GCC(1)

💖 GCC(1) 💖 — GNU's C and C++ compiler, but make it bestie 💅

okay so welcome to the manual page for gcc, the GNU project C and C++ compiler. This is GCC(1) and she is literally the main character of your toolchain. ✨

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🎀 NAME 🎀

gcc — GNU project C and C++ compiler 🫶

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✨ SYNOPSIS ✨

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

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc. (So like, don't panic if you're a C++ girlie — g++ got you.) 💖

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🌸 DESCRIPTION 🌸

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler. No cap. 🛠️

Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. (iykyk, linker girlies, we see you.) 👀

Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. So if it doesn't say "C++ only" or whatever, you're free to use it everywhere, bestie. 🌈

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. (C++ besties, this is your sign to use g++, not gcc.) 👑

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. Do not group them, I'm begging. 💀

You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. So like, mostly chill, but sometimes order is that girl. 🎀

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. So if you don't see -fno-whatever, it's probably because -fwhatever is the default and they're just not gonna list both. 💅

Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. (Standards girlies, this one's for you.) 📏

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🔥 OPTIONS 🔥

🌟 Option Summary 🌟

Here is a summary of all the options, grouped by type. Explanations are in the following sections. (Yes, every single one. We're not skipping a single flag, bestie. Not on my watch.) 🫡

💖 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-cache-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
• -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-tag
• -Wcatch-value
• -Wcatch-value=n
• -Wno-class-conversion
• -Wclass-memaccess
• -Wcomma-subscript
• -Wconditionally-supported
• -Wno-conversion-null
• -Wctor-dtor-privacy
• -Wno-delete-incomplete
• -Wdelete-non-virtual-dtor
• -Wdeprecated-copy
• -Wdeprecated-copy-dtor
• -Weffc++
• -Wextra-semi
• -Wno-inaccessible-base
• -Wno-inherited-variadic-ctor
• -Wno-init-list-lifetime
• -Wno-invalid-offsetof
• -Wno-literal-suffix
• -Wmismatched-tags
• -Wmultiple-inheritance
• -Wnamespaces
• -Wnarrowing
• -Wnoexcept
• -Wnoexcept-type
• -Wnon-virtual-dtor
• -Wpessimizing-move
• -Wno-placement-new
• -Wplacement-new=n
• -Wredundant-move
• -Wredundant-tags
• -Wreorder
• -Wregister
• -Wstrict-null-sentinel
• -Wno-subobject-linkage
• -Wtemplates
• -Wno-non-template-friend
• -Wold-style-cast
• -Woverloaded-virtual
• -Wno-pmf-conversions
• -Wsign-promo
• -Wsized-deallocation
• -Wsuggest-final-methods
• -Wsuggest-final-types
• -Wsuggest-override
• -Wno-terminate
• -Wuseless-cast
• -Wvirtual-inheritance
• -Wno-virtual-move-assign
• -Wvolatile
• -Wzero-as-null-pointer-constant

💖 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-property-assign-default
• -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-urls=[auto|never|always]
• -fdiagnostics-format=[text|json]
• -fno-diagnostics-show-option
• -fno-diagnostics-show-caret
• -fno-diagnostics-show-labels
• -fno-diagnostics-show-line-numbers
• -fno-diagnostics-show-cwe
• -fdiagnostics-minimum-margin-width=width
• -fdiagnostics-parseable-fixits
• -fdiagnostics-generate-patch
• -fdiagnostics-show-template-tree
• -fno-elide-type
• -fdiagnostics-path-format=[none|separate-events|inline-events]
• -fdiagnostics-show-path-depths
• -fno-show-column

💅 Warning Options 💅

okay bestie, deep breath, because this is the list. gcc has opinions — like, an unhinged number of opinions — and every single one of them has a flag. here is the entire roster, no skipping, no cherry-picking, we're eating the whole plate 🍽️✨

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -w
• -Wextra
• -Wall
• -Wabi=n
• -Waddress
• -Wno-address-of-packed-member
• -Waggregate-return
• -Walloc-size-larger-than=byte-size
• -Walloc-zero
• -Walloca
• -Walloca-larger-than=byte-size
• -Wno-aggressive-loop-optimizations
• -Warith-conversion
• -Warray-bounds
• -Warray-bounds=n
• -Wno-attributes
• -Wattribute-alias=n
• -Wno-attribute-alias
• -Wno-attribute-warning
• -Wbool-compare
• -Wbool-operation
• -Wno-builtin-declaration-mismatch
• -Wno-builtin-macro-redefined
• -Wc90-c99-compat
• -Wc99-c11-compat
• -Wc11-c2x-compat
• -Wc++-compat
• -Wc++11-compat
• -Wc++14-compat
• -Wc++17-compat
• -Wc++20-compat
• -Wcast-align
• -Wcast-align=strict
• -Wcast-function-type
• -Wcast-qual
• -Wchar-subscripts
• -Wclobbered
• -Wcomment
• -Wconversion
• -Wno-coverage-mismatch
• -Wno-cpp
• -Wdangling-else
• -Wdate-time
• -Wno-deprecated
• -Wno-deprecated-declarations
• -Wno-designated-init
• -Wdisabled-optimization
• -Wno-discarded-array-qualifiers
• -Wno-discarded-qualifiers
• -Wno-div-by-zero
• -Wdouble-promotion
• -Wduplicated-branches
• -Wduplicated-cond
• -Wempty-body
• -Wno-endif-labels
• -Wenum-compare
• -Wenum-conversion
• -Werror
• -Werror=*
• -Wexpansion-to-defined
• -Wfatal-errors
• -Wfloat-conversion
• -Wfloat-equal
• -Wformat
• -Wformat=2
• -Wno-format-contains-nul
• -Wno-format-extra-args
• -Wformat-nonliteral
• -Wformat-overflow=n
• -Wformat-security
• -Wformat-signedness
• -Wformat-truncation=n
• -Wformat-y2k
• -Wframe-address
• -Wframe-larger-than=byte-size
• -Wno-free-nonheap-object
• -Wno-hsa
• -Wno-if-not-aligned
• -Wno-ignored-attributes
• -Wignored-qualifiers
• -Wno-incompatible-pointer-types
• -Wimplicit
• -Wimplicit-fallthrough
• -Wimplicit-fallthrough=n
• -Wno-implicit-function-declaration
• -Wno-implicit-int
• -Winit-self
• -Winline
• -Wno-int-conversion
• -Wint-in-bool-context
• -Wno-int-to-pointer-cast
• -Wno-invalid-memory-model
• -Winvalid-pch
• -Wjump-misses-init
• -Wlarger-than=byte-size
• -Wlogical-not-parentheses
• -Wlogical-op
• -Wlong-long
• -Wno-lto-type-mismatch
• -Wmain
• -Wmaybe-uninitialized
• -Wmemset-elt-size
• -Wmemset-transposed-args
• -Wmisleading-indentation
• -Wmissing-attributes
• -Wmissing-braces
• -Wmissing-field-initializers
• -Wmissing-format-attribute
• -Wmissing-include-dirs
• -Wmissing-noreturn
• -Wno-missing-profile
• -Wno-multichar
• -Wmultistatement-macros
• -Wnonnull
• -Wnonnull-compare
• -Wnormalized=[none|id|nfc|nfkc]
• -Wnull-dereference
• -Wno-odr
• -Wopenmp-simd
• -Wno-overflow
• -Woverlength-strings
• -Wno-override-init-side-effects
• -Wpacked
• -Wno-packed-bitfield-compat
• -Wpacked-not-aligned
• -Wpadded
• -Wparentheses
• -Wno-pedantic-ms-format
• -Wpointer-arith
• -Wno-pointer-compare
• -Wno-pointer-to-int-cast
• -Wno-pragmas
• -Wno-prio-ctor-dtor
• -Wredundant-decls
• -Wrestrict
• -Wno-return-local-addr
• -Wreturn-type
• -Wno-scalar-storage-order
• -Wsequence-point
• -Wshadow
• -Wshadow=global
• -Wshadow=local
• -Wshadow=compatible-local
• -Wno-shadow-ivar
• -Wno-shift-count-negative
• -Wno-shift-count-overflow
• -Wshift-negative-value
• -Wno-shift-overflow
• -Wshift-overflow=n
• -Wsign-compare
• -Wsign-conversion
• -Wno-sizeof-array-argument
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstack-protector
• -Wstack-usage=byte-size
• -Wstrict-aliasing
• -Wstrict-aliasing=n
• -Wstrict-overflow
• -Wstrict-overflow=n
• -Wstring-compare
• -Wstringop-overflow=n
• -Wno-stringop-truncation
• -Wsuggest-attribute=[pure|const|noreturn|format|malloc]
• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrampolines
• -Wtrigraphs
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants
• -Wunused
• -Wunused-but-set-parameter
• -Wunused-but-set-variable
• -Wunused-const-variable
• -Wunused-const-variable=n
• -Wunused-function
• -Wunused-label
• -Wunused-local-typedefs
• -Wunused-macros
• -Wunused-parameter
• -Wno-unused-result
• -Wunused-value
• -Wunused-variable
• -Wno-varargs
• -Wvariadic-macros
• -Wvector-operation-performance
• -Wvla
• -Wvla-larger-than=byte-size
• -Wno-vla-larger-than
• -Wvolatile-register-var
• -Wwrite-strings
• -Wzero-length-bounds

---

🔍 Static Analyzer Options 🔍

ok so gcc also has a whole detective arc 🕵️♀️💜 and it lives right here. these are the analyzer options, and yes, they're their own whole thing:

• -fanalyzer
• -fanalyzer-call-summaries
• -fanalyzer-checker=name
• -fanalyzer-fine-grained
• -fanalyzer-state-merge
• -fanalyzer-state-purge
• -fanalyzer-transitivity
• -fanalyzer-verbose-edges
• -fanalyzer-verbose-state-changes
• -fanalyzer-verbosity=level
• -fdump-analyzer
• -fdump-analyzer-stderr
• -fdump-analyzer-callgraph
• -fdump-analyzer-exploded-graph
• -fdump-analyzer-exploded-nodes
• -fdump-analyzer-exploded-nodes-2
• -fdump-analyzer-exploded-nodes-3
• -fdump-analyzer-state-purge
• -fdump-analyzer-supergraph
• -Wno-analyzer-double-fclose
• -Wno-analyzer-double-free
• -Wno-analyzer-exposure-through-output-file
• -Wno-analyzer-file-leak
• -Wno-analyzer-free-of-non-heap
• -Wno-analyzer-malloc-leak
• -Wno-analyzer-null-argument
• -Wno-analyzer-null-dereference
• -Wno-analyzer-possible-null-argument
• -Wno-analyzer-possible-null-dereference
• -Wno-analyzer-stale-setjmp-buffer
• -Wno-analyzer-tainted-array-index
• -Wanalyzer-too-complex
• -Wno-analyzer-unsafe-call-within-signal-handler
• -Wno-analyzer-use-after-free
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame
• -Wno-analyzer-use-of-uninitialized-value

---

🌈 C and Objective-C-only Warning Options 🌈

these ones are ✨exclusive✨ — C and Objective-C only, no C++ invited to this party, bestie:

• -Wbad-function-cast
• -Wmissing-declarations
• -Wmissing-parameter-type
• -Wmissing-prototypes
• -Wnested-externs
• -Wold-style-declaration
• -Wold-style-definition
• -Wstrict-prototypes
• -Wtraditional
• -Wtraditional-conversion
• -Wdeclaration-after-statement
• -Wpointer-sign

---

🐛 Debugging Options 🐛

and now the part where you become a detective of your own chaos 🔎💀 these are the debugging options, and there are a LOT of them:

• -g
• -glevel
• -gdwarf
• -gdwarf-version
• -ggdb
• -grecord-gcc-switches
• -gno-record-gcc-switches
• -gstabs
• -gstabs+
• -gstrict-dwarf
• -gno-strict-dwarf
• -gas-loc-support
• -gno-as-loc-support
• -gas-locview-support
• -gno-as-locview-support
• -gcolumn-info
• -gno-column-info
• -gstatement-frontiers
• -gno-statement-frontiers
• -gvariable-location-views
• -gno-variable-location-views
• -ginternal-reset-location-views
• -gno-internal-reset-location-views
• -ginline-points
• -gno-inline-points
• -gvms
• -gxcoff
• -gxcoff+
• -gz[=type]
• -gsplit-dwarf
• -gdescribe-dies
• -gno-describe-dies
• -fdebug-prefix-map=old=new
• -fdebug-types-section
• -fno-eliminate-unused-debug-types
• -femit-struct-debug-baseonly
• -femit-struct-debug-reduced
• -femit-struct-debug-detailed[=spec-list]
• -fno-eliminate-unused-debug-symbols
• -femit-class-debug-always
• -fno-merge-debug-strings
• -fno-dwarf2-cfi-asm
• -fvar-tracking
• -fvar-tracking-assignments

---

and that's the vibe for this chunk 💖 still so much more to go, bestie — don't you dare close the tab 🌸🦋

🔥 Optimization Options 🔥

okay bestie this is the big one, the main character of the whole page 💅 this is the full roster of optimization flags — every single one of them, no skips, no cuts, no "you don't need this one". They're all here, in the order the page gives them, and the full tea on each lives further down. Buckle up.

• the alignment girlies (and friends): -faggressive-loop-optimizations, -falign-functions[=n[:m:[n2[:m2]]]], -falign-jumps[=n[:m:[n2[:m2]]]], -falign-labels[=n[:m:[n2[:m2]]]], -falign-loops[=n[:m:[n2[:m2]]]], -fno-allocation-dce, -fallow-store-data-races
• the math-y crew: -fassociative-math, -fauto-profile, -fauto-profile[=path], -fauto-inc-dec, -fbranch-probabilities, -fcaller-saves, -fcombine-stack-adjustments, -fconserve-stack, -fcompare-elim, -fcprop-registers, -fcrossjumping, -fcse-follow-jumps, -fcse-skip-blocks, -fcx-fortran-rules, -fcx-limited-range
• sections + dead code + devirtualization, oh my: -fdata-sections, -fdce, -fdelayed-branch, -fdelete-null-pointer-checks, -fdevirtualize, -fdevirtualize-speculatively, -fdevirtualize-at-ltrans, -fdse, -fearly-inlining, -fipa-sra, -fexpensive-optimizations, -ffat-lto-objects, -ffast-math, -ffinite-math-only, -ffloat-store, -fexcess-precision=style, -ffinite-loops, -fforward-propagate, -ffp-contract=style, -ffunction-sections
• the gcse clique (they roll deep): -fgcse, -fgcse-after-reload, -fgcse-las, -fgcse-lm, -fgraphite-identity, -fgcse-sm, -fhoist-adjacent-loads
• inline era ✨: -fif-conversion, -fif-conversion2, -findirect-inlining, -finline-functions, -finline-functions-called-once, -finline-limit=n, -finline-small-functions
• the ipa besties: -fipa-cp, -fipa-cp-clone, -fipa-bit-cp, -fipa-vrp, -fipa-pta, -fipa-profile, -fipa-pure-const, -fipa-reference, -fipa-reference-addressable, -fipa-stack-alignment, -fipa-icf
• register allocation and live patching, lowkey iconic: -fira-algorithm=algorithm, -flive-patching=level, -fira-region=region, -fira-hoist-pressure, -fira-loop-pressure, -fno-ira-share-save-slots, -fno-ira-share-spill-slots
• isolate those paths, babe: -fisolate-erroneous-paths-dereference, -fisolate-erroneous-paths-attribute, -fivopts, -fkeep-inline-functions, -fkeep-static-functions, -fkeep-static-consts, -flimit-function-alignment, -flive-range-shrinkage
• loop loop loop loop (say it five times fast): -floop-block, -floop-interchange, -floop-strip-mine, -floop-unroll-and-jam, -floop-nest-optimize, -floop-parallelize-all
• the lto situation: -flra-remat, -flto, -flto-compression-level, -flto-partition=alg
• merging + modulo + moving invariants: -fmerge-all-constants, -fmerge-constants, -fmodulo-sched, -fmodulo-sched-allow-regmoves, -fmove-loop-invariants
• the whole -fno- era, no notes 💀: -fno-branch-count-reg, -fno-defer-pop, -fno-fp-int-builtin-inexact, -fno-function-cse, -fno-guess-branch-probability, -fno-inline, -fno-math-errno, -fno-peephole, -fno-peephole2, -fno-printf-return-value, -fno-sched-interblock, -fno-sched-spec, -fno-signed-zeros, -fno-toplevel-reorder, -fno-trapping-math, -fno-zero-initialized-in-bss
• frame pointers, sibling calls, peeling loops: -fomit-frame-pointer, -foptimize-sibling-calls, -fpartial-inlining, -fpeel-loops, -fpredictive-commoning, -fprefetch-loop-arrays, -fprofile-correction, -fprofile-use, -fprofile-use=path, -fprofile-partial-training, -fprofile-values, -fprofile-reorder-functions
• reciprocal math + reorder everything: -freciprocal-math, -free, -frename-registers, -freorder-blocks, -freorder-blocks-algorithm=algorithm, -freorder-blocks-and-partition, -freorder-functions, -frerun-cse-after-loop, -freschedule-modulo-scheduled-loops, -frounding-math
• scheduling szn: -fsave-optimization-record, -fsched2-use-superblocks, -fsched-pressure, -fsched-spec-load, -fsched-spec-load-dangerous, -fsched-stalled-insns-dep[=n], -fsched-stalled-insns[=n]
• the heuristics lineup, they all ate: -fsched-group-heuristic, -fsched-critical-path-heuristic, -fsched-spec-insn-heuristic, -fsched-rank-heuristic, -fsched-last-insn-heuristic, -fsched-dep-count-heuristic, -fschedule-fusion, -fschedule-insns, -fschedule-insns2, -fsection-anchors, -fselective-scheduling, -fselective-scheduling2, -fsel-sched-pipelining, -fsel-sched-pipelining-outer-loops
• semantic interposition + shrink wrap + signaling nans: -fsemantic-interposition, -fshrink-wrap, -fshrink-wrap-separate, -fsignaling-nans, -fsingle-precision-constant, -fsplit-ivs-in-unroller, -fsplit-loops, -fsplit-paths, -fsplit-wide-types, -fsplit-wide-types-early, -fssa-backprop, -fssa-phiopt, -fstdarg-opt, -fstore-merging, -fstrict-aliasing, -fthread-jumps, -ftracer
• the tree girlies 🌳 (a whole forest): -ftree-bit-ccp, -ftree-builtin-call-dce, -ftree-ccp, -ftree-ch, -ftree-coalesce-vars, -ftree-copy-prop, -ftree-dce, -ftree-dominator-opts, -ftree-dse, -ftree-forwprop, -ftree-fre, -fcode-hoisting, -ftree-loop-if-convert, -ftree-loop-im, -ftree-phiprop, -ftree-loop-distribution, -ftree-loop-distribute-patterns, -ftree-loop-ivcanon, -ftree-loop-linear, -ftree-loop-optimize, -ftree-loop-vectorize, -ftree-parallelize-loops=n, -ftree-pre, -ftree-partial-pre, -ftree-pta, -ftree-reassoc, -ftree-scev-cprop, -ftree-sink, -ftree-slsr, -ftree-sra, -ftree-switch-conversion, -ftree-tail-merge, -ftree-ter, -ftree-vectorize, -ftree-vrp
• the closing lineup: -funconstrained-commons, -funit-at-a-time, -funroll-all-loops, -funroll-loops, -funsafe-math-optimizations, -funswitch-loops, -fipa-ra, -fvariable-expansion-in-unroller, -fvect-cost-model, -fvpt, -fweb, -fwhole-program, -fwpa, -fuse-linker-plugin, --param name=value
• and the icons themselves, the levels everyone actually types: -O, -O0, -O1, -O2, -O3, -Os, -Ofast, -Og 👑

---

🎀 Program Instrumentation Options 🎀

instrumentation bestie, this is how you spy on your own program (affectionately). every one of these:

• -p, -pg, -fprofile-arcs, --coverage, -ftest-coverage, -fprofile-abs-path, -fprofile-dir=path, -fprofile-generate, -fprofile-generate=path, -fprofile-note=path, -fprofile-prefix-path=path, -fprofile-update=method, -fprofile-filter-files=regex, -fprofile-exclude-files=regex, -fprofile-reproducible=[multithreaded|parallel-runs|serial]
• sanitizer szn 🧼: -fsanitize=style, -fsanitize-recover, -fsanitize-recover=style, -fasan-shadow-offset=number, -fsanitize-sections=s1,s2,..., -fsanitize-undefined-trap-on-error, -fbounds-check
• protection era (stay safe out there): -fcf-protection=[full|branch|return|none|check], -fstack-protector, -fstack-protector-all, -fstack-protector-strong, -fstack-protector-explicit, -fstack-check, -fstack-limit-register=reg, -fstack-limit-symbol=sym, -fno-stack-limit, -fsplit-stack
• vtable verification + instrumenting functions: -fvtable-verify=[std|preinit|none], -fvtv-counts, -fvtv-debug, -finstrument-functions, -finstrument-functions-exclude-function-list=sym,sym,..., -finstrument-functions-exclude-file-list=file,file,...

---

🌸 Preprocessor Options 🌸

the preprocessor, aka where all the ✨ magic ✨ (and all the macros) happen:

• -Aquestion=answer, -A-question[=answer], -C, -CC, -Dmacro[=defn], -dD, -dI, -dM, -dN, -dU
• charset + identifiers: -fdebug-cpp, -fdirectives-only, -fdollars-in-identifiers, -fexec-charset=charset, -fextended-identifiers, -finput-charset=charset, -fmacro-prefix-map=old=new, -fmax-include-depth=depth, -fno-canonical-system-headers, -fpch-deps, -fpch-preprocess, -fpreprocessed, -ftabstop=width, -ftrack-macro-expansion, -fwide-exec-charset=charset, -fworking-directory
• includes + dependency generation: -H, -imacros file, -include file, -M, -MD, -MF, -MG, -MM, -MMD, -MP, -MQ, -MT
• and the rest: -no-integrated-cpp, -P, -pthread, -remap, -traditional, -traditional-cpp, -trigraphs, -Umacro, -undef, -Wp,option, -Xpreprocessor option

---

🦋 Assembler Options 🦋

only two, but they're doing their best and we respect it:

• -Wa,option
• -Xassembler option

---

💜 Linker Options 💜

where all your stuff actually gets glued together, bestie:

• object-file-name, -fuse-ld=linker, -llibrary, -nostartfiles, -nodefaultlibs, -nolibc, -nostdlib, -e entry, --entry=entry, -pie, -pthread, -r, -rdynamic, -s, -static, -static-pie
• the static-lib era: -static-libgcc, -static-libstdc++, -static-libasan, -static-libtsan, -static-liblsan, -static-libubsan
• sharing + scripts + symbols: -shared, -shared-libgcc, -symbolic, -T script, -Wl,option, -Xlinker option, -u symbol, -z keyword

---

🍓 Directory Options 🍓

where GCC goes looking for stuff. path vibes only:

• -Bprefix, -Idir, -I-, -idirafter dir, -imacros file, -imultilib dir, -iplugindir=dir, -iprefix file, -iquote dir, -isysroot dir, -isystem dir, -iwithprefix dir, -iwithprefixbefore dir, -Ldir, -no-canonical-prefixes, --no-sysroot-suffix, -nostdinc, -nostdinc++, --sysroot=dir

---

🧚 Code Generation Options 🧚

the actual code-shape-shifting knobs:

• -fcall-saved-reg, -fcall-used-reg, -ffixed-reg, -fexceptions, -fnon-call-exceptions, -fdelete-dead-exceptions, -funwind-tables, -fasynchronous-unwind-tables, -fno-gnu-unique, -finhibit-size-directive, -fcommon, -fno-ident, -fpcc-struct-return
• pic/pie szn: -fpic, -fPIC, -fpie, -fPIE, -fno-plt, -fno-jump-tables, -frecord-gcc-switches, -freg-struct-return, -fshort-enums, -fshort-wchar, -fverbose-asm, -fpack-struct[=n], -fleading-underscore
• the rest of the roster: -ftls-model=model, -fstack-reuse=reuse_level, -ftrampolines, -ftrapv, -fwrapv, -fvisibility=[default|internal|hidden|protected], -fstrict-volatile-bitfields, -fsync-libcalls

---

⭐ Developer Options ⭐

for when you're deep in the compiler guts and you need to see everything. iykyk:

• -dletters, -dumpspecs, -dumpmachine, -dumpversion, -dumpfullversion, -fcallgraph-info[=su,da], -fchecking, -fchecking=n, -fdbg-cnt-list, -fdbg-cnt=counter-value-list
• disabling passes (you can turn off the drama): -fdisable-ipa-pass_name, -fdisable-rtl-pass_name, -fdisable-rtl-pass-name=range-list, -fdisable-tree-pass_name, -fdisable-tree-pass-name=range-list
• dump dump dump 🗑️✨: -fdump-debug, -fdump-earlydebug, -fdump-noaddr, -fdump-unnumbered, -fdump-unnumbered-links, -fdump-final-insns[=file], -fdump-ipa-all, -fdump-ipa-cgraph, -fdump-ipa-inline, -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename, -fdump-passes, -fdump-rtl-pass, -fdump-rtl-pass=filename, -fdump-statistics, -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename
• compare debug + enabling passes: -fcompare-debug[=opts], -fcompare-debug-second, -fenable-kind-pass, -fenable-kind-pass=range-list, -fira-verbose=n, -flto-report, -flto-report-wpa
• memory report era: -fmem-report-wpa, -fmem-report, -fpre-ipa-mem-report, -fpost-ipa-mem-report
• opt info + profile report: -fopt-info, -fopt-info-options[=file], -fprofile-report, -frandom-seed=string, -fsched-verbose=n, -fsel-sched-verbose, -fsel-sched-dump-cfg, -fsel-sched-pipelining-verbose, -fstats, -fstack-usage, -ftime-report, -ftime-report-details, -fvar-tracking-assignments-toggle, -gtoggle
• printing all the things 🖨️: -print-file-name=library, -print-libgcc-file-name, -print-multi-directory, -print-multi-lib, -print-multi-os-directory, -print-prog-name=program, -print-search-dirs, -Q, -print-sysroot, -print-sysroot-headers-suffix
• save your temps, babe: -save-temps, -save-temps=cwd, -save-temps=obj, -time[=file]

---

👑 Machine-Dependent Options 👑

these change based on what hardware you're on, bestie — it's giving "your mileage may vary":

AArch64 Options

• -mabi=name, -mbig-endian, -mlittle-endian, -mgeneral-regs-only, -mcmodel=tiny, -mcmodel=small, -mcmodel=large, -mstrict-align, -mno-strict-align, -momit-leaf-frame-pointer
• tls dials: -mtls-dialect=desc, -mtls-dialect=traditional, -mtls-size=size
• the cortex-a53 fixes (iconic): -mfix-cortex-a53-835769, -mfix-cortex-a53-843419, -mlow-precision-recip-sqrt, -mlow-precision-sqrt, -mlow-precision-div, -mpc-relative-literal-loads
• security szn 🔐: -msign-return-address=scope, -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti, -mharden-sls=opts
• target picking: -march=name, -mcpu=name, -mtune=name, -moverride=string, -mverbose-cost-dump
• stack protector guards: -mstack-protector-guard=guard, -mstack-protector-guard-reg=sysreg, -mstack-protector-guard-offset=offset, -mtrack-speculation, -moutline-atomics

Adapteva Epiphany Options 💅

okay bestie, Adapteva Epiphany said "here's the whole menu" and we're obsessed:

• -mhalf-reg-file
• -mprefer-short-insn-regs
• -mbranch-cost=num
• -mcmove
• -mnops=num
• -msoft-cmpsf
• -msplit-lohi
• -mpost-inc
• -mpost-modify
• -mstack-offset=num
• -mround-nearest
• -mlong-calls
• -mshort-calls
• -msmall16
• -mfp-mode=mode
• -mvect-double
• -max-vect-align=num
• -msplit-vecmove-early
• -m1reg-reg

AMD GCN Options 🎀

AMD GCN is lowkey iconic with these three:

• -march=gpu
• -mtune=gpu
• -mstack-size=bytes

ARC Options 🦋

ARC is giving so many flags, bestie. Periodt:

• -mbarrel-shifter
• -mjli-always
• -mcpu=cpu
• -mA6
• -mARC600
• -mA7
• -mARC700
• -mdpfp
• -mdpfp-compact
• -mdpfp-fast
• -mno-dpfp-lrsr
• -mea
• -mno-mpy
• -mmul32x16
• -mmul64
• -matomic
• -mnorm
• -mspfp
• -mspfp-compact
• -mspfp-fast
• -msimd
• -msoft-float
• -mswap
• -mcrc
• -mdsp-packa
• -mdvbf
• -mlock
• -mmac-d16
• -mmac-24
• -mrtsc
• -mswape
• -mtelephony
• -mxy
• -misize
• -mannotate-align
• -marclinux
• -marclinux_prof
• -mlong-calls
• -mmedium-calls
• -msdata
• -mirq-ctrl-saved
• -mrgf-banked-regs
• -mlpc-width=width
• -G num
• -mvolatile-cache
• -mtp-regno=regno
• -malign-call
• -mauto-modify-reg
• -mbbit-peephole
• -mno-brcc
• -mcase-vector-pcrel
• -mcompact-casesi
• -mno-cond-exec
• -mearly-cbranchsi
• -mexpand-adddi
• -mindexed-loads
• -mlra
• -mlra-priority-none
• -mlra-priority-compact
• mlra-priority-noncompact
• -mmillicode
• -mmixed-code
• -mq-class
• -mRcq
• -mRcw
• -msize-level=level
• -mtune=cpu
• -mmultcost=num
• -mcode-density-frame
• -munalign-prob-threshold=probability
• -mmpy-option=multo
• -mdiv-rem
• -mcode-density
• -mll64
• -mfpu=fpu
• -mrf16
• -mbranch-index

ARM Options 🌸

ARM options? The way that this list is so long, I can't even. But slay:

• -mapcs-frame
• -mno-apcs-frame
• -mabi=name
• -mapcs-stack-check
• -mno-apcs-stack-check
• -mapcs-reentrant
• -mno-apcs-reentrant
• -mgeneral-regs-only
• -msched-prolog
• -mno-sched-prolog
• -mlittle-endian
• -mbig-endian
• -mbe8
• -mbe32
• -mfloat-abi=name
• -mfp16-format=name
• -mthumb-interwork
• -mno-thumb-interwork
• -mcpu=name
• -march=name
• -mfpu=name
• -mtune=name
• -mprint-tune-info
• -mstructure-size-boundary=n
• -mabort-on-noreturn
• -mlong-calls
• -mno-long-calls
• -msingle-pic-base
• -mno-single-pic-base
• -mpic-register=reg
• -mnop-fun-dllimport
• -mpoke-function-name
• -mthumb
• -marm
• -mflip-thumb
• -mtpcs-frame
• -mtpcs-leaf-frame
• -mcaller-super-interworking
• -mcallee-super-interworking
• -mtp=name
• -mtls-dialect=dialect
• -mword-relocations
• -mfix-cortex-m3-ldrd
• -munaligned-access
• -mneon-for-64bits
• -mslow-flash-data
• -masm-syntax-unified
• -mrestrict-it
• -mverbose-cost-dump
• -mpure-code
• -mcmse
• -mfix-cmse-cve-2021-35465
• -mfdpic

AVR Options ✨

AVR said "I'm the main character" and gave us this:

• -mmcu=mcu
• -mabsdata
• -maccumulate-args
• -mbranch-cost=cost
• -mcall-prologues
• -mgas-isr-prologues
• -mint8
• -mdouble=bits
• -mlong-double=bits
• -mn_flash=size
• -mno-interrupts
• -mmain-is-OS_task
• -mrelax
• -mrmw
• -mstrict-X
• -mtiny-stack
• -mfract-convert-truncate
• -mshort-calls
• -nodevicelib
• -nodevicespecs
• -Waddr-space-convert
• -Wmisspelled-isr

Blackfin Options 💖

Blackfin options, bestie, are giving everything:

• -mcpu=cpu[-sirevision]
• -msim
• -momit-leaf-frame-pointer
• -mno-omit-leaf-frame-pointer
• -mspecld-anomaly
• -mno-specld-anomaly
• -mcsync-anomaly
• -mno-csync-anomaly
• -mlow-64k
• -mno-low64k
• -mstack-check-l1
• -mid-shared-library
• -mno-id-shared-library
• -mshared-library-id=n
• -mleaf-id-shared-library
• -mno-leaf-id-shared-library
• -msep-data
• -mno-sep-data
• -mlong-calls
• -mno-long-calls
• -mfast-fp
• -minline-plt
• -mmulticore
• -mcorea
• -mcoreb
• -msdram
• -micplb

C6X Options 🧚

C6X is short and sweet:

• -mbig-endian
• -mlittle-endian
• -march=cpu
• -msim
• -msdata=sdata-type

CRIS Options 🍓

CRIS came through with this list:

• -mcpu=cpu
• -march=cpu
• -mtune=cpu
• -mmax-stack-frame=n
• -melinux-stacksize=n
• -metrax4
• -metrax100
• -mpdebug
• -mcc-init
• -mno-side-effects
• -mstack-align
• -mdata-align
• -mconst-align
• -m32-bit
• -m16-bit
• -m8-bit
• -mno-prologue-epilogue
• -mno-gotplt
• -melf
• -maout
• -melinux
• -mlinux
• -sim
• -sim2
• -mmul-bug-workaround
• -mno-mul-bug-workaround

CR16 Options 💙

CR16 said "here's my whole vibe":

• -mmac
• -mcr16cplus
• -mcr16c
• -msim
• -mint32
• -mbit-ops
• -mdata-model=model

C-SKY Options 🌈

C-SKY options are so many, I'm obsessed:

• -march=arch
• -mcpu=cpu
• -mbig-endian
• -EB
• -mlittle-endian
• -EL
• -mhard-float
• -msoft-float
• -mfpu=fpu
• -mdouble-float
• -mfdivdu
• -melrw
• -mistack
• -mmp
• -mcp
• -mcache
• -msecurity
• -mtrust
• -mdsp
• -medsp
• -mvdsp
• -mdiv
• -msmart
• -mhigh-registers
• -manchor
• -mpushpop
• -mmultiple-stld
• -mconstpool
• -mstack-size
• -mccrt
• -mbranch-cost=n
• -mcse-cc
• -msched-prolog

Darwin Options 🍎

Darwin options, bestie, are a whole mood. And yes, some of these appear twice — we keep it exactly as written:

• -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 🔥

DEC Alpha is serving these:

• -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

eBPF Options 🦋

eBPF said "less is more":

• -mbig-endian
• -mlittle-endian
• -mkernel=version
• -mframe-limit=bytes
• -mxbpf

FR30 Options 🌿

FR30 is short:

• -msmall-model
• -mno-lsim

FT32 Options ⭐

FT32 came through:

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

FRV Options 💖

FRV options are so many, bestie:

• -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 🐧

GNU/Linux options, periodt:

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

H8/300 Options 🌸

H8/300 is giving:

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

HPPA Options 💙

HPPA options, no cap:

• -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 🔥

IA-64 said "I have so many flags" and we're here for it:

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

LM32 Options 🍑

LM32 is cute:

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

M32R/D Options 🦋

M32R/D options, slay:

• -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 ✨

M32C is short:

• -mcpu=cpu
• -msim
• -memregs=number

M680x0 Options 💅

M680x0 options are a whole era:

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

MCore Options 🎀

MCore options, bestie:

• -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 🌈

MeP is giving:

• -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 💙

MicroBlaze options, periodt:

• -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

---

okay bestie, welcome to the part of the manual where GCC is like "and for EVERY CPU ever, I have opinions" 💅 every one of these is just an option, no descriptions here, so I'm giving you the full roster. no skipping, no "lol close enough", we're reading the whole menu. 🫶

MIPS Options 💜

MIPS said "I contain multitudes" and then handed us the longest list in the whole building. iconic behavior honestly.

• -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

yes bestie, that's -mmno-mcu with the double m. it's written exactly like that in here. I don't make the rules, I just transcribe them with love. 💀

• -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

okay the whole -mfix-* family just showed up like a group chat with 9 people and everyone's typing. obsessed. 💅

• -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 ✨

short and sweet, we love a minimal queen.

• -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, -mtiny-printf

---

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
• -msoft-float, -mhard-float
• -mdouble-float, -munordered-float
• -mcmov, -mror, -mrori, -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 🫶

okay so this one is literally just a forwarding address. it's giving "ask my sister".

See RS/6000 and PowerPC Options.

---

PRU Options 🔥

• -mmcu=mcu, -minrt, -mno-relax, -mloop, -mabi=variant

---

RISC-V Options 💅

• -mbranch-cost=N-instruction
• -mplt, -mno-plt
• -mabi=ABI-string
• -mfdiv, -mno-fdiv
• -mdiv, -mno-div
• -march=ISA-string
• -mtune=processor-string
• -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
• -malign-data=type

---

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 👑

the main character. the one everybody keeps pointing at. deep breath bestie, we're doing all of it. 💖

• -mcpu=cpu-type, -mtune=cpu-type, -mcmodel=code-model, -mpowerpc64
• -maltivec, -mno-altivec
• -mpowerpc-gpopt, -mno-powerpc-gpopt
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt
• -mmfcrf, -mno-mfcrf
• -mpopcntb, -mno-popcntb
• -mpopcntd, -mno-popcntd
• -mfprnd, -mno-fprnd
• -mcmpb, -mno-cmpb
• -mhard-dfp, -mno-hard-dfp
• -mfull-toc, -mminimal-toc, -mno-fp-in-toc, -mno-sum-in-toc
• -m64, -m32
• -mxl-compat, -mno-xl-compat
• -mpe
• -malign-power, -malign-natural
• -msoft-float, -mhard-float
• -mmultiple, -mno-multiple
• -mupdate, -mno-update
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses
• -mfused-madd, -mno-fused-madd
• -mbit-align, -mno-bit-align
• -mstrict-align, -mno-strict-align
• -mrelocatable, -mno-relocatable
• -mrelocatable-lib, -mno-relocatable-lib
• -mtoc, -mno-toc
• -mlittle, -mlittle-endian
• -mbig, -mbig-endian
• -mdynamic-no-pic, -mswdiv, -msingle-pic-base
• -mprioritize-restricted-insns=priority
• -msched-costly-dep=dependence_type
• -minsert-sched-nops=scheme
• -mcall-aixdesc, -mcall-eabi, -mcall-freebsd
• -mcall-linux, -mcall-netbsd, -mcall-openbsd
• -mcall-sysv, -mcall-sysv-eabi, -mcall-sysv-noeabi
• -mtraceback=traceback_type
• -maix-struct-return, -msvr4-struct-return
• -mabi=abi-type
• -msecure-plt, -mbss-plt
• -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

not me noticing -mno-mpower8-fusion has an extra m in it. that's literally how it's printed here, so that's how it's staying. 💀

• -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
• -mprefixed, -mno-prefixed
• -mpcrel, -mno-pcrel
• -mmma, -mno-mmma

---

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
• -mtpf-trace-skip, -mno-tpf-trace-skip
• -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

---

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

okay so these are the Visium knobs, they're just names here, no essay attached:

• -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 next list is long. it's just the names, okay? the full tea on each one lives further down the page. deep breaths, we're going shopping:

• -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
• -mavx512bf16
• -menqcmd
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -mavx512vpopcntdq
• -mavx5124fmaps
• -mavx512vnni
• -mavx5124vnniw
• -mprfchw
• -mrdpid
• -mrdseed
• -msgx
• -mavx512vp2intersect
• -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
• -mharden-sls=choice**
• -mindirect-branch-cs-prefix

🪟 x86 Windows Options

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

⚡ Xstormy16 Options

• -msim

🧚 Xtensa Options

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

👑 zSeries Options

See S/390 and zSeries Options. (yes queen, that's the whole entry, it's just a lil forwarding address 🫶)

---

✨ Options Controlling the Kind of Output ✨

okay so compiling has up to four stages, and they go in exactly this order, no skipping the queue:

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

GCC can preprocess and compile several files either into several assembler input files, or into one assembler input file. Then each assembler input file produces an object file, and linking combines all the object files — both the ones that just got compiled and the ones you handed over as input — into an executable file. one big happy family fr 💖

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

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed. (yes bestie, file.mm and file.M showed up twice in this list with different meanings. we did not write the rules, we just report them 💀)
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f, file.for, file.ftn — Fixed form Fortran source code that should not be preprocessed.
• file.F, file.FOR, file.fpp, file.FPP, file.FTN — Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90, file.f95, file.f03, file.f08 — Free form Fortran source code that should not be preprocessed.
• file.F90, file.F95, file.F03, file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.go — Go source code.
• file.brig — BRIG files (binary representation of HSAIL).
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can also just say the language out loud with the -x option:

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

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

• -x none — Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

---

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. literally the compiler sitting there doing its little nothing burger 🙈

• -c — Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

• -S — Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

• -E — Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

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

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
• -### — Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines.
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
• --help={class|[^]qualifier}[,...] — prints a whole lil tour of the command-line options the compiler understands that fit into all the classes and qualifiers you asked for, straight to standard output. Here's the class menu bestie:
  • optimizers — every optimization option the compiler ships with, the whole glow-up toolbox ✨
  • warnings — all the options that control the warning messages the compiler produces
  • target — target-specific options. But unlike the --target-help option, target-specific options of the linker and assembler do not show up here, because those tools don't currently support the extended --help= syntax.
  • params — the values recognized by the --param option
  • language — the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, you need to select the common class instead.
  • common — the options that are common to all languages

And here are the qualifiers you can sprinkle on top 💅

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

So, for example, to display all the undocumented target-specific switches the compiler supports, you do:

--help=target,undocumented

You can flip the sense of any qualifier 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, you use:

--help=warnings,^joined,^undocumented

Heads up! The argument to --help= should not consist solely of inverted qualifiers. Like, it's just not a thing.

