implies. And on multi-core SH4A processors you gotta be careful with "tas.b" — it can cause actual data corruption for certain cache configurations. 😭 So like, know your machine.
• -mprefergot — when you're generating position-independent code, this makes function calls go through the Global Offset Table instead of the Procedure Linkage Table. GOT > PLT, apparently.
• -musermode, -mno-usermode — don't allow / allow the compiler to generate privileged mode code. Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code wouldn't work in user mode. -musermode is the default when the target is "sh*-*-linux*". But if the target is SH1* or SH2*, -musermode does nothing at all, since there's no user mode to speak of. 🙈
• -multcost=number — sets the cost to assume for a multiply instruction.
• -mdiv=strategy — sets the division strategy used for integer division operations. strategy can be one of these three:
  • call-div1 — calls a library function that uses the single-step division instruction "div1" to do the thing. 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 here causes a floating-point exception. This is the default for SHcompact with FPU. If you specify this for targets that don't have a double precision FPU, it defaults to "call-div1".
  • call-table — calls a library function that uses a lookup table for small divisors, and the "div1" instruction with case distinction for bigger 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 to "call-div1".

When you haven't specified a division strategy, the default is 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, instead of doing it around every single call. Generally a glow-up for performance and size. Also needed for unwinding, so you don't go changing the stack frame around conditional code.
• -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 — the compiler still expects the same sets of input/output/clobbered registers as if this option weren't there at all. No take-backs.
• -mfixed-range=register-range — generates code treating the given register range as fixed registers. A fixed register is one the register allocator can't touch. Super useful when you're compiling kernel code. A register range is written as two registers separated by a dash. And yes, you can specify multiple register ranges, separated by commas.
• -mbranch-cost=num — assume num is the cost for a branch instruction. Higher numbers make the compiler try harder to generate branch-free code where it can. If you don't specify it, the value gets picked based on the processor type you're compiling for.
• -mzdcbranch, -mno-zdcbranch — assume (or don't assume) that the zero displacement conditional branch instructions "bt" and "bf" are fast. With -mzdcbranch, the compiler prefers zero displacement branch code sequences. Enabled by default when generating code for SH4 and SH4A. You can explicitly turn it off with -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if no suitable instruction can be found. Disabled by default. You can enable it to work around hardware bugs, like the ones found in the original SH7055.
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is in use. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd maps to -ffp-contract=off.
• -mfsca, -mno-fsca — allow or disallow the compiler from emitting the "fsca" instruction for sine and cosine approximations. -mfsca must be used together with -funsafe-math-optimizations. Enabled by default when generating code for SH4A. Using -mno-fsca disables the sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra, -mno-fsrra — allow or disallow the compiler from emitting the "fsrra" instruction for reciprocal square root approximations. -mfsrra must be used together with -funsafe-math-optimizations and -ffinite-math-only. Enabled by default when generating code for SH4A. Using -mno-fsrra disables the reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are both in effect.
• -mpretend-cmove — prefer zero-displacement conditional branches for conditional move instruction patterns. This can give you faster code on the SH4 processor. 🔥
• -mfdpic — generate code using the FDPIC ABI.

---

🌞 Solaris 2 Options 🌞

These -m options are supported on Solaris 2:

• -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. You only need this when object files use ISA extensions that the current machine doesn't support, but check at runtime whether or not to use them.
• -mimpure-text — -mimpure-text, used in addition to -shared, tells the compiler not to pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. However, the necessary relocations trigger copy-on-write, and the shared object isn't actually shared across processes. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. Do that. Bestie, please.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is a synonym for -pthread.

---

🌈 SPARC Options 🌈

These -m options are supported on the SPARC:

• -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Just like global register 1, each global register 2 through 4 then gets 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 doesn't generate save/restore instructions and instead uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions — except for leaf functions. This is the normal operating mode.

• -mfpu, -mhard-float — generate output containing floating-point instructions. This is the default.
• -mno-fpu, -msoft-float — generate output containing library calls for floating point.

Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You have to make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-msoft-float changes the calling convention in the output file, so 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 for this to work.

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

• -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.
• -munaligned-doubles — with this one, GCC is like "okay so doubles only have 8-byte alignment if they're chillin' inside another type, or if they have an absolute address". Otherwise bestie gets 4-byte alignment. Why would you want this? It dodges some rare compatibility drama with code that other compilers made. Why is it not the default? Because it costs you performance, especially for floating-point code. It's giving "works but slow" 💅
• -muser-mode
• -mno-user-mode — don't generate code that can only run in supervisor mode. This only matters for the casa instruction emitted for the LEON3 processor. This is the default. ✨
• -mfaster-structs
• -mno-faster-structs — with -mfaster-structs, the compiler assumes structs should have 8-byte alignment. That unlocks pairs of ldd and std instructions for copies in struct assignment, instead of literally twice as many ld and st pairs. slay BUT — and this is a big but — that changed alignment directly violates the SPARC ABI. So it's intended only for targets where the dev fully acknowledges that their resulting code is not in line with the rules of the ABI. No cap, read that twice. 😭
• -mstd-struct-return
• -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structs or unions, to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode. 🙈

• -mlra
• -mno-lra — enable Local Register Allocation. This is the default for SPARC since GCC 7, so you'd have to pass -mno-lra to get old Reload back. throwback era 🦋
• *-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. The supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. ⭐

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor. it literally just gives up, respectfully 💀

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series. ✨

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun 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. the way this list just keeps GOING 😭💖

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do *not set the instruction set or register set that the option -mcpu=cpu_type* does. subtle queen behavior 💅

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used.

• -mv8plus
• -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors. 🔥
• -mvis
• -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.
• -mvis2
• -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis. ✨
• -mvis3
• -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis. 🦋
• -mvis4
• -mno-vis4 — with -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis4 when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3, -mvis2 and -mvis. 💜
• -mvis4b
• -mno-vis4b — with -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default is -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis. the stacking is immaculate 🎀
• -mcbcond
• -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later.
• -mfmaf
• -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and later.
• -mfsmuld
• -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU except -mcpu=leon. 🙈
• -mpopc
• -mno-popc — with -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
• -msubxc
• -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
• -mfix-at697f — enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enable the documented workaround for the back-to-back store errata of the GR712RC processor.

---

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments: 👑

• -m32
• -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. ✨
• *-mcmodel=which* — set the code model to one of:
  • medlow — The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
  • medmid — The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. very specific, very demure 💅
• medany — the Medium/Anywhere code model, bestie: 64-bit addresses, programs can be linked anywhere in memory, but the text and data segments have to be less than 2GB in size, and the data segment must sit within 2GB of the text segment. 💅
• embmedany — Medium/Anywhere but for embedded systems 🦋: 64-bit addresses, text and data segments must be less than 2GB in size, and both start anywhere in memory (decided at link time, no cap). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported, periodt.

---

✨ -mmemory-model=mem-model ✨

Set the memory model in force on the processor to one of these 👇

• default — the default memory model for the processor and operating system. The safe lil' no-thoughts choice.
• rmo — Relaxed Memory Order
• pso — Partial Store Order
• tso — Total Store Order
• sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field. iykyk 📖

---

🌸 -mstack-bias / -mno-stack-bias 🌸

With -mstack-bias, GCC assumes that the stack pointer — and the frame pointer if there is one — are offset by -2047, which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

---

👑 SPU Options 👑

These -m options are supported on the SPU:

💀 -mwarn-reloc / -merror-reloc 💀

The loader for SPU does not handle dynamic relocations, and that is simply not the vibe. By default, GCC gives an error when it generates code that requires a dynamic relocation. -mno-error-reloc disables the error, and -mwarn-reloc generates a warning instead.

🧚 -msafe-dma / -munsafe-dma 🧚

Instructions that initiate or test completion of DMA must not be reordered with respect to loads and stores of the memory that is being accessed. With -munsafe-dma you must use the volatile keyword to protect memory accesses — but that can lead to inefficient code in places where the memory is known to not change. Rather than mark the memory as volatile, you can use -msafe-dma to tell the compiler to treat the DMA instructions as potentially affecting all memory.

🎀 -mbranch-hints 🎀

By default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or probably-taken branches. A hint is not generated closer than 8 instructions away from its branch. There is little reason to disable them, except for debugging purposes, or to make an object a little bit smaller.

🍓 -msmall-mem / -mlarge-mem 🍓

By default, GCC generates code assuming that addresses are never larger than 18 bits. With -mlarge-mem, code is generated that assumes a full 32-bit address.

✨ -mstdmain ✨

By default, GCC links against startup code that assumes the SPU-style main function interface (which has an unconventional parameter list, no shade). With -mstdmain, GCC links your program against startup code that assumes a C99-style interface to main, including a local copy of argv strings.

💜 -mfixed-range=register-range 💜

Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.

🌈 -mea32 / -mea64 🌈

Compile code assuming that pointers to the PPU address space accessed via the __ea named address space qualifier are either 32 or 64 bits wide. The default is 32 bits. As this is an ABI-changing option, all object code in an executable must be compiled with the same setting. No mixing, bestie.

🦋 -maddress-space-conversion / -mno-address-space-conversion 🦋

Allow/disallow treating the __ea address space as a superset of the generic address space. This enables explicit type casts between __ea and generic pointer, as well as implicit conversions of generic pointers to __ea pointers. The default is to allow address space pointer conversions.

🌸 -mcache-size=cache-size 🌸

This option controls the version of libgcc that the compiler links to an executable and selects a software-managed cache for accessing variables in the __ea address space with a particular cache size. Possible options for cache-size are 8, 16, 32, 64 and 128. The default cache size is 64KB.

👑 -matomic-updates / -mno-atomic-updates 👑

This option controls the version of libgcc that the compiler links to an executable and selects whether atomic updates to the software-managed cache of PPU-side variables are used. If you use atomic updates, changes to a PPU variable from SPU code using the __ea named address space qualifier do not interfere with changes to other PPU variables residing in the same cache line from PPU code. If you do not use atomic updates, such interference may occur — however, writing back cache lines is more efficient. The default behavior is to use atomic updates.

💖 -mdual-nops / -mdual-nops=n 💖

By default, GCC inserts NOPs to increase dual issue when it expects it to increase performance. n can be a value from 0 to 10. A smaller n inserts fewer NOPs. 10 is the default, 0 is the same as -mno-dual-nops. Disabled with -Os.