Combining several classes is possible, although it usually restricts the output so much that there's nothing left to display 😭. One case where it does work, though, 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. And if --help is also specified anywhere on the command line, then it takes precedence over any --help= option. No cap.

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= changes. Instead of describing the displayed options, you get an indication of whether the option is enabled, disabled, or set to a specific value (assuming 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

Or 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

• --version — shows you the version number and the copyrights of the invoked GCC. Identity reveal moment 🪪
• -pass-exit-codes — normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. But if you specify -pass-exit-codes, gcc instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.
• -pipe — uses pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler can't read from a pipe, but the GNU assembler has no trouble at all. Iconic behavior honestly.
• -specs=file — processes file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they're processed in order, from left to right.
• -wrapper — invokes all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

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

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

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

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. It's giving inception, fr fr 🌀

---

🌸 Compiling C++ Programs 🌸

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

However — and this is the plot twist 🫠 — 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's 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're 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's 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 shortcuts bestie.

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++.

ok bestie, standards era, let's get into it 💅

The compiler can accept several base standards, like c90 or c++98, plus the GNU dialects of those standards, like gnu90 or gnu++98. When you specify a base standard, the compiler accepts all programs following that standard plus the ones using GNU extensions that don't contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, like the asm and typeof keywords — but not other GNU extensions that have no meaning in ISO C90, like omitting the middle term of a ?: expression. On the other hand, when you specify a GNU dialect of a standard, all features supported by the compiler get 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 what -Wpedantic uses to figure out which features count as GNU extensions for 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 — supports 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. The name gnu++1z is deprecated.
• c++20, c++2a — the next revision of the ISO C++ standard, planned for 2020. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀
• gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀

---

• -fgnu89-inline — tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions. ✨

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode. 🙅

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions.

• -fpermitted-flt-eval-methods=style — ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of FLT_EVAL_METHOD. As such, code conforming to C11 may not have been written expecting the possibility of the new values. 😭

-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of FLT_EVAL_METHOD specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.

style is either c11 or ts-18661-3 as appropriate.

The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.

• -aux-info filename — outputs to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. 🫶

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

• -fallow-parameterless-variadic-functions — accepts variadic functions without named parameters. Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. 🙈 This is only supported for C as this construct is allowed by C++.
• -fno-asm — do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm.

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

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

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory.

With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))

• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. 🧚
• -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. 💜
• -fopenacc — bestie, this turns ON handling of OpenACC directives, aka #pragma acc in C/C++ and !$acc in Fortran. Once you pass -fopenacc, the compiler starts generating accelerated code following the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. Heads up tho: this option implies -pthread, which means it's only supported on targets that have support for -pthread. ✨
• -fopenacc-dim=geom — sets the default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in order gang, worker, and vector. A size can be omitted, and then you just get a target-specific default value. 🎀
• -fopenmp — enables handling of OpenMP directives #pragma omp in C/C++ and !$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, so it's only supported on targets with support for -pthread. And also, -fopenmp implies -fopenmp-simd. 🔥
• -fopenmp-simd — enables handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are just ignored, like not even perceived. 💅
• -fgnu-tm — when you pass -fgnu-tm, 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, and its interface may change in future versions of GCC as the official specification changes. Also 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 — accepts some non-standard constructs used in Microsoft header files.

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

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

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

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

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

This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++. 🦋

• -fcond-mismatch — allows 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 — allows implicit conversions between vectors with differing numbers of elements and/or incompatible element types. Bestie, this option should not be used for new code. fr fr. 🚫
• -funsigned-char — lets the type char be unsigned, like unsigned char.

Every kind of machine has a default for what char should be. It's 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 soooo many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. 🫶

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

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

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. ✨

• -fsigned-bitfields
• -funsigned-bitfields
• -fno-signed-bitfields
• -fno-unsigned-bitfields — these options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types. 🎀
• -fsso-struct=endianness — sets the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. 💀

---

🎀 Options Controlling C++ Dialect 🎀

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

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

In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC. 💅

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

Here is a list of options that are only for compiling C++ programs:

• -fabi-version=n — use version n of the C++ ABI. The default is version 0.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.

Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

See also -Wabi. 🔥

• -fabi-compat-version=n — on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

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

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. ✨

• -fno-access-control — turn off all access checking. This switch is mainly useful for working around bugs in the access control code. 😭
• -faligned-new — enable support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t).

This flag is enabled by default for -std=c++17. 👑

• -fchar8_t
• -fno-char8_t — enable support for char8_t as adopted for C++2a. This includes the addition of a 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 enables functions to be overloaded for ordinary and UTF-8 strings:

int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f("text");     // Calls #1
int v2 = f(u8"text");   // Calls #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;

ok so this is still the -fchar8_t saga btw, we're mid-yap — and bestie, the way that changing the types of UTF-8 string and character literals literally introduces incompatibilities with ISO C++11 and later standards?? 💀 Like, the code below is well-formed under ISO C++11 but ill-formed the second you pass -fchar8_t:

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

---

• -fcheck-new — this one's giving safety goggles 🥽 it checks that the pointer operator new hands back is non-null before anybody tries to modify the storage it allocated. Ngl though, 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 anyway, even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion gets signalled by throwing std::bad_alloc instead. See also new (nothrow). 👑
• -fconcepts, -fconcepts-ts — okay so below -std=c++2a, -fconcepts enables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015). ✨

But with -std=c++2a and above, Concepts are part of the language standard, so -fconcepts defaults to on. However — and this is the plot twist 🎀 — the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled by -fconcepts-ts.

• *-fconstexpr-depth=n* — sets the max nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. 📏
• *-fconstexpr-cache-depth=n* — sets the max level of nested evaluation depth for C++11 constexpr functions that will be cached to n. It's a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you. 🧪
• *-fconstexpr-loop-limit=n* — sets the max 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 max number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). 😮‍💨
• -fcoroutines — enable support for the C++ coroutines extension (experimental). ⚠️ experimental, bestie, act accordingly
• -fno-elide-constructors — 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 causes G++ to call trivial member functions which otherwise would be expanded inline.

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

• -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.

🚨 It does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. Do not play with this one unless you know exactly what you're doing. 💀

• -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away — but when the use is in a different translation unit, there's significant overhead even if the variable doesn't actually need dynamic initialization.

If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option. 🌈

On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.

• -fno-gnu-keywords — don't recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc. 🙈
• -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.

💅 If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time.

• -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. 🎀
• -fno-implement-inlines — to save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. 🫣
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. 🪟
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified. ⭐
• -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17. 🦋
• -fno-nonansi-builtins — disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions. 🚫
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away.

The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. 💀

• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. 🔤
• -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. 🤫
• -fpermissive — downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile. 👀
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. 🧐
• -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes. ✨

🚨 Mixing code compiled with -frtti with that 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. This is a chaos combo, don't do it. 😭

• -fsized-deallocation — turns on the built-in global declarations

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

which is giving C++14 energy ✨. Bestie this is for when you write your own replacement deallocation functions and you wanna use the size of the object to make deallocation faster. It's on by default under -std=c++14 and above. And -Wsized-deallocation will warn you about places that might wanna add a definition. 👑

• -fstrict-enums — lets the compiler optimize based on the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). Heads up tho 💀 — this assumption may not be valid if your 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, exactly 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, and that's 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. Default is 10. 🎀
• *-ftemplate-depth=n* — set the maximum instantiation depth for template classes to n. Lowkey you need a limit on template instantiation depth 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). Default value is 900, because the compiler can run out of stack space before hitting 1024 in some situations. 😭
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this 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 do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO, 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 — this flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. 💅

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

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

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

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

• -fno-weak — do not use weak symbol support, even if it's provided by the linker. By default, G++ uses weak symbols if they are available. Bestie, this option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 🙅‍♀️
• -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). 🌸
• -nostdinc++ — do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🎀

---

In addition, these warning options have meanings only for C++ programs:

• -Wabi-tag (C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more info about ABI tags. ✨
• -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++2a. However, a comma expression wrapped in "( )" is not deprecated. Example:

void f(int *a, int b, int c) {
    a[b,c];     // deprecated
    a[(b,c)];   // OK
}

Enabled by default with -std=c++2a.

• -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. 🫠

• -Wno-literal-suffix (C++ and Objective-C++ only) — do not 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>

Okay bestie 💖 when you see something like this:

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

in this case, "PRId64" is treated as a separate preprocessing token. ✨

This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization. 🎀

These warnings are enabled by default. 👑

---

• -Wno-narrowing (C++ and Objective-C++ only) — ok so for C++11 and later standards, narrowing conversions are diagnosed by default, as the standard requires. A narrowing conversion from a constant? That's an error 💀. From a non-constant? That's a warning 😭. But -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 🦋

With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.

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

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

• -Wnoexcept (C++ and Objective-C++ only) — warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. "throw()" or "noexcept") but is known by the compiler to never throw an exception. 👑
• -Wnoexcept-type (C++ and Objective-C++ only) — warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat. 💅

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>". 🤯

• -Wclass-memaccess (C++ and Objective-C++ only) — warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call 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. 🫶

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. 😭
• -Wregister (C++ and Objective-C++ only) — warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. 💀
• -Wreorder (C++ and Objective-C++ only) — warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

                   struct A {
                     int i;
                     int j;
                     A(): j (0), i (1) { }
                   };

The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall. 🌈

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this warning warns when a call to "std::move" prevents copy elision. A typical scenario when 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. 😭

This warning is enabled by -Wall. ⭐

• -Wno-redundant-move (C++ and Objective-C++ only) — this warning 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. 💅

This warning is enabled by -Wextra. 🎀

• -Wredundant-tags (C++ and Objective-C++ only) — warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:

                   struct foo;
                   struct foo *p;   // warn that keyword struct can be eliminated

On the other hand, in this example there is no warning:

                   struct foo;
                   void foo ();   // "hides" struct foo
                   void bar (struct foo&);  // no warning, keyword struct is necessary

• -Wno-subobject-linkage (C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. 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. 🦋
• -Weffc++ (C++ and Objective-C++ only) — warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books: 📚
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have "operator=" return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload "&&", "||", or ",".

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. 👑

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

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

                   struct A {
                     virtual void f();
                   };

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

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

                   B* b;
                   b->f();

fails to compile. 🫶

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disable the diagnostic for converting a bound pointer to member function to a plain pointer. 💅
• -Wsign-promo (C++ and Objective-C++ only) — okay so this one yells at you when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. 💅
• -Wtemplates (C++ and Objective-C++ only) — 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. ✨
• -Wmismatched-tags (C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.

For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition.

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

It is not wrong to declare a class with the class-key struct as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. 🎀

• -Wmultiple-inheritance (C++ and Objective-C++ only) — 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. ✨
• -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. 😭
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. 🌸
• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. 💀
• -Wno-class-conversion (C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. 🫶
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. This includes postfix and prefix ++ and -- expressions of volatile-qualified types, using simple assignments where the left operand is a volatile-qualified non-class type for their value, compound assignments where the left operand is a volatile-qualified non-class type, volatile-qualified function return type, volatile-qualified parameter type, and structured bindings of a volatile-qualified type. This usage was deprecated in C++20. Enabled by default with -std=c++2a. ✨
• -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) — warns when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11. 💅
• -Waligned-new — warns 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. 🔥
• -Wno-placement-new
• -Wplacement-new=n — warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large.

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

This warning is enabled by default.

• -Wplacement-new=1 — this is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the 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]();

• -Wcatch-value
• -Wcatch-value=n (C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-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. 💅
• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs. 🌈
• -Wno-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. 😭
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons after in-class function definitions. ✨
• -Wno-inaccessible-base (C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option.

struct A { int a; };

struct B : A { };

struct C : B, A { };

• -Wno-inherited-variadic-ctor — suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🎀
• -Wno-invalid-offsetof (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. The restrictions on offsetof may be relaxed in a future version of the C++ standard. 💅
• -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. 🔥

• -Wsuggest-final-types — warns about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. ✨
• -Wsuggest-final-methods — warns about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. 💅
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. 🫶
• -Wuseless-cast (C++ and Objective-C++ only) — warns when an expression is casted to its own type. 💀
• -Wno-conversion-null (C++ and Objective-C++ only) — bestie, this one tells the compiler to chill and not warn you about conversions between NULL and non-pointer types. And heads up: -Wconversion-null is enabled by default, so the warnings are already on unless you turn them off. 💅

---

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

Heads up bestie: this manual does not describe the Objective-C and Objective-C++ languages themselves. We're just doing the flags here, okay? 🫶

So this whole section is about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. That said, you can also use most of the language-independent GNU compiler options. Like, for example, you might compile a file some_class.m like this:

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

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Also note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).

Here's the list of options that are only for compiling Objective-C and Objective-C++ programs, bestie, let's gooo:

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. ✨
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used. 👑
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
• -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

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

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

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 🚀
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.
• -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. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.
• -fno-local-ivars — by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues.
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. 📝
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector.
• -Wno-property-assign-default (Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified.
• -Wno-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
• -Wselector (Objective-C and Objective-C++ only) — warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used. 💀
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns you the second it spots a @selector(...) expression pointing at a selector nobody declared. And what counts as undeclared? No method with that name was declared before the @selector(...) shows up — not explicitly in an @interface or @protocol declaration, and not implicitly in an @implementation section either. Bestie, this option runs its checks immediately when it finds a @selector(...), whereas -Wselector waits until the final stage of compilation to do its thing. It also enforces that coding-style convention that methods and selectors gotta be declared before anybody uses them. 💅
• -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 have been formatted like they don't care about the output device's aspect — its width, that kind of thing. Rude, honestly. The options down here let you control the formatting algorithm for diagnostics: how many characters per line, how often the source location gets reported, all of it. Just know that some language front ends may simply not honor these options. 🙈

• -fmessage-length=n — tries to format error messages so they fit on lines of about n characters. If n is zero, no line-wrapping happens at all and each error message shows up on one single line. That zero behavior is the default for all front ends. 💖

Heads up — this option also affects how the #error and #warning pre-processor directives display, plus the deprecated function/type/variable attribute. It does not, however, touch the pragma GCC warning and pragma GCC error pragmas. Periodt.

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once — so if the message is too long for one physical line and has to wrap, the source location won't get re-emitted as a prefix over and over again on the continuation lines. This is the default behavior. 👑
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the reporter to emit that same source location info as a prefix on every physical line that results from breaking a message that was too long for one line. ✨
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler was configured, and 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 makes GCC use color only when the standard error is a terminal, and when it's not executing in an emacs shell. 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 get interpreted by the terminal or terminal emulator — check the section in your text terminal's documentation for permitted values and what they mean 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:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32

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

• "error=" — SGR substring for error: markers.
• "warning=" — SGR substring for warning: markers.
• "note=" — SGR substring for note: markers.
• "path=" — SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, like the identifiers of individual events and lines indicating interprocedural calls and returns.
• "range1=" — SGR substring for first additional range.
• "range2=" — SGR substring for second additional range.
• "locus=" — SGR substring for location information, file: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.
• -fdiagnostics-urls[=WHEN] — uses escape sequences to embed URLs inside diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, it embeds a URL for the documentation of that option. 🔗

WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when it's not executing in an emacs shell or any graphical terminal known to be incompatible with this feature — see below. 👀

The default depends on how the compiler was configured. It can be any of the above WHEN options.

GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

But okay — even with -fdiagnostics-urls=always, the behavior still depends on those environment variables: if GCC_URLS is set to empty or no, don't embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS isn't set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, and BEL is an ASCII character, CTRL-G, that usually sounds like a beep. 🔔

At this time GCC tries to detect a few terminals known to not implement the URL feature, and that have bugs — or at least had bugs in some versions that are still in use — where the URL escapes are likely to misbehave, i.e. print garbage all over your screen. That list is currently xfce4-terminal, certain known-to-be-buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with -fdiagnostics-urls=always. 💅

• -fno-diagnostics-show-option — by default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying this flag suppresses that behavior.
• -fno-diagnostics-show-caret — by default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses that info. The source line gets truncated to n characters if the -fmessage-length=n option is given. When output goes to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if that's not set, to the terminal width. ✂️
• -fno-diagnostics-show-labels — by default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, like the types of expressions:

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

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

• -fno-diagnostics-show-cwe — diagnostic messages can optionally have an associated @url{https://cwe.mitre.org/index.html, CWE} identifier. GCC itself only provides that kind of metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it gets printed with the diagnostic. This option suppresses the printing of that metadata.
• -fno-diagnostics-show-line-numbers — by default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed showing line numbers. This option suppresses 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, suitable for consumption by IDEs. 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 a half-open range, bestie — expressed as a count of bytes, starting at byte 1 for the initial column. So in the example above, bytes 3 through 20 of line 45 of "test.c" get 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 "\"", and non-printable characters as octal (e.g. vertical tab as "\013"). 💅

An empty replacement string means the given range is just… yeeted. Gone. Removed. And an empty range (e.g. "45:3-45:3") means the string is inserted at that position instead — iconic little insertion moment ✨

---

• -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, right after any diagnostics get printed. Like so:

--- 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 it's printing diagnostics about 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 turns on printing a tree-like structure showing the common and differing parts of the types, like:

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

The parts that differ get 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 are printed as "[...]" to simplify the error message. For example:

  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. fr fr, they're linked.

• -fdiagnostics-path-format=KIND — tells GCC how to print paths of control-flow events for diagnostics that have such a path attached to them.

KIND is none, separate-events, or inline-events, the default.

none means… don't print diagnostic paths. Simple as that, bestie 🙈

separate-events means print a separate "note" diagnostic for each event within the diagnostic. For example:

test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1

inline-events means print the events "inline" within the source code. This view tries to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source. 👑

For example, the same events as above might be printed as:

  'test': events 1-3
    |
    |   25 |   list = PyList_New(0);
    |      |          ^~~~~~~~~~~~~
    |      |          |
    |      |          (1) when 'PyList_New' fails, returning NULL
    |   26 |
    |   27 |   for (i = 0; i < count; i++) {
    |      |   ~~~
    |      |   |
    |      |   (2) when 'i < count'
    |   28 |     item = PyLong_FromLong(random());
    |   29 |     PyList_Append(list, item);
    |      |     ~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |     |
    |      |     (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
    |

Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped. 🦋

For example:

  'test': events 1-2
    |
    |  133 | {
    |      | ^
    |      | |
    |      | (1) entering 'test'
    |  134 |   boxed_int *obj = make_boxed_int (i);
    |      |                    ~~~~~~~~~~~~~~~~~~
    |      |                    |
    |      |                    (2) calling 'make_boxed_int'
    |
    +--> 'make_boxed_int': events 3-4
           |
           |  120 | {
           |      | ^
           |      | |
           |      | (3) entering 'make_boxed_int'
           |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
           |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |                                    |
           |      |                                    (4) calling 'wrapped_malloc'
           |
           +--> 'wrapped_malloc': events 5-6
                  |
                  |    7 | {
                  |      | ^
                  |      | |
                  |      | (5) entering 'wrapped_malloc'
                  |    8 |   return malloc (size);
                  |      |          ~~~~~~~~~~~~~
                  |      |          |
                  |      |          (6) calling 'malloc'
                  |
    <-------------+
    |
 'test': event 7
    |
    |  138 |   free_boxed_int (obj);
    |      |   ^~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (7) calling 'free_boxed_int'
    |
(etc)

• -fdiagnostics-show-path-depths — this option gives you extra info when printing control-flow paths associated with a diagnostic.

If this option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=separate-events. 🌸

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. iykyk — if you're not debugging GCC internals, this one's not for you, bestie.

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. 😭
• -fdiagnostics-format=FORMAT — picks a different format for printing diagnostics. FORMAT is text or json. The default is text.

The json format is a top-level JSON array containing JSON objects representing the diagnostics.

The JSON is emitted as one line, without formatting — the examples below have been formatted for clarity, so don't come for me 💅

Diagnostics can have child diagnostics. 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",
        "option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-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".

A diagnostic has a "kind". If this is "warning", then there is an "option" key describing the command-line option controlling the warning. 🫶

A diagnostic can contain zero or more locations. Each location has up to three positions within it: a "caret" position and 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, bestie. Its primary location is at the "+" token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, which 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 has a "fixits" array, consisting of half-open intervals, similar to 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. Deletions are expressed via an empty value for "string", insertions by having "start" equal "next". 💅

If the diagnostic has a path of control-flow events associated with it, it has a "path" array of objects representing the events. Each event object has a "description" string, a "location" object, along with a "function" string and a "depth" number for representing interprocedural paths. The "function" represents the current function at that event, and the "depth" represents the stack depth relative to some baseline: the higher, the more frames are within the stack.

For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:

"path": [
    {
        "depth": 0,
        "description": "when 'PyList_New' fails, returning NULL",
        "function": "test",
        "location": {
            "column": 10,
            "file": "test.c",
            "line": 25
        }
    },
    {
        "depth": 0,
        "description": "when 'i < count'",
        "function": "test",
        "location": {
            "column": 3,
            "file": "test.c",
            "line": 27
        }
    },
    {
        "depth": 0,
        "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
        "function": "test",
        "location": {
            "column": 5,
            "file": "test.c",
            "line": 29
        }
    }
]

---

✨ Options to Request or Suppress Warnings ✨

Okay so warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. Like, not a crime, but bestie... red flag energy. 🚩

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

• -fsyntax-only — Check the code for syntax errors, but don't do anything beyond that. Just vibes-checking the syntax and dipping. ✌️
• *-fmax-errors=n* — Caps the maximum number of error messages at n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. 👑
• -w — Inhibit all warning messages. Silencing them fr fr, no notes. 💀
• -Werror — Make all warnings into errors. Every little warning just got promoted to main character. 😭
• -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 audacity, honestly. 🙈

• -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. First L and it's over, no second chances. 🛑

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. Additional warnings can be produced by enabling the static analyzer;

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-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. It's giving inheritance hierarchy but for flags. 🌸

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 but thoughtful. ✨

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 does not warn at all unless optimization is enabled. Some warnings literally need the optimization glow-up to function. 💅

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

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected. Strict mode activated. 🎀

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system header files should use these escape routes; application programs should avoid them. Don't try the loophole in your own code bestie, it's not for you. 🫶

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

A feature that reports every single time your code doesn't conform to ISO C? Sounds cute in theory, but bestie, it would need a <em>ton</em> of extra work and it'd behave nothing like -Wpedantic. We have no plans to support such a feature in the near future. 💅

Okay so real talk: when the standard you picked with -std is a GNU extended dialect of C — think gnu90 or gnu99 — there's a base standard hiding underneath it, the version of ISO C that the GNU dialect is built on top of. Warnings from -Wpedantic show up wherever that base standard requires them. And lowkey it wouldn't even make sense to warn only about features that aren't in the GNU C dialect you specified, because by definition the GNU dialects of C include literally every feature the compiler supports with that option, so there'd be nothing left to warn about. 😭

• -pedantic-errors — Throw an error whenever the base standard (peep -Wpedantic for that one) demands a diagnostic. That covers some cases where there's undefined behavior at compile-time, and some other cases that don't even stop compilation of programs that are perfectly valid per the standard. 🚨

Heads up: this is not the same as -Werror=pedantic, bestie. There are errors this option enables that the other one doesn't, and vice versa. Not interchangeable, no cap.

• -Wall — Turns on all the warnings about constructions that some users consider questionable and that are easy to dodge (or tweak to shut the warning up), even when macros are in the mix. It also switches on some language-specific warnings described over 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++)
-Wenum-conversion in C/ObjC;
-Wformat
-Wformat-overflow
-Wformat-truncation
-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
-Wzero-length-bounds

Not every warning flag is implied by -Wall, bestie. Some of them flag constructions users generally don't think twice about but you might occasionally wanna check for; others flag stuff that's necessary or genuinely hard to avoid in certain cases, with no simple way to rewrite the code and silence the warning. Some of them get switched on by -Wextra, but a whole bunch of them you gotta enable individually, one by one. 😮‍💨

• -Wextra — Switches on some extra warning flags that -Wall doesn't cover. (This option used to be called -W. The old name still works, but the newer one is way more descriptive, periodt.) 💖

-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)
-Wstring-compare
-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.

---

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — Warns you about code affected by ABI changes. That includes code that might not play nice with the vendor-neutral C++ ABI, plus the psABI for the particular target. 🦋

Since G++ now defaults to updating the ABI with every major release, normally -Wabi only warns about C++ ABI compatibility problems if a check got added later in a release series for an ABI issue discovered after the initial release. -Wabi warns about way more stuff if an older ABI version is selected (with -fabi-version=n).

-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level — like -Wabi=2 to warn about changes relative to -fabi-version=2.

If you hand it an explicit version number and -fabi-compat-version isn't specified, the version number from this option gets used for compatibility aliases. If you don't give an explicit version number with this option, but -fabi-compat-version is specified, then that version number is what gets used for C++ ABI warnings.

Even though an effort has been made to warn about all such cases, there are probably some cases that slip through unwarned even though G++ is generating incompatible code. There may also be cases where warnings get emitted even though the code that's generated is compatible. It's giving ✨imperfect✨, bestie.

You should rewrite your code to avoid these warnings if you're worried 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.

This option also enables warnings 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. Prior to GCC 4.4, this was not the case. For example:

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

"union U" is now always passed in memory.

• -Wchar-subscripts — Warns if an array subscript has type "char". This is a super common cause of error, because programmers keep forgetting that this type is signed on some machines. 😭 This warning is enabled by -Wall.
• -Wno-coverage-mismatch — Warns if feedback profiles don't match when you're using the -fprofile-use option. If a source file gets changed between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC can't use the profile feedback information at all. By default this warning is enabled and it's treated as an error. -Wno-coverage-mismatch disables the warning, or -Wno-error=coverage-mismatch disables the error. 🫶

Disabling the error for this warning can result in poorly optimized code and is useful only for very minor changes, like bug fixes to an existing code-base. Completely disabling the warning is not recommended. Don't do it, bestie, I'm begging. 🙈

• -Wno-cpp — (C, Objective-C, C++, Objective-C++ and Fortran only) Suppresses warning messages emitted by "#warning" directives. Quiet. 🤫
• -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — bestie, this one yells at you when a float gets sneakily promoted to double behind your back 💅

Here's the tea: CPUs with a 32-bit "single-precision" floating-point unit do float in actual hardware, but they have to emulate double in software. And on a machine like that, doing your math with double values is way more expensive because of all the software-emulation overhead. Not the vibe. 😭

And it's soooo easy to do this by accident, because floating-point literals are implicitly double type. Like, look at this:

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

The compiler does the entire computation with double here, because that floating-point literal is a double. Caught in 4K. 🫶

• -Wduplicate-decl-specifier (C and Objective-C only) — warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. Why are you saying it twice, girl, we heard you the first time 🙈 This warning is enabled by -Wall.
• -Wformat
• -Wformat=n — checks calls to printf and scanf and friends, making sure the arguments you supplied have types that actually match the format string, and that the conversions in the format string make sense. Slay. ✨

This covers standard functions, plus others specified by format attributes, across the printf, scanf, strftime and strfmon families — and strfmon is an X/Open extension, not in the C standard, iykyk — plus other target-specific families.

Which functions get checked without format attributes being specified depends on the standard version you selected. And those checks of functions without the attribute specified get disabled by -ffreestanding or -fno-builtin.

Formats are checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features, and GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, you get warnings about format features not in the selected standard version — but not for strfmon formats, since those aren't in any version of the C standard. 🎀

• -Wformat=1
• -Wformat — -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. 👑
• -Wformat=2 — enables -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

• -Wno-format-contains-nul — if -Wformat is specified, don't warn about format strings that contain NUL bytes. 🌸
• -Wno-format-extra-args — if -Wformat is specified, don't warn about excess arguments to a printf or scanf format function. The C standard says such arguments are just ignored. 💅

Now, where the unused arguments sit between used arguments that are specified with $ operand number specifications, normally warnings are still given, because the implementation couldn't know what type to pass to va_arg to skip over the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says such unused arguments are allowed. The more you know 🦋

• -Wformat-overflow
• -Wformat-overflow=level — warns about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. And yeah, enabling 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, takes 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 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, this call to sprintf gets 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:

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

Increasing the size of the buffer by a single byte is enough to avoid the warning — though it may not be enough to avoid the overflow. 🙈

• -Wformat-overflow=2 — Level 2 also warns 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 can't be assumed 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 diagnosed again, 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 growing 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 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, don't warn about zero-length formats. The C standard specifies that zero-length formats are allowed. Yes bestie, an empty format is legal, we don't bully it 🎀
• -Wformat-nonliteral — if -Wformat is specified, also warn if the format string is not a string literal and so can't be checked — unless the format function takes its format arguments as a va_list. 👀
• -Wformat-security — if -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, like printf (foo);.

This may be a security hole if the format string came from untrusted input and contains %n. Do not ignore this one, bestie. 🚨

(This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that aren't included in -Wformat-nonliteral.)

• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. Pick a lane, babe 💅
• -Wformat-truncation
• -Wformat-truncation=level — warns about calls to formatted input/output functions like snprintf and vsnprintf that might result in output truncation. 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. Enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives. Except as noted otherwise, the option uses the same logic as -Wformat-overflow. 🦋
  • -Wformat-truncation
  • -Wformat-truncation=1 — Level 1 of -Wformat-truncation, enabled by -Wformat, takes a conservative approach that only warns about calls to bounded functions whose return value is unused and that will most likely result in output truncation. 🍓
  • -Wformat-truncation=2 — Level 2 also warns about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. Y2K called, she wants her bug back 💀
• -Wnonnull — warns about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute. Bestie said "this cannot be null" and you handed her null. Rude. 😤

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

• -Wnonnull-compare — warns when you compare an argument marked with the nonnull function attribute against null inside the function. Girl. You literally promised it wouldn't be null. 💀

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

• -Wnull-dereference — warns you when the compiler clocks a path that's gonna hit erroneous or undefined behavior from dereferencing a null pointer 👀. Heads up bestie: this one only works when -fdelete-null-pointer-checks is active, and that's turned on by optimizations in most targets. How precise the warnings get depends on which optimization options you're running. iykyk.
• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warns about uninitialized variables that are literally initialized with themselves 😭 not me doing int i = i. Note this option can only be used together with the -Wuninitialized option.

Like, GCC only warns about i being uninitialized in this lil snippet when -Winit-self has been specified:

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

This warning is enabled by -Wall in C++.

• -Wno-implicit-int (C and Objective-C only) — controls warnings for when a declaration just... doesn't specify a type 🙈. This warning is enabled by default in C99 and later dialects of C, and also by -Wall.
• -Wno-implicit-function-declaration (C and Objective-C only) — controls warnings for when a function gets used before it's been declared 💀. This warning is enabled by default in C99 and later dialects of C, and also by -Wall. And the warning gets upgraded into an error by -pedantic-errors.
• -Wimplicit (C and Objective-C only) — same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
• -Wimplicit-fallthrough — this is the same as -Wimplicit-fallthrough=3, and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. slay.
• -Wimplicit-fallthrough=n — warns when a switch case falls through 😳 like:

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

It does not warn when the last statement of a case can't fall through, e.g. when there's a return statement or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes control flow statements into account, like ifs, and only warns when it's appropriate. E.g.

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

Since sometimes a switch case fall through is genuinely desirable, GCC gives you an attribute, __attribute__ ((fallthrough)), to use alongside a null statement so you can suppress the warning that would normally show up:

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

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 you can use [[gnu::fallthrough]];, which is a GNU extension. And instead of these attributes, it's also possible to drop a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below 👇. The option argument n specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches ".*" regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches ".*falls?[ \t-]*thr(ough|u).*" regular expression.
• -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
• -Wimplicit-fallthrough=4 case sensitively matches one of the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t]*FALLTHR(OUGH|U)[ \t]*"
• -Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

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

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

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. 💅

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default.
• -Wignored-qualifiers (C and C++ only) — warns if the return type of a function has a type qualifier such as const. For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 😤

This warning is also enabled by -Wextra.

• -Wno-ignored-attributes (C and C++ only) — controls warnings when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default.
• -Wmain — 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 is 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. the audacity 😭

In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional.

  if (some_condition ())
    foo ();
    bar ();  /* Gotcha: this is not guarded by the "if".  */

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

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

The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

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

• -Wmissing-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 is 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 💀. 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 is missing one of the attributes.

template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);

template <>
void* __attribute__ ((malloc))   // missing alloc_size
allocate<void> (size_t);

• -Wmissing-braces — warns if an aggregate or union initializer isn't fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.

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

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only) — warns if a user-supplied include directory does not exist. 🫶
• -Wno-missing-profile — okay bestie, this one controls warnings if feedback profiles are missing when you're using -fprofile-use. It diagnoses those cases where a new function or a new file gets added between compiling with -fprofile-generate and with -fprofile-use, and you never regenerated the profiles. In those cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, if profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to tell you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. You can use -Wno-missing-profile to disable the warning, but that is not recommended and should be done only when non-existent profile data is justified. 💅
• -Wmultistatement-macros — Warn about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, where only the first statement is actually guarded after the macro is expanded.

For example:

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

will increment y unconditionally, not just when c holds. This can usually be fixed 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 — Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.

Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.

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 — Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

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

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <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 is considered returning without a value).

For C only, warn about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used. Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.

Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used.

This warning is enabled by default in C++ and by -Wall otherwise. 💀

• -Wno-shift-count-negative — Controls warnings if a shift count is negative. This warning is enabled by default. ⭐
• -Wno-shift-count-overflow — Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default. ⭐
• -Wshift-negative-value — Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes. 🌈
• -Wno-shift-overflow, -Wshift-overflow=n — These options control warnings about left shift overflows.
  • -Wshift-overflow=1 — This is the warning level of -Wshift-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.) 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. 💖
• -Wno-switch-bool — Do not warn when 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. ✨

• -Wno-switch-outside-range — This option controls warnings when a switch case has a value that is outside of its respective type range. This warning is enabled by default for C and C++ programs. 🌸
• -Wno-switch-unreachable — Do not warn when 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). 🧚

okay bestie so if the compiler is being a lil dramatic about a variable you genuinely don't use, you can just be like "girl, chill" and slap the "unused" attribute on it. ✨

• -Wunused-but-set-variable — warns you whenever a local variable gets assigned to but is otherwise never used (declaring it doesn't count, bestie 💅). This one is switched on by -Wall.

Wanna shush it? Use the "unused" attribute. 🙈

It's also enabled by -Wunused, which — you guessed it — -Wall turns on.

• -Wunused-function — screams at you when a static function is declared but never defined, or when a non-inline static function is just... sitting there unused. Enabled by -Wall. 🎀
• -Wunused-label — warns whenever a label is declared but never used. Also enabled by -Wall.

To suppress it, use the "unused" attribute. 🫶

• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef defined locally inside a function never gets used. Enabled by -Wall. 🦋
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration.

To suppress this one, use the "unused" attribute. 💜

• -Wno-unused-result — tells GCC not to warn when someone calls a function marked with the warn_unused_result attribute and then ignores the return value. The default here is -Wunused-result. 👑
• -Wunused-variable — warns whenever a local or static variable is unused aside from its declaration. Heads up: this option implies -Wunused-const-variable=1 for C, but not for C++. Enabled by -Wall.

To suppress it, use the "unused" attribute. ✨

• -Wunused-const-variable
• -Wunused-const-variable=n — warns 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++. Why the drama? Because in C this declares variable storage, but in C++ it's not an error since const variables take the place of #defines. 🧚

To suppress this warning, use the "unused" attribute.

• -Wunused-const-variable=1 — this is the warning level that -Wunused-variable enables for C. It only warns about unused static const variables defined in the main compilation unit — it does not warn about static const variables declared in any header that gets included. 🌸
• -Wunused-const-variable=2 — this level also warns for unused constant static variables in headers (system headers excluded, don't worry 💅). This is the warning level of plain -Wunused-const-variable, and you have to ask for it explicitly, since in C++ this isn't an error and in C it might be a whole struggle to clean up every header you included.

• -Wunused-value — warns whenever a statement computes a result that is explicitly not used. To suppress it, cast the unused expression to void. This includes an expression-statement, or the left-hand side of a comma expression that has no side effects. Example time: an expression like 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 but real talk: to actually get a warning about an unused function parameter, you have to either specify -Wextra -Wunused (and yes, -Wall implies -Wunused), or separately specify -Wunused-parameter. No shortcuts, bestie.

• -Wuninitialized — warns if an automatic variable is used without first being initialized. In C++, it warns if a non-static reference or non-static const member shows up in a class without constructors. 😭

If you want to warn about code that uses the uninitialized value of a variable in its own initializer, use the -Winit-self option.

These warnings happen for individual uninitialized elements of structure, union or array variables, as well as for variables that are uninitialized as a whole. They do not happen for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements you get warnings for depend on the precise optimization options and the version of GCC you're running. 🌈

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 data flow analysis can just delete those computations before the warnings ever get printed. 💀

• -Wno-invalid-memory-model — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that's 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 code gets diagnosed:

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

-Winvalid-memory-model is enabled by default. 🔥

• -Wmaybe-uninitialized — for an automatic (i.e. local) variable, if there's a path from the function entry to a use of the variable that is initialized, but there are some other paths where the variable isn't initialized, the compiler emits a warning if it can't prove the uninitialized paths are never executed at run time. 🦋

These warnings are only even possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables. Periodt.

They're made optional because GCC may not be able to figure out when the code is actually correct despite looking like it has an error. Here's one example of how that can happen:

        {
          int x;
          switch (y)
            {
            case 1: x = 1;
              break;
            case 2: x = 4;
              break;
            case 3: x = 5;
            }
          foo (x);
        }

If the value of y is always 1, 2 or 3, then x is always initialized — but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code. 💅

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler only sees 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's genuinely no problem, because longjmp can't actually be called at the place that would cause a problem. 😭

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. 🫶

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warns when a #pragma directive is encountered that GCC doesn't understand. And if this command-line option is used, warnings get issued even for unknown pragmas in system header files. That's not the case if the warnings are only enabled by the -Wall command-line option. 👀
• -Wno-pragmas — stops warning about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. ✨
• -Wno-prio-ctor-dtor — stops warning if a priority from 0 to 100 is used for a constructor or destructor. The constructor and destructor attributes let you 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, because the compiler reserves priority values between 0–100 for the implementation. 🎀
• -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 it does attempt to catch the more common pitfalls. It's included in -Wall. It is equivalent to -Wstrict-aliasing=3. 💜
• -Wstrict-aliasing=n — again, only active when -fstrict-aliasing is active. Warns about code that might break the strict aliasing rules the compiler is using for optimization. Higher levels mean higher accuracy (fewer false positives). Higher levels also mean more effort — kinda like how -O works. And -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3. 🌸

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, since it has very few false negatives. But 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 (though not as many as level 1), 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. But note: it does not warn about all cases where the code might overflow — it only warns about cases where the compiler implements some optimization. So this warning depends on the optimization level. ✨

okay so real talk bestie, an optimization that assumes signed overflow never happens is totally safe when the values in play genuinely never overflow. but the way that means this warning can just… hand you a false positive about code that is not actually a problem. so to keep your attention on the stuff that matters, there are several warning levels. and 🫶 no warnings get issued for undefined signed overflow when the compiler is estimating how many iterations a loop needs — especially when it's figuring out whether the loop even runs at all.

---

• -Wstrict-overflow=1 — yells about cases that are both questionable and easy to avoid. like when the compiler simplifies "x + 1 > x" down to 1. this level is switched on by -Wall; the higher levels are not, you gotta ask for them explicitly, periodt.
• -Wstrict-overflow=2 — also yells about other cases where a comparison gets simplified to a constant. 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. and -Wstrict-overflow with no level at all = -Wstrict-overflow=2. 💅
• -Wstrict-overflow=3 — also yells about other cases where a comparison gets simplified. example: "x + 1 > 1" gets simplified to "x > 0".
• -Wstrict-overflow=4 — also yells about other simplifications not covered by the cases above. example: "(x * 10) / 5" gets simplified to "x * 2".
• -Wstrict-overflow=5 — also yells about cases where the compiler shrinks the magnitude of a constant involved in a comparison. example: "x + 2 > y" gets simplified to "x + 1 >= y". this one is only reported at the highest warning level because the simplification applies to sooo many comparisons, which means this level gives you a truly unhinged number of false positives. 😭

---

• -Wstring-compare — yells for calls to strcmp and strncmp whose result is determined to be either zero or non-zero in equality tests, because the length of one argument is greater than the size of the array the other argument is stored in (or the bound, in the strncmp case). those calls could be mistakes fr fr. like, the strcmp call below gets diagnosed because its result is necessarily non-zero no matter what's in the array a. 🦋

extern char a[4];
void f (char *d)
{
  strcpy (d, "string");
  ...
  if (0 == strcmp (a, d))   // cannot be true
    puts ("a and d are the same");
}

-Wstring-compare is enabled by -Wextra.

• -Wstringop-overflow
• -Wstringop-overflow=type — yells for calls to string manipulation functions like memcpy and strcpy that are determined to overflow the destination buffer. the optional argument is one greater than the type of Object Size Checking used to figure out the size of the destination. that argument only means anything for functions operating on character arrays — raw memory functions like memcpy always use Object Size type-0, no exceptions bestie. the option also yells for calls that specify a size bigger than the largest possible object, or at most SIZE_MAX / 2 bytes. it produces the best results with optimization on, but it can still catch a small subset of simple buffer overflows even with no optimization, in calls to GCC built-ins like __builtin_memcpy that correspond to the standard functions. either way, the option only warns about a subset of the buffer overflows caught by the corresponding overflow-checking built-ins. for example, it warns about the strcpy call below because it copies at least 5 characters (the string "blue", terminating NUL included 👀) into a 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
}

Option -Wstringop-overflow=2 is enabled by default.

• -Wstringop-overflow
• -Wstringop-overflow=1 — uses type-zero Object Size Checking to determine the sizes of destination objects. this is the default setting of the option. at this setting it does 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. when the destination could be one of several objects, it's assumed to be the biggest one. on Linux systems, with optimization enabled at this setting, the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. 🐧
• -Wstringop-overflow=2 — uses type-one Object Size Checking to determine the sizes of destination objects. at this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. but it does not warn for excessive writes to those same members of unknown objects referenced by pointers, since they might point to arrays holding an unknown number of elements. 🙈
• -Wstringop-overflow=3 — uses type-two Object Size Checking to determine the sizes of destination objects. at this setting the option warns about overflowing the smallest object or data member. this is the most restrictive setting of the option, and heads up: it may result in warnings for code that's actually safe.
• -Wstringop-overflow=4 — uses type-three Object Size Checking to determine the sizes of destination objects. at this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to warn. just like -Wstringop-overflow=3, this setting may result in warnings for benign code. 💀

---

• -Wno-stringop-truncation — do not warn for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that may either truncate the copied string or leave the destination unchanged.

in this next example, the call to strncat specifies a bound that's smaller than the length of the source string. so the copy of the source gets truncated, and the call is diagnosed. to dodge the warning, use bufsize - strlen (buf) - 1) as the bound. 💅

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

as another example, the strncpy call below copies into d just the characters before 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. to avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead. 🎀

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

in the following example, the call to strncpy uses the size of the destination buffer as the bound. if the length of the source string is equal to or greater than that size, the result of the copy will not be NUL-terminated. so yeah, that 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 nonstring attribute to avoid this warning. but bestie, those arrays are not suitable arguments to functions that expect NUL-terminated strings. to help catch accidental misuse of such arrays, GCC issues warnings unless it can prove the use is safe. 🫶

---

• -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] — warns for cases where adding an attribute might be a glow-up. the attributes currently supported are listed below. 👑
  • -Wsuggest-attribute=pure
  • -Wsuggest-attribute=const
  • -Wsuggest-attribute=noreturn
  • -Wmissing-noreturn
  • -Wsuggest-attribute=malloc — warn about functions that might be candidates for the pure, const, noreturn, or malloc attributes. the compiler only warns for functions visible in other compilation units, or (for pure and const) if it can't prove the function returns normally. a function returns normally if it contains no infinite loop and doesn't 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 make the analysis more accurate. 🔥
  • -Wsuggest-attribute=format
  • -Wmissing-format-attribute — warn about function pointers that might be candidates for format attributes. ngl these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes 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 carry a format attribute to avoid the warning. 💖

okay so GCC also lowkey warns about function definitions that might be candidates for format attributes. bestie, these are only possible candidates. GCC is like, "hmm, this function calls something like vprintf or vscanf, so maybe it needs a format attribute?" but that's not always the case, and some functions that do need format attributes might not get detected. 💅

---

-Wsuggest-attribute=cold

Warn about functions that might be candidates for cold attribute. This is based on static detection and generally only warns about functions which always leads to a call to another cold function such as wrappers of C++ throw or fatal error reporting functions leading to abort. 🥶

-Walloc-zero

Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated) relying on it may result in subtle portability bugs and should be avoided. 🚫

-Walloc-size-larger-than=byte-size

Warn about calls to functions decorated with attribute alloc_size that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-alloc-size-larger-than. 📏

-Wno-alloc-size-larger-than

Disable -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. 🔇

-Walloca

This option warns on all uses of alloca in the source. 👀

-Walloca-larger-than=byte-size

This option warns on calls to alloca with an integer argument whose value is either zero, or that is not 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. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range.

For example, a bounded case of alloca could be:

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

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

Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example:

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

If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking. 🚨

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

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

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

This option also warns when alloca is used in a loop. 🔁

-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. The option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. 🔕

-Warith-conversion

Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion. 🔄

void f (char c, int i)
{
  c = c + i; // warns with -Wconversion
  c = c + 1; // only warns with -Warith-conversion
}

-Warray-bounds, -Warray-bounds=n

This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall. 🚧

• -Warray-bounds=1 — This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Warray-bounds=2 — This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default.

-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. 📋

-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.) ⚠️

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;

-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. 🔇

-Wzero-length-bounds

Warn about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined. See

For example, the first two stores in function bad are diagnosed because the array elements overlap the subsequent members b and c. The third store is diagnosed by -Warray-bounds because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

void bad (void)
{
  x.a[0] = 0;   // -Wzero-length-bounds
  x.a[1] = 1;   // -Wzero-length-bounds
  x.a[2] = 2;   // -Warray-bounds
}

Option -Wzero-length-bounds is enabled by -Warray-bounds. 🚫

• -Wno-div-by-zero — bestie, this one tells GCC to not warn about compile-time integer division by zero. But floating-point division by zero? It's not warned about, because that can be a totally legit way of obtaining infinities and NaNs. 💅
• -Wsystem-headers — print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they 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. 🎀
• -Wfloat-equal — warn if floating-point values are used in equality comparisons. The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. 😭
• -Wtraditional (C and Objective-C only) — warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. Here's the whole list, bestie:
  • Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion. 💖
• -Wdeclaration-after-statement (C and Objective-C only) — warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. 🙈
• -Wshadow — warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global. 👑
• -Wno-shadow-ivar (Objective-C only) — do not warn whenever a local variable shadows an instance variable in an Objective-C method.
• -Wshadow=global — warn for any shadowing. Same as -Wshadow.
• -Wshadow=local — warn when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example:

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

Since the two variable i in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing const char *i by char *i does not emit a warning.

This warning is also 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 do not get a warning. In addition, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. -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 — bestie, this one yells at you if a function's stack usage might go over byte-size. And listen, the math it does to figure out that stack usage? It's conservative, like it assumes the worst. Anything you alloca-ed, any variable-length arrays, anything in that whole vibe — the compiler counts it all when it's deciding whether to drag you or not.

The message it spits out matches -fstack-usage's energy exactly. So:

• If the stack usage is fully static but goes over the limit you set, it's giving:

warning: stack usage is 1120 bytes

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

warning: stack usage might be 1648 bytes

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

warning: stack usage might be unbounded

And heads up: -Wstack-usage=PTRDIFF_MAX is on by default ✨. Wanna silence the warnings this option controls? You can either pass a byte-size of SIZE_MAX or bigger, or just hit it with -Wno-stack-usage.

• -Wno-stack-usage — turns off all the -Wstack-usage= warnings. Literally equivalent to passing -Wstack-usage=SIZE_MAX or anything larger. Same thing, different font 💅.
• -Wunsafe-loop-optimizations — warns you when a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And if you pair it with -funsafe-loop-optimizations? Then it warns when the compiler does make those assumptions. So either way you're getting notified, iconic honestly.
• -Wno-pedantic-ms-format (MinGW targets only, bestie) — when you use this alongside -Wformat and -pedantic with no GNU extensions, it shuts off the warnings about non-ISO printf/scanf format width specifiers — that's I32, I64, and I — used on Windows targets, which depend on the MS runtime. Niche but valid.
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. GNU C assigns these types a size of 1, for convenience when you're doing calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. Also enabled by -Wpedantic, so you might already have it and not even know 🫶.
• -Wno-pointer-compare — stops warning you when a pointer gets compared to a zero character constant. Because usually, that means you meant to dereference that pointer. Like:

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

Heads up though: that code is invalid in C++11. Not cute of it. This warning is on by default.

• -Wtype-limits — warns when a comparison is always true or always false because the data type's range is just too limited for it to be anything else. But it does not warn for constant expressions, okay? So for example, if an unsigned variable gets compared against zero with < or >=, it'll flag that. Also enabled by -Wextra.
• -Wabsolute-value (C and Objective-C only) — warns when you call a standard absolute-value function but there's a better standard function for that exact argument. Example: calling abs(3.14) triggers it, because the correct call for the absolute value of a double is fabs. It also fires when the argument to such a function has an unsigned type (which is just... pointless math). You can suppress it with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* shows up inside a /* comment, or whenever a backslash-newline appears inside a // comment. Enabled by -Wall.
• -Wtrigraphs — warns if any trigraphs show up that might change what your program means. Trigraphs inside comments don't get warned about, except the ones that would form escaped newlines.

This one's implied by -Wall. And if -Wall isn't given, it's still enabled unless trigraphs are enabled. Want trigraph conversion without the warnings but with all the other -Wall goodies? Do this:

-trigraphs -Wall -Wno-trigraphs

• -Wundef — warns if an undefined identifier gets evaluated in an #if directive. Those identifiers get replaced with zero, fyi, which is why it matters.
• -Wexpansion-to-defined — warns whenever defined shows up in the expansion of a macro (yes, including when the macro gets expanded by an #if directive). That usage is not portable, so like... be careful. Also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that never get used. A macro counts as "used" if it's expanded or tested for existence at least once. And the preprocessor also warns if a macro hadn't been used by the time it gets redefined or undefined. Rude, but fair.

Built-in macros, macros defined on the command line, and macros defined in include files? Not warned about. They're exempt, they're the favorites.

Note: if a macro is actually used but only inside skipped conditional blocks, the preprocessor still reports it as unused. To dodge the warning in that case, you could improve the scope of the macro's definition — like, move it into the first skipped block. Or you could just give it a dummy use, something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — stops warning you whenever an #else or an #endif is followed by text. This happens sometimes in older programs with code shaped like:

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

That second and third FOO should be in comments, bestie. Anyway, this warning is on by default.

• -Wbad-function-cast (C and Objective-C only) — warns when a function call gets cast to a non-matching type. Like, warn if you call a function that returns an integer type and then cast it to a pointer type. Sir, why.
• -Wc90-c99-compat (C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. For instance: variable length arrays, the long long type, the 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) — warns about features not in ISO C99 but present in ISO C11. For instance: anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, and so on. Also independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc11-c2x-compat (C and Objective-C only) — warns about features not in ISO C11 but present in ISO C2X. For instance: omitting the string in _Static_assert, use of [[]] syntax for attributes, use of decimal floating-point types, and so on. Independent of the standards mode, again. Warnings are disabled in the expression that follows __extension__.
• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++. Example: requesting an implicit conversion from void * to a pointer to a non-void type.
• -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning changed between ISO C++ 1998 and ISO C++ 2011. Like, identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This one turns on -Wnarrowing and is enabled by -Wall.
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by -Wall.
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall.
• -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall.
• -Wcast-qual — warns whenever a pointer is cast in a way that strips a type qualifier off the target type. Example: warn if a const char * gets cast to an ordinary char *.

It also warns when you make a cast that introduces a type qualifier unsafely. Example: casting char ** to const char ** is unsafe, like in this masterpiece of a disaster:

/* 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 — warns whenever a pointer is cast so that the target's required alignment goes up. 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 — same deal, warns whenever a pointer cast increases the target's required alignment. Example: warn if a char * gets cast to an int * — except this time, regardless of the target machine. No mercy, no exceptions 💅.
• -Wcast-function-type — warns when a function pointer gets cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of the initial arguments that are provided get considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types get ignored, like int vs. long on ILP32 targets. Type qualifiers get ignored too, same energy. The function type void (*) (void) is special — it matches everything, which you can use to suppress this warning. And in a cast involving pointer-to-member types, this warning fires whenever the type cast changes the pointer to member type. Enabled by -Wextra.
• -Wwrite-strings — when compiling C, this gives string constants the type const char[length] so that copying one's address into a non-const char * pointer produces a warning. These warnings help you find code at compile time that might try to write into a string constant — but only if you've been super careful about using const in declarations and prototypes. Otherwise it's just a nuisance. Which is literally why -Wall doesn't request these warnings.

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

• -Wclobbered — warns for variables that might get changed by longjmp or vfork. Also enabled by -Wextra.

---

• -Wconversion — bestie, this is the vibe check for implicit conversions that might secretly change a value 💅 That 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). But no cap, it will NOT warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or when the value isn't changed by the conversion at all, like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled with -Wno-sign-conversion.

For C++, it also warns for confusing overload resolution for user-defined conversions; and for 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.

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. tiny type going right back to itself, so real

• -Wdangling-else — the "wait, which if does that else belong to??" warning 😭 Warns about constructions where there may be confusion about which if statement an else branch belongs to. Here's the classic crime scene:

{
  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, as the indentation in the example above clearly shows. When there's potential for this confusion, GCC issues a warning when this flag is specified. To make the warning go away, 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. it comes free with the parentheses package, iconic

• -Wdate-time — Warns when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered, because they might prevent bit-wise-identical reproducible compilations. timestamps are lowkey the enemy of "same build every time" 🕐
• -Wempty-body — Warns if an empty body occurs in an if, else or do while statement. bro the body is empty... what are we even doing This warning is also enabled by -Wextra.
• -Wno-endif-labels — Do not warn about stray tokens after #else and #endif. just minding your own business, nothing to see here
• -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. comparing apples to oranges but make it enums 🍎🍊
• -Wenum-conversion (C, Objective-C only) — Warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra. enum says "not my type" 💜
• -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. It 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, fr fr.

-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. signed vs unsigned beef, the oldest rivalry in the game 😤
• -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. cast it or zip it ✨
• -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. the precision is leaking bestie 💧
• -Wno-scalar-storage-order — Do not warn on suspicious constructs involving reverse scalar storage order. backwards bytes? we simply do not look 🫣
• -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. It 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 math is mathing but the pointer said nope
• -Wsizeof-pointer-memaccess — Warns for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. It 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));. it literally hands you the fix, so polite of it 🫶

-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.

• -Wno-sizeof-array-argument — Do not warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs. array parameters are basically little liars, so this one's on by default 💅
• -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. That indicates the user has omitted a multiplication by the element size. you forgot to multiply bestie, the elements are NOT the bytes 😭 This warning is enabled by -Wall.
• -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. you flipped them. you flipped them!! 🙈 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 its use in a conditional usually indicates 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. you wanted strcmp, not ==. we've all been there 🫶 This warning is enabled by -Wall.
• -Wno-address-of-packed-member — Do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. unaligned pointers, my beloathed
• -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. It also warns when the operands of a logical operator are the same:

extern int a;
if (a < 0 && a < 0) { ... }

saying the exact same thing twice and expecting a different answer?? girl 😭

• -Wlogical-not-parentheses — Warns about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its whole purpose is to detect suspicious code like this:

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

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

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

This warning is enabled by -Wall. parentheses: the solution to (almost) everything ✨

• -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.) returning a whole struct? bold move, honestly
• -Wno-aggressive-loop-optimizations — Warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of the iterations. the compiler found UB in your loop. screaming. crying. throwing up. 💀
• -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. unknown attribute? we ignore. broken supported attribute? we still yell 🔥
• -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, it also warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-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. just #include the header bestie, it's right there 🎀

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 — bestie, this one tells GCC to chill about certain built-in macros getting redefined. It suppresses the warnings you'd get for redefining "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__". like, let them live 💅
• -Wstrict-prototypes (C and Objective-C only) — warns when a function is declared or defined without specifying the argument types. Tiny lil plot twist tho: an old-style function definition is allowed to slide without a warning if it's preceded by a declaration that does specify the argument types. so like, cover for it and it's fine ✨
• -Wold-style-declaration (C and Objective-C only) — warns about obsolescent usages in a declaration, per the C Standard. Example: it yells if storage-class specifiers like "static" aren't the first things in a declaration. this one is also enabled by -Wextra. ngl it's giving "put your keys by the door" energy.
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. And it warns even if there's a previous prototype, no mercy. Little detail: a definition using () is not considered old-style in C2X mode, because in that case it's equivalent to (void) — but for older standards it totally is old-style. standards change, vibes stay 🦋
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions, like this:

void foo(bar) { }

this warning is also enabled by -Wextra.

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. And yeah, it warns even if the definition itself provides a prototype. Use this to catch 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. For C++, use -Wmissing-declarations to detect missing declarations instead. iykyk 🌸
• -Wmissing-declarations — warns if a global function is defined without a previous declaration, and does so even if the definition itself provides a prototype. Use it to detect 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 that. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. three whole exclusions, we love a nuanced queen ✨
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. Example: this code 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, so this modification doesn't trigger anything:

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

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

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

Likewise, in C++ it doesn't warn about the empty { } initializer, for example:

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

This warning is included in -Wextra. To get the other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. mix and match, it's giving build-your-own-warning 🎀

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually they mean someone made a typo in their code, because they have implementation-defined values and shouldn't be used in portable code. fr fr, leave those in the drafts folder 🙈
• -Wnormalized=[none|id|nfc|nfkc] — okay this one is so cool actually. In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when characters outside the basic ASCII character set are used, you can have two different character sequences that look identical. To avoid confusion, the ISO 10646 standard sets 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 — this option controls that warning.

There are four levels of warning GCC supports. 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, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. Like, 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. It's hoped that future versions of the standards involved will fix this — which is exactly why this option isn't the default. manifesting a fix for the girlies 🕯️

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should 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. like, terrifying. no cap.

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 of these into a standard form too, 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, and 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. a whole aesthetic choice, honestly 🦋

• -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. two doors, same hallway 💅
• -Wno-deprecated — do not warn about usage of deprecated features. let the past rest ✨
• -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. math said no and we're not discussing it 🎀
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default. yes, the "no" prefix is doing something different here, don't look at it too long 💀
• -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. negotiation, but make it compilers 🌸
• -Woverride-init (C and Objective-C only) — 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. the classic swap ✨

• -Wno-override-init-side-effects (C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default. side effects get a pass, we love a boundary 💅
• -Wpacked — warns if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for basically no benefit. all risk, zero reward, toxic honestly For instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" doesn't itself have the packed attribute:

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

• -Wnopacked-bitfield-compat — history lesson time 🎀: the 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example, there'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 it. turning it off is a choice and it's yours 👑

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warns if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning gets 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. the four-language girlies stay winning 🌈

• -Wpadded — warns if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens, it's possible to rearrange the fields of the structure to reduce the padding and make the structure smaller. interior design but for memory 🎀
• -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. say it again for the people in the back, but also, please don't 💅
• -Wrestrict — 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. Example: the call to the "strcpy" function below tries 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. optimization is its glow-up era ✨

• -Wnested-externs (C and Objective-C only) — warns if an "extern" declaration is encountered within a function. extern said "why am I inside a function" 😭
• -Winline — bestie, this one yells at you when a function you swore was inline... literally cannot be inlined. 😭 But real talk: even with this on, the compiler stays quiet about failures to inline stuff declared in system headers. Not a peep.

So how does it decide? The compiler runs a whole bunch of heuristics — like, it looks at the size of the function being inlined, and how much inlining has already gone down in the current function. Which means the tiniest, most insignificant little change in your source can make -Winline warnings pop up out of nowhere or vanish into the void. It's giving chaotic energy, fr fr. 🙈

• -Wint-in-bool-context — warns you when you're being sus with integer values where boolean values are supposed to be. Like conditional expressions (?:) using non-boolean integer constants in a boolean context — for example if (a <= b ? 2 : 3). Or left shifting signed integers in a boolean context, like for (a = 0; 1 << a; a++);. Same deal for every kind of multiplication, no matter 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. Heads up though: in C++, casting to a pointer type of smaller size is straight-up 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. C and Objective-C only, bestie, don't try this elsewhere.
• -Winvalid-pch — warns you when a precompiled header is found in the search path but... cannot be used. It's there, it's just not serving. 💀
• -Wlong-long — warns if the long long type is used. Enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. Wanna mute it? 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. Enabled by either -Wpedantic or -Wtraditional. To shut it up, use -Wno-variadic-macros.
• -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments, like va_start. These warnings are enabled by default, so this is you asking for peace and quiet. 🫶
• -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. Here's the tea on how a vector operation can be implemented:
  • "piecewise" — the scalar operation is performed on every vector element.
  • "in parallel" — the vector operation is implemented using scalars of wider type, which normally is more performance efficient.
  • "as a single scalar" — the vector fits into a scalar type.
• -Wvla — warns if a variable-length array is used in the code. And -Wno-vla prevents the -Wpedantic warning of the variable-length array. 🌈
• -Wvla-larger-than=byte-size — with this on, the compiler warns 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. It's similar to how -Walloca-larger-than=byte-size works, but for variable-length arrays.

Ngl though, GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning might not even get triggered for those. 🙈

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

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

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. This option is equivalent to -Wvla-larger-than=SIZE_MAX or larger. 💅
• -Wvolatile-register-var — warns if a register variable is declared volatile. And the plot twist: the volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. So don't get too comfortable. 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. Your code is just too much, and honestly? Relatable. 😭
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. It's implied by -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. Stay safe out there. 🛡️
• -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-lto-type-mismatch — during the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default.
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute.
• -Wno-hsa — do not warn when HSAIL cannot be emitted for the compiled function or OpenMP construct. These warnings are enabled by default.

---

👑 Options That Control Static Analysis 👑

• -fanalyzer — okay this one is the main character. 💅 This option enables a static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them.

Heads up though: this analysis is much more expensive than other GCC warnings. It's giving luxury pricing. 💀

Enabling this option effectively enables the following warnings:

-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-array-index
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame

This option is only available if GCC was configured with analyzer support enabled. No analyzer support, no analyzer. Periodt.

• -Wanalyzer-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code.

By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns if this occurs. So like, at least now you'd know. 🙈

• -Wno-analyzer-double-fclose — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. Once is a vibe, twice is a problem. 😭

• -Wno-analyzer-double-free — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to disable it.

This diagnostic warns for paths through the code in which a pointer can have free called on it more than once.

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Bestie. NOT the password in the log file. 💀

• -Wno-analyzer-file-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it.

This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked.

• -Wno-analyzer-free-of-non-heap — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global).

• -Wno-analyzer-malloc-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.

This diagnostic warns for paths through the code in which a pointer allocated via malloc is leaked.

• -Wno-analyzer-possible-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

• -Wno-analyzer-possible-null-dereference — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced.

• -Wno-analyzer-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-argument to disable it.

This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

• -Wno-analyzer-null-dereference — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to disable it.

Okay so this one's the last of the analyzer-warning besties, and then we pivot to debug info. Stay with me 💖

• It's giving: paths through the code where a value you know is NULL gets dereferenced. The analyzer is not letting that slide, and honestly? fair.
• -Wno-analyzer-stale-setjmp-buffer — heads up, this warning only exists if -fanalyzer is on, because -fanalyzer is what enables it; use -Wno-analyzer-stale-setjmp-buffer to turn it back off.

So like, it flags paths through the code where longjmp gets called to rewind to a jmp_buf that belongs to a setjmp call inside a function that has already returned. 🫠

When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. And bestie, that stack frame becomes invalid the moment the function containing the setjmp call returns. Tryna rewind to it via longjmp means referencing a stack frame that literally does not exist anymore, and that's gonna lead to a crash (or worse). 😭

• -Wno-analyzer-tainted-array-index — this one needs both -fanalyzer and -fanalyzer-checker=taint to even be enabled; use -Wno-analyzer-tainted-array-index to disable it.

It warns about paths through the code where a value that could be under an attacker's control is used as the index of an array access without being sanitized. 🚨

• -Wno-analyzer-unsafe-call-within-signal-handler — requires -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it.

It warns about paths through the code where a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. 💀

• -Wno-analyzer-use-after-free — requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it.

It warns about paths through the code where a pointer is used after free is called on it. Classic. We hate to see it.

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

It warns about paths through the code where a pointer is dereferenced that points to a variable in a stale stack frame. ✨

---

The parameters that actually control the exploration vibes are: --param analyzer-bb-explosion-factor=value, --param analyzer-max-enodes-per-program-point=value, --param analyzer-max-recursion-depth=value, and --param analyzer-min-snodes-for-call-summary=value.

And the following options control the analyzer itself. 🎀

• -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring all the paths through the function from callsite to each possible return.

If it's enabled, call summaries only get used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value). iykyk.

• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it.

Some checkers are off by default (even with -fanalyzer), like the taint checker that implements -Wanalyzer-tainted-array-index, and this option is what you need to enable them. 👑

• -fanalyzer-fine-grained — this option is for analyzer developers, fr.

Internally the analyzer builds an exploded graph that combines control flow graphs with data flow information. 🌈

By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge.

• -fanalyzer-show-duplicate-count — for analyzer developers: if multiple diagnostics got detected as duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic.
• -fno-analyzer-state-merge — for analyzer developers.

By default the analyzer tries to simplify analysis by merging sufficiently similar states at each program point as it builds its exploded graph. With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.

• -fno-analyzer-state-purge — for analyzer developers.

By default the analyzer tries to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant, e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.

• -fanalyzer-transitivity — enables transitivity of constraints within the analyzer.
• -fanalyzer-verbose-edges — for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.
• -fanalyzer-verbose-state-changes — for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
• -fanalyzer-verbosity=level — controls the complexity of the control flow paths that get emitted for analyzer diagnostics.

The level can be one of:

1. At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-free diagnostic, both calls to free will be shown.
2. As per the previous level, but also show events for the entry to each function.
3. As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. true path taken at a conditional).

This level is the default. ⭐

4. As per the previous level, but show all control flow events, not just significant ones.
5. This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. This option is overridden by -fdump-analyzer-stderr.
• -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.
• -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot.
• -fdump-analyzer-exploded-graph — dumps a representation of the exploded graph suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. 🦋
• -fdump-analyzer-exploded-nodes — emits diagnostics showing where nodes in the exploded graph are in relation to the program source.
• -fdump-analyzer-exploded-nodes-2 — dumps a textual representation of the exploded graph to file.eg.txt.
• -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the exploded graph to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files, just so you know. 🙈
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dumps a representation of the supergraph suitable for viewing with GraphViz, but annotates the graph with info on what state will be purged at each node. The graph is written to file.state-purge.dot.
• -fdump-analyzer-supergraph — dumps representations of the supergraph suitable for viewing with GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. These show all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the exploded graph and diagnostics associated with them. 💅

---

🌸 Options for Debugging Your Program 🌸

To tell GCC to emit extra info for use by a debugger, in almost all cases you need only to add -g to your other options. That's it. That's the whole vibe.

GCC lets you use -g with -O. The shortcuts taken by optimized code may occasionally be surprising, and I need you to be seated for this: 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're not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect info useful for debugging don't run at all, so -Og may result in a better debugging experience. 🫶

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

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra info 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 — produces debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
• -gdwarf
• -gdwarf-version — produces debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 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 (yes, with the stray parenthesis, we keep it exactly as written 💀) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF.

• -gstabs — bestie, this makes debugging info in stabs format (if your setup even supports it), and nope, no GDB extensions. It's the format DBX rocks on most BSD systems. But heads up 👀 on MIPS, Alpha and System V Release 4 systems, this option makes stabs debugging output that DBX literally cannot understand. And on System V Release 4 systems it needs the GNU assembler. Not a vibe if you're on one of those, fr.
• -gstabs+ — same stabs format (if supported) but with GNU extensions that only the GNU debugger (GDB) gets. The way these extensions will have other debuggers crashing or flat-out refusing to read your program... it's giving chaos 🫠.
• -gxcoff — debugging info in XCOFF format (if that's supported, key phrase). This is the format the DBX debugger uses on IBM RS/6000 systems. Niche but iconic.
• -gxcoff+ — XCOFF format (if supported) using GNU extensions that only GDB understands. Using these extensions is likely to make other debuggers crash or refuse to read your program, AND it may cause assemblers other than the GNU assembler (GAS) to fail with an error. So like... be careful 💀.
• -gvms — debugging info in Alpha/VMS debug format (if supported). It's the format DEBUG uses on Alpha/VMS systems. Respect the elders ✨.
• -glevel
• -ggdblevel
• -gstabslevel
• -gxcofflevel
• -gvmslevel — okay so this one asks for debugging info AND uses level to say how much info you want. Default level? 2. Let's break the levels down, bestie:
  • Level 0 produces no debug information at all. So -g0 fully negates -g. Gone. Poof.
  • Level 1 produces minimal info — just enough for backtraces in the parts of the program you're not planning to debug. It includes descriptions of functions and external variables, plus line number tables, but literally nothing about local variables.
  • Level 3 includes extra info, like all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. Slay.

If you use multiple -g options, with or without level numbers, the last such option is the one that's effective. Last one wins, no takebacks.

Also, real talk: -gdwarf does not accept a concatenated debug level, so you don't get it confused with -gdwarf-level. Instead, add an extra -glevel option to change the debug level for DWARF. Note it down 📝.

• -fno-eliminate-unused-debug-symbols — by default, no debug info gets produced for symbols that aren't actually used. Use this option if you want debug info for all the symbols. Completionist energy 🎀.
• -femit-class-debug-always — instead of emitting debugging info for a C++ class in only one object file, this emits it in all object files that use the class. Use this only with debuggers that can't handle the way GCC normally emits debugging info for classes, because using it increases the size of your debugging info by as much as a factor of two. Double. Twice. It's a lot 😭.
• -fno-merge-debug-strings — tells the linker to not merge together strings in the debugging info that are identical across different object files. Not every assembler or linker supports merging, jsyk. Merging shrinks the size of the debug info in the output file, but it costs you increased link processing time. And it's enabled by default. So this flag is the "no thanks, I'll take the slower link" option.
• -fdebug-prefix-map=old=new — when compiling files that live in directory old, this records debugging info describing them as if they lived in directory new instead. You can use it to swap a build-time path for an install-time path in the debug info. You can also use it to turn an absolute path into a relative path by setting new to .. This can give you more reproducible builds, which are location independent — but it may need an extra command to tell GDB where to find the source files. See also -ffile-prefix-map. Chef's kiss for reproducible builds 🧚.
• -fvar-tracking — runs the variable tracking pass. It computes where variables are stored at each position in the code, and then better debugging info gets generated (if the debugging info format supports that info, that is). It's enabled by default when you compile with optimization (-Os, -O, -O2, ...), debugging information (-g), and the debug info format supports it. So like, it just shows up when it can 💅.
• -fvar-tracking-assignments — annotates assignments to user variables early in compilation, then tries to carry those annotations all the way through compilation to the end, all in the name of improving debug info while optimizing. Pairing it with -gdwarf-4 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. The teamwork 🤝.
• -gsplit-dwarf — separates as much DWARF debugging info as possible into a separate output file with the extension .dwo. This lets the build system avoid linking files with debug info. But to actually be useful, this option needs a debugger capable of reading .dwo files. No .dwo-reading debugger, no benefit, periodt.
• -gdescribe-dies — adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. Little glow-up for your DIEs ✨.
• -gpubnames — generates DWARF .debug_pubnames and .debug_pubtypes sections. That's it, that's the whole thing.
• -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. Specific, but we love specificity 🎀.
• -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, since the linker can then remove duplicates. But plot twist: not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging info. So it's giving "it depends" 💀.
• -grecord-gcc-switches
• -gno-record-gcc-switches — this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging info. The options get concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into 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, so this is you being strict about it. Boundaries, bestie 🚧.
• -gno-strict-dwarf — allows using extensions from a later DWARF standard version than the one selected with -gdwarf-version. Live a little 🌈.
• -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. Free glow-up if your assembler's got it 🫶.
• -gno-as-loc-support — forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are going to be generated at all.
• -gas-locview-support — informs the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives. 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 info into DWARF debugging info, rather than just file and line. Enabled by default. We love detail here 💅.
• -gstatement-frontiers
• -gno-statement-frontiers — this one makes GCC create markers in the internal representation at the beginning of statements, and keeps 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 info 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 if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will make the compiler emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so debug info consumers that aren't aware of these augmentations can still consume them — they just won't derive any benefit from them either. Fair trade 🤷.

This is enabled by default when outputting DWARF 2 debug info 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 will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views. Silent. Mismatching. The audacity 😭.

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 enable it with -gvariable-location-views=incompat5. Heads up: this option may be removed in the future, 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 would be rendered unable to decode location lists using it. So like, don't be surprised when things can't read it 💀.

• -ginternal-reset-location-views, -gno-internal-reset-location-views — bestie, this one tries to sniff out which location views can just be skipped from the location view lists. The catch? It needs the compiler to have ✨extremely✨ accurate insn length estimates, which like... isn't always the vibe. And if you're using an assembler that doesn't do location view lists, it might silently generate wrong view lists 😭. The GNU assembler will call that error out as a "view number mismatch", so at least someone's paying attention. It's only turned on for ports that define a reliable estimation function. iykyk.
• -ginline-points, -gno-inline-points — this one generates extended debug info for inlined functions 🫶. Location view tracking markers get inserted at inlined entry points so address and view numbers can be computed and written out in debug info. Fun fact: you can turn this on independently of location views — in which case the view numbers just won't be output — but it can only be enabled along with statement frontiers. And it's only on by default if location views are enabled. Teamwork, honestly. 💅
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. If you don't give type, the default depends on what your assembler and linker can actually do. type can be one of none (no compression, we're rawdogging the debug sections), zlib (zlib compression in ELF gABI format), or zlib-gnu (zlib compression in traditional GNU format). If your linker can't write compressed debug sections, the option gets rejected, periodt. Otherwise, if the assembler doesn't support them, -gz is just silently ignored while producing object files. 🙈
• -femit-struct-debug-baseonly — emit 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 struct is defined. This seriously shrinks your debugging information, but at a significant potential loss in type information to the debugger 💀. Want something less aggressive? See -femit-struct-debug-reduced. Want full control? See -femit-struct-debug-detailed. Also: this only works with DWARF debug output, no exceptions.
• -femit-struct-debug-reduced — emit 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 defined in a system header. This significantly reduces debugging info size with some potential loss of type info to the debugger. More aggressive version? -femit-struct-debug-baseonly. More detailed control? -femit-struct-debug-detailed. And yeah, DWARF debug output only. ✨
• -femit-struct-debug-detailed[=spec-list] — this is where you get to specify exactly which struct-like types the compiler generates debug info for 👑. The whole intent is to cut down duplicate struct debug info between different object files in the same program. Think of it as the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, covering 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 or member. Indirect uses come through pointers to structs — so when using 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 you want debug info for. The values none and any mean the normal thing. 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, compiling foo.c means debug info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to find the best settings for your application, fr fr. The default is -femit-struct-debug-detailed=all. And this works only with DWARF debug output. 💖

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler-generated ".eh_frame" section instead of using GAS ".cfi_*" directives. Simple as that. 🌸
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC skips producing debug symbol output for types that are nowhere used in the source file being compiled. But sometimes it's genuinely useful to have GCC emit debug info for all types declared in a compilation unit, whether or not they're actually used — like when you want to cast a value to a type that isn't used in your program but is declared. More often, though, this results in a significant amount of wasted space. So like... use it with intention. 🦋

---

🚀 Options That Control Optimization 🚀

These options control various sorts of optimizations. Buckle up bestie. 💅

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. It's giving predictable. 😌

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, always trade-offs. ✨

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. Team effort fr fr. 🫶

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. Iconic little trick, ngl. 🔥

• -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 do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. Main character energy honestly. 👑

-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
-ffinite-loops -fgcse  -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp  -fipa-cp  -fipa-icf -fipa-ra  -fipa-sra  -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-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. Just saying. 👀

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 -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model -fvect-cost-model=dynamic
-fversion-loops-for-strides

• -O0 — Reduce compilation time and make debugging produce the expected results. This is the default. 🎀
• -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 turns on -finline-functions, makes the compiler tune for code size instead of execution speed, and does even more optimizations that are all about shrinking your code. tiny code, big vibes ✨

• -Ofast — bestie, this one just disregards strict standards compliance, no cap. -Ofast turns on every single -O3 optimization. It also flips on optimizations that are not valid for all standard-compliant programs 🙈. Specifically it enables -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays (unless -fmax-stack-var-size is specified), plus -fno-protect-parens.
• -Og — optimize the debugging experience 💅. -Og should honestly be your go-to for the standard edit-compile-debug cycle — it gives you a reasonable level of optimization while keeping compilation fast and debugging actually pleasant. It's a better choice than -O0 for producing debuggable code, because some compiler passes that collect debug info are just disabled at -O0.