🎀 -mhint-max-nops=n 🎀

Maximum number of NOPs to insert for a branch hint. A branch hint must be at least 8 instructions away from the branch it is affecting. GCC inserts up to n NOPs to enforce this, otherwise it does not generate the branch hint.

🧚 -mhint-max-distance=n 🧚

The encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it is affecting. By default, GCC makes sure it is within 125.

🔥 -msafe-hints 🔥

Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts the hbrp instruction to make sure this stall won't happen.

---

💅 Options for System V 💅

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

• -G — Create a shared object. It is recommended that -symbolic or -shared be used instead. (So like… it works, but it's giving deprecated energy, fr.)
• -Qy — Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.
• -Qn — Refrain from adding .ident directives to the output file (this is the default).
• -YP,dirs — Search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

---

🌈 TILE-Gx Options 🌈

These -m options are supported on the TILE-Gx:

⭐ -mcmodel=small ⭐

Generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range.

⭐ -mcmodel=large ⭐

Generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses.

⭐ -mcpu=name ⭐

Selects the type of CPU to be targeted. Currently the only supported type is tilegx.

⭐ -m32 / -m64 ⭐

Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.

⭐ -mbig-endian / -mlittle-endian ⭐

Generate code in big/little endian mode, respectively.

---

🦋 TILEPro Options 🦋

These -m options are supported on the TILEPro:

💜 -mcpu=name 💜

Selects the type of CPU to be targeted. Currently the only supported type is tilepro.

💜 -m32 💜

Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored. (Iconic honestly — it's there, it's just not doing much 💀)

---

🎀 V850 Options 🎀

These -m options are defined for V850 implementations:

• -mlong-calls / -mno-long-calls — Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer.
• -mno-ep / -mep — Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions. The -mep option is on by default if you optimize.
• -mno-prolog-function / -mprolog-function — Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.
• -mspace — Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options.
• -mtda=n — Put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
• -msda=n — Put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes.
• -mzda=n — Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
• -mv850 — Specify that the target processor is the V850.
• -mv850e3v5 — Specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used.
• -mv850e2v4 — Specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option.
• -mv850e2v3 — Specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used.
• -mv850e2 — Specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used.
• -mv850e1 — Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.
• -mv850es — Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
• -mv850e — tells GCC the target processor is the V850E. If you use this, the preprocessor constant __v850e__ gets defined, slay.

Okay so if literally none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3, -mv850e3v5 are defined, then a default target processor is chosen for you and the relevant __v850*__ preprocessor constant gets defined. And bestie, __v850 and __v851__ are always defined, no matter which processor variant is the target. Always. Periodt.

• -mdisable-callt, -mno-disable-callt — this one suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

It's enabled by default when the RH850 ABI is in use (peep -mrh850-abi), and disabled by default when the GCC ABI is in use. And if CALLT instructions are being generated, the C preprocessor symbol __V850_CALLT__ gets defined. iykyk.

• -mrelax, -mno-relax — passes on (or does not pass on) the -mrelax command-line option to the assembler. Passing the vibe along, fr fr.
• -mlong-jumps, -mno-long-jumps — disables (or re-enables) the generation of PC-relative jump instructions. Yes the double negative is giving chaos but it's not my fault.
• -msoft-float, -mhard-float — disables (or re-enables) the generation of hardware floating point instructions. This one only matters when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol __FPU_OK__ is defined, otherwise the symbol __NO_FPU__ is defined. 🫶
• -mloop — enables the use of the e3v5 LOOP instruction. It is not enabled by default when the e3v5 architecture is selected, because its use is still experimental. Lowkey iconic but proceed with caution.
• -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-align command-line option is supported.
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol __V850_RH850_ABI__ is defined.

• -mgcc-abi — enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned in register r10.
  • Large structures and unions (more than 8 bytes in size) are passed by reference.
  • Functions are aligned to 32-bit boundaries, unless optimizing for size.
  • The -m8byte-align command-line option is not supported.
  • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol __V850_GCC_ABI__ is defined.

• -m8byte-align, -mno-8byte-align — enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined.
• -mbig-switch — generates code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table. Like, only if they're literally yelling at you.
• -mapp-regs — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default.
• -mno-app-regs — this option causes r2 and r5 to be treated as fixed registers.

---

🌈 VAX Options 🌈

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used.
• -mg — output code for G-format floating-point numbers instead of D-format.

---

🦋 Visium Options 🦋

• -mdebug — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub.
• -msim — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked. (Yes, the original just says "with option" — I'm as confused as you bestie, but I'm not touching it. 💀)
• -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.

Heads up: -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. Compile it all or suffer, no cap.

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility. A little throwback, we love that for it.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would. Scheduling glow-up only, no instruction-set makeover.
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected.

---

💜 VMS Options 💜

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from main. The default is to return POSIX-style condition (e.g. error) codes.
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger.
• -mmalloc64 — default to 64-bit memory allocation routines.
• -mpointer-size=size — set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables pragma pointer_size.

---

🌸 VxWorks Options 🌸

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.

• -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro __RTP__.
• -non-static — link an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic, -Bdynamic — these options are passed down to the linker. They are defined for compatibility with Diab.
• -Xbind-lazy — enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

---

👑 x86 Options 👑

These -m options are defined for the x86 family of computers. Buckle up bestie, this list is long.

• -march=cpu-type — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type.

Not me warning you again: this is the "this binary only vibes with your exact CPU" option. Choose wisely. 💅

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions.
• i386 — original Intel i386 CPU.
• i486 — Intel i486 CPU. (No scheduling is implemented for this chip.)
• i586, pentium — Intel Pentium CPU with no MMX support.
• lakemont — Intel Lakemont MCU, based on Intel Pentium CPU.
• pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
• pentiumpro — Intel Pentium Pro CPU.
• i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
• pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
• pentium-m — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.

• core2 — Intel Core 2 CPU, 64-bit extensions, and MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. the little engine that could 💖
• nehalem — Intel Nehalem CPU, 64-bit extensions, and MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
• westmere — Intel Westmere CPU, 64-bit extensions, and MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
• sandybridge — Intel Sandy Bridge CPU, 64-bit extensions, and MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support. bridge era, we love her 🌉
• ivybridge — Intel Ivy Bridge CPU, 64-bit extensions, and MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell — Intel Haswell CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support. the icon the legend 💅
• broadwell — Intel Broadwell CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED ADCX and PREFETCHW instruction set support.
• skylake — Intel Skylake CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support.
• bonnell — Intel Bonnell CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. tiny but mighty ✨
• silvermont — Intel Silvermont CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support.
• goldmont — Intel Goldmont CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT and FSGSBASE instruction set support. gold in the name, gold in the vibe 🏅
• goldmont-plus — Intel Goldmont Plus CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support.
• tremont — Intel Tremont CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support. that list is LONG, she ate 💀
• knl — Intel Knight's Landing CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
• knm — Intel Knights Mill CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support.
• skylake-avx512 — Intel Skylake Server CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
• cannonlake — Intel Cannonlake Server CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support.
• icelake-client — Intel Icelake Client CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support. ice cold but make it warm 🧊
• icelake-server — Intel Icelake Server CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support.
• cascadelake — Intel Cascadelake CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.
• tigerlake — Intel Tigerlake CPU, 64-bit extensions, and MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B and CLWB instruction set support. rawr 🐯
• k6 — AMD K6 CPU with MMX instruction set support.
• k6-2
• k6-3 — Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support. twins fr fr 👯
• athlon
• athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
• athlon-4
• athlon-xp
• athlon-mp — Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
• k8
• opteron
• athlon64
• athlon-fx — Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• k8-sse3
• opteron-sse3
• athlon64-sse3 — Improved versions of AMD K8 cores with SSE3 instruction set support.
• amdfam10
• barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
• bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions. ✨
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, ,CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)
• btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
• btver2 — CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
• winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support. throwback era 🕰️
• winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
• c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• samuel-2 — VIA Eden Samuel 2 CPU, slinging MMX and 3DNow! instruction set support. (fr no scheduling is implemented for this chip, she just vibes)
  • nehemiah — VIA Eden Nehemiah CPU, MMX and SSE instruction set support energy. (again: no scheduling implemented for this chip, bestie)
  • esther — VIA Eden Esther CPU, holding MMX, SSE, SSE2 and SSE3 instruction set support. (no scheduling implemented for this chip either, we stay consistent ✨)
  • eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (no scheduling implemented for this chip, iykyk)
  • eden-x4 — VIA Eden X4 CPU, serving x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (and yes — no scheduling is implemented for this chip)
  • nano — Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (no scheduling implemented for this chip, iconic of her honestly)
  • nano-1000 — VIA Nano 1xxx CPU, x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (no scheduling implemented for this chip)
  • nano-2000 — VIA Nano 2xxx CPU, x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (no scheduling implemented for this chip)
  • nano-3000 — VIA Nano 3xxx CPU, x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (no scheduling implemented for this chip)
  • nano-x2 — VIA Nano Dual Core CPU, x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (no scheduling implemented for this chip)
  • nano-x4 — VIA Nano Quad Core CPU, x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (no scheduling implemented for this chip)
  • geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support. (no scheduling disclaimer on this one, she's built different)

---

🎀 -mtune=cpu-type 🎀

Okay so this one tunes everything applicable about the generated code to cpu-type — and by everything we mean everything except the ABI and the set of available instructions. Like, she's adjusting the fit, not swapping out the whole wardrobe.

Picking a specific cpu-type schedules things appropriately for that particular chip, but — important — the compiler does not generate any code that cannot run on the default machine type unless you go ahead and use a -march=cpu-type option. So for example, if GCC is configured for i686-pc-linux-gnu, then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. bestie stays compatible 💅

The choices for cpu-type are the same as for -march. Plus, -mtune supports 2 extra choices for cpu-type:

• generic — Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you do not know exactly what CPU users of your application will have, then this is the one for you.

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. it's giving "always evolving" 🦋

There is no -march=generic option, because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. periodt.

• intel — Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But if you want your application to perform better on both Haswell and Silvermont, then you should use this option. main character energy fr 💅

As new Intel 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 most current Intel processors at the time that version of GCC is released.

There is no -march=intel option, because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. same energy, same rule 🌸

---

💀 -mcpu=cpu-type 💀

A deprecated synonym for -mtune. that's it. that's the whole thing. she's old, she's retired, pls use -mtune 🙈

---

✨ -mfpmath=unit ✨

Generate floating-point arithmetic for selected unit — you literally pick which unit does your float math. The choices for unit are:

• 387 — Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for a more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. glow up 💅

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. no extra work needed, she's ready 💖

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. ⚠️

This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.