Like -O0, -Og completely disables a bunch of optimization passes, so the individual options controlling them have zero effect. Otherwise, -Og enables all the -O1 optimization flags except the ones that might mess with debugging:

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

If you use multiple -O options, with or without level numbers, the last such option is the one that's effective. Last one wins, periodt. 👑

---

Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you'd typically use. You can figure out the other form by either removing no- or adding it. iykyk 🦋

The following options control specific optimizations. They're either activated by -O options or related to ones that are. You can use these flags in the rare cases when you want to "fine-tune" the optimizations being performed. fancy ✨

• -fno-defer-pop — for machines that have to pop arguments after a function call, this always pops the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; it lets the compiler let arguments pile up on the stack across several function calls and pop them all at once. 🫶
• -fforward-propagate — performs 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 are performed and the second one is scheduled after loop unrolling.

This option is 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 it's allowed by the language standard — this is currently not implemented and 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 avoids the instructions to save, set up and restore the frame pointer; on many targets it also frees up an extra register. 🌈

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted. 😭

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -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 (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. Enabled at levels -O2, -O3. 🔥
• -fno-inline — do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute.

• -finline-small-functions — integrates functions into their callers when their body is smaller than the expected function call code (so the overall program size gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline. 🌸

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

• -findirect-inlining — also inlines indirect calls that are discovered 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 are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile. 💜

• -finline-functions-called-once — considers all static functions called once for inlining into their caller, even if they aren't marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og. 🙅

• -fearly-inlining — inlines functions marked by always_inline and functions whose body seems smaller than the function call overhead, early on — before doing -fprofile-generate instrumentation and the real inlining pass. Doing this makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. 🧚

Enabled by default.

• -fipa-sra — performs interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference 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 allows coarse control of that limit. n is the size of functions that can be inlined, in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters like this:

• 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 these 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. In no way does it 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 that are 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. In addition to -fmerge-constants, this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable — including multiple instances of the same variable in recursive calls — to have distinct locations, so using this option results in non-conforming behavior. 🚨

• -fmodulo-sched — performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
• -fmodulo-sched-allow-regmoves — performs more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependence edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled. 🔥
• -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 that were introduced by other optimization passes. 🦋

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

• -fno-function-cse — bestie, this one says: do not stash function addresses in registers. Every single instruction that calls a constant function has to carry that function's address right there, explicitly, out in the open. 💅

Which means the code it spits out is less efficient, yeah — but plot twist, some weird little hacks out there that mess with the assembler output can get fully confused by the optimizations that happen when this flag is off. So it's a trade, okay? 🫶

The default is -ffunction-cse.

• -fno-zero-initialized-in-bss — okay so if your target has a BSS section, GCC's whole thing by default is to yeet variables that get initialized to zero into BSS. Saves space in the resulting code. very slay, very efficient. ✨

This option turns that behavior off, because some programs literally rely on their variables landing in the data section — like, so the resulting executable can find the beginning of that section and/or make assumptions based on it. so, yeah, they need it. 🙈

The default is -fzero-initialized-in-bss.

• -fthread-jumps — this one does optimizations that check whether a jump branches to a location where another comparison, one that's subsumed by the first, is hanging out. If so, the first branch gets 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.

• -fsplit-wide-types — when you're using a type that takes up multiple registers (think long long on a 32-bit system, iykyk), this splits those registers apart and allocates them independently. Normally this generates better code for those types, but heads up — it may make debugging more difficult. 💀

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

• -fsplit-wide-types-early — fully split wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on. no cap. 🦋

This is the default on some targets.

• -fcse-follow-jumps — in common subexpression elimination (CSE), scan through jump instructions when the target of the jump isn't reached by any other path. Example time: 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 — similar to -fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks makes CSE 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. round two, bestie. 💅

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

• -fgcse — perform a global common subexpression elimination pass. This pass also does global constant and copy propagation. iconic behavior. 👑

Note: when compiling a program using computed gotos — that's 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. the way that works is so clean. ✨

Enabled by default when -fgcse is enabled.

• -fgcse-sm — when -fgcse-sm is enabled, a store motion pass runs 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). bye redundant loads 🫶

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. literally just tidying up. 🧚

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. Which means it assumes that loop code doesn't invoke undefined behavior — for example, by causing signed integer overflows or out-of-bound array accesses. 😭 The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
• -funconstrained-commons — this option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds. 🙈
• -fcrossjumping — perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping. no promises, bestie. 🤷‍♀️

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 don't 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's 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 — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones — which means two copies of the function. two. copies. 😭 With this option, the base and complete variants are changed to be thunks that call a common implementation. so much better.

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're converted back into original form. it's giving "trust but verify". 💅
• -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. the little things that cost a lot. 🎀

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. slay. ✨

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 takes advantage of this. But 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. small registers, big vibes. 🌈
• -fira-algorithm=algorithm — picks the coloring algorithm for the integrated register allocator (IRA, our beloved ✨). algorithm can be either priority (that's Chow's priority coloring) or CB (Chaitin-Briggs coloring, the fancy one 💅). Heads up bestie: Chaitin-Briggs is not implemented for all architectures, but for the targets that do support it, it's the default — because it literally generates better code. no cap.
• -fira-region=region — sets which regions IRA uses. region is one of these three, and they're all iconic in their own way:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set. it's giving "tiny messy register file? say less."
  • 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, ...). the people's princess fr fr 🎀
  • 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. small code size queen 👑
• -fira-hoist-pressure — tells IRA to evaluate register pressure in the code hoisting pass, so it can decide whether to hoist expressions. Usually gives you smaller code, but it can slow the compiler down. worth it? maybe. 💀

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

• -fira-loop-pressure — makes IRA evaluate register pressure in loops so it can decide whether to move loop invariants. Usually results in faster and smaller code on machines with large register files (>= 32 registers — yeah, that many, we love a big register file 🌸), but it can slow the compiler down.

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

• -fno-ira-share-save-slots — disables sharing of stack slots used for saving call-used hard registers that live through a call. So each hard register gets its own separate stack slot, and as a result function stack frames are larger. bloated. not the vibe.
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Every pseudo-register that does not get a hard register gets its own separate stack slot, and again function stack frames end up larger. same energy, different registers 🙈
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) those values if it's profitable. recalc > reload, we love efficiency ✨

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

• -fdelayed-branch — if supported for the target machine, it attempts to reorder instructions to exploit instruction slots available after delayed branch instructions. filling the gaps, main character scheduling energy 💅

Enabled at levels -O, -O2, -O3, -Os, but not at -Og. (-Og said not today.)

• -fschedule-insns — if supported for the target machine, it attempts to reorder instructions to eliminate execution stalls caused by required data being unavailable. This helps machines with slow floating point or memory load instructions, by letting other instructions be issued until the result of the load or floating-point instruction is actually required. it's giving "keep the pipeline busy bestie" 🔥

Enabled at levels -O2, -O3.

• -fschedule-insns2 — similar to -fschedule-insns, but it requests an additional pass of instruction scheduling after register allocation has been done. Especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. double the scheduling, double the slay 🌈

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

• -fno-sched-interblock — disables instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, 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 insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Using this can improve the generated code and decrease its size, by preventing register pressure from rising above the number of available hard registers and the subsequent spills in register allocation. protect the registers, protect the peace 🫶
• -fsched-spec-load — allows speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions. yeah it's literally called dangerous, we're being dramatic and honest 💀 This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns, -fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list, during the second scheduling pass. Let's break it down:
  • -fno-sched-stalled-insns means no insns are moved prematurely.
  • -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely.
  • -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This only has an effect during the second scheduling pass, and only if -fsched-stalled-insns is used. The details:
  • -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0.
  • -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. ⚠️ This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

• -fsched-group-heuristic — enables the group heuristic in the scheduler. It 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. It 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. It 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. It favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-last-insn-heuristic — enables the last-instruction heuristic in the scheduler. It 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. It 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 may change its schedule. Use this option to control that behavior. keep your scheduled loops scheduled, bestie 🎀
• -fselective-scheduling — schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
• -fselective-scheduling2 — schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on. (iykyk 🦋)
• -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, allow interposing of symbols by the dynamic linker. What that means: for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change. While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it is expensive in 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.

Heads up: the flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak.

• -fshrink-wrap — emits function prologues only before the parts of the function that need it, rather than at the top of the function. no unnecessary preamble, it's giving efficiency 💅 This flag is enabled by default at -O and higher.
• -fshrink-wrap-separate — shrink-wraps separate parts of the prologue and epilogue separately, so those parts are only executed when needed. This option is on by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports this. two options, one vibe ✨
• -fcaller-saves — okay so this one lets GCC put values into registers that get clobbered by function calls 💅, and it pulls that off by emitting extra instructions to save and restore those registers around such calls. It only does the allocation when it looks like it'll actually give you better code — no wasted effort, bestie. On certain machines this is always enabled by default, usually the ones that have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os. 🫶
• -fcombine-stack-adjustments — it tracks stack adjustments (that's pushes and pops, babe) plus stack memory references, and then tries to find ways to combine them. 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 at all — iconic. 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 — which happens on targets that don't expose prologues and epilogues in RTL. 😭
• -fconserve-stack — attempt to minimize stack usage 🍓. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
• -ftree-reassoc — perform reassociation on trees 🌸. This flag is enabled by default at -O and higher.
• -fcode-hoisting — perform code hoisting. Code hoisting tries to move the evaluation of expressions that are executed on all paths to the function exit as early as possible. It's especially useful as a code size optimization, but it often helps for code speed as well — double win, no cap. This flag is enabled by default at -O2 and higher. 🔥
• -ftree-pre — perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3. 💅
• -ftree-forwprop — perform forward propagation on trees. This flag is enabled by default at -O and higher. ✨
• -ftree-fre — perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
• -ftree-phiprop — perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher. 🦋
• -fhoist-adjacent-loads — speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher. 🎀
• -ftree-copy-prop — perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher.
• -fipa-pure-const — discover which functions are pure or constant. Enabled by default at -O and higher. 💜
• -fipa-reference — discover which static variables do not escape the compilation unit. Enabled by default at -O and higher.
• -fipa-reference-addressable — discover read-only, write-only and non-addressable static variables. Enabled by default at -O and higher.
• -fipa-stack-alignment — reduce stack alignment on call sites if possible. Enabled by default.
• -fipa-pta — perform interprocedural pointer analysis and interprocedural modification and reference analysis. Careful though — this option can cause excessive memory and compile-time usage on large compilation units 😵. It is not enabled by default at any optimization level.
• -fipa-profile — perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also, functions executed once (such as cold, noreturn, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher. 🧊
• -fipa-cp — perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile. 🔥
• -fipa-cp-clone — perform 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 ipa-cp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. ⭐
• -fipa-vrp — when enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf — perform Identical Code Folding for functions and read-only variables 👑. The optimization reduces code size and may disturb unwind stacks by replacing a function by an equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold. 🌈

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

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

If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter 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 function, 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 — detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer 💀. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled.
• -fisolate-erroneous-paths-attribute — detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute 💀. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future. 🙈
• -ftree-sink — perform forward store motion on trees. This flag is enabled by default at -O and higher.
• -ftree-bit-ccp — perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled. 🎀
• -ftree-ccp — perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.
• -fssa-backprop — propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at -O and higher. ✨
• -fssa-phiopt — bestie this one does pattern matching on SSA PHI nodes to optimize conditional code ✨ It's on by default at -O1 and higher, except for -Og though, so watch out.
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array 🎀 This flag is enabled by default at -O2 and higher.
• -ftree-tail-merge — it's giving detective energy 🔍 it looks for identical code sequences, and when it finds them, replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
• -ftree-dce — 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. Enabled by default at -O2 and higher if -Os is not also specified.
• -ffinite-loops — assume that a loop with an exit will eventually take the exit and not loop indefinitely 🌀 This lets the compiler remove loops that otherwise have no side-effects, not considering eventual endless looping as such.

This option is enabled by default at -O2 for C++ with -std=c++11 or higher.

• -ftree-dominator-opts — performs a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 🌳 It also performs jump threading (to reduce jumps to jumps). Enabled by default at -O and higher.
• -ftree-dse — performs dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads 👀 in this case the earlier store can be deleted. Enabled by default at -O and higher.
• -ftree-ch — performs loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations ✨ It also saves one jump. Enabled by default at -O and higher. It is not enabled for -Os, since 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 — perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure 🧚
• -fgraphite-identity — 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. Some minimal optimizations are also performed by the code generator isl, 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. 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 without checking that it is profitable to parallelize the loops 👀
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🫶 This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise.
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops to branch-less equivalents ✨ The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place 🌸 For example, the loop

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

is transformed to

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

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 -O2 and higher, 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 flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to 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. This flag is 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 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, replace uses of the final value 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_ — parallelizes loops, i.e., splits 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 — okay so this one does basic block vectorization on trees 💫 and it's on by default at -O3, plus whenever you turn on -ftree-vectorize, -fprofile-use, or -fauto-profile. slay.
• -fvect-cost-model=model — this changes up the cost model that vectorization uses, bestie. model has to be one of unlimited, dynamic, or cheap:
  • unlimited — the vectorized code-path is just assumed to be profitable, periodt.
  • dynamic — a runtime check guards the vectorized code-path so it only kicks in for iteration counts that'll likely run faster than the plain scalar loop.
  • cheap — turns off vectorization for loops where it'd be cost prohibitive (like when you'd need runtime checks for data dependence or alignment), but otherwise it's the same as dynamic.

The default cost model depends on your other optimization flags and is either dynamic or cheap. iykyk.

• -fsimd-cost-model=model — same idea, but for vectorizing loops marked with the OpenMP simd directive 🌈. model is one of unlimited, dynamic, cheap, and every value means exactly what it does in -fvect-cost-model. By default, whatever cost model you set with -fvect-cost-model is the one that gets used here. 💅
• -ftree-vrp — does Value Range Propagation on trees, which is giving constant propagation but make it ranges instead of values. This lets the optimizers yeet unnecessary range checks like array bound checks and null pointer checks. On by default at -O2 and up ✨. Heads up tho: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled.
• -fsplit-paths — splits paths that lead to loop backedges 🦋. This can make dead code elimination and common subexpression elimination better. On by default at -O3 and above.
• -fsplit-ivs-in-unroller — lets you express the values of induction variables in later iterations of the unrolled loop using the value from the first iteration 🌀. This breaks those long dependency chains, which makes the scheduling passes more efficient. chef's kiss.

Fun fact: a combo of -fweb and CSE is often enough to get the same effect. BUT it's not reliable when the loop body is more complicated than a single basic block, and it straight-up doesn't work on some architectures because of restrictions in the CSE pass. 🙈

This optimization is enabled by default.

• -fvariable-expansion-in-unroller — with this, the compiler makes multiple copies of some local variables when it unrolls a loop, which can result in superior code 👑.

This one's enabled by default for PowerPC targets, but disabled by default everywhere else.

• -fpartial-inlining — inlines parts of functions 🫶. It only does anything when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

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

• -fpredictive-commoning — does the predictive commoning optimization, aka reusing computations (especially memory loads and stores) that were performed in previous iterations of loops 🔁.

Enabled at level -O3, and also enabled by -fprofile-use and -fauto-profile.

• -fprefetch-loop-arrays — if your target machine supports it, this generates instructions to prefetch memory so loops that access big arrays run faster 🚀.

Real talk tho: this option may generate better or worse code — results are highly dependent on the structure of the loops in 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, bestie, read carefully 💀). This transformation lets GCC optimize or even 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 figuring out 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 gives its 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, and -fpeephole2 is enabled at levels -O2, -O3, -Os.

• -fno-guess-branch-probability — do not guess branch probabilities using heuristics, no cap 🚫.

So here's the tea: GCC uses heuristics to guess branch probabilities if they're not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics get 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. 💡

You can also 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 to reduce the number of taken branches and improve code locality ✨.

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

• -freorder-blocks-algorithm=algorithm — picks the algorithm used for basic block reordering 🎀. The algorithm argument can be simple, which doesn't increase code size (except sometimes from secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all the 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 🔥.

Lowkey important: this optimization is automatically turned off when there's 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 doesn't 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 by using special subsections .text.hot for the most frequently executed functions and .text.unlikely for unlikely-executed functions. The reordering is done by the linker, so the object file format must support named sections AND the linker has to place them in a reasonable way.

Heads up: this option isn't effective unless you either provide profile feedback (peep -fprofile-arcs for the deets) 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 live 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. And a character type may alias any other type.

Pay special attention to code like this, bestie:

union a_union {
  int i;
  double d;
};

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

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So the code above works as expected 💅. However, this code might not:

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

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, like:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

⚠️ that last one is undefined behavior, fr fr. don't do it.

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 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes 🎯. This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary.

If m isn't specified, it defaults to n.

Examples, so you can see it in action: -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. 🌸

...so the second pair of n2:m2 values is where you get to specify a secondary alignment, bestie 💅. Like, -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, and otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. And if m2 isn't specified? It defaults to n2. Lowkey elegant.

Some assemblers only support this flag when n is a power of two; in that case, it gets rounded up. 🙃

-fno-align-functions and -falign-functions=1 are equivalent, and both mean functions are not aligned. Periodt.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536. ✨

Enabled at levels -O2, -O3.

---

-flimit-function-alignment

If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. 🎀

-falign-labels 🌸

-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2

Align all branch targets to a power-of-two boundary. 🦋

Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead. (Yeah, they outrank it. main character energy.)

If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

-falign-loops 🔥

-falign-loops
-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. (Investment! ✨)

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-jumps 👑

-falign-jumps
-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. 🫶

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

-fno-allocation-dce

Do not remove unused C++ allocations in dead code elimination. 🧚

-fallow-store-data-races

Allow the compiler to introduce new data races on stores. 😭 (Yeah. It says the quiet part out loud.)

Enabled at level -Ofast.

-funit-at-a-time

This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. 💀

Enabled by default.

-fno-toplevel-reorder

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

-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.

-flto[=n] 💖

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

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

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

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. 🦋

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

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

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

The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. 💅

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

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects. This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. 😭

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. 🌸

Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.

If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. 🙃

There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. 🚫

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time. 💜

Diagnostic options like -Wstringop-overflow get passed straight through to the link stage, and their setting matches the compile-step setting at function granularity. Okay so heads up: this only matters for diagnostics that get emitted during optimization. And also? Code transforms like inlining can make warnings get enabled or disabled for regions of code that aren't consistent with the compile-time setting. iykyk. 💅

When you need to pass options to the assembler via -Wa or -Xassembler, bestie, make sure you either compile those translation units with -fno-lto or consistently use the same assembler options on <em>all</em> translation units. Alternatively you can also specify assembler options at LTO link time. No stress either way, just pick one and commit. ✨

To enable debug info generation you need to supply -g at compile time. If <em>any</em> of the input files at link time were built with debug info generation enabled, the link will enable debug info generation too. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. 🚫

If LTO encounters objects with C linkage declared with incompatible types in separate translation units that are being linked together (that's undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time though — the warning doesn't save you. Similar diagnostics may be raised for other languages too.

Another feature of LTO? You can apply interprocedural optimizations on files written in different languages together. It's giving multilingual queen 👑:

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 you're mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Consistency is the vibe, periodt.

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it is possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process. Slay 🎀:

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

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. The way that it just hands them over to GCC... iconic. 💖

If you're <em>not</em> using a linker with plugin support and/or you don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for <em>both</em> LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Two vibes, one file. 🦋

Link-time optimizations do <em>not</em> require the presence of the whole program to operate. If the program doesn't require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when the linker plugin is active (see -fuse-linker-plugin). Lowkey the optimizer eats when you let it. 😌

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there'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. It's giving exclusive, no gatekeeping allowed but also no backwards compat 💀.

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.

If you specify the optional <em>n</em>, the optimization and code generation done at link time is executed in parallel using <em>n</em> parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. Parallel bestie mode activated 🏃‍♀️💨

You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You <strong>must</strong> prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. Not me being nosy and finding your job server 🙈

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

-flto-partition=alg

Specify the partitioning algorithm used by the link-time optimizer. The value is one of these cuties:

• 1to1 — specifies a partitioning mirroring the original source files.
• balanced — specifies partitioning into equally sized chunks (whenever possible). This is the default.
• max — creates a new partition for every symbol where possible. This one is intended for internal testing only, don't get cute with it in prod.
• none — disables partitioning and streaming completely.
• one — specifies that exactly one partition should be used.

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. So many options, pick your fighter ✨

-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. Goldilocks energy fr fr 🍓

-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 and thriving 💅

-ffat-lto-objects

Fat LTO objects are object files that contain <em>both</em> the intermediate language <em>and</em> 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. Annoying but worth it, bestie 😭

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). Same energy, less typing ✨

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. She's efficient, she's clever 💅

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. Copy that? Not today 🎀

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. She's not mad, she's just disappointed 💀

This option is enabled by -fauto-profile.

-fprofile-partial-training

With -fprofile-use, all portions of programs not executed during the train run are optimized aggressively for <em>size</em> rather than <em>speed</em>. In some cases it's not practical to train all possible hot paths in the program. (For example, the program may contain functions specific for a given hardware and training may not cover all hardware configurations the program is run on.) With -fprofile-partial-training, profile feedback will be ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when the train run isn't representative, but also leads to significantly bigger code. Tradeoffs, bestie, always tradeoffs ⚖️

-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. The glow-up list incoming ✨:

• -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

okay bestie, real talk: you literally cannot use this option until you've generated profiling information first. No profile, no party. 💀

By default, GCC throws an error at you if the feedback profiles don't match the source code. You can downgrade that whole situation into just a warning by using -Wno-error=coverage-mismatch. Heads up though — doing that may leave you with poorly optimized code. And also, by default, GCC will warn you if the feedback profiles don't exist at all (peep -Wmissing-profile for that one).

If path is specified, GCC goes looking in that path for the profile feedback data files. See -fprofile-dir. 🫶

✨ -fauto-profile / -fauto-profile=path ✨

Turn on sampling-based feedback-directed optimizations, plus the following optimizations — many of which are lowkey only worth it when you've got 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

not me pointing out the glow-up: the list is almost the same as the one above, except this one swaps -fprofile-reorder-functions out for -fprofile-correction. Iconic. Keep them straight, bestie. 💅

path is the name of a file containing AutoFDO profile information. If you just leave it out, it defaults to fbdata.afdo in the current directory. 📁

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

E.g.

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

Then you use the create_gcov tool to convert that raw profile data into a format GCC can actually eat. You've also gotta hand over 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

---

🦋 floating-point arithmetic, aka the speed vs. correctness drama 🦋

The following options control how the compiler behaves regarding floating-point arithmetic. These options trade off between speed and correctness. And listen — all of them must be specifically enabled. No autopilot here. ✨

-ffloat-store

Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value gets taken from a register or from memory. 🚫

This option prevents that undesirable excess precision situation on machines like the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same deal for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for those programs — after you've modified them to store all pertinent intermediate computations into variables. 👑

-fexcess-precision=style

This option lets you have 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 — which 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's basically 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.

real talk warning: -fexcess-precision=standard is not implemented for languages other than C. And 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 don't 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. ⚡

bestie, listen to me: 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.

heads up: 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 your code assuming that IEEE signaling NaNs might throw user-visible traps during floating-point ops. Turning this on disables the optimizations that could change how many exceptions you actually see with signaling NaNs. It also implies -ftrapping-math — they're besties, you can't have one without the other. 💅

It makes the preprocessor macro "__SUPPORT_SNAN__" get defined, so your code can check for it.

Default? -fno-signaling-nans. So off, unless you say otherwise.

🫶 real talk: this option is experimental and currently does not guarantee to disable all GCC optimizations that affect signaling NaN behavior. It's giving "we're working on it", not "fully cooked". 😭

• -fno-fp-int-builtin-inexact — do not let the built-in functions "ceil", "floor", "round" and "trunc" — plus their "float" and "long double" variants — generate code that raises the "inexact" floating-point exception for noninteger arguments. So if you pass like 2.7 to ceil, no inexact flag drama.

The lore: ISO C99 and C11 are fine with these functions raising "inexact", but ISO/IEC TS 18661-1:2014 (the C bindings to IEEE 754-2008, which got folded into ISO C2X) says nope, they can't do that.

Default is -ffp-int-builtin-inexact, which lets the exception be raised — unless C2X or a later C standard is selected. Also, this option literally does nothing unless -ftrapping-math is in effect. iykyk.

And even if you do use -fno-fp-int-builtin-inexact, if the functions end up generating a call to a library function, the "inexact" exception may still be raised if that library implementation doesn't follow TS 18661. Not me exposing the library like that 💀

• -fsingle-precision-constant — treat floating-point constants as single precision instead of quietly converting them into double-precision constants. Stop the glow-up, they're fine as they are ✨
• -fcx-limited-range — when it's on, this option is basically saying "a range reduction step is not needed when performing complex division." Also, no checking whether the result of a complex multiplication or division is "NaN + I*NaN", and no attempt to rescue the situation if it is. The default is -fno-cx-limited-range, but -ffast-math turns it on for you.

This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. But also — it applies to all languages, so don't think you can dodge it by switching languages bestie.

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

Default is -fno-cx-fortran-rules.

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. Heads up: this section includes experimental options that may produce broken code. Read before you leap, babes. 🙈

• -fbranch-probabilities — so you ran a program compiled with -fprofile-arcs, right? Now compile it a second time using -fbranch-probabilities to improve optimizations based on how many times each branch was actually taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file.

⚠️ the info in that data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. No mixing and matching, that's not the vibe.

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're 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. Iconic tbh.

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

• -fprofile-values — if you combine it with -fprofile-arcs, it adds code so some data about the values of expressions in the program gets gathered. 🕵️

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions so it can be used in optimizations.

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

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects the first time of execution of a function and orders those functions in ascending order.

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option tells the compiler to add code that gathers information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on it. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. So like, it peeks at your denominator and goes "oh I know her" 💅

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

• -frename-registers — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register". 😭 so maybe don't do this while you're debugging, fr fr.

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently if they're adjacent to each other in the instruction flow. besties standing next to each other, slaying together 🫶

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

• -ftracer — perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, allowing other optimizations to do a better job.

Enabled by -fprofile-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. it's giving "bigger but not necessarily better", choose wisely ✨

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's enough information that they don't 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. two loops, one condition, zero overlap ✂️

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's 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. everybody 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.

• -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 reduce the number of GOT entries and GOT accesses on some targets. less math, more slay 💅

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 ✨

okay so GCC has a bunch of internal constants that control how much optimization it does. Like, GCC won't inline functions that have more than a certain number of instructions. You can control some of those constants from the command line using the --param option. 💅

The names of specific parameters and the meaning of their values are tied to the compiler's internals, and they can change without notice in future releases. so don't get attached, bestie. 🙈

To get the minimal, maximal, and default value of a parameter, you can use the --help=param -Q options. 🫶

In each case, the value is an integer. The following choices of name are recognized for all targets:

• predictable-branch-outcome — When a branch is predicted to be taken with probability lower than this threshold (in percent), then it's 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 that 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 maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. 😭
• min-crossjump-insns — The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. 🎀
• max-grow-copy-bb-insns — The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. 🦋
• max-goto-duplication-insns — The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. 🌈
• max-delay-slot-insn-search — The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. ⭐
• max-delay-slot-live-search — When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. 💀
• max-gcse-memory — The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done. 🧚
• max-gcse-insertion-ratio — If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. 💜
• max-pending-list-length — The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. 🍓
• max-modulo-backtrack-attempts — The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. 😵
• max-inline-insns-single — Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). 👑
• max-inline-insns-auto — When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto). 💅
• max-inline-insns-small — This is bound applied to calls which are considered relevant with -finline-small-functions. ✨
• max-inline-insns-size — This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. 🎀
• uninlined-function-insns — Number of instructions accounted by inliner for function overhead such as function prologue and epilogue. 🦋
• uninlined-function-time — Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. 🌸
• inline-heuristics-hint-percent — The scale (in percents) applied to inline-insns-single, inline-insns-single-O2, inline-insns-auto when inline heuristics hints that inlining is very profitable (will enable later optimizations). 🔥
• 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 estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. 🌈
• large-function-insns — The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. 💀
• large-function-growth — Specifies maximal growth of 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 large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. 👑
• inline-unit-growth — Specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. 🧚
• ipa-cp-unit-growth — Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size. 💜
• large-stack-frame — The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. 🍓
• large-stack-frame-growth — Specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size. 😭
• max-inline-insns-recursive, max-inline-insns-recursive-auto — Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. --param max-inline-insns-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 maximum recursion depth used for recursive inlining. ✨

...and just so we're clear, bestie: --param max-inline-recursive-depth only applies to functions that are declared inline. For functions that are not declared inline, recursive inlining only happens when -finline-functions (which is included in -O3) is enabled — and in that case it's --param max-inline-recursive-depth-auto that takes over instead. 💅

• min-inline-recursive-probability — recursive inlining is only worth it for functions that have deep recursion on average, and it can genuinely hurt functions with shallow recursion depth, because it inflates the prologue size or makes the function body more complex for all the other optimizers to deal with. 😭 When profile feedback is available (see -fprofile-generate), the actual recursion depth can be guessed from the probability that a function recurses via a given call expression. This parameter limits inlining to only those call expressions whose probability exceeds the given threshold (in percents).
• early-inlining-insns — says how much growth the early inliner is allowed to make. Effectively it increases the amount of inlining for code that has a big abstraction penalty. ✨
• max-early-inliner-iterations — the iteration limit for the early inliner. Basically it bounds how many nested indirect calls the early inliner can resolve. Deeper chains still get handled by late inlining, no worries. 🫶
• comdat-sharing-probability — the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.
• profile-func-internal-id — a parameter controlling whether to use the function internal id in profile database lookup. If the value is 0, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes like function reordering and so on. 👑
• min-vect-loop-bound — the minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. After vectorization the iteration count needs to be greater than the value you set here for vectorization to be allowed.
• gcse-cost-distance-ratio — a scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. Right now it's only supported in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting gets with simple expressions, i.e. the expressions whose cost is less than gcse-unrestricted-cost. Setting it to 0 disables hoisting of simple expressions. 💀
• gcse-unrestricted-cost — the cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining how far an expression can travel. Also only supported in the code hoisting pass right now. The lesser the cost, the more aggressive code hoisting is. Setting it to 0 lets all expressions travel unrestricted distances.
• max-hoist-depth — how deep we search in the dominator tree for expressions to hoist. This exists to dodge quadratic behavior in the hoisting algorithm. A value of 0 doesn't limit the search, but it might slow down compilation of huge functions. 🐌
• max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. Used to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. Used to cap compilation time in tree tail merging. ⏳
• store-merging-allow-unaligned — lets the store merging pass introduce unaligned stores, if it's legal to do so.
• max-stores-to-merge — the maximum number of stores the store merging pass will try to merge into wider stores.
• max-unrolled-insns — the maximum number of instructions a loop is allowed to have to be unrolled. If a loop is unrolled, this parameter also decides how many times the loop code gets unrolled.
• max-average-unrolled-insns — the maximum number of instructions, biased by the probabilities of their execution, that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
• max-unroll-times — the maximum number of unrollings of a single loop. 🔁
• max-peeled-insns — the maximum number of instructions a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code gets peeled.
• max-peel-times — the maximum number of peelings of a single loop.
• max-peel-branches — the maximum number of branches on the hot path through the peeled sequence.
• max-completely-peeled-insns — the maximum number of insns of a completely peeled loop.
• max-completely-peel-times — the maximum number of iterations of a loop for it to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling.
• max-unswitch-insns — the maximum number of insns of an unswitched loop.
• max-unswitch-level — the maximum number of branches unswitched in a single loop.
• lim-expensive — the minimum cost of an expensive expression in loop invariant motion. 💸
• min-loop-cond-split-prob — when FDO profile information is available, min-loop-cond-split-prob specifies the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split.
• iv-consider-all-candidates-bound — a bound on the number of candidates for induction variables, below which all candidates get considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered, so we don't blow up into quadratic time complexity. 🌀
• iv-max-considered-uses — the induction variable optimizations 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 — average number of iterations of a loop.
• dse-max-object-size — maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. 😮‍💨
• dse-max-alias-queries-per-store — maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores.
• scev-max-expr-size — 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. 🐢
• max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with simd pragma.
• vect-max-version-for-alignment-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
• vect-max-version-for-alias-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for the vectorizer. A value of -1 means no limit. 🔥
• max-iterations-to-track — the maximum number of iterations of a loop that the brute-force algorithm for analyzing the number of iterations of the loop tries to evaluate.
• hot-bb-count-fraction — the denominator n of fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have in order to be considered hot. The default is 10000, which means a basic block counts as hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means it is never considered hot. Used in non-LTO mode.
• hot-bb-count-ws-permille — the number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that a basic block's execution count must be part of in order for it to be considered hot. The default is 990, which means a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means it is never considered hot. Used in LTO mode. 🌈
• hot-bb-frequency-fraction — the denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is 1000, which means a basic block is considered hot in a function if it's executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means it is never considered hot.
• unlikely-bb-count-fraction — the denominator n of fraction 1/n of the number of profiled runs of the entire program below which the execution count of a basic block must be in order for that basic block to be considered unlikely executed. The default is 20, which means a basic block is considered unlikely executed if it's executed in fewer than 1/20, or 5%, of the runs of the program. 0 means it is always considered unlikely executed.
• max-predicted-iterations — the maximum number of loop iterations we predict statically. This is useful when a function contains a single loop with a known bound plus 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 ends up looking artificially cold next 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 maximum length of a constant string for a builtin string cmp call eligible for inlining. 🎀
• align-threshold — select fraction of the maximal frequency of executions of a basic block in a function to align the basic block. 💅
• align-loop-iterations — a loop expected to iterate at least the selected number of iterations is aligned. 🔥
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion. The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value. 😭
• tracer-max-code-growth — stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth. 💀
• tracer-min-branch-ratio — stop reverse growth when the reverse probability of best edge is less than this threshold (in percent). 🌸
• tracer-min-branch-probability, tracer-min-branch-probability-feedback — stop forward growth if the best edge has probability lower than this threshold. Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective. 🦋
• stack-clash-protection-guard-size — specify 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. ⚠️ danger: yeah that last bit is the important part, bestie. 😭
• stack-clash-protection-probe-interval — stack clash protection involves probing stack space as it is allocated. This param controls the maximum 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. ⚠️ same warning energy: don't go past the OS guard. 💅
• max-cse-path-length — the maximum number of basic blocks on path that CSE considers. 🧚
• max-cse-insns — the maximum number of instructions CSE processes before flushing. 🍓
• ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation. The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of RAM is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging. 🐢✨
• ggc-min-heapsize — minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation. The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. 🗑️💖
• max-reload-search-insns — the maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. 🔍
• max-cselib-memory-locations — the maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. 🧠
• max-sched-ready-insns — the maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. 😴
• max-sched-region-blocks — the maximum number of blocks in a region to be considered for interblock scheduling. 🧩
• max-pipeline-region-blocks — the maximum number of blocks in a region to be considered for pipelining in the selective scheduler. 🚰
• 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. 🎲
• max-sched-extend-regions-iters — the maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions. 🚫
• max-sched-insn-conflict-delay — the maximum conflict delay for an insn to be considered for speculative motion. ⏳
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns are scheduled. 🎯
• sched-state-edge-prob-cutoff — the minimum probability an edge must have for the scheduler to save its state across it. 💾
• sched-mem-true-dep-cost — minimal distance (in CPU cycles) between store and load targeting same memory locations. ⚡
• selsched-max-lookahead — the maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. 🔭
• 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 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, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. 🔢
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-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. 🛡️
• 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 — estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches). 🚀
• simultaneous-prefetches — maximum number of prefetches that can run at the same time. 🧚‍♀️
• l1-cache-line-size — the size of cache line in L1 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. 🗄️
• 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 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. 🚧

okay so this one? it's for processors that already have hardware prefetchers doing their lil thing on their own — because sometimes the hardware prefetchers and the software prefetchers are lowkey beefing with each other. 😭 So if your hardware prefetchers have a maximum stride they can actually handle, put that number here, and it makes your software prefetchers way better at their job.