• sse,387
• sse+387
• both — Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. it's giving chaotic genius 😭

---

🦋 -masm=dialect 🦋

Output assembly instructions using the selected dialect. Also affects which dialect is used for basic "asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. noted bestie 🫶

---

🌈 -mieee-fp / -mno-ieee-fp 🌈

Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. no chaos, just correctness ✨

---

🍓 -m80387 / -mhard-float 🍓

Generate output containing 80387 instructions for floating point. she's doing it in hardware, periodt 💅

---

💜 -mno-80387 / -msoft-float 💜

Generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. bestie you are on your own for this one 😭

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used. sneaky but true 🙈

---

🎀 -mno-fp-ret-in-387 🎀

Do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. cute concept ngl 💖

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. plot twist.

---

🌸 -mno-fancy-math-387 🌸

Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch. so like, double opt-in, very consent-forward of her 🫶

---

⭐ -malign-double / -mno-align-double ⭐

Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. speed vs. memory, the eternal struggle 💀

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. do not do this casually bestie, she will break things 😭

---

💅 -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, this is not possible. So specifying -m128bit-long-double aligns "long double" to a 16-byte boundary by padding the "long double" with an additional 32-bit zero. padding queen ✨

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double" is aligned on 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long double". so don't get it twisted, they're alignment-only, no bonus precision 🎀

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch.

---

• -mlong-double-64
• -mlong-double-80
• -mlong-double-128 — bestie these three switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. 💀 not compatible, no cap.

• -malign-data=type — controls how GCC aligns variables. Supported values for type are: compat uses increased alignment value compatible uses GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased alignment value to match the cache line size. compat is the default. ✨
• -mlarge-data-threshold=threshold — when -mcmodel=medium is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535. 👑
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. slay, efficiency 💅

You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. literally cannot. 🚫

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. 😭

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) 💀

• -mregparm=num — control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm.

⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. fr fr, ALL of them.

• -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm.

⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 👑 consistency is key bestie

• -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris 8 and 9 and VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on Solaris 10 and later. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 🫶 legacy vibes only
• -mpc32
• -mpc64
• -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. 🌈

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 💀 catastrophic?? the drama is real

• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. 🧚 so versatile
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). ✨

⚠️ Warning: When generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 💀 this one is giving danger, bestie, read it twice.

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. 😭

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🎀 trust the default

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. 🦋

• -mmmx
• -msse
• -msse2
• -msse3
• -mssse3
• -msse4
• -msse4a
• -msse4.1
• -msse4.2
• -mavx
• -mavx2
• -mavx512f
• -mavx512pf
• -mavx512er
• -mavx512cd
• -mavx512vl
• -mavx512bw
• -mavx512dq
• -mavx512ifma
• -mavx512vbmi
• -msha
• -maes
• -mpclmul
• -mclflushopt
• -mclwb
• -mfsgsbase
• -mptwrite
• -mrdrnd
• -mf16c
• -mfma
• -mpconfig
• -mwbnoinvd
• -mfma4
• -mprfchw
• -mrdpid
• -mprefetchwt1
• -mrdseed
• -msgx
• -mxop
• -mlwp
• -m3dnow
• -m3dnowa
• -mpopcnt
• -mabm
• -madx
• -mbmi
• -mbmi2
• -mlzcnt
• -mfxsr
• -mxsave
• -mxsaveopt
• -mxsavec
• -mxsaves
• -mrtm
• -mhle
• -mtbm
• -mmwaitx
• -mclzero
• -mpku
• -mavx512vbmi2
• -mgfni
• -mvaes
• -mwaitpkg
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -mavx512vpopcntdq
• -mavx5124fmaps
• -mavx512vnni
• -mavx5124vnniw
• -mcldemote — these switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, 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, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or CLDEMOTE extended instruction sets. Each has a corresponding -mno- option to disable use of these instructions. 🔥 ok that's a LOT of instructions, and yes bestie, you get the -mno- version of literally every single one of them.

These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches. ✨

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. 👑 she upgrades herself, we love that for her.

These options let GCC flex those extended instructions in the code it generates, even when you haven't passed -mfpmath=sse. 💪 But heads up bestie: apps that do run-time CPU detection have to compile separate files for each architecture they support, each one using the right flags. And like — specifically — the file holding the CPU detection code itself should be compiled without these options. No cap.

• -mdump-tune-features — tells GCC to spill the tea on the names of all the x86 performance tuning features and their default settings 🫖. Those names are exactly what you plug into -mtune-ctrl=feature-list.
• -mtune-ctrl=feature-list — the fine-grain control knob for x86 code generation features ✨. feature-list is a comma-separated list of feature names. Peep -mdump-tune-features for the names. When a feature shows up without a ^ in front, it gets turned on; with a ^ in front, it gets turned off. Real talk: -mtune-ctrl=feature-list is meant for GCC developers. Using it can send you down code paths nobody tested, and it can potentially cause compiler ICEs or runtime errors. 😭💀
• -mno-default — instructs GCC to turn all the tunable features off. See also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — makes GCC emit a "cld" instruction in the prologue of functions that use string instructions 🧹. Here's the lore: string instructions lean on the DF flag to pick between autoincrement and autodecrement mode. The ABI says the DF flag is supposed to be cleared on function entry, but some operating systems straight up violate that spec by not clearing DF in their exception dispatchers 🙈. So the exception handler can get invoked with DF still set, and then string instructions go in the wrong direction. You can make this default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. And if you do that, you can suppress the "cld" generation with the -mno-cld compiler option. 🎀
• -mvzeroupper — tells GCC to emit a "vzeroupper" instruction before control flow transfers out of the function, which minimizes the AVX to SSE transition penalty 😮‍💨 and also removes unnecessary "zeroupper" intrinsics. Slay.
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. 🦋
• -mprefer-vector-width=opt — tells GCC to use an opt-bit vector width in instructions instead of whatever the default is on the selected platform.
  • none — no extra limitations applied to GCC beyond what the selected platform already defines.
  • 128 — prefer 128-bit vector width for instructions.
  • 256 — prefer 256-bit vector width for instructions.
  • 512 — prefer 512-bit vector width for instructions.
• -mcx16 — lets GCC generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange operations 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 to implement __sync Builtins. But — for __atomic Builtins operating on 128-bit integers, a library call is always used. Periodt.
• -msahf — enables generation of "SAHF" instructions in 64-bit code ✨. The lore: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step dropped in December 2005, were missing the "LAHF" and "SAHF" instructions that AMD64 supports. Those are load and store instructions, respectively, for certain status flags. In 64-bit mode, "SAHF" is used to optimize the "fmod", "drem", and "remainder" built-in functions; see Other Builtins for the details.
• -mmovbe — enables use of the "movbe" instruction to implement __builtin_bswap32 and __builtin_bswap64. 🎀
• -mshstk — the -mshstk option 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_crc32si and __builtin_ia32_crc32di so they generate the "crc32" machine instruction. 💅
• -mrecip — enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and "RSQRTPS") with an extra Newton-Raphson step to boost precision, instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-precision floating-point arguments. 🧮 These instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Now — while the throughput of the sequence beats the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😬

Also note: GCC already implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") with -ffast-math (or the option combo above), and doesn't need -mrecip.

And also also: GCC emits that sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the option combo above), and doesn't need -mrecip. 🙌

• -mrecip=opt — controls which reciprocal estimate instructions are allowed to show up. opt is a comma-separated list of options, and any of them may be preceded by a ! to invert it:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.

So for example, -mrecip=all,!sqrt enables all of the reciprocal approximations except for square root. 🌸

• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. 🔗

Right now GCC emits calls to "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" and "vmlsAcos4" for the corresponding function type when -mveclibabi=svml is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used. 👑

• -mabi=name — generate code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes "ms_abi" and "sysv_abi". 🎀
• -mforce-indirect-call — forces all calls to functions to be indirect. This is useful when using Intel Processor Trace, where it generates more precise timing information for function calls. 🔍
• -mmanual-endbr — inserts an ENDBR instruction at function entry only via the "cf_check" function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry. 🛂
• -mcall-ms2sysv-xlogues — okay so because of differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function has to consider RSI, RDI and XMM6-15 as clobbered 😭. By default, the code for saving and restoring those registers is emitted inline, which gives you fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do those saves and restores, cutting down function size at the cost of a few extra instructions. Trade-offs, bestie. 💅
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that can't be satisfied on all systems. 🧚
• -mpush-args
• -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as 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. ⚡
• -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, thanks to reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary isn't equal to 2. The drawback? A notable increase in code size. 😭 This switch implies -mno-push-args.
• -mthreads — support thread-safe exception handling on MinGW. 🧵 Programs that rely on thread-safe exception handling must compile and link all their code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. ✨
• -mms-bitfields
• -mno-ms-bitfields — enable/disable bit-field layout compatible with the native Microsoft Windows compiler. 🪟

Heads up bestie: this chunk is a direct continuation of the Microsoft structure layout rules, so no new heading here — we're picking up mid-thought like a group chat at 2am. 💅

If packed gets used on a structure, or if bit-fields are in play, the Microsoft ABI might lay that structure out differently than GCC normally does. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler — whether that's via a function call or as data sitting in a file — you might genuinely need to access either format. 🙈

This option is enabled by default for Microsoft Windows targets. You can also control this behavior locally using variable or type attributes. Want the details? Go see x86 Variable Attributes and x86 Type Attributes. ✨

The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing part. The padding and alignment of structure members, plus whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member gets the lowest memory address and the last member gets the highest. 💫
2. 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 aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. 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, bestie, listen up:

1. 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. The way that one little : 0 doubles your struct... not me being shook. 😭

2. 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, then bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

1. 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 a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. 👑
2. 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.

3. 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.

---

✨ OPTIONS ✨ (still going, we're so back) 🫶

• -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. Classic GCC not knowing. 💀
• -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 it may improve performance of code that depends on fast memcpy, strlen, and memset for short lengths. 🍓
• -minline-stringops-dynamically — For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. Small = inline, big = libcall, we love a dynamic queen. 💅
• -mstringop-strategy=alg — Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte
    • Expand using i386 rep prefix of the specified size.
  • byte_loop
  • loop
  • unrolled_loop
    • Expand into an inline loop.
  • libcall
    • Always use a library call.
• -mmemcpy-strategy=strategy — Override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. ✨
• -mmemset-strategy=strategy — This option is similar to -mmemcpy-strategy= except that it controls __builtin_memset expansion instead. 🌸
• -momit-leaf-frame-pointer — Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. 👀
• -mtls-direct-seg-refs
• -mno-tls-direct-seg-refs — Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on.