A value of -1 means no threshold at all — so prefetch hints can be issued for any constant stride. 💅

and bestie, hear me: this setting is only useful for strides that are known and constant. If your stride is out here being mysterious and unpredictable, this param can't help you. 🫶

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• loop-interchange-max-num-stmts — the max number of stmts in a loop that are allowed to get interchanged. that's it, that's the ceiling. ✨
• loop-interchange-stride-ratio — the minimum ratio between the stride of two loops for an interchange to actually be profitable. below that ratio, don't bother. 💅
• min-insn-to-prefetch-ratio — the minimum ratio between the number of instructions and the number of prefetches needed before prefetching gets enabled in a loop. 📊
• prefetch-min-insn-to-mem-ratio — basically the same vibe but with memory references: the minimum ratio between the number of instructions and the number of memory references before prefetching gets enabled in a loop. 🌸
• use-canonical-types — should the compiler use the "canonical" type system? Should always be 1, periodt — 1 uses a way more efficient internal mechanism for comparing types in C++ and Objective-C++. BUT 😭 if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. emergency escape hatch, iykyk.
• 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. so like, a size cap but make it ~ratio~. 🎀
• max-partial-antic-length — the maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. And the tea ☕: for some sorts of source code, the enhanced partial redundancy elimination optimization can run away, consuming all the memory available on your host machine. 💀 This parameter puts a limit on the length of the sets that get computed, which stops that runaway behavior. And if you set it to 0? unlimited set length, godspeed bestie. 🙈
• rpo-vn-max-loop-depth — the maximum loop depth that gets value-numbered optimistically. When the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones... don't. 🤷‍♀️
• sccvn-max-alias-queries-per-access — the maximum number of alias-oracle queries we perform when hunting for redundancies in loads and stores. If this limit gets hit, the search is aborted and the load or store is just not considered redundant. fyi 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. But if a function contains more loops than the number you give here, only at most that many of the most frequently-executed loops form regions for regional register allocation. popularity contest but for loops. 👑
• ira-max-conflict-table-size — okay so IRA uses a fancy sophisticated algorithm to compress the conflict table, but the table can still eat 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 doesn't require building a pseudo-register conflict table. trade-off city. 💅
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops, for decisions about moving loop invariants (see -O3). This parameter gives the number of available registers reserved for some other purposes. Default of the parameter is the best found from numerous experiments. ✨ science ✨
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values that got reloaded into registers in subsequent insns. this optimization is called inheritance. 💖 EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to the inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. no cap.
• loop-invariant-max-bbs-in-loop — loop invariant motion can be very expensive, both in compilation time and in the amount of compile-time memory it needs, especially with very large loops. 😭 So: loops with more basic blocks than this parameter simply won't have loop invariant motion optimization performed on them. sorry babe, too big.
• loop-max-datarefs-for-datadeps — building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are no handled by the optimizations using 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 with variable tracking at assignments enabled, the analysis for that function is retried without it, after removing all debug insns from the function. And if the limit gets exceeded even without debug insns, var tracking analysis is completely disabled for that function. 💀 Setting the parameter to zero makes it unlimited. 🌈
• max-vartrack-expr-depth — sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If it's set too low, value expressions that are available and could be represented in debug information may end up not being used 😔; setting it higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. pick your struggle. ✨
• max-debug-marker-count — sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will be 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 replaces a pointer to an aggregate with one or more new parameters, but only when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. it's giving ✨budget✨.
• ipa-sra-max-replacements — the maximum pieces of an aggregate that IPA-SRA tracks. And as a consequence, it's also the maximum number of replacements of a formal parameter. two facts, one param. 💅
• sra-max-scalarization-size-Ospeed and sra-max-scalarization-size-Osize — okay so the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) want to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate that's considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. speed or size bestie, you gotta choose. 🔥
• sra-max-propagations — the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation. 👑
• tm-max-aggregate-size — when making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions, as opposed to save/restore code sequence pairs. This option only applies when using -fgnu-tm. 🧚
• graphite-max-nb-scop-params — to avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of 0 can be used to lift the bound. And just so we're clear: 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, 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 attempts 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 whose scores exceed ipa-cp-eval-threshold. over the bar or it's not happening. ⭐
• ipa-cp-max-recursive-depth — maximum depth of recursive cloning for a self-recursive function. 🔁
• ipa-cp-min-recursive-probability — recursive cloning only happens when the probability of the call being executed exceeds the parameter. 🎲
• ipa-cp-recursion-penalty — the percentage penalty that recursive functions will receive when they're evaluated for cloning. 😬
• ipa-cp-single-call-penalty — the percentage penalty that functions containing a single call to another function will receive when they're evaluated for cloning. same energy, different crime. 💅
• 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 clocks that a cloning candidate would make the loop iteration count a known quantity, it slides a lil bonus of ipa-cp-loop-hint-bonus onto that candidate's profitability score. free points bestie ✨
• ipa-max-aa-steps — so while IPA-CP is out here analyzing function bodies, it runs alias analysis to track values pointed to by function parameters. but it's not about to spend its whole life on one gigantic function, so once it's examined ipa-max-aa-steps statements that modify memory, it simply gives up and treats all memory as clobbered. lowkey relatable 💅
• ipa-max-switch-predicate-bounds — the max number of boundary endpoints of case ranges in a switch statement. if a switch goes over this limit, IPA-CP just won't construct a cloning cost predicate (the thing it uses to estimate cloning benefit) for that switch's default case. no predicate, no problem 🎀
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter, to estimate the benefit of cloning on a certain constant value. but if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, that expression gets labeled "complicated" and IPA analysis just won't touch it. respect the boundaries 🦋
• lto-partitions — pick your desired number of partitions produced during WHOPR compilation. heads up: the number of partitions should exceed the number of CPUs you're compiling with. more partitions than CPUs, periodt 🔥
• lto-min-partition — the size of the minimal partition for WHOPR (measured in estimated instructions). this exists so tiny lil programs don't get shredded into a million partitions and cost you more than they're worth.
• lto-max-partition — the size of the max partition for WHOPR (again, in estimated instructions), giving you an upper bound on how big any individual partition can be. meant to be used only with balanced partitioning, iykyk 👑
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming. that's the whole vibe.
• cxx-max-namespaces-for-diagnostic-help — the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. it's the compiler being like "did you mean…?" 💖
• sink-frequency-threshold — the maximum relative execution frequency (in percents) of the target block compared to a statement's original block, for statement sinking of that statement to be allowed. bigger numbers = more aggressive sinking. and a small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink. ✨
• max-stores-to-sink — the max number of conditional store pairs that can be sunk. set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. 🍓
• case-values-threshold — the smallest number of different values for which it's actually better to use a jump-table instead of a tree of conditional branches. if the value is 0, use the default for the machine. let the machine decide, main character energy 💅
• jump-table-max-growth-ratio-for-size — the maximum code size growth ratio when expanding into a jump table (in percent). this one's the parameter you get when optimizing for size.
• jump-table-max-growth-ratio-for-speed — the maximum code size growth ratio when expanding into a jump table (in percent). and this one's used when optimizing for speed. same ratio, different agenda 🌈
• tree-reassoc-width — set the max number of instructions executed in parallel in a reassociated tree. if it's non-zero, this parameter overrides the target dependent heuristics that'd otherwise be used by default.
• sched-pressure-algorithm — pick between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is more likely to stop instructions from being reordered. Algorithm 2 was designed as a compromise between algorithm 1's relatively conservative approach and the default scheduler's rather aggressive approach — it leans way harder on having a regular register file and accurate register pressure classes. for more details, see haifa-sched.c in the GCC sources. the default choice depends on the target. ⭐
• max-slsr-cand-scan — set the max number of existing candidates that get 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 on by default if you're using the -fsanitize=address option. to turn global objects protection off, use --param asan-globals=0. 🛡️
• asan-stack — enable buffer overflow detection for stack objects. on by default when using -fsanitize=address. to disable stack protection, use the --param asan-stack=0 option.
• asan-instrument-reads — enable buffer overflow detection for memory reads. on by default when using -fsanitize=address. to disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — enable buffer overflow detection for memory writes. on by default when using -fsanitize=address. to disable memory writes protection, use the --param asan-instrument-writes=0 option.
• asan-memintrin — enable detection for built-in functions. on by default when using -fsanitize=address. to disable built-in functions protection, use --param asan-memintrin=0.
• asan-use-after-return — enable detection of use-after-return. on by default when using the -fsanitize=address option. to disable it, use --param asan-use-after-return=0.

Note: by default the check is disabled at run time. to enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. 😭 don't skip this one bestie

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than or equal to this number, use callbacks instead of inline checks. e.g. to disable inline code, use --param asan-instrumentation-with-call-threshold=0.
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. 🧚
• max-fsm-thread-path-insns — maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
• max-fsm-thread-length — maximum number of basic blocks on a finite state automaton jump thread path.
• max-fsm-thread-paths — maximum number of new jump thread paths to create for a finite state automaton.
• parloops-chunk-size — chunk size of the omp schedule for loops parallelized by parloops.
• parloops-schedule — schedule type of the omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime). 🌸
• parloops-min-per-thread — the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.
• max-ssa-name-query-depth — maximum depth of recursion when querying properties of SSA names in things like fold routines. one level of recursion corresponds to following a use-def chain. 💀
• hsa-gen-debug-stores — enable emission of special debug stores within HSA kernels, which then get read and reported by the libgomp plugin. generation of these stores is disabled by default, so use --param hsa-gen-debug-stores=1 to enable it.
• max-speculative-devirt-maydefs — the maximum number of may-defs we analyze when looking for a must-def that specifies the dynamic type of an object which invokes a virtual call we might 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's considered unpredictable.
• max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is used, 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 a split region (in percent, relative to the entry BB of the function) for partial inlining to 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 permits performing redundancy elimination after reload.
• max-loop-header-insns — the maximum number of insns in a loop header duplicated by the copy loop headers pass.
• vect-epilogues-nomask — enable loop epilogue vectorization using a smaller vector size. ✨
• slp-max-insns-in-bb — maximum number of instructions in a 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 that the swing modulo scheduler actually bothers to consider.
• sms-dfa-history — how many cycles the swing modulo scheduler looks back on when it's checking conflicts using DFA.
• max-inline-insns-recursive-auto — the max 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 is on.
• sms-max-ii-factor — a factor for tuning the upper bound the swing modulo scheduler uses when scheduling a loop.
• lra-max-considered-reload-pseudos — the max number of reload pseudos considered while 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 used 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.
• 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.
• 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 decides it's 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 decides it's 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.
• store-merging-max-size — maximum size of a single store merging region in bytes.
• hash-table-verification-limit — the number of elements for which hash table verification is done for each searched element.
• max-find-base-term-values — maximum number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.
• analyzer-max-recursion-depth — the maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block which is considered hot. The parameter is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the maximum number of 'after supernode' exploded nodes within the analyzer per supernode, before terminating analysis.

---

The following choices of name are available on AArch64 targets:

• aarch64-sve-compare-costs — when vectorizing for SVE, consider using "unpacked" vectors for smaller elements and use the cost model to pick the cheapest approach. Also use the cost model to choose between SVE and Advanced SIMD vectorization.

Using unpacked vectors includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.

• aarch64-float-recp-precision — the number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1.
• aarch64-double-recp-precision — the number of Newton iterations for calculating the reciprocal for double type. The precision of division is proportional to this param when division approximation is enabled. The default value is 2.
• aarch64-autovec-preference — force an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4, inclusive.
  • 0 — use the default heuristics.
  • 1 — use only Advanced SIMD for auto-vectorization.
  • 2 — use only SVE for auto-vectorization.
  • 3 — use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
  • 4 — use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.

The default value is 0.

---

✨ Program Instrumentation Options ✨

okay 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 collecting profiling statistics so you can find program hot spots, do code coverage analysis, or do profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking 😭. There'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.

• -p, -pg — generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking 🫶.

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options.

• -fprofile-arcs — add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable.

When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

• --coverage — this option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details 🌸.
  1. 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.
  2. Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
  3. Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  4. Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting.
  5. For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  6. For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information 💖.

So like, with -fprofile-arcs, for every single function in your program GCC builds a whole program flow graph ✨, then it goes and finds a spanning tree for that graph. The tea is: only the arcs that are not on the spanning tree actually need instrumenting, so the compiler drops in code to count how many times those arcs get executed. And when an arc is the only way out or the only way into a block, the instrumentation code can just chill inside the block itself — but if not, a brand new basic block has to be created to hold it. 💅

---

• -ftest-coverage — bestie this one 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. If you wanna know what auxname means and how to actually generate test coverage data, go peek at the -fprofile-arcs option above 👆. Also lowkey important: your coverage data matches the source files way more closely if you don't optimize. 🎀
• -fprofile-abs-path — automatically converts relative source file names into absolute path names inside the .gcno files. This lets gcov find the correct sources in projects where the compilations happen with different working directories. no cap, it's a lifesaver. 🦋
• -fprofile-dir=path — sets the directory to search for the profile data files in, and that directory is path. Heads up: 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. And to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. See the similar option -fprofile-note.

When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:

• %p — process ID.
• %q{VAR} — value of environment 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, okay?

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.

And to optimize the program based on the collected profile information, use -fprofile-use. 🔥

• -fprofile-note=path — if path is specified, GCC saves the .gcno file into the path location. But ngl, if you combine this option with multiple source files, the .gcno file will be overwritten. 💀
• -fprofile-prefix-path=path — this one can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where the base directory of the built source tree is. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. That's not it when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because then the profile data won't be found during the optimized build. 😭 In such setups, -fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. slay.
• -fprofile-update=method — alters 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 an detached thread), a profile file can be still corrupted. 😱

Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line. 👑

• -fprofile-filter-files=regex — instrument 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 — instrument 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 /usr/ folder. 🙈

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful, for example, for distribution packages. 🌈

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before fork function is invoked.

Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible.

With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with make -j). This reduces quality of gathered data, in particular of indirect call profiling. 💅

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. And psst — this option cannot be combined with -fsanitize=thread. 🚫
• -fsanitize=kernel-address — enables AddressSanitizer for Linux kernel. See https://github.com/google/kasan/wiki for more details. 🐧
• -fsanitize=pointer-compare — instruments comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option 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. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option 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 are instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak. 🚫

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 is 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. The option cannot be combined with -fsanitize=thread. 💧
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are 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. Current suboptions are:

-fsanitize=shift

This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent. ✨

-fsanitize=shift-exponent

This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument. 🔍

• -fsanitize=shift-base — bestie, if the second arg of a shift op is in range, this checks that the result of the shift isn't undefined. Heads up tho: what counts as "undefined" is lowkey different between C and C++, and also between ISO C90 and C99, etc. iykyk ✨
• -fsanitize=integer-divide-by-zero — catches integer division by zero AND the cursed INT_MIN / -1 division. Both of those? Yeah, bad. 💀
• -fsanitize=unreachable — with this on, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. When you actually reach that __builtin_unreachable call, the behavior is undefined. So like… don't. 🙈
• -fsanitize=vla-bound — tells the compiler to check that the size of a variable length array is positive. Positivity check for your VLAs, slay. 🎀
• -fsanitize=null — enables pointer checking! An app built with this turned on will issue 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 pointed to by a NULL pointer. NULL said no thank you. 🫶
• -fsanitize=return — enables return statement checking. Programs built with this turned on will issue an error message when the end of a non-void function is reached without actually returning a value. Note: this one works in C++ only, bestie.
• -fsanitize=signed-integer-overflow — enables signed integer overflow checking. We check that the result of +, *, and both unary and binary - does not overflow in signed arithmetics. But wait — integer promotion rules must be taken into account. So like, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — enables instrumentation of array bounds. Various out of bounds accesses get detected. BUT: flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.
• -fsanitize=bounds-strict — enables strict instrumentation of array bounds. 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, tho.
• -fsanitize=alignment — enables checking of alignment of pointers when they're dereferenced, or when a reference gets bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. Alignment is not a vibe, it's a requirement. ✨
• -fsanitize=object-size — enables instrumentation of memory references using the __builtin_object_size function. Various out of bounds pointer accesses get detected. builtin said catch them all.
• -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 be a legitimate way of obtaining infinities and NaNs. So it's not always a crime, okay? 🌈
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also, this option does not work well with FE_INVALID exceptions enabled. Noted.
• -fsanitize=nonnull-attribute — enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute. Respect the attribute bestie. 💅
• -fsanitize=returns-nonnull-attribute — enables instrumentation of return statements in functions marked with the returns_nonnull function attribute, to detect returning null values from such functions. Returning null from a returns_nonnull function? Caught.
• -fsanitize=bool — enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. bool has exactly two moods, bestie. 0 or 1, that's it. 🎀
• -fsanitize=enum — enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued. Stay in your enum lane.
• -fsanitize=vptr — enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. C++ said identity verification. 👑
• -fsanitize=pointer-overflow — enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math with pointers said no overflow allowed.
• -fsanitize=builtin — enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. Zero is not the moment for those builtins, fr fr. 💀

---

Okay so while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead. This currently works only for the C family of languages. 🌸

• -fno-sanitize=all — this option disables all previously enabled sanitizers. Heads up: -fsanitize=all is not allowed, because some sanitizers cannot be used together. So don't try it. 🙈
• -fasan-shadow-offset=number — this option forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. Experimental vibes only. 🧚
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. si may contain wildcards. Wildcards said let me in. ✨
• -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code.

Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable. Runtime side said don't forget about me.

Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none. Scope said stay in your lane. 💅
• -fsanitize-undefined-trap-on-error — the -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine. The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. No libubsan? No problem. 🦋
• -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block. Every single one. ✨
• -fsanitize-coverage=trace-cmp — enable dataflow guided fuzzing code instrumentation. 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. So many trace functions bestie, but every one of them matters. 👑
• -fcf-protection=[full|branch|return|none|check] — enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Security said not today, ROP. 🛡️

The value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value return implements checking of validity at the point of returning from a function. The value full is an alias for specifying both branch and return. The value none turns off instrumentation. Pick your fighter.

The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value check is ignored at the compile time. LTO said consistency check.

The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return. Bits said 1 and 1. ⭐

You can also use the nocf_check attribute to tag which functions and calls get skipped from instrumentation. Very "not me, not today" of them. 💅

Currently the x86 GNU/Linux target ships an implementation based on Intel Control-flow Enforcement Technology (CET), and it works for i686 processors 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. Yes bestie, it's on by default, you didn't even ask. 😭

---

✨ the stack protection girlies ✨

• -fstack-protector — emits extra code that checks for buffer overflows, like stack smashing attacks. It does this by adding a guard variable to functions with vulnerable objects. That includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack count here — optimized-away variables or variables living in registers don't. Periodt. 👑
• -fstack-protector-all — same energy as -fstack-protector except that all functions are protected. No exceptions, everyone gets a guard. 🛡️
• -fstack-protector-strong — like -fstack-protector but it includes extra functions to protect — the ones that have local array definitions, or that have references to local frame addresses. And same as before: only variables actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count. 🫶
• -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the stack_protect attribute. Hand-picked icons only. ✨

---

🧚 stack checking 🧚

• -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 you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems if there's only one stack.

Note that this switch doesn't actually cause checking to be done — the operating system or the language runtime has to do that. The switch causes generation of code to ensure that they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler, but bestie, it comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are 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 is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. 💀

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-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. 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 stops allocations from jumping over any stack guard page provided by the operating system. No hopping the fence, bestie. 🚫

Most targets do not 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. 💖

---

📏 stack limits & split stacks 📏

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit

These generate code to ensure that the stack does not grow beyond a certain value — either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it'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. Currently this is 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. 🔥

---

👑 vtable verification (C++ girlies, this one's for you) 👑

• -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 has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. 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 on the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. Ranked choice voting, 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 is defined, or the current working directory otherwise. 🌸

Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. (bestie please delete the old one, it's giving hoarder 😭)

• -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 is defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. 📊

Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.

---

🔍 instrumenting functions 🔍

• -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 does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) 🦋

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 🍓

Okay so bestie — a function can also be handed the no_instrument_function attribute, and if it is? That instrumentation just does not happen. No notes. This is exactly the move for the profiling functions we listed above 👆, for high-priority interrupt routines, and honestly for any function where you literally cannot safely call the profiling functions — think signal handlers, if your profiling routines generate output or allocate memory. Danger zone, don't do it there. 💀

• -finstrument-functions-exclude-file-list=file,file,... — Sets up the list of functions that get excluded from instrumentation (peep the description of -finstrument-functions for the whole story). If the file holding a function definition matches one of file, that function does not get instrumented. And the match is done on substrings — so if the file parameter is a substring of the file name, that counts as a match, periodt.

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

That excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. Substring matching is so unserious but it works 💅

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). iykyk, quoting is the whole game ✨

• -finstrument-functions-exclude-function-list=sym,sym,... — Same energy as -finstrument-functions-exclude-file-list, but this one sets the list of function names to exclude from instrumentation. The function name you're matching against is its user-visible name, like "vector<int> blah(const vector<int> &)", NOT the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). Not the cursed one, the cute one. Match is 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 Mth NOP. If M is omitted, it defaults to 0, so the function entry points to the address just at the first NOP. ✨ The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time — provided that the code segment is writable. Writable code segment, bestie. Remember that part. 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 basically: it depends, always has, always will 💅

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. She's documented AND organized.

Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If "N=0", no pad location is recorded. 👑

The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue. 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. The preprocessor is the opening act and honestly she deserves her flowers 🌸

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. Unsuitable. As in, do not feed it to the compiler and act surprised. 😭

In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options. Go read those too, bestie, they're part of the lore 🫶

• -D name — Predefine name as a macro, with definition 1. Simple. Clean. Iconic.
• -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. Truncated. By newlines. Watch out. 💀

If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. Quoting is not optional, it's a lifestyle ✨

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works. Noted and memorized 💅

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Order matters, always has 🌈

• -U name — Cancel any previous definition of name, either built in or provided with a -D option. Undefined. Unbothered. Unbothered and undefined.
• -include file — Process file as if #include "file" appeared as the first line of the primary source file. However — the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the #include "..." search chain as normal. 🦋

If multiple -include options are given, the files are included in the order they appear on the command line. In order. Do not test her.

• -imacros file — Exactly like -include, except that any output produced by scanning file is thrown away. Thrown. Away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. Macros only, no baggage. We love a low-commitment relationship 💅

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. Standard ones stay, the extras get the boot 🥾
• -pthread — Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. Both. Not one. Both. 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. Dependency tea, served hot ☕

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands. No commands. Just the rule. She's minimal like that.

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. Do not let them mix, it's giving chaos 💀

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. Silent but deadly.

• -MM — Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header. System headers stay out of the group chat.

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. It's not about the brackets, bestie.

• -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. Same place. You've been warned.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. Overrides. Takes precedence. Main character energy 👑

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, as a missing header file renders this useless. No path prepending, no preprocessed output, no drama.

This feature is used in automatic updating of makefiles. iykyk 🫶

• -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. Dummy rules doing the Lord's work 🙏

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. Automatic. Convenient. But override-able.

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. Exactly. The. String. No edits.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

• -MQ target — Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. Auto-quoted, as a treat 💅

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix. Sneaky little .d situation.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file. Same flag, different meaning depending on context. Yes really. 😭

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Side effect? More like side quest ✨

• -MMD — Like -MD except mention only user header files, not system header files. User headers only, system headers can stay home 🎀
• -fpreprocessed — bestie, this one's a heads-up to the preprocessor like "chill, we already did this." It tells it the input file has already been preprocessed, which shuts down macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. 💅 The preprocessor still recognizes and removes comments though, so you can pass it a file that was preprocessed with -C without anything exploding. In this mode the integrated preprocessor is basically just a tokenizer for the front ends. Also: -fpreprocessed is implicit if your input file has one of the extensions .i, .ii or .mi — those are exactly the extensions GCC uses for preprocessed files created by -save-temps. ✨
• -fdirectives-only — when preprocessing, handle directives, but do not expand macros. No cap.

The way this option behaves depends on the -E and -fpreprocessed options:

• With -E, preprocessing is limited to handling directives such as #define, #ifdef, and #error. Other preprocessor operations, like macro expansion and trigraph conversion, are not performed. Bonus: -dD 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. iconique, honestly.
• -fextended-identifiers — accept universal character names and extended characters in identifiers. This one's 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.
• *-fmax-include-depth=depth* — set the maximum depth of the nested #include. The default is 200. 🌀
• -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs show up on the line. If the value is less than 1 or greater than 100, the option is **ignored. The default is 8. lowkey so specific but okay.
• -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 happens inside a macro expansion. Heads up though: using this option makes the preprocessor and the compiler consume more memory. 😭 The level parameter picks how precise the token location tracking is, so you can dial memory consumption down if you need to:
  • 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. 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. You can use this to change an absolute path into 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 one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of wchar_t and the big-endian or little-endian byte order being used for code generation. 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 doesn't specify it, or GCC can't get this info 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 makes the dependency-output flags also list the files from the precompiled header's dependencies. If you don't specify it, only the precompiled header gets listed and not the files that were used to create it, because those files aren't consulted when a precompiled header is used.
• -fpch-preprocess — this option lets you use a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename", into the output to mark the place where the precompiled header was found, plus 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 gets 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 you can inhibit it with the negated form -fno-working-directory. If the -P flag is present on 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 over the older form -A predicate(answer), which is still supported, because it doesn't use shell special characters. slay, shell-safe.
• *-A -predicate*=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments get passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

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 you're running the preprocessor on something that is not C code, and it's going to be sent to a program that might get 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. throwback energy fr fr.
• -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're found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . ⭐
• -d letters — says to make debugging dumps during compilation as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 💀
  • -dM — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. yes bestie that's really the rule, don't come for me

• -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. 💖
• -dN — like -dD, but emit only the macro names, not their expansions. just the names, no tea
• -dI — output #include directives in addition to the result of preprocessing. 🎀
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time. it's giving selective, and honestly? valid

---

💅 -fdebug-cpp

This option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. every single token. no skips. 🦋

When used from GCC without -E, this option has no effect. literally nothing. icon of doing the least.

🫶 -Wp,option

You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. comma-split, periodt

However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.

bestie please — undocumented and subject to change. that's the vibe check failing in real time. avoid if you can. 😭

🌸 -Xpreprocessor option

Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. for the weird niche stuff only your platform understands, iykyk

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. twice. not once. twice. 💅

🧚 -no-integrated-cpp

Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing.

If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only and once for actual compilation of the preprocessed input.

This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. two passes, one glow-up ✨

---

✨ Passing Options to the Assembler ✨

You can pass options to the assembler. yes queen, the assembler gets a say too

• -Wa,option — pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas. 🌈
• -Xassembler option — pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument. again: twice!!

---

👑 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. no link, no vibes

• 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. the linker stays home. it's not coming.

🌟 -flinker-output=type

This option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. For debugging the compiler and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually.

If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled. both. off. gone.

If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This allows to build shared libraries without position-independent code on architectures where this is possible, i.e. on x86. preserved ≠ enabled, bestie, read it again

If type is pie, code generation produces an -fpie executable. This results 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, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library assuming that the majority of objects in the library are used. 🔥

Finally nolto-rel configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of benefits of whole program optimizations are lost. trade-offs, always trade-offs 😭

During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of object files in incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it 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.

warning incoming — mixing LTO and non-LTO objects in an incremental link? not today. 🫶

🌈 -fuse-ld=bfd

Use the bfd linker instead of the default linker. swap!

⭐ -fuse-ld=gold

Use the gold linker instead of the default linker.

🦋 -fuse-ld=lld

Use the LLVM lld linker instead of the default linker.

🎀 -fuse-ld=mold

Use the Modern Linker (mold) instead of the default linker. mold era 💅

💜 -llibrary

💜 -l library

Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.) posix said "make it legal", not "make it good"

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. plus your -L dirs, don't forget those

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. shared wins by default. static needs -static to show up.

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. 😭

order matters!! — this is THE classic linker heartbreak. put your libs last, bestie. 🫶

🍓 -lobjc

You need this special case of the -l option in order to link an Objective-C or Objective-C++ program. special case, special treatment ✨

🌸 -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. startup files? skipped. libs? still there.

🧚 -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. your problem now, bestie

💅 -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. bare metal babes rise up 🔥

👑 -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 full no. absolute nothing.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. libgcc.a stays behind, and it's not happy about it 😭

Okay so, bestie, real talk: in most cases you need libgcc.a even when you're trying to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc too. Why? Because that's how you make sure you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, which is used to ensure C++ constructors are called. iconic of it, honestly.)

• -e entry — like, the classic.
• --entry=entry — specify that the program entry point is entry. The argument gets interpreted by the linker; the GNU linker accepts either a symbol name or an address. ✨
• -pie — produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. no cap, do both or it's giving chaos.
• -no-pie — don't produce a dynamically linked position independent executable. periodt.
• -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. don't half-do it bestie 💅
• -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. main character energy for your symbol table, fr.
• -s — remove all symbol table and relocation information from the executable. 💀
• -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. slay.

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.

• -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. 🧊
• -symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
• -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. ⚠️
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. 🙈

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.

• -Wl,option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.

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. 💖
• -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. iykyk.

---

🗂️ Options for Directory Search 🗂️

These options specify directories to search for header files, for libraries and for parts of the compiler. like a little map for your include hunt 🗺️✨

• -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 specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

🌸 and that's the vibe check on include paths — the order matters, bestie, don't skip a step 🌸

You can totally 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. 💅 But bestie, do not use this option to add directories that hold vendor-supplied system header files — use -isystem for that instead, okay?

The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that's applied to the standard system directories. ✨

And here's the tea: if a standard system include directory, or a directory you specified with -isystem, is also specified with -I, then the -I option gets ignored. 🙈 The directory is still searched, but as a system directory at its normal position in the system include chain. Why? To make sure GCC's procedure to fix buggy system headers and the ordering for the #include_next directive don't get inadvertently changed. If you really, truly need to change the search order for system directories, use the -nostdinc and/or -isystem options.

• -I- — Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before the -I-, and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

On top of that, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". And there is no way to override this effect of -I-. 💀

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 🎀
• -iwithprefix dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This one puts it where -idirafter would.
• -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This one puts it in the same place -I 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 info. 🦋
• -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. 👑
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
• -iplugindir= dir — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🤫
• -L dir — Add directory dir to the list of directories to be searched for -l.
• -B prefix — 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. ✨

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. Easy peasy. 💅

• -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. 🔥
• -fwrapv — This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 💜
• -fwrapv-pointer — okay so bestie, this one tells the compiler to assume that pointer arithmetic overflow on addition and subtraction just wraps around, twos-complement style 🌀. Which sounds chaotic but it's a promise, not a bug. Heads up tho: this flag disables some optimizations that assume pointer overflow is invalid. So you're trading some speed for some defined-ness. Your call 💅
• -fstrict-overflow — this one implies -fno-wrapv -fno-wrapv-pointer, and when it's negated it implies -fwrapv -fwrapv-pointer. It's basically the "no wrapping allowed, we're being strict about overflow" vibe ✨
• -fexceptions — turn on exception handling, babes. This generates the extra code you need to propagate exceptions 🎀. On some targets that means GCC generates frame unwind information for all functions, which can cause significant data size overhead — it does not affect execution speed tho, so breathe 😌. If you don't specify it, GCC turns it on by default for languages like C++ that normally need exception handling, and turns it off for languages like C that normally don't. But like — you might still need to enable it when you're compiling C code that has to interoperate properly with exception handlers written in C++ 🤝. You might also want to disable it if you're compiling older C++ programs that don't use exception handling at all.
• -fnon-call-exceptions — generate code that lets trapping instructions throw exceptions 🔥. Ngl, this requires platform-specific runtime support that does not exist everywhere, so vibe-check your target first. Also, it only lets trapping instructions throw exceptions — we're talking memory references or floating-point instructions. It does not let exceptions be thrown from arbitrary signal handlers like "SIGALRM" 🚫
• -fdelete-dead-exceptions — the idea here is that instructions which may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away ✂️. 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 it just generates any needed static data and doesn't affect the generated code in any other way 🧚. You normally do not need to enable this one; instead, a language processor that needs this handling enables it on your behalf. So like, let the professionals handle it 💜
• -fasynchronous-unwind-tables — generate an unwind table in DWARF format, if it's supported by the target machine 🦋. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events — think debugger or garbage collector showing up uninvited 😭
• -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 stay 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 — and this is the plot twist — it causes "dlclose" to be ignored for affected DSOs. So 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 the glow-up is that it allows 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. And "short" structures and unions are those whose size and alignment match that of some integer type.

⚠️ Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.

• -freg-struct-return — return "struct" and "union" values in registers when possible ✨. This is more efficient for small structures than -fpcc-struct-return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there's no standard convention, GCC defaults to -fpcc-struct-return — except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative 👑

⚠️ Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

• -fshort-enums — allocate to an "enum" type only as many bytes as it needs for the declared range of possible values 🌸. Specifically, the "enum" type is equivalent to the smallest integer type that has enough room. Efficient! Tiny! Cute!

⚠️ 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.

• -fcommon — in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard 📚. Tentative definitions are distinct from declarations of a variable with the "extern" keyword, which do not allocate storage. The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker, so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit 💥. The -fcommon places uninitialized global variables in a common block, which lets the linker resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition 🤝. This behavior is inconsistent with C++, and on many targets it implies a speed and code size penalty on global variable references. It's mainly useful to enable legacy code to link without errors — iykyk 🧓
• -fno-ident — ignore the "#ident" directive 🙈
• -finhibit-size-directive — don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle and the two halves are placed at locations far apart in memory ✂️. This option is used when compiling crtstuff.c; you should not need to use it for anything else. Periodt.
• -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

The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change 🔮 — so don't go parsing them, bestie.

• -frecord-gcc-switches — this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created 📼. It's only implemented on some targets, and the exact format of the recording is target and binary file format dependent — but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file 🙃. See also -grecord-gcc-switches for another way of storing compiler options into the object file.

-fpic 💖

Okay bestie, listen — this one makes position-independent code (PIC), which is what you want for a shared library, if the target machine supports it. What does "position-independent" even mean? It means the code reaches all constant addresses through a global offset table (GOT). The dynamic loader resolves those GOT entries when the program starts — and heads up, the dynamic loader is not part of GCC, it's part of your operating system. Not our department, we just vibe here.

So if the GOT size for the linked executable goes over some machine-specific maximum, the linker is gonna yell at you and tell you -fpic does not work. In that case, recompile with -fPIC instead. The maximums, for the record:

• 8k on the SPARC
• 28k on AArch64
• 32k on the m68k and RS/6000
• x86 has no such limit, iconic, we love that for them

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. And code generated for the IBM RS/6000 is always position-independent anyway, so.

When this flag is set, the macros __pic__ and __PIC__ get defined to 1.

---

-fPIC 🎀

If the target machine supports it, this emits position-independent code suitable for dynamic linking, and — the glow-up — it avoids any limit on the size of the global offset table. This option actually makes a difference on AArch64, m68k, PowerPC and SPARC.

Same deal as before though: PIC requires special support, so it works only on certain machines, no cap.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2.

---

-fpie, -fPIE ✨

These are basically -fpic and -fPIC's little siblings, except the position-independent code they generate can only be linked into executables. Usually you use them to compile code that's gonna be linked using the -pie GCC option. That's the whole bit.

Both -fpie and -fPIE define the macros __pie__ and __PIE__. Those macros have the value 1 for -fpie and 2 for -fPIE. 💅

---

-fno-plt 🔥

Bestie. Do not use the PLT for external function calls in position-independent code. Instead, load the callee address at the call sites straight from the GOT and branch to it. Why is this a slay? Because it makes code more efficient by eliminating PLT stubs and by exposing GOT loads to optimizations. On architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this also gives the compiler more register allocation freedom. We love freedom.

But okay, the trade-off, because there always is one: lazy binding requires use of the PLT. With -fno-plt, all external symbols are resolved at load time. No sneaky deferred resolution for you.

Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions — so you can do it per-function instead of globally.

And one more thing: in position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

---

-fno-jump-tables 💀

Do not use jump tables for switch statements, even where it would be more efficient than other code generation strategies. So yes, you're telling GCC to be less efficient on purpose — and there's a reason.

This option is useful in conjunction with -fpic or -fPIC when you're building code that forms part of a dynamic linker and therefore cannot reference the address of a jump table. Some targets don't need a GOT for jump tables at all, so on those, this option isn't even needed. Situational, but when you need it you need it. 🦋

---

-ffixed-reg 🌸

Treat the register named reg as a fixed register. Generated code should never refer to it — except perhaps as a stack pointer, frame pointer, or in some other fixed role, because those are kind of unavoidable.

reg must be the name of a register. The register names that are accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. So look there, bestie.

Heads up: this flag has no negative form, because it specifies a three-way choice. There's no -fno-fixed-reg, that's not a thing.

---

-fcall-used-reg 🫶

Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. So, like, if you were counting on it surviving, you weren't.

It is an error to use this flag with the frame pointer or stack pointer. And using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Not dramatic, just accurate. 😭

And again — 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. Very polite of them.

It is an error to use this flag with the frame pointer or stack pointer. And using it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

Also, a different sort of disaster results from using this flag for a register in which function values may be returned. Yeah. Don't.

This flag has no negative form, because it specifies a three-way choice. (See the pattern? It's a trilogy. 👑)

---

-fpack-struct[=n] 🍓

Without a value specified, pack all structure members together without holes. Just squish 'em. When a value is specified — and it must be a small power of two — pack structure members according to this value, which represents the maximum alignment. Meaning: objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location. Yeah, we're in the weeds now.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface — that's the only reason you'd be here.

---

-fleading-underscore 🧚

This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code — you know, the old stuff, the ancestral vibes.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Also, not all targets provide complete support for this switch, so it's a bit of a gamble.

---

-ftls-model=model 🌈

Alter the thread-local storage model to be used. The model argument should be one of:

• global-dynamic
• local-dynamic
• initial-exec
• local-exec

Now, note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. So the compiler might just be smarter than your request. It happens. 💅

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

---

-ftrampolines 🎀

For targets that normally need trampolines for nested functions, this always generates them instead of using descriptors. Otherwise — for targets that don't need them, like for example HP-PA or IA-64 — it does nothing. Literally nothing. Cute.

So what even is a trampoline? It's a small piece of code that's 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. Which means — and this is the important part — it requires the stack to be made executable in order for the program to work properly. Yes, executable stack. Yes, that's a thing. 🙈

-fno-trampolines is enabled by default on a language by language basis, to let the compiler avoid generating them if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. Bestie, I mean it. Be careful.

---

-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. And why would you care? Because using this feature can:

• very substantially improve linking and load times of shared object libraries 📈
• produce more optimized code
• provide near-perfect API export
• prevent symbol clashes

It is strongly recommended that you use this in any shared objects you distribute. Like, please.

Despite the nomenclature, default always means public — i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.

A good explanation of the benefits of ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper, which can be found at https://www.akkadia.org/drepper/. That said, there's a superior solution that this option makes possible: instead of marking things hidden when the default is public, you make the default hidden and mark things public. This is the norm with DLLs on Windows, and with -fvisibility=hidden plus __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 🫶

For those adding visibility support to existing code, you may find #pragma GCC visibility of use. It works by you enclosing the declarations you wish to set visibility for with (for example):

#pragma GCC visibility push(hidden)
#pragma GCC visibility pop

Bear in mind that symbol visibility should be viewed as part of the API interface contract, and thus all new code should always specify visibility when it is not the default. I.e., declarations only for use within the local DSO should always be marked explicitly as hidden, so as to avoid PLT indirection overheads — and making this abundantly clear also aids readability and self-documentation of the code. Iconic. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

Be aware that headers from outside your project — in particular system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. Don't get caught out, bestie. 💜

extern declarations are literally immune to -fvisibility, bestie — which means a huge chunk of code can just be recompiled with -fvisibility=hidden and nothing breaks. 💅 But (and this is the plot twist 🎀) that also means calls to extern functions with no explicit visibility go through the PLT, so the actual move is to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden. 🫶

Also!! Important: -fvisibility does affect C++ vague linkage entities. So for example, an exception class that gets thrown between DSOs has to be explicitly marked with default visibility so that the type_info nodes get unified between the DSOs. no cap. 🧚

There's a whole overview of these techniques, their benefits, and how to actually use them at <http://gcc.gnu.org/wiki/Visibility>. 📚

---

-fstrict-volatile-bitfields

Use this one when 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. ✨

For example — 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 way more efficient 32-bit access. 😤

If this option is disabled, the compiler uses the most efficient instruction. So in that same 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. 💀 (yeah. rude.)

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. In that case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 🫡

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. 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 one 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 is enabled, which means the only actually useful form of the option is -fno-sync-libcalls. And btw this option is used in the implementation of the libatomic runtime library. 💖

---

GCC Developer Options 👩‍💻✨

Okay so this section is for the GCC developers — the main characters behind the curtain 🎭 — including options that support compiler testing and investigation of compiler bugs and compile-time performance problems. It covers options that produce debug dumps at various points in the compilation; options that print statistics like memory use and execution time; and options that print info about GCC's configuration, like where it searches for libraries. 🔍

Ngl bestie, you should rarely need any of these for ordinary compilation and linking tasks. This is behind-the-scenes lore. 🙈

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 the 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. 🗂️

---

-fcallgraph-info

-fcallgraph-info=MARKERS

Makes the compiler output callgraph information for the program, on a per-object-file basis. The info is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information if a list of comma-separated markers is additionally specified. ✨

• When the su marker is specified, the callgraph is decorated with stack usage information; it's equivalent to -fstack-usage.
• When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects.