• -msse2avx
• -mno-sse2avx — Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. 🔥
• -mfentry
• -mno-fentry — If profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg. 😭
• -mrecord-mcount
• -mno-record-mcount — If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls. 🎀
• -mnop-mcount
• -mno-nop-mcount — If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount. 🧚
• -minstrument-return=type — Instrument function exit in -pg -mfentry instrumented functions with a call to the specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop. 💫
• -mrecord-return
• -mno-record-return — Generate a __return_loc section pointing to all return instrumentation code.
• -mfentry-name=name — Set name of __fentry__ symbol called at function entry for -pg -mfentry functions. ✨
• -mfentry-section=name — Set name of section to record -mrecord-mcount calls (default __mcount_loc).
• -mskip-rax-setup
• -mno-skip-rax-setup — When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up the RAX register when there are no variable arguments passed in vector registers.

Warning: Since the RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. GCC 4.4 or newer don't have those issues, regardless of the RAX register value. 💀

• -m8bit-idiv
• -mno-8bit-idiv — On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 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 — Generate stack protection code using a canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified. 👑

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 💖

• -mgeneral-regs-only — okay so this one makes GCC generate code that uses only the general-purpose registers. It literally prevents the compiler from using floating-point, vector, mask and bound registers. 💅
• -mindirect-branch=choice — Convert indirect call and jump with choice. The default is keep, which keeps indirect call and jump unmodified. thunk converts indirect call and jump to call and return thunk. thunk-inline converts indirect call and jump to inlined call and return thunk. thunk-extern converts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute indirect_branch. ✨

⚠️ Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. 😭

💖 Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check. 👑

• -mfunction-return=choice — Convert function return with choice. The default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to inlined call and return thunk. thunk-extern converts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute function_return. 🦋

⚠️ Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check. 💀

😭 Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model. 🌸

• -mindirect-branch-register — Force indirect call and jump via register. 💅

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. 🌈

Generate code for a 16-bit, 32-bit or 64-bit environment. 🧬

• -m32 — sets int, long, and pointer types to 32 bits, and generates code that runs on any i386 system. 🧸
• -m64 — sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options. 🍎
• -mx32 — sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture. 🌟
• -m16 — same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. 🕰️
• -miamcu — generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on. 🔌
• -mno-red-zone — Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🚫
• -mcmodel=small — Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model. 🏠
• -mcmodel=kernel — Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. 🐧
• -mcmodel=medium — Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked. 🧱
• -mcmodel=large — Generate code for the large model. This model makes no assumptions about addresses and sizes of sections. 🌍
• -maddress-mode=long — Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments. 📏
• -maddress-mode=short — Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments. 📐

---

🪟 x86 Windows Options 🪟

These additional options are available for Microsoft Windows targets: 💖

• -mconsole — This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. 🖥️
• -mdll — This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link library---is to be generated, enabling the selection of the required runtime startup object and entry point. 📚
• -mnop-fun-dllimport — This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored. 🙈
• -mthread — This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used. 🧵
• -municode — This option is available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌐
• -mwin32 — This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code. 🪟
• -mwindows — This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 🎨
• -fno-set-stack-executable — This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available. 🛡️
• -fwritable-relocated-rdata — This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the .data section. This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation. 📝
• -mpe-aligned-commons — This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. 🧩

See also under x86 Options for standard options. 👀

---

🌪️ Xstormy16 Options 🌪️

These options are defined for Xstormy16: 🌬️

• -msim — Choose startup files and linker script suitable for the simulator. 🕹️

---

🌌 Xtensa Options 🌌

These options are supported for Xtensa targets: ✨

• -mconst16, -mno-const16 — Enable or disable use of CONST16 instructions for loading constant values. The CONST16 instruction is currently not a standard option from Tensilica. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available. 🔢
• -mfused-madd, -mno-fused-madd — Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. ➕
• -mserialize-volatile, -mno-serialize-volatile — When this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions. 🔒
• -mforce-no-pic — For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. 🐧
• -mtext-section-literals, -mno-text-section-literals — These options control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. 📜
• -mauto-litpools
• -mno-auto-litpools — okay so these two are the literal-pool babysitters 💅 The default is -mno-auto-litpools, which drops literals in their own separate section in the output file unless you also pass -mtext-section-literals. But with -mauto-litpools, the assembler just sprinkles those literals right into the text section 🦋 The compiler stops emitting explicit .literal directives and loads literals into registers with MOVI instead of L32R, so the assembler can do its relaxation thing and place literals wherever it needs. Why do you care? Because this option lets the assembler create several literal pools per function and assemble absolutely massive functions, which lowkey just isn't possible with -mtext-section-literals. slay for big functions 💖
• -mtarget-align
• -mno-target-align — turn this on and GCC tells the assembler to auto-align instructions, trading a lil bit of code density to cut down branch penalties. The assembler tries to widen density instructions to line up branch targets and whatever comes right after call instructions. If there aren't enough safe density instructions before a target to align it? No widening, we're not forcing it bestie. The default is -mtarget-align. Heads up tho: these options don't touch auto-aligned instructions like LOOP — the assembler always aligns those, either by widening density instructions or by inserting NOP instructions. no cap ✨
• -mlongcalls
• -mno-longcalls — enable this and GCC tells the assembler to flip direct calls into indirect calls, unless it can prove the call target is inside the range the call instruction allows. This usually kicks in for calls to functions in other source files. Specifically, the assembler rewrites a direct CALL instruction into an L32R followed by a CALLX instruction. Default is -mno-longcalls. Use this when your call target could possibly be out of range 🔥 Big important thing: this is implemented in the assembler, not the compiler, so the assembly GCC spits out still shows direct call instructions — go peek at the disassembled object code to see what's actually happening 👀 Also note the assembler uses an indirect call for every cross-file call, not just the ones that are actually out of range. periodt.