When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files. 🦋

---

-dletters

-fdump-rtl-pass

-fdump-rtl-pass=filename

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. 🔧

Some -dletters switches have different meaning when -E is used for preprocessing. 👀

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings — buckle up bestie, it's a long list but every single one slays in its own way: 💖

• -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.
• -fdump-rtl-shorten — Dump after shortening branches.
• -fdump-rtl-sibling — Dump after sibling call optimizations.
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — These options enable dumping after five rounds of instruction splitting. ✂️
• -fdump-rtl-sms — Dump after modulo scheduling. This pass is only run on some architectures.
• -fdump-rtl-stack — Dump after conversion from GCC's flat register file registers to the x87's stack-like registers. This pass is only run on x86 variants. 🧱
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
• -fdump-rtl-unshare — Dump after all rtl has been unshared.
• -fdump-rtl-vartrack — Dump after variable tracking.
• -fdump-rtl-vregs — dumps the whole vibe after virtual registers get converted into hard registers 💅
• -fdump-rtl-web — dump time, but it happens after live range splitting. slay.
• -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*… but they always produce empty files 💀 they exist and they do absolutely nothing. no cap. main character energy with zero plot.
• -da, -fdump-rtl-all — the "give me literally everything" button 🎀 produces all the dumps listed above.
• -dA — annotates the assembler output with miscellaneous debugging information. sprinkle sprinkle ✨
• -dD — dumps all macro definitions, at the end of preprocessing, on top of the normal output.
• -dH — produces a core dump whenever an error occurs. dramatic much? but honestly valid 😭
• -dp — annotates the assembler output with a comment showing which pattern and alternative is used. the length and cost of each instruction also get printed.
• -dP — dumps the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation while it's at it. two for one bestie 🫶
• -dx — just generates RTL for a function instead of compiling it. usually used with -fdump-rtl-expand.

---

• -fdump-debug — dumps the debugging information generated during the debug generation phase.
• -fdump-earlydebug — dumps the debugging information generated during the early debug generation phase. yes it's a different phase, no they're not the same, moving on 💖
• -fdump-noaddr — when doing debugging dumps, suppresses address output. this makes it way 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. iykyk 🌸
• -freport-bug — collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs. like a little emergency kit for when things go sideways 🧚
• -fdump-unnumbered — when doing debugging dumps, suppresses instruction numbers and address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
• -fdump-unnumbered-links — when doing debugging dumps (see the -d option above), suppresses instruction numbers for the links to the previous and next instructions in a sequence.
• -fdump-ipa-switch
• -fdump-ipa-switch-options — controls the dumping at various stages of inter-procedural analysis language tree to a file. the file name gets generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. here's what 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.

also, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined.

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -fdump-lang-all
• -fdump-lang-switch
• -fdump-lang-switch-options
• -fdump-lang-switch-options=filename — controls the dumping of language-specific information. the options and filename portions behave exactly as described in the -fdump-tree option. these are the switch values that 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 — prints on stderr the list of optimization passes that are turned on and off by the current command-line options. like a little receipt of what's happening 🧾
• -fdump-statistics-option — enables and controls dumping of pass statistics in a separate file. the file name gets generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled.
• -fdump-tree-all
• -fdump-tree-switch
• -fdump-tree-switch-options
• -fdump-tree-switch-options=filename — controls 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. these are the options available:
  • address — print the address of each node. usually this is not meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment.
  • asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file.
  • slim — when dumping front-end intermediate representations, inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

• raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.
• details — enable more detailed dumps (not honored by every dump option). also include information from the optimization passes.
• stats — enable dumping various statistics about the pass (not honored by every dump option).
• blocks — enable showing basic block boundaries (disabled in raw dumps).
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

this option currently only works for RTL dumps, and the RTL is always dumped in slim form.

• vops — enable showing virtual operands for every statement.
• lineno — enable showing line numbers for statements.
• uid — enable showing the unique ID ("DECL_UID") for each variable.
• verbose — enable showing the tree dump for each statement.
• eh — enable showing the EH region number holding each statement.
• scev — enable showing scalar evolution analysis details.
• 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.
• optall — turn on all optimization options, i.e., optimized, missed, and note.

to figure out what tree dumps are available, or to find the dump for a pass you're obsessed with, follow these steps bestie:

1. invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you're 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 it 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 ⚠️

• -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 split into three groups:

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 options from each group can be freely mixed since they're non-overlapping. however, in case of any conflicts, the later options override the earlier options on the command line. 👑

these options control which kinds of messages should be emitted:

• optimized — print information when an optimization is successfully applied. it's up to a pass to decide which information is relevant. for example, the vectorizer passes print the source location of loops which are successfully vectorized.
• missed — print information about missed optimizations. individual passes control which information to include in the output.

• note — spills the verbose tea about optimizations: which transformations went down, more detailed deets about the decisions made, etc. gossip but for compilers ✨
• all — prints all the detailed optimization info, the whole buffet: optimized, missed, and note. no crumbs left. 💅

The next option is the vibe-check dial for how much dump detail you get:

• internals — by default GCC only emits the "high-level" messages, like it's keeping things cute and concise. This option unlocks the extra, way-more-detailed messages that are probably only gonna be interesting to actual GCC developers. iykyk 🙈

---

You can drop one or more of these option keywords to describe a whole 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 one is a superset of every optimization group listed above, so it's giving "I contain multitudes". 👑

If options is omitted, it defaults to optimized-optall, which means: dump messages about successful optimizations from all the passes, while omitting the messages that get treated as "internals". sensible default bestie ✨

If the filename is provided, then the dumps from all the applicable optimizations get concatenated into that filename. Otherwise the dump goes out onto stderr. And here's the catch: even though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. only the first one gets the aux, the rest get ghosted 💀

Note that the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, you should use stderr instead. no take-backs, it's just gone 😭

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 the missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. order-agnostic? she's flexible, she's chill, she's iconic 💅

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt. 📝

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict, since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities. one file only, she does NOT share 🚫

---

✨ -fsave-optimization-record ✨

Write 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 within the compressed JSON file is subject to change. she's in her experimental era, don't get too attached 🧪

It's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, plus the following additional data for each message:

• the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness 🔥
• the function name of the code being optimized, where applicable
• the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies 🧬
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location
• the GCC pass that emitted the message
• 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. nested like matryoshka dolls but make it compiler internals 🪆

---

✨ -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 dial goes up and the tea gets spicier 🌶️

---

✨ -fenable-kind-pass and -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. devs only, bestie — this is the backstage pass, not the front door 🎟️

• -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. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes. 📋
• -fdisable-tree-pass
• -fdisable-tree-pass=range-list — disable tree pass pass. See -fdisable-rtl for the description of option arguments.
• -fenable-ipa-pass — enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fenable-rtl-pass
• -fenable-rtl-pass=range-list — enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass
• -fenable-tree-pass=range-list — enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options. 🍓

# disable ccp1 for all functions
   -fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
   -fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
   -fdisable-rtl-gcse2=foo,foo2
# disable early inlining
   -fdisable-tree-einline
# disable ipa inlining
   -fdisable-ipa-inline
# enable tree full unroll
   -fenable-tree-unroll

---

✨ -fchecking ✨

✨ -fchecking=n ✨

Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. double-checking the double-check, and yes she knows it might change your output 👀

---

✨ -frandom-seed=string ✨

This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It'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. 🎲

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. no copy-paste, give each file her own little fingerprint 🫶

---

✨ -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. 🗃️

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps. she's considerate, she would never overwrite your source like that 💖

If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories, or the same source file compiled for multiple output destinations, it's 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, collision, not the cute kind of parallel play 😭

---

✨ -save-temps=obj ✨

Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps. 🎀

For example:

gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar

…creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o. bestie it really said "one flag, six files, no thoughts" 💅

---

• -time[=file] — ok so this one is literally a stopwatch for your build ⏱️ It reports the CPU time taken by each subprocess in the compilation sequence. For C source files, that means the compiler proper and the assembler (plus the linker, if linking is actually happening).

If you don't give it 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, periodt.

But if you do name an output file, the output gets appended to that file instead, 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 that later you can tell what file was being compiled, and with which options. it's giving receipts 📝

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If you leave the optional argument out (or if file is .), the name of the dump file is determined by appending .gkd to the compilation output file name. cute little .gkd suffix, slay ✨
• -fcompare-debug[=opts] — okay this one is lowkey paranoid in the best way 🫶 If no error occurs during compilation, it runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. It dumps the final internal representation in both compilations, and prints an error if they differ.

If you omit the equal sign, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. 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. bye felicia 🙈

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). And if you just want a warning instead of a whole rejection, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. no cap.

• -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. considerate queen 👑

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. that's it, that's the whole use case 💀

• -gtoggle — turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument in the command line 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. mainly intended to be used with -fcompare-debug. one and done, iconic 🌈
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the exact same way that -gtoggle toggles -g. twins fr fr 👯
• -Q — makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. it's chatty. we love a talkative compiler 🗣️
• -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. the details!! the LORE!! 🔍
• -fira-verbose=n — controls 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. so like, n=12 behaves like 2 but on stderr. math 💅
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. Heads up bestie: 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 prints for the WPA phase of link-time optimization. ✂️
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. memory tea ☕
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
• -fpre-ipa-mem-report
• -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. before and after pics 📸
• -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. The auxname is generated from the name of the output file, if explicitly specified and it is not an executable; otherwise it is the basename of the source file. An entry is made up of three fields:
  • The name of the function.
  • A number of bytes.
  • One or more qualifiers: static, dynamic, bounded.

The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function — for example, to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 🫠

• -fstats — emits 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. so like, not for us, but go off devs 💜
• -fdbg-cnt-list — prints the name and the counter upper bound for all debug counters. list them all bestie 📋
• -fdbg-cnt=counter-value-list — sets the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of

name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]

tuples which sets the name of the counter and list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. And for dbg_cnt(tail_call) true is returned for the first 10 invocations. intervals ate and left no crumbs 🎯

• -print-file-name=library — prints the full absolute name of the library file library that would be used when linking — and doesn't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. just the name, nothing more, respect the boundary 🛑
• -print-multi-directory — prints the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib — prints 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 — prints 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. pick your path bestie 🛤️
• -print-multiarch — prints 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`

nesting commands like that is giving main character energy ngl 👑

• -print-search-dirs — prints the name of the configured installation directory and a list of program and library directories gcc searches — and doesn't do anything else.

This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. do NOT forget it, I'm begging 🙏

• -print-sysroot — prints the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. silence is an answer too 🤫
• -print-sysroot-headers-suffix — prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler is not configured with such a suffix — and doesn't do anything else. ✨
• -dumpmachine — prints the target machine your compiler is aimed at (like i686-pc-linux-gnu) and then does absolutely nothing else, bestie. That's the whole job. 💅
• -dumpversion — prints the compiler version (like 3.0, 6.3.0 or 7) and nothing else, periodt. Heads up tho: this is the version that shows up in filesystem paths and specs, so depending on how the compiler was configured it could be a single number (major version), two numbers with a dot (major and minor), or three numbers with dots (major, minor and patchlevel). Depends on config, iykyk. 🎀
• -dumpfullversion — prints the full compiler version and nothing else. This one is always three numbers separated by dots: major, minor and patchlevel. No surprises, no crumbs. ✨
• -dumpspecs — prints the compiler's built-in specs and nothing else. This one is literally only used when GCC itself is being built, so unless you're doing that, it's giving "backstage pass". 🙈

---

🛠️ Machine-Dependent Options 🛠️

Every target machine GCC supports can bring its own options to the party — like, to let you compile for a specific processor variant or ABI, or to control optimizations that are specific to that machine. By convention, machine-specific option names all start with -m. Cute, consistent, iconic. 💜

Some configurations of the compiler also support extra target-specific options, usually so they play nice with other compilers on the same platform. Besties supporting besties. 🫶

💖 AArch64 Options 💖

These options are defined for AArch64 implementations:

• -mabi=name — generates 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. And warning bestie: the LP64 and ILP32 ABIs are not link-compatible. You have to 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 — generates big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it will not impose any restrictions on the assembler. Know the difference, bestie.
• -mlittle-endian — generates little-endian code. This is the default when GCC is configured for an aarch64-*-* target but not an aarch64_be-*-* target.
• -mcmodel=tiny — generates code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generates code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.
• -mcmodel=large — generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. Also, -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC. Not them fighting, just not compatible, periodt. 🚫
• -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. Choose your fighter. 🥊
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. The supported locations are global for a global canary or sysreg for a canary in an appropriate system register.

With the sysreg choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which system register to use as the base register for reading the canary, and what offset from that base register to read from. There is no default register or offset — this is entirely for use within the Linux kernel, so it's giving "you already know what you're doing" energy. 🧚

• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. The supported locations are global for a global canary or sysreg for a canary in an appropriate system register.

With the sysreg choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which system register to use as the base register for reading the canary, and what offset from that base register to read from. There is no default register or offset — this is entirely for use within the Linux kernel. Yes, it's listed twice in the source, no I'm not fixing it, it's canon. 😭

• -mtls-dialect=desc — uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
• -mtls-dialect=traditional — uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. The old-school route. 🌸
• -mtls-size=size — specifies the bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer, so check your binutils before you get your hopes up bestie. 💅
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enables or disables 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. Silly little bug, silly little fix. 🎀
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enables or disables the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time, and this option will only pass the corresponding flag to the linker. So it's not the compiler doing the work, it's just passing the note along like a good friend. 📝
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enables or disables the reciprocal square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. Fast and loose, babes. ⚡
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enables or disables the square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it 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. Package deal. 🎁
• -mlow-precision-div, -mno-low-precision-div — enables or disables the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enables or disables generation of additional code to track speculative execution through conditional branches. That 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. Security girlies stay winning. 👑
• -moutline-atomics, -mno-outline-atomics — enables or disables 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'll use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. Runtime decision-making, main character energy fr fr. ✨

This option is only applicable when compiling for the base ARMv8.0 instruction set. If you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. Same deal when using -mcpu= and the selected cpu supports the lse feature. This option is on by default.

• -march=name — specifies the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they enable by default:

• armv8-a : Armv8-A : includes by default +fp, +simd
• armv8.1-a : Armv8.1-A : includes by default armv8-a, +crc, +lse, +rdma
• armv8.2-a : Armv8.2-A : includes by default armv8.1-a
• armv8.3-a : Armv8.3-A : includes by default armv8.2-a
• armv8.4-a : Armv8.4-A : includes by default armv8.3-a, +fp16fml, +dotprod
• armv8.5-a : Armv8.5-A : includes by default armv8.4-a, +sb, +ssbs, +predres
• armv8.6-a : Armv8.6-A : includes by default armv8.5-a, +bf16, +i8mm

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. Last one wins, sorry to the lefties. 🙈

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.

• -mtune=name — specifies the name of the target processor for which GCC should tune the performance of the code. The permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, ares, exynos-m1, emag, falkor, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, neoverse-512tvb, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, ampere1, ampere1a, 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 specify that GCC should tune for a big.LITTLE system. Two cores, one compiler, iconic. 🦋

The value neoverse-512tvb says GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Like, translating that: the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that 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 a specific core like Neoverse V1, but more specific than the default tuning described below. 💅

Plus, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. 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.

This option cannot be suffixed by feature modifiers. Periodt.

• -mcpu=name — Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. 👑

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead 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. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

• -moverride=string — Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. 🫠

This option is only intended to be useful when developing GCC. Like, it's a dev-only vibe, bestie.

• -mverbose-cost-dump — Enable 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 — Select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection. 💀
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti — Select 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. 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. The optional argument b-key can be used to sign the functions with the B-key instead of the A-key. bti turns on branch target identification mechanism. 🛡️
• -mharden-sls=opts — Enable 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 — Specify 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 allows GCC to make assumptions about the vector length when it is 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 for big-endian targets. 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. ✨

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✨ -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. 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. Use of this option with architectures prior to Armv8.5-A is not supported.
• 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.
• sve2 — Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
• sve2-bitperm — Enable SVE2 bitperm instructions. This also enables SVE2 instructions.
• sve2-sm4 — Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
• sve2-aes — Enable SVE2 aes instructions. This also enables SVE2 instructions.
• sve2-sha3 — Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
• tme — Enable the Transactional Memory Extension.
• i8mm — Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• f32mm — Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• f64mm — Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• bf16 — Enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.

Feature crypto drags in aes, sha2, and simd with it — and simd drags in fp. And the reverse tea? nofp pulls in nosimd, which pulls in nocrypto, noaes and nosha2. It's giving ✨consequences✨, bestie. 💅

---

🦋 Adapteva Epiphany Options 🦋

These -m options are the whole personality for Adapteva Epiphany:

• -mhalf-reg-file — don't hand out a single register in the r32...r63 range, so your code can still run on hardware variants that just don't have those registers. gatekeeping the good registers fr 🙈
• -mprefer-short-insn-regs — preferentially allocate the registers that let short instructions be generated. Sounds cute, but it can bump up the instruction count, so overall code size might go down or up. It's giving chaos 🎀
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic, no cap, and it is not guaranteed to give you consistent results across releases. 💀
• -mcmove — turn on generation of conditional moves. slay ✨
• -mnops=num — emit num NOPs before every other generated instruction. Padding queen 👑
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags. That's faster than a software comparison, but it can get incorrect results when NaNs are around, or when two different small numbers are compared such that their difference comes out as zero. The default is -msoft-cmpsf, which uses slower but IEEE-compliant software comparisons. Pick your fighter 😭
• -mstack-offset=num — set the offset between the top of the stack and the stack pointer. 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 heads up: this option changes the ABI — compiling a program with a different stack offset than the libraries were compiled with generally does not work. It's useful if you wanna evaluate whether a different stack offset would give you better code, but to actually build working programs with a different stack offset, the recommended move is configuring the toolchain with the appropriate --with-stack-offset=num option. 🫶
• -mno-round-nearest — make the scheduler assume the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — unless an attribute says otherwise, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing an (otherwise direct) call. This is the default.
• -mshort-calls — unless an attribute says otherwise, assume all direct calls are in range of the b / bl instructions, so use those instructions for direct calls. The default is -mlong-calls.
• -msmall16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, for which -mlong-calls semantics are in effect.
• -mfp-mode=mode — set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. main character energy for your FPU 💅

mode can be set to one of the following values:

• caller — any mode at function entry is valid, and it's retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to 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. flexible icon ✨
• truncate — the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller.

• -mno-split-lohi, -mno-postinc, -mno-postmodify — code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify. (yes bestie that first one is written exactly like that, no dash, we don't make the rules 🎀)
• -mnovect-double — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as the preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. ⚠️
• -msplit-vecmove-early — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. bestie the theory did not eat 😭
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means no register is used for this purpose. The default is -m1reg-none.

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🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port, no cap.

• -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).
  • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
• -mstack-size=bytes — specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may be many threads and limited memory available 😭 The size of the stack allocation may also have an impact on run-time performance. The default 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 — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture. 💅
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on. she's got it all 💅
  • 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 — generate double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr — disable lr and sr instructions from using FPX extension aux registers.
• -mea — generate extended arithmetic instructions. Currently only divaw, adds, subs, and sat16 are supported. Only valid for -mcpu=ARC700.
• -mno-mpy — do not generate mpy-family instructions for ARC700. This option is deprecated. 🚩
• -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions.
• -mmul64 — generate mul64 and mulu64 instructions. Only valid for -mcpu=ARC600.
• -mnorm — generate norm instructions. This is the default if -mcpu=ARC700 is in effect.
• -mspfp, -mspfp-compact — generate single-precision FPX instructions, tuned for the compact implementation.
• -mspfp-fast — generate single-precision FPX instructions, tuned for the fast implementation.
• -msimd — enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
• -msoft-float — this option is ignored; it's provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can be overridden by FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision. it's just here vibing, doing nothing 🫶
• -mswap — makes the compiler spit out swap instructions. ✨
• -matomic — turns on the locked load/store conditional extension so atomic memory built-in functions actually work. Not available for ARC 6xx or ARC EM cores, bestie, so don't come crying to me if you try. 💀
• -mdiv-rem — enables the div and rem instructions for ARCv2 cores.
• -mcode-density — enables code density instructions for ARC EM. For ARC HS this one is on by default, no cap.
• -mll64 — enables double load/store operations for ARC HS cores. two at once, she's efficient 💅
• *-mtp-regno=regno* — tells it which register number to use as the thread pointer register.
• *-mmpy-option=multo* — compiles 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. sad little meow meow 😭
  • 1, w — 16x16 multiplier, fully pipelined. Enables these instructions: mpyw and mpyuw.
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. 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. 🫶

• *-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. The supported values for fpu are:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. Single-precision floating-point is switched on too. Not available for ARC EM.
  • fpuda — enables double-precision floating-point hardware extensions using double-precision assist instructions. Single-precision floating-point is also enabled. Only available for ARC EM.
  • fpuda_div — same double-precision-assist situation, and it also turns on single-precision floating-point, square-root, and divide extensions. Only available for ARC EM.
  • fpuda_fma — double-precision assist again, plus single-precision floating-point and fused multiply and add hardware extensions. Only available for ARC EM.
  • fpuda_all — double-precision assist, and all single-precision floating-point hardware extensions get enabled too. Only available for ARC EM.
  • fpus_div — enables single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables double-precision floating-point, square-root and divide hardware extensions. This option includes fpus_div. Not available for ARC EM.
  • fpus_fma — enables single-precision floating-point and fused multiply and add hardware extensions.
  • fpud_fma — enables double-precision floating-point and fused multiply and add hardware extensions. This option includes fpus_fma. Not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions. the whole buffet 🍓
  • fpud_all — enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
• *-mirq-ctrl-saved=register-range*, blink, lp_count — specifies the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is written as two registers separated by a dash. The 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 a fast interrupt is entered. Fast interrupts are the ones with the highest priority level, P0. They only save the PC and STATUS32 registers, so no memory transactions happen during interrupt entry and exit. Use this when you're running fast interrupts on an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
• *-mlpc-width=width* — sets the width of the lp_count register. Valid width values are 8, 16, 20, 24, 28 and 32 bits. Default is fixed at 32 bits. If the width is under 32, the compiler won't try to turn your loops into the zero-delay loop mechanism unless it knows for sure that the lp_count register can hold the loop-counter value it needs. Depending on the width you pick, the compiler and run-time library might still use the loop mechanism for other stuff. This option defines the macro __ARC_LPC_WIDTH__ with the value of width. 💅
• -mrf16 — tells the compiler to generate code for a 16-entry register file. Defines the __ARC_RF16__ preprocessor macro.
• -mbranch-index — enables use of bi or bih instructions to implement jump tables.

---

The following options get passed straight through to the assembler, and they also define preprocessor macro symbols. iykyk ✨

• -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 next bunch controls how the assembly code gets annotated 👑:

• -misize — annotates assembler instructions with estimated addresses.
• -mannotate-align — explains what alignment considerations led to the decision to make an instruction short or long.

---

And these ones get 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.

---

These ones control the semantics of the generated code, which is a fancy way of saying what the code actually means, bestie 🧚:

• -mlong-calls — generates calls as register indirect calls, which gives you access to the full 32-bit address range.
• -mmedium-calls — don't use less than a 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 the 25-bit range can be used 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 — puts 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 double load/store operations are available.
• -mno-sdata — doesn't generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache — uses ordinarily cached memory accesses for volatile references. This is the default.
• -mno-volatile-cache — enables cache bypass for volatile references.

---

And these last ones are the fine-tuning knobs, the little finishing touches 💅:

• -malign-call — does alignment optimizations for call instructions.
• -mauto-modify-reg — enables the use of pre/post modify with register displacement.
• -mbbit-peephole — enables bbit peephole2.
• -mno-brcc — disables a target-specific pass in arc_reorg that generates compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass.
• -mcase-vector-pcrel — switch case tables go PC-relative, which lets the compiler shorten them 💅. This is the default when you're on -Os.
• -mcompact-casesi — turns on the compact "casesi" pattern. Default for -Os, and it's only available on ARCv1 cores. Heads up bestie: this option is deprecated.
• -mno-cond-exec — kills the ARCompact-specific pass that generates conditional execution instructions.

okay so here's the tea 🍵 — because of delay slot scheduling and all the messy interactions between operand numbers, literal sizes, instruction lengths and conditional execution support, the target-independent pass for generating conditional execution is often lacking. So the ARC port keeps a special little pass around that hunts for more conditional-execution opportunities after register allocation, branch shortening and delay slot scheduling have all happened. This pass generally (but not always, bestie, not always) improves performance and code size — at the cost of extra compile time, which is literally why there's an option to switch it off. And if you're having problems with call instructions blowing past their allowable offset range because they got conditionalized, you should consider -mmedium-calls instead. 🫶

• -mearly-cbranchsi — enables pre-reload use of the "cbranchsi" pattern.
• -mexpand-adddi — expands "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. Deprecated, bestie.
• -mindexed-loads — lets you use indexed loads. Ngl this one can be problematic 😭 because some optimizers then assume indexed stores exist, which... they do not. Not the case. At all.
• -mlra — enables 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 optimizing for size (using -Os), prologues and epilogues that have to save or restore a lot of registers often get shortened by calling a special function in libgcc — and that's what we call a millicode call ✨. But since these calls can cause performance issues and/or linking issues when you're linking in a nonstandard way, this option exists to turn millicode call generation on or off.
• -mcode-density-frame — lets the compiler emit "enter" and "leave" instructions. These are only valid for CPUs with the code-density feature, iykyk 💅.
• -mmixed-code — tweaks register allocation to help 16-bit instruction generation. The general effect: average instruction size goes down while instruction count goes up. It's giving trade-off energy.
• -mq-class — this option is deprecated. It enables q instruction alternatives. Default for -Os.
• -mRcq — enables Rcq constraint handling. Most short code generation depends on this. It's the default. 🎀
• -mRcw — enables 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, if you don't set it, the behavior 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* — the cost to assume for a multiply instruction, where 4 is equal to a normal instruction.
• *-munalign-prob-threshold=probability* — set the 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 indicates 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 they are now deprecated and will be removed in a future release, so maybe don't build your whole personality around them 💀:

• -margonaut — obsolete FPX.
• -mbig-endian, -EB — compile code for big-endian targets. Using these 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. Using these is now deprecated too. 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* — the 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, buckle up 💖:

• *-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 the code to actually execute correctly. If you also specify -fomit-frame-pointer with this option, stack frames won't be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated.
• -mapcs — this is a synonym for -mapcs-frame, and it's 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 can't be reliably used inside one program. The default is -mno-thumb-interwork, because slightly larger code gets generated when you specify -mthumb-interwork. In AAPCS configurations this option is meaningless.
• -mno-sched-prolog — prevents reordering of instructions in the function prologue, and prevents merging those instructions with instructions in the function's body. So all functions start with a recognizable set of instructions (or honestly 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.
  • soft makes GCC generate output containing library calls for floating-point operations.
  • softfp allows 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 (and this part matters, fr fr): the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing, no matching, no exceptions. 😤

• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it won't 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 simply ignored. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32. Also, BE32 format has been deprecated by ARM.
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it's allowed to emit when generating assembly code. You can use this option 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, armv8.6-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.

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 ✨, and you can add them 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 that it depends on — for example, the +crypto extension will always enable the +simd extension. The one exception to this additive construction is extensions prefixed with +no...: those extensions disable the specified option and any other extensions that may depend on the presence of that extension.

ok so real talk bestie, the order you stack these extensions in literally matters, because a later +nofoo can just ✨delete✨ an earlier +foo. Case in point:

-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. 💀 yeah. order. matters.

---

Also most extension names are generically named, but their actual effect depends on which architecture you slap them onto. Like, the +simd option works on both armv7-a and armv8-a — but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a. same name, different vibe, iykyk. 🦋

The list below is the whole rundown of supported extensions for each architecture. Architectures not mentioned do not support any extensions. periodt. 💅

---

🎀 armv5te

🎀 armv6

🎀 armv6j

🎀 armv6k

🎀 armv6kz

🎀 armv6t2

🎀 armv6z

🎀 armv6zk

All of these cuties share the same two extensions:

• +fp — the VFPv2 floating-point instructions. You can also write +vfpv2 as an alias for this one. 🫶
• +nofp — turns the floating-point instructions off.

---

🌸 armv7

This one is the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures. (the shared bestie trio 💖)

• +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. +vfpv3-d16 works as an alias for this. Heads up: floating-point is not supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — turns the floating-point instructions off.

---

🌈 armv7-a

• +mp — the multiprocessing extension.
• +sec — the security extension.
• +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. +vfpv3-d16 can be used as an alias for this extension.
• +simd — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. +neon and +neon-vfpv3 can both 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 — disables the Advanced SIMD instructions (this does not disable floating point, ok? they're different things 🫠).
• +nofp — disables the floating-point and Advanced SIMD instructions.

---

🦋 armv7ve

This is the extended version of the ARMv7-A architecture, with support for virtualization. main character energy fr.

• +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. +vfpv4-d16 can be used as an alias for this extension.
• +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. +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. +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 — disables the Advanced SIMD instructions (again: does not disable floating point 👀).
• +nofp — disables 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 — disables the cryptographic instructions.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions. yeah, all three. 😭
• +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 — disables the cryptographic instructions.
• +nofp — disables 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

These two share the exact same roster, bestie:

• +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 — enables the Dot Product extension. This also enables Advanced SIMD instructions.
• +nocrypto — disables the cryptographic extension.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

---

💖 armv8.4-a

• +fp16 — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension. (yeah it's doing a lot. iconic behavior 💅)
• +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 — disables the cryptographic extension.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

---

🌟 armv8.5-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 — disables the cryptographic extension.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
• +fp16 — the half-precision floating-point data processing instructions 💖 this one also turns on the Advanced SIMD and floating-point instructions, plus the Dot Product extension, plus the half-precision floating-point fmla extension. a whole bundle, no cap.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions, and also the Dot Product extension. package deal bestie ✨
• +crypto — the cryptographic instructions 🔐 this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — turns the cryptographic extension off. bye 👋
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions. everything. gone. 💀
• +i8mm — the 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — the Brain half-precision floating-point instructions 🧠 this also enables Advanced SIMD and floating-point instructions.

---

armv7-r</p>

• +fp.sp — the single-precision VFPv3 floating-point instructions. the extension +vfpv3xd works as an alias for this one, iykyk.
• +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 — disables the floating-point extension.
• +idiv — the ARM-state integer division instructions.
• +noidiv — disables 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 — disables the floating-point extensions.

---

armv8.1-m.main

• +dsp — the DSP instructions.
• +mve — the M-Profile Vector Extension (MVE) integer instructions.
• +mve.fp — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions.
• +nofp — disables the floating-point extension.
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7.

---

armv8-m.main

• +dsp — the DSP instructions.
• +nodsp — disables the DSP extension.
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions.
• +nofp — disables the floating-point extension.
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7.

---

armv8-r

• +crc — the Cyclic Redundancy Check (CRC) instructions.
• +fp.sp — the single-precision FPv5 floating-point instructions.
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions.
• +nocrypto — disables the cryptographic instructions.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.

---

-march=native makes the compiler auto-detect the architecture of the build computer 🕵️♀️ rn this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect doesn't work out, the option just has no effect. it's giving "we tried" 😭

---

-mtune=name

this option tells GCC the name of the target ARM processor it should tune the performance of the code for 🎯 for some ARM implementations you can get better performance by using this option. the 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, cortex-a76ae, cortex-a77, 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-m35p, cortex-m55, 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.

additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system 👑 the permissible names are:

cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.

-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch 🌈 the goal is 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 giving "trend cycle" fr fr.

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. noted 📝

-mtune=native makes the compiler auto-detect the CPU of the build computer 🔍 rn 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, we move 💅

---

-mcpu=name[+extension...]

this one specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type to tune for performance (as if specified by -mtune). the way it does double duty?? multitasking icon ✨

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 the extension syntax can be used 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. so many little rules, but we keep them all bestie 💖

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.

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, cortex-m35p.
• +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, cortex-m33 and cortex-m35p. 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 the 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 is equivalent to -march=arch -mtune=generic-arch. see -mtune for more information 👀

-mcpu=native tells the compiler to auto-detect the CPU of the build computer. At present this feature is only supported on GNU/Linux, and not every architecture gets recognized. And if the auto-detect doesn't work out? The option just... does nothing. Has no effect. It's giving "tried my best" energy. 🙈

---

• -mfpu=name — this one says what floating-point hardware (or hardware emulation) is available on the target. The 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. Heads up bestie: neon is an alias for neon-vfpv3, and vfp is an alias for vfpv2. ✨

The setting auto is the default and it's special. It makes the compiler pick the floating-point and Advanced SIMD instructions based on whatever -mcpu and -march are set to.

If the floating-point hardware you selected includes the NEON extension (like, say, -mfpu=neon), then listen up: floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (specifically, denormal values are treated as zero), so using NEON instructions may lead to a loss of precision. 😭 The math girlies need to know.

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. 💅

• -mfp16-format=name — specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative. The default is none, in which case the __fp16 type is not defined at all. Not me giving you the option to simply not have the type 🫶
• -mstructure-size-boundary=n — the sizes of all structures and unions get rounded up to a multiple of the number of bits you set here. 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 here's the spicy part: 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. It's giving incompatible exes. 💔

This option is deprecated. She's retiring, okay?

• -mabort-on-noreturn — generate a call to the function abort at the end of a noreturn function. It gets executed if the function tries to return. Like a little security guard at the exit door. 🚪
• -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 that register. You need this switch if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.

Even with this switch on, not all function calls are turned into long calls. The heuristic is: static functions, functions that have the short_call attribute, functions inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions within the scope of a #pragma long_calls directive are always turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, and so does placing the function calls within the scope of a #pragma long_calls_off directive. Note bestie: these switches have no effect on how the compiler generates code to handle function calls via function pointers. 💀

• -msingle-pic-base — treat the register used for PIC addressing as read-only, instead of 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 it's a "you handle it" situation. 🫡
• -mpic-register=reg — specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by the compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.
• -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When it's disabled on such targets, it will enable -msingle-pic-base by default. Iconic chain reaction behavior. 🌈
• -mpoke-function-name — write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

                     t0
                         .ascii "arm_poke_function_name", 0
                         .align
                     t1
                         .word 0xff000000 + (t1 - t0)
                     arm_poke_function_name
                         mov     ip, sp
                         stmfd   sp!, {fp, ip, lr, pc}
                         sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). Detective work fr 🔍

• -mthumb / -marm — select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option.

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. Per-function vibes, we love to see it. ✨

• -mflip-thumb — switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code. It's a test-only girlie, don't invite her to the party. 🙈
• -mtpcs-frame — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. 👑
• -mcallee-super-interworking — gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. So in AAPCS land she simply doesn't exist. 💀
• -mcaller-super-interworking — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. Same deal as above, bestie. 🫶
• -mtp=name — specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to __aeabi_read_tp; cp15, which fetches the thread pointer from cp15 directly (supported in the arm6k architecture); and auto, which uses the best available method for the selected processor. The default setting is auto. Main character default behavior honestly. 👑
• -mtls-dialect=dialect — specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. 🌸
• -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. Pick a lane, girl. 😤
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. Thank you for your service. 🎀
• -munaligned-access / -mno-unaligned-access — enables (or disables) reading and writing of 16- and 32-bit values from addresses that are not 16- or 32-bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time.

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ✨

• -mneon-for-64bits — this option is deprecated and has no effect. She's literally just standing there. Not a single thing. 💀
• -mslow-flash-data — assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and is off by default. It conflicts with -mword-relocations. 🍓
• -masm-syntax-unified — bestie, this one says "assume the inline assembler is rolling with unified asm syntax." The default rn is off, which means it's giving divided syntax energy. Heads up tho: this option does nothing for Thumb2. And it might change in future GCC releases, so don't get too comfy. Also, divided syntax? Consider it deprecated, periodt. 💅
• -mrestrict-it — restricts how IT blocks get generated so they play by ARMv8-A's rules. IT blocks can only hold a single 16-bit instruction from a select little set of instructions. This is on by default for ARMv8-A Thumb mode. ✨
• -mprint-tune-info — prints CPU tuning info as a comment in the assembler file. Real talk: this is only for regression testing the compiler, not for normal code-compiling use. Disabled by default. 🙈
• -mverbose-cost-dump — turns on verbose cost model dumping in the debug dump files. It exists purely for debugging the compiler, iykyk. 🔍
• -mpure-code — do not let constant data sit in code sections. On top of that, when you're compiling for ELF object format, it gives every text section the ELF processor-specific section attribute SHF_ARM_PURECODE. Only available when generating non-pic code for M-profile targets. 🌸
• -mcmse — generates secure code following the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives at <https://developer.arm.com/documentation/ecm0359818/latest/>. 🛡️
• -mfix-cmse-cve-2021-35465 — mitigates a potential security issue with the VLLDM instruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when -mcpu= is used with cortex-m33, cortex-m35p or cortex-m55. Want it off? -mno-fix-cmse-cve-2021-35465 disables the mitigation. 🔥
• -mfdpic, -mno-fdpic — picks the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this is on by default and implies -fPIE if you didn't pass any PIC/PIE-related options. On other targets it only switches on the FDPIC-specific code generation features, and you gotta supply the PIC/PIE-related options yourself as needed. 👑

Ngl, static linking is not supported here, because it would still drag in the dynamic linker when the program self-relocates. If that behavior is chill with you, use -static and -Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and honestly required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain you used for the userland programs. 🫶

---

🌈 AVR Options 🌈

These options are defined for AVR implementations, bestie:

• -mmcu=mcu — specifies the Atmel AVR instruction set architecture (ISA) or MCU type. The default for this option is avr2. GCC supports the following AVR devices and ISAs, so buckle up, it's a lot 🎀:
  • 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.
  • avr5 — "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, 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 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
  • avrxmega2 — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5.
  • 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 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. This one only does anything on reduced Tiny devices like ATtiny40. Also peep the absdata AVR Variable Attributes, variable attribute. 🧚
• -maccumulate-args — accumulates outgoing function arguments and acquires/releases the needed stack space for outgoing function arguments once, in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards.

Popping the arguments after the call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, since arguments don't need to be removed from the stack after such a function call.

This can lead to reduced code size for functions that make several calls to functions that get their arguments on the stack, like calls to printf-like functions. 🌟

• -mbranch-cost=cost — sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0. 💖
• -mcall-prologues — prologues and epilogues get expanded into calls to the appropriate subroutines. It's giving smaller code size, bestie. ✨
• -mdouble=bits
• -mlong-double=bits — sets the size, in bits, of the double or long double type respectively. The possible values for bits are 32 and 64. Whether a specific value for bits is even allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the same energy applies to the default values of these options. 💅
• -mgas-isr-prologues — interrupt service routines (ISRs) are allowed to use the __gcc_isr pseudo instruction that GNU Binutils supports. If this option is on, you can still turn the feature off for individual ISRs by using the AVR Function Attributes no_gccisr function attribute. It's activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils supports PR21683 (https://sourceware.org/PR21683). 🎀
• -mint8 — assume int is an 8-bit integer. This affects the sizes of literally all the types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Just know that this option does not conform to the C standards, but it results in smaller code size. 🙈
• -mmain-is-OS_task — don't save registers in main. The effect is the same as attaching the AVR Function Attributes OS_task attribute to main. It's activated per default if optimization is on. 🔥
• -mn-flash=num — assume the flash memory has a size of num times 64 KiB.
• -mno-interrupts — the generated code is not compatible with hardware interrupts. Code size is smaller, no cap.
• -mrelax — try to replace a CALL or JMP instruction with the shorter RCALL or 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 the code is located. So the assembler code generated by the compiler is the same, but the instructions in the executable may differ from the instructions in the assembler code. That's just how it is.

Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below. 👑

• -mrmw — assume 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. fr fr.

• -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 either.

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 gets 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. tiny but mighty 💖
• -mfract-convert-truncate — allow using 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. This means the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu. 🫶

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu.

• -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 the __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 a word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

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 as follows by the compiler and are subject to some limitations:

• The compiler never sets EIND.
• The compiler uses EIND implicitly in EICALL/EIJMP instructions, or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue.
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. 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 the high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing.

• The startup code initializes the RAMP special function registers with zero. 🌸
• If a AVR Named Address Spaces, named address space other than generic or "__flash" is used, then "RAMPZ" gets set as needed before the operation. Like, it's just handled for you bestie.
• 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. No skipping this one, fr fr.

🦋 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 it depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6", respectively, and

100, 102, 103, 104, 105, 106, 107

for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.

• __AVR_Device__ — Setting -mmcu=device defines this built-in macro, which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro "__AVR_ATmega8__", -mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.

The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_DEVICE_NAME__ — Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to "atmega8".

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_XMEGA__ — The device / architecture belongs to the XMEGA family of devices.
• __AVR_HAVE_ELPM__ — The device has the "ELPM" instruction.
• __AVR_HAVE_ELPMX__ — The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
• __AVR_HAVE_MOVW__ — 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. slay.
• __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 — Some devices support reading from flash memory by means of "LD*" instructions. The flash memory is seen in the data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put ".rodata" into RAM. This is handled by the default linker description file, and is currently available for "avrtiny" and "avrxmega3". Even more convenient, there is no need to use address spaces like "__flash" or features like attribute "progmem" and "pgm_read_*".
• __WITH_AVRLIBC__ — The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.
• __HAVE_DOUBLE_MULTILIB__ — Defined if -mdouble= acts as a multilib option.
• __HAVE_DOUBLE32__, __HAVE_DOUBLE64__ — Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.
• __DEFAULT_DOUBLE__ — The size in bits of "double" if -mdouble= is not set. To test the layout of "double" in a program, use the built-in macro "__SIZEOF_DOUBLE__".
• __HAVE_LONG_DOUBLE32__, __HAVE_LONG_DOUBLE64__, __HAVE_LONG_DOUBLE_MULTILIB__, __DEFAULT_LONG_DOUBLE__ — Same as above, but for "long double" instead of "double". 💅
• __WITH_DOUBLE_COMPARISON__ — Reflects the --with-double-comparison={tristate|bool|libf7} configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3.
• __WITH_LIBF7_LIBGCC__, __WITH_LIBF7_MATH__, __WITH_LIBF7_MATH_SYMBOLS__ — Reflects the --with-libf7={libgcc|math|math-symbols} configure option (https://gcc.gnu.org/install/configure.html#avr).

---

🖤 Blackfin Options 🖤

• -mcpu=cpu[-sirevision] — Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The "__SILICON_REVISION__" macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision is any, the "__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

Without this option, bf532 is used as the processor by default.

Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 😭

• -msim — Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
• -momit-leaf-frame-pointer — Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. little win, big vibe.
• -mspecld-anomaly — When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
• -mno-specld-anomaly — Don't generate extra code to prevent speculative loads from occurring.
• -mcsync-anomaly — When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
• -mno-csync-anomaly — Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
• -mlow64k — When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.
• -mno-low64k — Assume that the program is arbitrarily large. This is the default.
• -mstack-check-l1 — Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
• -mid-shared-library — Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim.
• -mno-id-shared-library — okay so this one tells the compiler to generate code that doesn't assume ID-based shared libraries are being used. And plot twist: this is the default! So it's giving "we already do this" energy. 💅
• -mleaf-id-shared-library — bestie, this one generates code that supports shared libraries via the library ID method, BUT it assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. Speedy vibes only. 🚀
• -mno-leaf-id-shared-library — opposite energy: it does not assume that the code being compiled won't link against any ID shared libraries. Result? Slower code is generated for jump and call insns. Sad. 😭
• -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 (slay); 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. So like, do with that what you will. 🤷
• -msep-data — this generates 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. Iconic for embedded. 🌈
• -mno-sep-data — generates code that assumes that the data segment follows the text segment. And yes, this is the default. So if you don't say anything, this is what you get. 💁
• -mlong-calls, -mno-long-calls — so this tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction. This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note: these switches have no effect on how the compiler generates code to handle function calls via function pointers. K thanks. 🫶
• -mfast-fp — links 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. So like, speed over strictness. ⚡
• -minline-plt — enables inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. So don't bother unless you have that. 🙃
• -mmulticore — builds a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision]. This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main. If this option is not used, the single-core application programming model is used. 🧠
• -mcorea — builds a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore. 🅰️
• -mcoreb — builds a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. This option can only be used in conjunction with -mmulticore. 🅱️
• -msdram — builds a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application. 💾
• -micplb — assumes that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off. 🔧

---

✨ C6X Options ✨

• -march=name — specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x. So many choices, bestie! 🏗️
• -mbig-endian — generates code for a big-endian target. 🐘
• -mlittle-endian — generates code for a little-endian target. This is the default. 🐭
• -msim — chooses startup files and linker script suitable for the simulator. 🎮
• -msdata=default — puts small global and static data in the .neardata section, which is pointed to by register B14. Puts small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Puts small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const. So basically: small stuff goes near, big stuff goes far. 🏠
• -msdata=all — puts all data, not just small objects, into the sections reserved for small data, and uses addressing relative to the B14 register to access them. Everything is small now, okay? 🤏
• -msdata=none — makes no use of the sections reserved for small data, and uses absolute addresses to access all data. Puts all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Puts all constant data into the .const section. No small data energy here. 🚫

---

✨ CRIS Options ✨

These options are defined specifically for the CRIS ports.

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10. So many versions! 🏛️
• -mtune=architecture-type — tunes 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 — warns 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. So just shortcuts, bestie. ✨
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the muls and mulu instructions for CPU models where it applies. This option is active by default. So bug be gone! 🐛
• -mpdebug — enables CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file. Debug vibes. 🔍
• -mcc-init — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes. Safety first! 🛡️
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment. So no sneaky side effects. 🙅
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. So alignment tweaks, but don't touch my ABI. 📐
• -m32-bit, -m16-bit, -m8-bit — similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. Pick your bit width. 🔢
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated.

⚠️ bestie, this is dangerous: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. So you better inspect the compiled code visually! 👀

• -mno-gotplt, -mgotplt — with -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt. So default is to generate. 🎯
• -melf — legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets. So it does nothing, but it's there. 👻
• -mlinux — legacy no-op option only recognized with the cris-axis-linux-gnu target. Another ghost option. 👻
• -sim — this option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. Simulator time! 🕹️
• -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. Specific addresses, bestie. 📍

---

✨ CR16 Options ✨

These options are defined specifically for the CR16 ports.

• -mmac — enables the use of multiply-accumulate instructions. Disabled by default. So MAC it up! ✖️➕
• -mcr16cplus, -mcr16c — generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default. So default is the plus one. ➕
• -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. 32 bits, baby. 🔢
• -mbit-ops — generates sbit/cbit instructions for bit manipulations. Bit magic! ✨
• -mdata-model=model — choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model. So default is medium, but no far for CR16C. 🗺️

---

✨ 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. So many choices, default is ck810. 🏗️
• -mcpu=cpu — picks the C-SKY target processor for you bestie 💅 valid cpu values are, like, a whole saga:

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.

• -mbig-endian, -EB, -mlittle-endian, -EL — choose big- or little-endian code. the default is little-endian 🌸
• -mhard-float, -msoft-float — pick hardware or software floating-point implementations. default is soft float, no cap 🫶
• -mdouble-float, -mno-double-float — when -mhard-float is in effect, this enables generation of double-precision float instructions. it's the default except when compiling for CK803 ✨
• -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, enables generation of "frecipd", "fsqrtd", and "fdivd" instructions. default except when compiling for CK803 💫
• -mfpu=fpu — select the floating-point processor. lowkey this one 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. defaults to on for CK801 and off otherwise, periodt 💅
• -mistack, -mno-istack — enable interrupt stack instructions; default is off. the -mistack option is required to handle the "interrupt" and "isr" function attributes 🦋
• -mmp — enable multiprocessor instructions; default is off.
• -mcp — enable coprocessor instructions; default is off.
• -mcache — enable coprocessor instructions; default is off. (yes bestie, it says coprocessor, that's literally what the page says, i don't make the rules 😭)
• -msecurity — enable C-SKY security instructions; default is off 🔥
• -mtrust — enable C-SKY trust instructions; default is off 👑
• -mdsp, -medsp, -mvdsp — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. all of these 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, default is off ⭐
• -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. this one is not supported on CK801, CK802, or CK803, and it's 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. defaults to on 🧚
• -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — generate code using "stm" and "ldm" instructions. not supported on CK801 but 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 it's optional on other processors 💜
• -mstack-size, -mno-stack-size — emit .stack_size directives for each function in the assembly output. defaults to off.
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. defaults to off.
• -mbranch-cost=n — set the branch costs to roughly n instructions. default is 1.
• -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences.

⚠️ bestie listen — using this can result in code that is not compliant with the C-SKY V2 ABI prologue requirements, and that code cannot be debugged or backtraced. it is disabled by default for a reason 😭

---

✨ 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 🫶

• -Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. these directories are interleaved with those specified by -I options and are scanned in a left-to-right order 🌸

a framework directory is a directory with frameworks in it. a framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. the name of a framework is the name of this directory excluding the .framework. headers associated with the framework are found in one of those two directories, with Headers being searched first. a subframework is a framework directory that is in a framework's Frameworks directory. includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. two subframeworks are siblings if they occur in the same framework. a subframework should not have the same name as a framework; a warning is issued if this is violated. currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. the standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. an example include looks like "#include <Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory 🦋

• -iframeworkdir — like -F except the directory is treated as a system directory. the main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. this option is valid only for the C family of languages 💖
• -gused — emit debugging information for symbols that are used. for stabs debugging format, this enables -feliminate-unused-debug-symbols. this is by default ON ✅
• -gfull — emit debugging information for all symbols and types.
• -mmacosx-version-min=version — the earliest version of MacOS X that this executable will run on is version. typical values of version include 10.1, 10.2, and 10.3.9 ⭐

if the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible ✨

• -mkernel — enable kernel development mode. the -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. this mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets 👑
• -mone-byte-bool — override the defaults for "bool" so that "sizeof(bool)==1". by default "sizeof(bool)" is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86 🤔

⚠️ warning: the -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. using this switch may require recompiling all other modules in a program, including system libraries. use this switch to conform to a non-default data model 💀

• -mfix-and-continue, -ffix-and-continue, -findirect-data — generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility 💜
• -all_load — loads all members of static archive libraries. see man ld(1) for more information 📚
• -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal 🔥
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — produce a Mach-o bundle format file. see man ld(1) for more information 🎀
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. see man ld(1) for more information 🌈
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command 🧚
• -force_cpusubtype_ALL — this causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option 💅

okay so real talk bestie — the next batch isn't even GCC's own thing 💅 you're literally just passing these straight through to the Darwin linker. One giant relay race of flags:

• -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 moment: GCC is just the messenger here 🫶 if you wanna know what any of these actually do, you gotta go read the Darwin linker man page, bestie. Not our lore.

---

🦋 DEC Alpha Options 🦋

These -m options are defined for the DEC Alpha implementations — vintage energy, but she still serves:

• -mno-soft-float, -msoft-float — use (or 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 emulation routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have floating-point registers. 💀 yeah, that's just how it is.

• -mfp-reg, -mno-fp-regs — generate code that uses (or 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 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 precision can be selected:
  • p — program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
  • f — function precision. The trap handler can determine the function that caused a floating-point exception.
  • i — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.

Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — this option marks the generated code as IEEE conformant. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.

no cap, read that one twice bestie — -mieee-conformant on its own is a no-no. You need the trap-precision and trap-mode buddies along for the ride. 🎀

• -mbuild-constants — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it outputs the constant as a literal and generates code to load it from the data segment at run time.

Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

You typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none is specified.
• -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.
• -mexplicit-relocs, -mno-explicit-relocs — older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — when -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.

The default is -mlarge-text.

• -mcpu=cpu_type — set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are

• ev4
• ev45
• 21064

Schedules as an EV4 and has no instruction set extensions.

• ev5, 21164 — schedules like an EV5, and it's got literally no instruction set extensions. plain and proud 💅
  • ev56, 21164a — schedules like an EV5, and it does support the BWX extension. glow up unlocked ✨
  • pca56, 21164pc, 21164PC — schedules like an EV5, and supports both the BWX and MAX extensions. she's got range 🦋
  • ev6, 21264 — schedules like an EV6, and supports BWX, FIX, and MAX. full stack bestie 👑
  • ev67, 21264a — schedules like an EV6, and supports BWX, CIX, FIX, and MAX. the whole alphabet of extensions fr fr 🔥

Native toolchains also support the value native, which picks the best architecture option for whatever processor the host is running. quick heads up tho: -mcpu=native has no effect if GCC doesn't recognize the processor. it just kinda shrugs 😭

• -mtune=cpu_type — this one sets only the instruction scheduling parameters for the machine type cpu_type. the instruction set itself does not change. so you're tuning the vibe, not swapping the whole outfit 🎀

Native toolchains also support the value native, which selects the best architecture option for the host processor. and same deal as before: -mtune=native has no effect if GCC doesn't recognize the processor. 💀

• -mmemory-latency=time — sets the latency the scheduler should assume for typical memory references as seen by the application. bestie, this number is highly dependent on the memory access patterns your app uses and the size of the external cache on the machine. it's giving "it depends" energy, sorry 🙈

Valid options for time are:

• number — a decimal number representing clock cycles.
• L1, L2, L3, main — the compiler has built-in 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), plus main memory. note that L3 is only valid for EV5. ⭐

---

🫧 eBPF Options 🫧

• -mframe-limit=bytes — specifies the hard limit for frame sizes, in bytes. currently, the value you can specify should be less than or equal to 32767. it defaults to whatever limit the version of the Linux kernel you're targeting imposes. so the kernel kinda sets the ceiling 🧚
• -mkernel=version — specifies the minimum version of the kernel that will run the compiled program. GCC uses this version to figure out which instructions to use, what kernel helpers to allow, etc. currently, version can be one of 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, 4.11, 4.12, 4.13, 4.14, 4.15, 4.16, 4.17, 4.18, 4.19, 4.20, 5.0, 5.1, 5.2, latest and native. that's a whole lot of options, no cap 🌈
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. this one is the default. 💖
• -mxbpf — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • Save and restore callee-saved registers at function entry and exit, respectively.

---

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port. iykyk 🫶

• -msmall-model — use the small address space model. this can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. so, like, small code, small assumptions 🌱
• -mno-lsim — assume that runtime support has been provided and 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.

bestie 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. real hardware, real responsibilities 😤

• -mlra — enable Local Register Allocation. this is still experimental for FT32, so by default the compiler uses standard reload. science is still sciencing here 🧪
• -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.
• -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". keeping it fixed, keeping it simple 💅
• -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. ✨
• -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 you're 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. (yes, the capitalization is the whole difference, stay sharp bestie 👀)
• -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 isn't, -mno-gprel-ro can be used to disable it. math is mathing 🔥
• -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. it's giving "please don't touch" 🙈
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. this option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. this allows the functions to be placed anywhere within the 32-bit address space. the whole map is yours 🗺️
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. this option only has an effect when VLIW packing is enabled. it doesn't create new packets; it merely adds NOPs to existing ones.
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers.
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (default). this switch is mainly for debugging the compiler and will likely be removed in a future version. so like… enjoy it while it lasts ⏳
• -mno-cond-move — disable the use of conditional-move instructions. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mscc — enable the use of conditional set instructions (default). this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-scc — disable the use of conditional set instructions. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mcond-exec — enable the use of conditional execution (default). this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-cond-exec — disable the use of conditional execution. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mvliw-branch — run a pass to pack branches into VLIW instructions (default). this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mmulti-cond-exec — enable optimization of "&&" and "||" in conditional execution (default). this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-multi-cond-exec — disable optimization of "&&" and "||" in conditional execution. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mnested-cond-exec — enable nested conditional execution optimizations (default). this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -mno-nested-cond-exec — disable nested conditional execution optimizations. this switch is mainly for debugging the compiler and will likely be removed in a future version.
• -moptimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. it is enabled by default.
• -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code.
• -mtomcat-stats — cause gas to print out tomcat statistics. numbers, but make it tomcat 🐱
• -mcpu=cpu — select the processor type for which to generate code. 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, and they're basically a "which C library are we vibing with today" situation 💅

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use the 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 the Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid — compile code that's compatible with the Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option turns on -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options over to the linker. And finally, this option causes the preprocessor macro "__ANDROID__" to be defined. ✨

• -tno-android-cc — disable the compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable the linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

---

✨ H8/300 Options 🦋

These -m options are defined for the H8/300 implementations, and they're giving very niche-but-iconic energy, ngl:

• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are stored on the stack before execution of a function with the monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — extended registers are <em>not</em> stored on the stack before execution of a function with the monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — make int data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

---

🔥 HPPA Options 👑

These -m options are defined for the HPPA family of computers — retro tech, main character energy, we love to see it 🌈

• -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 figure out the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around. 🙈
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0

Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

• -mcaller-copies — the caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP.
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only. 💀
• -mdisable-fpregs — prevent floating-point registers from being used in any manner whatsoever. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and then attempt to perform floating-point operations, the compiler aborts. Not a drill.
• -mdisable-indexing — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
• -mno-space-regs — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

Such code is suitable for level 0 PA systems and kernels.

• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.

This option does not work in the presence of shared libraries or nested functions.

• -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems.
• -mgas — enable the use of assembler directives only GAS understands.
• -mschedule=cpu-type — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.
• -mlinker-opt — enable the optimization pass in the HP-UX linker. Note that this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. 😭
• -msoft-float — generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile <em>all</em> of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• -msio — generate the predefine "_SIO" for server IO. The default is -mwsio. This generates the predefines "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
• -mgnu-ld — use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mhp-ld — use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mlong-calls — generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

• -munix=unix-std — generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 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.

It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option. 🚨

Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.

• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — okay so HP-UX's setlocale in libc is lowkey dependent on libdld.sl, and there's literally no archive version of libdld.sl?? so when you pass -static, you need special link options to resolve that dependency, it's giving messy 💀

On HP-UX 10 and later, the GCC driver just adds the necessary options to link with libdld.sl when -static is specified. Which means… the resulting binary ends up dynamic. 🫠 And on the 64-bit port, the linkers generate dynamic binaries by default anyway, so. If you wanna stop the GCC driver from adding those link options, -nolibdld is your girl.

• -threads — adds multithreading support with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker, no cap. ✨

---

🧚 IA-64 Options 🧚

These are the -m options defined for the Intel IA-64 architecture. Strap in 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. Heads up: 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 doesn't 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 — if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled.
• -mno-sched-prefer-non-control-spec-insns, -msched-prefer-non-control-spec-insns — if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled.
• -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.
• -msched-spec-ldc — use a simple data speculation check. This option is on by default. ✨
• -msched-control-spec-ldc — use a simple check for control speculation. This option is on by default. ✨
• -msched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. This option is on by default. ✨
• -msched-fp-mem-deps-zero-cost — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
• -msel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default.
• -msched-max-memory-insns=_max-insns_ — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
• -msched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled. 🦋

---

🌈 LM32 Options 🌈

These -m options are defined for the LatticeMico32 architecture:

• -mbarrel-shift-enabled — enable barrel-shift instructions.
• -mdivide-enabled — enable divide and modulus instructions.
• -mmultiply-enabled — enable multiply instructions.
• -msign-extend-enabled — enable sign extend instructions.
• -muser-enabled — enable user-defined instructions. 💅

---

🍓 M32C Options 🍓

• -mcpu=_name_ — select the CPU for which code is generated. _name_ may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. 🫶

Do NOT use this option when generating programs that will run on real hardware. You must provide your own runtime library for whatever I/O functions are needed. fr fr, this one matters.

• -memregs=_number_ — specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries. 😭

---

💜 M32R/D Options 💜

These -m options are defined for Renesas M32R/D architectures:

• -m32r2 — generate code for the M32R/2.
• -m32rx — generate code for the M32R/X.
• -m32r — generate code for the M32R. This is the default. 👑
• -mmodel=small — assume every object lives down in the lower 16MB of memory (so its address can just be loaded with the ld24 instruction), and assume every subroutine is reachable with bl. This one's the default, bestie. ✨

You can set the addressability of a particular object with the model attribute.

• -mmodel=medium — assume objects could be anywhere in the 32-bit address space (so the compiler generates seth/add3 instructions to load their addresses), but still assume every subroutine is reachable with bl.
• -mmodel=large — assume objects could be anywhere in the 32-bit address space (compiler generates seth/add3 to load addresses), and assume subroutines might not be reachable with bl — so the compiler emits the much slower seth/add3/jl instruction sequence. it's giving "correct but chunky" 😭
• -msdata=none — disable the small data area entirely. Variables go into one of .data, .bss, or .rodata (unless the section attribute was specified). This is the default. 🎀

The small data area is made of the sections .sdata and .sbss. Objects can be explicitly dropped into the small data area with the section attribute using one of those 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 generate special instructions to reference them. slay 💅
• -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. Default value of num is 8. The -msdata option has to 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 spit out some statistics that might help with debugging programs. 🐛
• -malign-loops — align all loops to a 32-byte boundary.
• -mno-align-loops — don't enforce that 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, branches are preferred over conditional code; if it's 2, 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 isn't 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's optimized for a particular microarchitecture. ✨

• -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 *cpu*s are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire *cpu*s are given by the table below, which also classifies the CPUs into families:

Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484
5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚫

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above.

• -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.

• -m68000, -mc68000 — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It's equivalent to -march=68000.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It's equivalent to -march=68010.
• -m68020, -mc68020 — generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It's equivalent to -march=68020.
• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It's equivalent to -march=68030.
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It's equivalent to -march=68040.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 doesn't have code to emulate those instructions.

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It's equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 doesn't have code to emulate those instructions.

• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It's equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It's equivalent to -mcpu=5206, and is now deprecated in favor of that option.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
• -m68020-40 — generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40.

• -m68020-60 — generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

The option is equivalent to -march=68020 -mtune=68020-60.

• -mhard-float, -m68881 — generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets. 🔥
• -msoft-float — bestie, do not generate floating-point instructions; call the library instead 🫶. This is the default for 68000, 68010, and 68832 targets, and it's also the default for ColdFire devices that don't even have an FPU. No FPU, no problem.
• -mdiv, -mno-div — generate (or don't generate) ColdFire hardware divide and remainder instructions ✨. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one you specified with -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

When this option is enabled, GCC defines the macro "__mcfhwdiv__" 💅.

• -mshort — consider type "int" to be 16 bits wide, like "short int". Also, parameters passed on the stack get 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. Iconic little optimization, ngl.

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 using more memory.

Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k 🙈.

Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

• -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system 💅.
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment 🌸. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC.
• -mno-sep-data — generate code that assumes 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 💖.

• -mhardlit, -mno-hardlit — inline constants into the code stream if it can be done in two instructions or less.
• -mdiv, -mno-div — use the divide instruction. (Enabled by default) 👑.
• -mrelax-immediate, -mno-relax-immediate — allow arbitrary-sized immediates in bit operations.
• -mwide-bitfields, -mno-wide-bitfields — always treat bit-fields as "int"-sized.
• -m4byte-functions, -mno-4byte-functions — force all functions to be aligned to a 4-byte boundary.
• -mcallgraph-data, -mno-callgraph-data — emit callgraph information.
• -mslow-bytes, -mno-slow-bytes — prefer word access when reading byte quantities.
• -mlittle-endian, -mbig-endian — generate code for a little-endian target 🌈.
• -m210, -m340 — generate code for the 210 processor.
• -mno-lsim — assume that runtime support has been provided, and so omit the simulator library (libsim.a) from the linker command line.
• -mstack-increment=size — set the maximum amount for a single stack increment operation 📏. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much 💀. The default value is 0x1000.

---

🌸 MeP Options 🌸

• -mabsdiff — enables the "abs" instruction, which is the absolute difference between two registers ✨.
• -mall-opts — enables all the optional instructions---average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation 👑.
• -maverage — enables the "ave" instruction, which computes the average of two registers.
• -mbased=n — variables of size n bytes or smaller are placed in the ".based" section by default 🎀. Based variables use the $tp register as a base register, and there is a 128-byte limit to the ".based" section.
• -mbitops — enables the bit operation instructions---bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-and-set ("tas").
• -mc=name — selects which section constant data is placed in 🦋. name may be tiny, near, or far.
• -mclip — enables the "clip" instruction. Note that -mclip is not useful unless you also provide -mminmax 💅.
• -mconfig=name — selects one of the built-in core configurations 🌟. Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The "MeP-Integrator" tool, not part of GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is default.
• -mcop — enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the -mconfig= option.
• -mcop32 — enables the 32-bit coprocessor's instructions.
• -mcop64 — enables the 64-bit coprocessor's instructions.
• -mivc2 — enables IVC2 scheduling 🔥. IVC2 is a 64-bit VLIW coprocessor.
• -mdc — causes constant variables to be placed in the ".near" section.
• -mdiv — enables the "div" and "divu" instructions.
• -meb — generate big-endian code.
• -mel — generate little-endian code.
• -mio-volatile — tells the compiler that any variable marked with the "io" attribute is to be considered volatile 👑.
• -ml — causes variables to be assigned to the ".far" section by default.
• -mleadz — enables the "leadz" (leading zero) instruction ✨.
• -mm — causes variables to be assigned to the ".near" section by default.
• -mminmax — enables the "min" and "max" instructions.
• -mmult — enables the multiplication and multiply-accumulate instructions.
• -mno-opts — disables all the optional instructions enabled by -mall-opts 💀.
• -mrepeat — enables the "repeat" and "erepeat" instructions, used for low-overhead looping 🎀.
• -ms — causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to this section. Accesses to these variables use the %gp base register.
• -msatur — enables the saturation instructions. Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like "as" 🙈.
• -msdram — 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 — makes every function default to the .far section. Skip it and your functions all cozy up in .near by default. 💅
• -mtiny=n — variables that are n bytes or smaller get yeeted into the .tiny section, and those cuties use the $gp base register. Default is 4. But bestie, listen: the .tiny section has a hard 65536-byte limit. ✨

---

🌸 MicroBlaze Options 🌸

• -msoft-float — do floating point with software emulation. This is the default, btw.
• -mhard-float — actually use the hardware floating-point instructions. Fancy. 💅
• -mmemcpy — don't optimize block moves, just call memcpy like a normal person.
• -mno-clearbss — this one's deprecated, bestie. Use -fno-zero-initialized-in-bss instead.
• -mcpu=cpu-type — use the features of, and schedule code for, the given CPU. The supported values are formatted vX.YY.Z, where X is the major version, YY is the minor version, and Z is compatibility code. Example values: v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a. 👑
• -mxl-soft-mul — use software multiply emulation. Default behavior.
• -mxl-soft-div — use software emulation for divides. Also the 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.
• -mxl-stack-check — deprecated alert 🚨 use -fstack-check instead.
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections.
• -mxl-multiply-high — use multiply high instructions for the high part of a 32x32 multiply.
• -mxl-float-convert — use hardware floating-point conversion instructions.
• -mxl-float-sqrt — use the hardware floating-point square root instruction.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target.
• -mxl-reorder — use reorder instructions (that's 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 the displacement between the text and data segments is fixed at static link time. This lets data be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing isn't supported.
  • xmdstub — for use with the 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 loaded using a bootloader. This model uses startup file crt2.o, which does not contain a processor reset vector handler. Perfect for handing control on a processor reset to the bootloader instead of the application. 🦋
  • novectors — for applications that don't need any of the MicroBlaze vectors. Might be handy for apps running inside a monitoring application. This model uses crt3.o as a startup file.

Heads up: -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.

---

👑 MIPS Options 👑

• -EB — generate big-endian code.
• -EL — generate little-endian code. This is the default for mips*el-*-* configurations.
• -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, 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 native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC doesn't recognize the processor. 😭

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.

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of the target architecture as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000. ✨

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.

• -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 isn't used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it's possible to generate code that runs on a family of processors but optimize the code for one particular member of that family. 💅

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -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 don't generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.

MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes. 🎀

• -mflip-mips16 — generate MIPS16 code on alternating functions. This option exists for regression testing of mixed MIPS16/non-MIPS16 code generation, and it is not intended for ordinary use in compiling user code. Just so you know.
• -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 can't jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump isn't compressed.

• -minterlink-mips16
• -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility.
• -mabi=32
• -mabi=o64
• -mabi=n32
• -mabi=64
• -mabi=eabi — generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead. 💫

For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.

The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved.

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 🌈

• -mabicalls
• -mno-abicalls — generate (or don't generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems.
• -mshared, -mno-shared — okay so this one decides whether the code we generate is fully position-independent (that's -mshared) or not (-mno-shared), meaning it can be linked into shared libraries. Heads up bestie: this option only touches -mabicalls. 💅

So here's the lore: all -mabicalls code has traditionally been position-independent, no matter what -fPIC or -fpic are doing. But plot twist — as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. That whole mode? 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 lowkey it doesn't affect the ABI of the final executable — it only affects the ABI of relocatable objects. And using -mno-shared generally makes executables both smaller and quicker. 🌸

-mshared is the default. periodt.

• -mplt, -mno-plt — assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. This 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 causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. 😭

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do. 🙈

These options have no effect unless GCC is generating position independent code.

• -mgp32 — assume that general-purpose registers are 32 bits wide.
• -mgp64 — assume that general-purpose registers are 64 bits wide.
• -mfp32 — assume that floating-point registers are 32 bits wide.
• -mfp64 — assume that floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers. mystery vibe, iykyk. 🦋
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
• -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. 💀
• -msingle-float — assume that the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — 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. (yes bestie, they just flip-flopped the bits, iconic honestly)

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 — use (or don't use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (or don't use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled.
• -mdmx, -mno-mdmx — use (or don't use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled. 🎀
• -mips3d, -mno-mips3d — use (or don't use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
• -mmicromips, -mno-micromips — generate (or don't generate) microMIPS code. 💅

MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes.

• -mmt, -mno-mt — use (or don't use) MT Multithreading instructions.
• -mmcu, -mno-mcu — use (or don't use) the MIPS MCU ASE instructions.
• -meva, -mno-eva — use (or don't use) the MIPS Enhanced Virtual Addressing instructions.
• -mvirt, -mno-virt — use (or don't use) the MIPS Virtualization (VZ) instructions.
• -mxpa, -mno-xpa — use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.
• -mcrc, -mno-crc — use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.
• -mginv, -mno-ginv — use (or don't use) the MIPS Global INValidate (GINV) instructions.
• -mloongson-mmi, -mno-loongson-mmi — use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
• -mloongson-ext, -mno-loongson-ext — use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.
• -mloongson-ext2, -mno-loongson-ext2 — use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
• -mlong64 — force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined. 🌟
• -mlong32 — force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. 💖

• -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses. 🔥
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🫶

• -mextern-sdata, -mno-extern-sdata — assume (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 bestie, if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that's no bigger than num bytes — you have GOT to make sure Var lands in a small data section. Periodt. If Var is defined by another module, you've got two moves: 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 have to link the application with a high-enough -G setting. No skipping this.

The easiest way to satisfy all these restrictions is honestly just to compile and link every module with the same -G option. Simple. Iconic. But maybe you're feeling fancy and you wanna build a library that supports several different small data limits. You can absolutely do that — compile the library with the highest supported -G setting and additionally use -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 💅

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• -mgpopt, -mno-gpopt — Use (or don't use, bestie) GP-relative accesses for symbols that are known to be in a small data section. See -G, -mlocal-sdata and -mextern-sdata for the lore. -mgpopt is the default for all configurations, every single one.

-mno-gpopt is your bestie for cases where the $gp register might not hold the value of "_gp". Like what? Okay so imagine the code is part of a library that might get used in a boot monitor — programs that call boot monitor routines pass an unknown value in $gp. (In those situations, the boot monitor itself is usually compiled with -G0.) 🫶

Heads up: -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, so it may be the move for some embedded systems. 🌸
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — Put uninitialized "const" variables in the read-only data section. Just so you know, this option is only meaningful in conjunction with -membedded-data. They're a package deal. 🎀
• -mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings, and yes we're listing all three:
  • -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 one can be useful on targets configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
• -msplit-addresses, -mno-split-addresses — Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. Ngl, this option has been superseded by -mexplicit-relocs but it's retained for backwards compatibility. Old but still here. 💀
• -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 do not trap) on integer division by zero. The default is -mcheck-zero-division.
• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them, and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. And you can completely disable divide-by-zero checks using -mno-check-zero-division.

• -mload-store-pairs, -mno-load-store-pairs — Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. So like, it's on, but it's not doing anything unless the vibes are right. ✨
• -mmemcpy, -mno-memcpy — Force (do not force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.
• -mlong-calls, -mno-long-calls — Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment. 256MB bestie, that's the rule.

This option has no effect on abicalls code. The default is -mno-long-calls.

• -mmad, -mno-mad — Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
• -mimadd, -mno-imadd — Enable (disable) use of the "madd" and "msub" integer instructions. The default is -mimadd on architectures that support "madd" and "msub" — except for the 74k architecture, where it was found to generate slower code. Not it being the problem child. 🙈
• -mfused-madd, -mno-fused-madd — Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is -mfused-madd.

Okay so important detail: on the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.
• -mfix-24k, -mno-fix-24k — Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC. Not GCC's problem, it's the assembler's. 💅
• -mfix-r4000, -mno-fix-r4000 — Work around certain R4000 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
  • A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
  • An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
• -mfix-r4400, -mno-fix-r4400 — Work around certain R4400 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
• -mfix-r10000, -mno-fix-r10000 — Work around certain R10000 errata:
  • "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.

Heads up: this option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

• -mfix-r5900, -mno-fix-r5900 — Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule 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.

So here's the tea. 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. like bestie that's on the kernel, not on us 💅

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.

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• -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. three whole arguments, no notes ✨
• 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. heuristic queen, she's not making any promises 😭
• -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. deprecated, bestie, we said what we said 💀
• -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. three flavours, one vibe check, we love options 🫶

• -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. two pipes vs one pipe, that's the whole tea ☕

• -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. faster but thiccer, the eternal tradeoff 😌

• -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. undefined behavior alert, bestie, do NOT play with this on SMP 🚨

• -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. off at EVERY level, she's committed to not helping 🙈

• -mlxc1-sxc1
• -mno-lxc1-sxc1 — When applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default.
• -mmadd4
• -mno-madd4 — When applicable, enable (disable) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default.

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🌸 MMIX Options 🌸

These options are defined for the MMIX:

• -mlibfuncs
• -mno-libfuncs — Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. no matter the size, periodt ✨
• -mepsilon
• -mno-epsilon — Generate floating-point comparison instructions that compare with respect to the rE epsilon register.
• -mabi=mmixware
• -mabi=gnu — Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up. two ABIs, two whole personalities 💅
• -mzero-extend
• -mno-zero-extend — When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
• -mknuthdiv
• -mno-knuthdiv — Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used. both valid, one's just the main character 😌
• -mtoplevel-symbols
• -mno-toplevel-symbols — Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
• -mbranch-predict
• -mno-branch-predict — Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
• -mbase-addresses
• -mno-base-addresses — Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require -mno-base-addresses. short and fast but limited, the way that everything's a tradeoff fr fr 😭
• -msingle-exit
• -mno-single-exit — Force (do not force) generated code to have a single exit point in each function. one exit door only, we're enforcing it 👑

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🎀 MN10300 Options 🎀

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
• -mno-mult-bug — Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
• -mam33 — Generate code using features specific to the AM33 processor.
• -mno-am33 — Do not generate code using features specific to the AM33 processor. This is the default.
• -mam33-2 — Generate code using features specific to the AM33/2.0 processor.
• -mam34 — Generate code using features specific to the AM34 processor.
• -mtune=cpu-type — Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34. pick your fighter: mn10300, am33, am33-2, am34 🦋
• -mreturn-pointer-on-d0 — when you're generating a function that hands back a pointer, return that pointer in both a0 and d0. If you don't, the pointer comes back only in a0, and any attempt to call such a function without a prototype straight up errors out. Heads up bestie: this one is on by default, so use -mno-return-pointer-on-d0 if you wanna turn it off. 💅
• -mno-crt0 — do not link in the C run-time initialization object file. That's it, that's the whole vibe. 🚫
• -mrelax — tells the linker "go off, do a relaxation optimization pass" to shorten branches, calls and absolute memory addresses. Only does anything when you use it on the command line for the final link step, so don't expect magic earlier in the pipeline.

⚠️ Real talk: this option makes symbolic debugging impossible. Like, fully impossible. Choose wisely. 💀

• -mliw — lets the compiler generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. It defines the preprocessor macro __LIW__. ✨
• -mno-liw — nope, no Long Instruction Word instructions. This one defines the preprocessor macro __NO_LIW__.
• -msetlb — allows the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. Also the default. Defines the preprocessor macro __SETLB__. 🎀
• -mno-setlb — don't generate SETLB or Lcc instructions. Defines the preprocessor macro __NO_SETLB__.

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🌈 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. (Yes, it's here again. Same energy.) 🙈

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🦋 MSP430 Options 🦋

These ones are defined for the MSP430, bestie:

• -masm-hex — forces assembly output to always use hex constants. Normally those constants would be signed decimals, but this option exists for testsuite and/or aesthetic purposes. 💅
• -mmcu= — pick 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 __. That symbol is then used by the msp430.h header file to pick 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 one gets selected — otherwise the 430X ISA is selected. There's also a generic MCU name of msp430 you can use to select the 430 ISA, and 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 makes the linker go search for a script called xxx.ld. 🗂️

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond the hard-coded stuff.

GCC searches for the devices.csv file using these methods in the given precedence order — where the first method takes precedence over the second, which takes precedence over the third:

• Include path specified with -I and -L — devices.csv will be searched for in each of the directories specified by include paths and linker library search paths.
• Path specified by the environment variable MSP430_GCC_INCLUDE_DIR — set the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory also gets registered as an include path, and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line.
• The msp430-elf{,bare}/include/devices directory — finally, GCC will examine msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if you've created it and copied devices.csv there, then the MCU data will be read. As above, this directory will also be registered as an include path, and linker library path.

If none of the above search methods find devices.csv, then the hard-coded MCU data is used.