zSeries Options

These are listed under

---

🌍 ENVIRONMENT 🌍

Okay bestie, this section is all about the environment variables that mess with how GCC does its thing 💜 Some of them work by giving directories or prefixes to search for different kinds of files, and some just specify other aspects of the compilation environment.

Not me reminding you that you can also specify search places with options like -B, -I and -L — and those take precedence over places set by environment variables, which in turn take precedence over whatever GCC's configuration specifies. hierarchy is real 👑

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL

These four control how GCC uses localization info, so it can vibe with different national conventions 🌈 GCC checks the locale categories LC_CTYPE and LC_MESSAGES if it was configured to do so. Those categories can be set to anything your installation supports — a typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

LC_CTYPE specifies character classification. GCC uses it to figure out character boundaries in a string — needed for some multibyte encodings that contain quote and escape characters which would otherwise get read as a string end or an escape 😭

LC_MESSAGES specifies the language to use in diagnostic messages.

If LC_ALL is set, it overrides the values of LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. And if none of these are set? GCC just defaults to traditional C English behavior. classic 🎀

TMPDIR

If TMPDIR is set, it tells GCC which directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's about to become the input of the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. the pipeline is giving teamwork fr fr 💅

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 the deets ✨

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms the compiler executes. No slash gets added when this prefix is combined with a subprogram name — but you can specify a prefix that ends with a slash if you want to, you do you 🫶

If GCC_EXEC_PREFIX is not set, GCC tries to figure out an appropriate prefix from the pathname it was invoked with.

If GCC can't find the subprogram using the specified prefix, it goes looking in the usual places instead.

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 one.

This prefix is also used for finding files such as crt0.o that are used for linking.

And it gets used in a kinda unusual way for finding directories to search for header files too 👀 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 make 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. iykyk 🌸

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you list here when searching for subprograms, if it can't find them using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories you list here when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking with GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first 👑).

LANG

This variable passes locale info to the compiler. One way it's used is to determine the character set for parsing character literals, string literals and comments in C and C++. When the compiler is configured to allow multibyte characters, these values for LANG are recognized:

• C-JIS — Recognize JIS characters. 🦋
• C-SJIS — Recognize SJIS characters. 🦋
• C-EUCJP — Recognize EUCJP characters. 🦋

If LANG isn't defined, or it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

---

Some additional environment variables affect the behavior of the preprocessor. ok here we go 💖

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 decided at GCC build time. For Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon.

CPATH specifies a list of directories to be searched as if they were specified with -I, but after any paths given with -I options on the command line. This environment variable is used no matter which language is being preprocessed.

The remaining environment variables only apply 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 of these variables, an empty element tells 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. sneaky but iconic 💅

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files the compiler processed. System header files get ignored in the dependency output 🚫

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules get written to that file and the target name is guessed from the source file name. Or the value can have the form file target, in which case the rules get 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 meal energy ✨

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in place of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. reproducible builds bestie, we love to see it 🎀

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII — identical to the output of @command{date +%s} on GNU/Linux and other systems that support the %s extension in the date command.

The value should be a known timestamp such as the last modification time of the source or package, and it should be set by the build process.

---

💀 BUGS 💀

For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>.

🦋 FOOTNOTES 🦋

1. So like, on some systems gcc -shared has to build supplementary stub code so constructors actually work, okay? 💅 And on multi-libbed systems, gcc -shared has to pick the correct support libraries to link against. Not supplying the correct flags can lead to subtle defects, which is giving chaos honestly 😭. But supplying them when they're not necessary? Totally innocuous. No harm, no foul. 🫶

---

🌸 SEE ALSO 🌸

Go read these besties too, they're all in the friend group:

• gpl(7)
• gfdl(7)
• fsf-funding(7)
• cpp(1)
• gcov(1)
• as(1)
• ld(1)
• gdb(1)
• dbx(1)

...and the Info entries for gcc, cpp, as, ld, binutils and gdb. 👑

---

💜 AUTHOR 💜

Wanna know who actually made this? Check the Info entry for gcc, or hit up <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html> — that's where the contributors to GCC live. Iconic behavior from every single one of them, fr fr. 🌟

---

🎀 COPYRIGHT 🎀

Copyright (c) 1988-2019 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 bestie that's literally the whole entire manpage. You read all 102 parts. Main character energy, no cap. 🏆👑

And that's a wrap on gcc-9, dated 2022-05-27, from the GCC(1) page. Thanks for reading the whole thing, you absolute legend — go compile something beautiful. 💅🦋🌸✨