• -mwarn-mcu
• -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. 🔔
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. Heads up: this option is deprecated. The -mmcu= option should be used to select the ISA instead.
• -msim — link to the simulator runtime libraries and linker script. This overrides any scripts that would have 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 gets passed to the assembler and linker, and allows the linker to perform certain optimizations that literally cannot be done until the final link.
• mhwmult= — describes the type of hardware multiply supported by the target. (And yes, it's written without the leading dash in the original, we don't make the rules. 🫠) Accepted values:
  • none — no hardware multiply
  • 16bit — the original 16-bit-only multiply supported by early MCUs
  • 32bit — the 16/32-bit multiply supported by later MCUs
  • f5series — the 16/32-bit multiply supported by F5-series MCUs
  • auto — tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified, or if the MCU name 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. This saves space in the generated code. When compiling at -O3 or higher, however, the hardware multiplier is invoked inline. This makes for bigger, but faster code. ⚡

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 🛡️

• -minrt — enables the use of a minimum runtime environment — no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
• -mtiny-printf — enables reduced code size printf and puts library functions. The tiny implementations of these functions are not reentrant, so they must be used with caution in multi-threaded applications.

Support for streams has been removed and the string to be printed will always be sent to stdout via the write syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• -mcode-region=
• -mdata-region= — these options tell the compiler where to place functions and data that do not have one of the lower, upper, either or section attributes. Possible values are lower, upper, either or any. The first three behave like the corresponding attribute. The fourth possible value — any — is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text, .data, etc) to the memory regions. 📦
• -msilicon-errata= — passes on a request to the assembler to enable the fixes for the named silicon errata.
• -msilicon-errata-warn= — passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.
• -mwarn-devices-csv
• -mno-warn-devices-csv — warn if devices.csv is not found or there are problems parsing it (default: on).

---

🌸 NDS32 Options 🌸

These options are defined for NDS32 implementations:

• -mbig-endian — generate code in big-endian mode.
• -mlittle-endian — generate code in little-endian mode.
• -mreduced-regs — use reduced-set registers for register allocation.
• -mfull-regs — use full-set registers for register allocation.
• -mcmov — generate conditional move instructions.
• -mno-cmov — do not generate conditional move instructions.
• -mext-perf — generate performance extension instructions.
• -mno-ext-perf — do not generate performance extension instructions.
• -mext-perf2 — generate performance extension 2 instructions.
• -mno-ext-perf2 — do not generate performance extension 2 instructions.
• -mext-string — generate string extension instructions.
• -mno-ext-string — do not generate string extension instructions.
• -mv3push — generate v3 push25/pop25 instructions.
• -mno-v3push — do not generate v3 push25/pop25 instructions.
• -m16-bit — generate 16-bit instructions.
• -mno-16-bit — do not generate 16-bit instructions.
• *-misr-vector-size=num* — specify the size of each interrupt vector, which must be 4 or 16.
• *-mcache-block-size=num* — specify the size of each cache block, which must be a power of 2 between 4 and 512.
• *-march=arch* — specify the name of the target architecture.
• *-mcmodel=code-model* — set the code model to one of:
  • small — all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.
  • large — all the text and data segments can be within 4GB addressing space.
• -mctor-dtor — enable constructor/destructor feature.
• -mrelax — guide linker to relax instructions.

---

🧚 Nios II Options 🧚

okay so these are the options that live for the Altera Nios II processor, and bestie, there are so many of them, but we're gonna get through it together 💖

• -G num — puts 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 for num is 8. 🎀
• -mgpopt=option, -mgpopt, -mno-gpopt — generate (or don't generate, if you're in your no era) GP-relative accesses. Here's the tea on the option names it recognizes:
  • none — do not generate GP-relative accesses. periodt.
  • local — generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. 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 also generates GP-relative accesses for small data objects that are external, weak, or common. > If you use this option, you must make sure all parts of your program (libraries included, no exceptions) are compiled with the same -G setting.
  • data — generates GP-relative accesses for all data objects in the program. > If you use this, 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. no cap.
  • all — generates GP-relative addresses for function pointers as well as data pointers. > If you use this one, the entire text, data, and BSS segments of your program must fit in 64K of memory, and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. it's giving high stakes 🫶

-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

The default is -mgpopt — except when -fpic or -fPIC is specified to generate position-independent code. Also note that the Nios II ABI does not permit GP-relative accesses from shared libraries, so don't even try it 😭

You may need to specify -mno-gpopt explicitly when you're 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. rude, honestly 💀

• -mgprel-sec=regexp — this one specifies additional section names that can be accessed via GP-relative addressing. It's most useful when paired 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 sections you specify are in addition to the standard .sdata and .sbss small-data sections that -mgpopt already recognizes.

• -mr0rel-sec=regexp — 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. Most useful alongside "section" attributes on variable declarations and a custom linker script. Again, the regexp is a POSIX Extended Regular Expression. 🦋

In contrast to using 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. so it's a whole manual vibe, iykyk.

---

• -mel, -meb — generate little-endian (that's the default, bestie) or big-endian (experimental 👀) code, respectively.
• -march=arch — names the target Nios II architecture. GCC uses this name to figure out 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 cache by using I/O variants of the instructions. The default is not to bypass the cache. 🙈
• -mno-cache-volatile, -mcache-volatile — volatile memory accesses bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache. (yes, same default, different flag, we love consistency 💅)
• -mno-fast-sw-div, -mfast-sw-div — do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3 and above. ⭐
• -mno-hw-mul, -mhw-mul, -mno-hw-mulx, -mhw-mulx, -mno-hw-div, -mhw-div — enable or disable emitting the "mul", "mulx" and "div" family of instructions by the compiler. The default is to emit "mul" and not emit "div" and "mulx". 💜
• -mbmx, -mno-bmx, -mcdx, -mno-cdx — enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. > Enabling these also requires -march=r2. Since these 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. iconic tbh 🔥

Here's every value of insn that's supported. Except where noted otherwise, 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 the only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. 🧚

• fwrx — write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — write src1 into Y.
• frdxhi, frdxlo — read the most or least (respectively) significant half of X and store it in dest.
• frdy — read the value of Y and store it into dest.

Note that you can gain more local control over generation of Nios II custom instructions by using the "target("custom-insn=N")" and "target("no-custom-insn")" function attributes or pragmas. 🎀

• -mcustom-fpu-cfg=name — enables a predefined, named set of custom instruction encodings (peep -mcustom-insn above for the deets). Currently the following sets are defined:
  • -mcustom-fpu-cfg=60-1 is equivalent to:

-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

• -mcustom-fpu-cfg=60-2 is equivalent to:

-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant

• -mcustom-fpu-cfg=72-3 is equivalent to:

-mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. the more specific one wins, that's the rule 💅

Note that you can gain more local control over selection of a FPU configuration by using the "target("custom-fpu-cfg=name")" function attribute or pragma. ✨

---

These additional -m options are available for the Altera Nios II ELF (bare-metal) target: 🌈

• -mhal — link with HAL BSP 💫. This one suppresses linking with the GCC-provided C runtime startup and termination code, and you'd typically use it alongside -msys-crt0= to point at the alternate startup code that the HAL BSP provides.
• -msmallc — link against a limited version of the C library, -lsmallc, instead of Newlib. small library, big vibes.
• -msys-crt0=startfile — here startfile is the file name of the startfile (crt0) to use when linking. Heads up bestie: this option is only useful in conjunction with -mhal.
• -msys-lib=systemlib — systemlib is the library name of the library that hands over the low-level system calls the C library needs, like "read" and "write". This 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 the given specified PTX ISA (like sm_35). ISA strings must be lower-case, no exceptions. 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 execution instead of offloading execution.
• -moptimize — apply partitioned execution optimizations. This is the default whenever 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, it 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 so you can test the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
• -muniform-simt — switch to a code generation variant that lets you execute all threads in each warp, while keeping memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. 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. iconic behavior honestly ✨
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant.

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🌸 OpenRISC Options 🌸

These options are defined for OpenRISC:

• -mboard=name — configure a board specific runtime. This gets passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — this option is ignored; it exists for compatibility purposes only. It used to select linker and preprocessor options for use with newlib. rip to its former job 💀
• -msoft-div, -mhard-div — select software or hardware divide ("l.div", "l.divu") instructions. The default is hardware divide.
• -msoft-mul, -mhard-mul — select software or hardware multiply ("l.mul", "l.muli") instructions. The default is hardware multiply.
• -msoft-float, -mhard-float — select software or hardware for floating point operations. The default is software.
• -mdouble-float — when -mhard-float is selected, this enables generation of double-precision floating point instructions. By default functions from libgcc are used to perform double-precision floating point operations.
• -munordered-float — when -mhard-float is selected, this enables generation of unordered floating point compare and set flag ("lf.sfun*") instructions. By default functions from libgcc are used to perform unordered floating point compare and set flag operations.
• -mcmov — enable generation of conditional move ("l.cmov") instructions. By default the equivalent gets generated using set and branch.
• -mror — enable generation of rotate right ("l.ror") instructions. By default functions from libgcc are used to perform rotate right operations.
• -mrori — enable generation of rotate right with immediate ("l.rori") instructions. By default functions from libgcc are used to perform rotate right with immediate operations.
• -msext — enable generation of sign extension ("l.ext*") instructions. By default memory loads are used to perform sign extension.
• -msfimm — enable generation of compare and set flag with immediate ("l.sf*i") instructions. By default extra instructions get generated to store the immediate to a register first.
• -mshftimm — enable generation of shift with immediate ("l.srai", "l.srli", "l.slli") instructions. By default extra instructions get generated to store the immediate to a register first.

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💜 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, using 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's 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

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🍓 PRU Options 🍓

These command-line options are defined for PRU target:

• -minrt — link with a minimum runtime environment, with no support for static initializers and constructors. Using this option can significantly reduce the size of the final ELF binary. ⚠️ Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features.
• -mmcu=mcu — specify the PRU MCU variant to use. Check Newlib for the exact list of supported MCUs.
• -mno-relax — make GCC pass the --no-relax command-line option to the linker instead of the --relax option.
• -mloop — allow (or do not allow) GCC to use the LOOP instruction.
• -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences:

Function Pointer Size

TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.

Optional Return Value Pointer

Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.

---

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti.

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

---

🌈 RISC-V Options 🌈

These command-line options are defined for RISC-V targets:

• -mbranch-cost=n — sets the cost of branches to roughly n instructions. Like, tell the compiler branches cost about this many instructions, bestie. 💅
• -mplt, -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt. So if you're not doing PIC, it's literally not the vibe. ✨
• -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 or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions. 💚
• -march=ISA-string — generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf. 🌈
• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture name. Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, and size.

When -mtune= is not specified, the default is rocket.

The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but 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 is not 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. No cap, bestie, this is a whole-toolchain commitment. 💀

• -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 are optimizing for supports fast unaligned access or not. 🌿
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model. 👑
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked. ⭐
• -mexplicit-relocs, -mno-exlicit-relocs — use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. (Yes, -mno-exlicit-relocs is spelled exactly like that in the original, iykyk.) 🙈
• -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. 🎀
• -malign-data=type — control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen which uses x register width as the alignment value, and natural which uses natural alignment. xlen is the default. 💅

---

✨ 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, -mhard-dfp, -mno-hard-dfp — you use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above.

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. 💅✨

---

okay so these are the PowerPC/POWER4+ instruction-vibe toggles, and they are so specific to which processor generation you're talking to, bestie.

The -mmfcrf option lets GCC generate the move from condition register field instruction — that one lives on the POWER4 processor and any other processor supporting the PowerPC V2.01 architecture. The -mpopcntb option lets GCC generate the popcount and double-precision FP reciprocal estimate instruction, which is a POWER5 thing and belongs to processors supporting the PowerPC V2.02 architecture. The -mpopcntd option lets GCC generate the popcount instruction implemented on the POWER7 processor and other processors supporting the PowerPC V2.06 architecture. The -mfprnd option lets GCC generate the FP round to integer instructions implemented on the POWER5+ processor and other processors supporting the PowerPC V2.03 architecture. The -mcmpb option lets GCC generate the compare bytes instruction implemented on the POWER6 processor and other processors supporting the PowerPC V2.05 architecture. The -mhard-dfp option lets GCC generate the decimal floating-point instructions implemented on some POWER processors. 🎀

The -mpowerpc64 option lets GCC generate the additional 64-bit instructions found in the full PowerPC64 architecture, and treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

---

✨ OPTIONS ✨

• -mcpu=cpu_type — set the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. The 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, power10, 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 doing 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 -mprefixed -mpcrel -mmma

The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — it doesn't necessarily reflect the actual hardware's capabilities. If you want to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec. 💅

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment.

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type or register usage, the way -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. 🦋
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux.
• -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space. 🌈
• -maltivec, -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 matches array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. ✨

• -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.
• -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.
• -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) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux.

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 — bestie, this one flips on the 64-bit AIX ABI and calling convention, meaning 64-bit pointers, a 64-bit long type, and literally all the infrastructure needed to make that work. Saying -maix64 implies -mpowerpc64, and -maix32 disables that 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32, so it's giving 32 by default unless you say otherwise. 💅
• -mxl-compat, -mno-xl-compat — this produces code that conforms more closely to IBM XL compiler semantics when you're using the AIX-compatible ABI. What it does, specifically:
  • Passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, in addition to argument FPRs.
  • Does not assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double.
  • Uses XL symbol names for long double support routines.

Okay so the lore: 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. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — it's only necessary when you're calling subroutines compiled by IBM XL compilers without optimization. 🙈

• -mpe — support for the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so the app can actually 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 point at the right directory location. Big heads up: the Parallel Environment does not support threads, so -mpe and -pthread are incompatible. pick one, bestie. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types (like floating-point doubles) onto their natural size-based boundary. Meanwhile -malign-power tells 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 there. 💅
• -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 you gotta pass the option to GCC when linking too. 🫶
• -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These get generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. ✨
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. But bestie, if you use -mno-update, there is a small window between the time 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. not the vibe. 😭
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. 🔥
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used. Plot twist: 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. glow-up tbh. ✨
• -mmulhw, -mno-mulhw — generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. 👑
• -mdlmzb, -mno-dlmzb — generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. 🦋
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

Example time, bestie: 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. tiny. iconic. 🎀

• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (do) assume that unaligned memory references are handled by the system.
• -mrelocatable, -mno-relocatable — generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section — which is 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. 💅
• -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. The slay part: objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. mix and match, bestie. 🫶
• -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems, do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle. 🌸
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig. 🌈
• -mdynamic-no-pic — on Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. iykyk. 😌
• -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 the runtime, not on us. 💅
• *-mprioritize-restricted-insns=priority* — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the 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. two for one. 💅
• -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). ✨

okay bestie we are DEEP in PowerPC territory now and this part is giving ancient ABI scrolls 💀 but we're gonna get through it together, one flag at a time. grab a snack 🍓

---

• -mabi=abi-type — this one extends your current ABI with a specific extension, or yeets an extension back out. the valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and, for AIX specifically: vec-extabi, vec-default.
• -mabi=ibmlongdouble — switches the current ABI so long double uses IBM extended-precision. heads up bestie: this is not likely to work if your system already defaults to IEEE extended-precision long double. and if you change the long double type away from IEEE extended-precision, the compiler will warn you unless you pass -Wno-psabi. also it requires -mlong-double-128 to be enabled. 💅
• -mabi=ieeelongdouble — same energy but flipped: switches the current ABI to use IEEE extended-precision long double. also not likely to work if your system defaults to IBM extended-precision long double. and if you change long double away from IBM extended-precision, the compiler warns unless you use -Wno-psabi. also requires -mlong-double-128 to be enabled.
• -mabi=elfv1 — switches the current ABI to 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. iconic honestly 🔥
• -mabi=elfv2 — switches the current ABI to the ELFv2 ABI. this is the default ABI for little-endian PowerPC 64-bit Linux. and yeah, overriding the default ABI also needs special system support and is likely to fail in spectacular ways. same chaos, different endianness 🦋
• -mgnu-attribute, -mno-gnu-attribute — emit (or don't emit) .gnu_attribute assembly directives that set tag/value pairs in a .gnu.attributes section. those pairs specify ABI variations in function parameters or return values. ✨
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, this assumes all calls to variable argument functions are properly prototyped. if they're not, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) — that bit tells the world 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 that bit. so like, less bookkeeping 💅
• -msim — on embedded PowerPC systems, assume the startup module is called sim-crt0.o and the standard C libraries are libsim.a and libc.a. this is the default for powerpc-*-eabisim configurations.
• -mmvme — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
• -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
• -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. (yes it's really called yellowknife 🍓)
• -mvxworks — on System V.4 and embedded PowerPC systems, this says you're compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, sets the PPC_EMB bit in the ELF flags header to say that eabi extended relocations are being used.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, these do (or don't) adhere to the Embedded Applications Binary Interface (EABI), which is a whole set of modifications to the System V.4 specs.

picking -meabi means the stack is aligned to an 8-byte boundary, a function __eabi gets 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 is aligned to a 16-byte boundary, no EABI initialization function gets 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, which is pointed to by register r2. small initialized non-const global and static data goes in .sdata, pointed to by register r13. small uninitialized global and static data goes in .sbss, which sits right next to .sdata.

warning 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 again: -msdata=sysv is incompatible with -mrelocatable. 🙈

• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code exactly like -msdata=eabi; otherwise compile code exactly like -msdata=sysv. ✨
• -msdata=data — on System V.4 and embedded PowerPC systems: small global data goes in the .sdata section, small uninitialized global data goes in .sbss, but do not use register r13 to address small data. this is the default behavior unless other -msdata options are used.
• -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. clean, simple, no small-data drama 💅
• -mreadonly-in-sdata — puts read-only objects in the .sdata section too. this is the default.
• -mblock-move-inline-limit=num — inline all block moves (think 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, then all inline expansion of block compare — both non-loop and loop — 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 isn't constant, at most num bytes will be compared before memcmp gets called to compare the rest 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 by the end of that inline compare, a call to strcmp or strncmp handles the rest of the comparison. the default is 64 bytes.
• -G num — on embedded PowerPC systems, this puts global and static items that are 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 also gets passed to the linker.

bestie listen: all modules should be compiled with the same -G num value. no cap. 🔥

• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, these do (or don't) 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 actually needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. a short call gets generated if the compiler knows the call can't be that far away.

you can override this setting with the shortcall function attribute, or with #pragma longcall(0).

some linkers can detect out-of-range calls and generate glue code on the fly. on those systems, long calls are unnecessary and just generate slower code. as of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. there's a plan to add this feature to the GNU linker for 32-bit PowerPC systems too. 👑

on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (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 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 to emit the glue for every direct call, and the Darwin linker decides whether to use it or discard it.

in the future, GCC may ignore all longcall specifications when the linker is known to generate glue. 💜

• -mpltseq, -mno-pltseq — implement (or don't implement) -fno-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 they're enabled by default if that support is detected when configuring GCC — and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt.

heads up: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. 😭

• -mtls-markers, -mno-tls-markers — mark (or don't mark) calls to __tls_get_addr with a relocation specifying the function argument. that relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. slay 🌈
• -mrecip, -mno-recip — okay so this one turns on the reciprocal estimate + reciprocal square root estimate instructions, paired with extra Newton-Raphson steps to crank the precision, instead of actually doing a divide (or a sqrt-then-divide) for floating-point args. 💅 Bestie, if you're reaching for -mrecip, you should also have -ffast-math on — or at minimum -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math. And yes, the tea is that even though the throughput of this sequence is generally higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp (like, the inverse of 1.0 equals 0.99999994) for reciprocal square roots. 😭 Not a vibe for precision girlies.
• -mrecip=opt — this is the one where you get to pick which reciprocal estimate instructions are allowed in. opt is a comma-separated list of options, and any of them can be prefixed with a ! to invert it (so !rsqrtd means "not rsqrtd"). 🙈
  • all — enable all estimate instructions. go big or go home. ✨
  • default — enable the default instructions, which is literally equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip. the ✨nothing✨ era.
  • 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. slay, exclusion list era. 🎀

• -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions give higher-precision estimates than the PowerPC ABI says they have to. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. 🫶 Heads up though: the double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't provide an estimate that converges after three steps.
• -mveclibabi=type — tells GCC which ABI type to use for vectorizing intrinsics with an external library. type — rn the only supported value is mass, which says "use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries." 💖

When 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, and tanhf4.

ok but listen 😭 both -ftree-vectorize and -funsafe-math-optimizations must also be enabled, and the MASS libraries must be specified at link time. no skipping steps, bestie.

• -mfriz, -mno-friz — generate (or don't generate) the friz instruction when -funsafe-math-optimizations is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. ⚠️ But friz does not return the same value if the floating-point number is too large to fit in an integer. so like, be aware.
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, 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. 💜

fr fr: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. that's a hard no.

• -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. The -mno-save-toc-indirect option is the default.
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. ✨

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that is compatible with functions compiled with older versions of GCC. lowkey a time capsule situation. 🕰️

The -mno-compat-align-parm option 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. The supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (that's the default with GNU libc version 2.4 or later). 🐤

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. 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. 💅

• -mpcrel, -mno-pcrel — generate (or don't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. no shortcuts bestie 🫶
• -mprefixed, -mno-prefixed — generate (or don't generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled. ⭐
• -mmma, -mno-mma — generate (or don't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled.

---

🌸 RX Options 🌸

These command-line options are defined for RX targets, bestie — a whole different flavor of slay: 🦋

• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. Note that RX floating-point hardware only works on 32-bit values, which is literally why the default is -m32bit-doubles. 💀
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series. ✨

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

⚠️ Note: if the -fpu option is enabled, then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. not me enabling danger mode by accident 😭

• -mcpu=name — selects the type of RX CPU to be targeted. name — 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 does not have a hardware floating-point unit, and so -nofpu is enabled by default when this type is selected. 💅

• -mbig-endian-data, -mlittle-endian-data — store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format. 🌈
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited and it is up to the programmer to make sure the area does not overflow. 😭 Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for use pointing to this area, so it's no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.

Note that common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable. 🎀

The default value is zero, which disables this feature. Note that this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It's up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. 🔍

• -msim, -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime. 🧚
• -mas100-syntax, -mno-as100-syntax — when generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default. 🦋
• -mmax-constant-size=N — sets the max size (in bytes) of a constant that can be an operand in an RX instruction ✨. Okay so the RX instruction set does let you use constants up to 4 bytes long in instructions, but a longer value = a longer instruction, so sometimes it's lowkey better to cap how big the constants can get 🎀. Any constant that's too big gets yeeted into a constant pool and referenced via register indirection instead. N can be between 0 and 4, and a value of 0 (the default) or 4 means all sizes are fair game 💖.
• -mrelax — turns on linker relaxation 🦋. That's the process where the linker tries to shrink your program by finding shorter versions of various instructions. Disabled by default, bestie 💅.
• -mint-register=N — picks how many registers get reserved for fast interrupt handler functions 👑. N can be between 0 and 4. A value of 1 means register "r13" is reserved exclusively for 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 nothing 🌸.
• -msave-acc-in-interrupts — says interrupt handler functions should preserve the accumulator register 💜. This is only needed if normal code might actually use the accumulator register, like if it does 64-bit multiplications. The default is to just ignore the accumulator, because that makes the interrupt handlers faster 🫶.
• -mpid
• -mno-pid — enables generation of position independent data ✨. When it's on, any access to constant data happens via an offset from a base address held in a register, which means the location of constant data can be figured out at run time without needing to relocate the executable — big win for embedded apps with tight memory constraints 💖. Data that can be modified is not affected by this option, ngl.

Heads up: using this feature reserves a register, usually "r13", for the constant data base address 😭. That can make code slower and/or larger, 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 get allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. So yes, it's possible for the small data area register to be "r8" if both -mint-register=4 and -mpid are specified on the command line 🦋.

By default this feature is not enabled, and the default can be restored via the -mno-pid command-line option 💅.

• -mno-warn-multiple-fast-interrupts
• -mwarn-multiple-fast-interrupts — this one stops 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 🌸.
• -mallow-string-insns
• -mno-allow-string-insns — enables or disables use of the string manipulation instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL" "SWHILE" and also the "RMPA" instruction 🎀. These instructions may prefetch data, which is not safe to do if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info 💖.)

The default is to allow these instructions, but GCC literally cannot reliably detect all the circumstances where a string instruction might be used to access an I/O register, so their use can't be disabled automatically 😭. Instead it relies on the programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

When the instructions are enabled, GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol "__RX_DISALLOW_STRING_INSNS__" ✨.

• -mjsr
• -mno-jsr — use only (or not only) "JSR" instructions to access functions 👑. You'd reach for this option when code size exceeds the range of "BSR" instructions. Note that -mno-jsr does not mean "don't use "JSR"" — it means 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 💅. That attribute flags a function intended to process fast interrupts. GCC makes sure it only uses the registers "r10", "r11", "r12" and/or "r13", and only provided that the normal use of the corresponding registers has been restricted via the -ffixed-reg or -mint-register command-line options 🌸.

---

✨ S/390 and zSeries Options ✨

These are the -m options defined for the S/390 and zSeries architecture 💖.

• -mhard-float
• -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations 🎀. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. That one's 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 a backchain pointer into the callee's stack frame 🌸. A backchain may be needed so you can debug using tools that don't 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 all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space, which is kind of a waste ngl. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area, and unused space gets 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 lets GCC, in particular, generate 64-bit instructions ✨. For the s390 targets, the default is -m31, while the s390x targets default to -m64 👑.
• -mzarch
• -mesa — 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 — bestie, -mzvector turns on the vector language extensions and the builtins that use the instructions from the vector extension facility that showed up with the IBM z13 machine generation. This one adds support for vector as a keyword so you can define vector-type variables and arguments. Heads up tho: vector is only available when GNU extensions are enabled — it will not be expanded when you're asking for strict standard compliance, like with -std=c99. And on top of the GCC low-level builtins, -mzvector also enables a whole set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To actually use those builtins you need to include the header file vecintrin.h. Oh and -mzvector is disabled by default. 💅
• -mmvcle, -mno-mvcle — generate (or don't generate) code using the "mvcle" instruction to do block moves. When you pass -mno-mvcle, it uses a "mvc" loop instead. This is the default unless you're optimizing for size. ✨
• -mdebug, -mno-debug — print (or don't print) additional debug information when compiling. The default is to not print debug info. 🐛
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. The possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, and native.

The default is -march=z900.

Specifying native as the cpu type can be used to select the best architecture option for the host processor. Lowkey important: -march=native has no effect if GCC does not recognize the processor. 😭

• *-mtune=cpu-type* — tune everything applicable about the generated code to cpu-type, 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) TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS. fr fr.
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (does not change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines that give you the ability to selectively skip function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace. 🙈
• -mfused-madd, -mno-fused-madd — generate code that uses (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's intended to identify functions that most probably cause a stack overflow. Super useful in an environment with limited stack size, e.g. the linux kernel. 👑
• -mwarn-dynamicstack — emit a warning if the function calls "alloca" or uses dynamically-sized arrays. Ngl this is generally a bad idea with a limited stack size. 💀
• -mstack-guard=stack-guard, *-mstack-size=stack-size* — if these options are provided, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows *downward). If the *stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are meant to help you debug stack overflow problems. The additionally emitted code causes only little overhead, so it can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. And the stack-guard option can only be used in conjunction with stack-size. 🧚
• *-mhotpatch=pre-halfwords*,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions 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. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign. 🫶
• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
• -mbitops — enable the use of bit manipulation instructions on SH2A.
• -mfmovd — enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
• -mrenesas — comply with the calling conventions defined by Renesas.
• -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
• -mnomacsave — mark the "MAC" register as call-clobbered, even if -mrenesas is given.
• -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee. ✨
• -minline-ic_invalidate — ok so this one inlines the code that invalidates instruction cache entries after nested function trampolines get set up 🧚. It does nothing if -musermode is in effect and the code generation option you picked (like -m4) doesn't allow the icbi instruction. And if your selected code generation option doesn't allow icbi and -musermode is not in effect? Then the inlined code goes and manipulates the instruction cache address array directly with an associative write 😳. That's not just needs privileged mode at run time — it also fails if the cache line had been mapped via the TLB and has become unmapped. Messy. Be aware.
• -misize — dumps instruction size and location right into the assembly code. cute little breadcrumb trail ✨
• -mpadstruct — 🚨 this option is deprecated. It pads structures to a multiple of 4 bytes, which is incompatible with the SH ABI. so like, don't.
• -matomic-model=model — sets the model of atomic operations plus any additional parameters, all as a comma separated list 🍓. For the deets on the atomic built-in functions, go peek at __atomic Builtins. Here's what's on the menu:
  • none — disables compiler generated atomic sequences and just emits library calls for atomic operations instead. This is the default if the target is not sh*-*-linux*.
  • soft-gusa — generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions 💅. Heads up: the generated atomic sequences need extra support from the system's interrupt/exception handling code, and they're only suitable for SH3* and SH4* single-core systems. This one's enabled by default when the target is sh*-*-linux* and SH3* or SH4*. And when the target is SH4A, it also partially utilizes the hardware atomic instructions movli.l and movco.l to cook up more efficient code — unless strict is specified.
  • soft-tcb — generates software atomic sequences that use a variable in the thread control block 🧵. It's a variation of the gUSA sequences that can also be used on SH1* and SH2* targets. Same deal though: the generated atomic sequences need extra support from the system's interrupt/exception handling code, and they're only suitable for single-core systems. When you use this model, you have to also specify the gbr-offset= parameter.
  • soft-imask — generates software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111 🙈. This model works only when the program runs in privileged mode, and it's only suitable for single-core systems. Good news: no extra support from the system's interrupt/exception handling code needed. It's enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
  • hard-llcs — generates hardware atomic sequences using only the movli.l and movco.l instructions 🔥. Only available on SH4A, and suitable for multi-core systems. Since the hardware instructions only support 32 bit atomic variable access, access to 8 or 16 bit variables gets emulated with 32 bit accesses. Code compiled with this is also compatible with other software atomic model interrupt/exception handling systems if it's executed on an SH4A system. And for this model, no additional support from the system's interrupt/exception handling code is required 👑.
  • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that the generated atomic sequences should use when the soft-tcb model has been selected. For other models, this parameter is just ignored, whatever 💀. The value you give must be an integer multiple of four and in the range 0-1020.
  • strict — this parameter prevents mixed usage of multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model only. no blending, periodt.
• -mtas — generates the tas.b opcode for __atomic_test_and_set. But ⚠️ notice: depending on your particular hardware and software configuration, this can degrade overall performance, because of the operand cache line flushes that the tas.b instruction implies. And on multi-core SH4A processors, tas.b must be used with caution since it can result in data corruption for certain cache configurations 😭. be careful bestie.
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table 🌈.
• -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 would not work in user mode. -musermode is the default when the target is sh*-*-linux*. And if the target is SH1* or SH2*, -musermode has no effect, since there is no user mode 💅.
• -multcost=number — sets the cost to assume for a multiply insn.
• -mdiv=strategy — sets the division strategy used for integer division operations ✨. strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. 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 larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. If you specify this for targets that don't have dynamic shift instructions, it defaults to call-div1.

When a division strategy hasn't been specified, the default strategy is selected based on the current target 🎀. For SH2A, the default strategy is to use the divs and divu instructions instead of library function calls.

• -maccumulate-outgoing-args — reserves space once for outgoing arguments in the function prologue rather than around each call 👑. Generally beneficial for performance and size. Also needed for unwinding, to avoid changing the stack frame around conditional code.
• -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 were not present.
• -mfixed-range=register-range — generates code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use 🚫. Super useful when compiling kernel code. A register range is specified as two registers separated by a dash. You can specify multiple register ranges separated by a comma.
• -mbranch-cost=num — assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible 🌸. If you don't specify it, the value is selected depending on the processor type being compiled for.
• -mzdcbranch, -mno-zdcbranch — assume (do not assume) that zero displacement conditional branch instructions bt and bf are fast ⚡. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction can't be found 🫶. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055.
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions ✨. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
• -mfsca, -mno-fsca — allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations 🌀. The -mfsca option must be used in combination with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra, -mno-fsrra — allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations 💜. The -mfsrra option must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It's enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
• -mpretend-cmove — prefers zero-displacement conditional branches for conditional move instruction patterns 👀. This can result in faster code on the SH4 processor.
• -mfdpic — generates code using the FDPIC ABI 🌈.

---

🎀 Solaris 2 Options 🎀

These -m options are supported on Solaris 2, bestie:

• -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler 🙈. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
• -mimpure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object 🫶. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. But bestie, plot twist: the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. 😭 So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. periodt.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is a synonym for -pthread. same vibe, different name, iykyk.

---

✨ SPARC Options ✨

These -m options are supported on the SPARC:

• -mno-app-regs
• -mapp-regs — say -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default. 🫶

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.

• -mflat
• -mno-flat — with -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.

• -mfpu
• -mhard-float — generate output containing floating-point instructions. This is the default.
• -mno-fpu
• -msoft-float — generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-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. no half measures bestie 💅

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

• -mno-unaligned-doubles
• -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code.

• -muser-mode
• -mno-user-mode — do not generate code that can only run in supervisor mode. This is relevant only for the "casa" instruction emitted for the LEON3 processor. This is the default.
• -mfaster-structs
• -mno-faster-structs — with -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. 🚨 breaking the rules on purpose, iconic but risky.
• -mstd-struct-return
• -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

• -mlra
• -mno-lra — enable Local Register Allocation. This is the default for SPARC since GCC 7 so -mno-lra needs to be passed to get old Reload.
• *-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations. 🌈

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• 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. 💖

• *-mtune=cpu_type* — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type does.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used.

• -mv8plus
• -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
• -mvis
• -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.
• -mvis2
• -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -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 flexes version 4.0 of the UltraSPARC Visual Instruction Set extensions, and honestly? iconic. Default is -mvis4 when you're targeting a cpu that supports those instructions, like niagara-7 and later. Setting -mvis4 also sets -mvis3, -mvis2 and -mvis — it's giving package deal 💅
• -mvis4b, -mno-vis4b — with -mvis4b, GCC uses version 4.0 of the UltraSPARC VIS extensions plus the extra VIS instructions Oracle dropped in the SPARC Architecture 2017. Default is -mvis4b when targeting a cpu that supports them, like m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis. chain reaction, obsessed ✨
• -mcbcond, -mno-cbcond — with -mcbcond, GCC takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. Default is -mcbcond when targeting a CPU that supports them, like Niagara-4 and later. one branch, two jobs, main character energy 🎀
• -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. Default is -mfmaf when targeting a CPU that supports them, like 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. 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. Default is -mpopc when targeting a CPU that supports such an instruction, like Niagara-2 and later.
• -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. Default is -msubxc when targeting a CPU that supports such an instruction, like Niagara-7 and later.
• -mfix-at697f — turns on the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor). we love a fix 🙈
• -mfix-ut699 — turns on the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — turns on the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — turns on the documented workaround for the back-to-back store errata of the GR712RC processor.

---

So these -m options are supported in addition to all the ones above, but only on SPARC-V9 processors in 64-bit environments. glow up era 🌈

• -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 these besties:
  • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
  • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • medany — the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • embmedany — the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported.
• -mmemory-model=mem-model — set the memory model in force on the processor to one of:
  • default — the default memory model for the processor and operating system.
  • rmo — Relaxed Memory Order
  • pso — Partial Store Order
  • tso — Total Store Order
  • sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field. no cap, it's in the manual 📖

• -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

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💖 Options for System V 💖

These extra options are available on System V Release 4 for compatibility with other compilers on those systems, iykyk:

• -G — create a shared object. It is recommended that -symbolic or -shared be used instead. (like, please use the new thing bestie 😭)
• -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.

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🦋 TILE-Gx Options 🦋

These -m options are supported on the TILE-Gx:

• -mcmodel=small — generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range.
• -mcmodel=large — generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses.
• -mcpu=name — selects the type of CPU to be targeted. Currently the only supported type is tilegx.
• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mbig-endian, -mlittle-endian — generate code in big/little endian mode, respectively.

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🌸 TILEPro Options 🌸

These -m options are supported on the TILEPro:

• -mcpu=name — selects the type of CPU to be targeted. Currently the only supported type is tilepro.
• -m32 — generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored. (a lil decorative ngl 💀)

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🎀 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 — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. always on, they said 💅

• -mdisable-callt, -mno-disable-callt — this option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined.

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler.
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions.
• -msoft-float, -mhard-float — turns the generation of hardware floating point instructions off (or back on). Heads up: this one only actually means anything when the target architecture is V850E2V3 or higher. When hardware floating point instructions are being generated, the C preprocessor symbol __FPU_OK__ gets defined; otherwise you get __NO_FPU__ instead. 💅
• -mloop — enables use of the e3v5 LOOP instruction. It's not on by default when you pick the e3v5 architecture, because bestie... that instruction is still experimental. 🧪
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default, periodt. With this ABI version, here's the rules:
  • 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 ABI version 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 ABI version, the rules are:
  • 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 ABI version is enabled, the C preprocessor symbol __V850_GCC_ABI__ is defined. 🌸

• -m8byte-align, -mno-8byte-align — lets double and long long types 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. Only reach for this if the assembler/linker start complaining about out of range branches within a switch table. Like, only then, fr fr.
• -mapp-regs — this option makes r2 and r5 get used in the code the compiler generates. This setting is the default. 👑
• -mno-app-regs — this option makes r2 and r5 be treated as fixed registers.

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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.

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Visium Options 🌈

• -mdebug — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub. 🐛
• -msim — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked.
• -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default. ✨
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

Careful bestie: -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. 😭

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would.
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected. 🙈

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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. 🎀

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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. 💅

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x86 Options 👑

These -m options are defined for the x86 family of computers.

• -march=cpu-type — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=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.
• x86-64-v2, x86-64-v3, x86-64-v4 — these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.

Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.

• 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 with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• nehalem — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.

• sandybridge — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
• ivybridge — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell — Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
• broadwell — Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED ADCX and PREFETCHW instruction set support.
• skylake — Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support.
• bonnell — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• silvermont — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support.
• goldmont — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT and FSGSBASE instruction set support.
• goldmont-plus — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support.
• tremont — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support.
• knl — Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
• knm — Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support.
• skylake-avx512 — Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
• cannonlake — Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support.
• icelake-client — Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support.
• icelake-server — Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support.
• cascadelake — Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.
• cooperlake — Intel cooperlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
• tigerlake — Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT and KEYLOCKER instruction set support.

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and now the AMD side of the family, bestie 💖 they said "team red rights" and honestly? they ate.

• k6 — AMD K6 CPU with MMX instruction set support.
• k6-2, k6-3 — Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
• athlon, athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
• athlon-4, athlon-xp, athlon-mp — Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
• k8, opteron, athlon64, athlon-fx — Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• 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, PCLMUL, 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, PCLMUL, 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, PCLMUL, 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, PCLMUL, 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, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)
• znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)
• 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, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
• winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
• winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
• c3 — VIA C3 CPU, slinging MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip, so don't come for it about the scheduling, okay? It's not there. Periodt.)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (Again: no scheduling is implemented for this chip. Not a single vibe of scheduling.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip. She has range but no scheduler, bestie.)
• samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip. 💀)
• nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip. Yes, still. Every single time.)
• esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip. Not one line of scheduling. Moving on.)
• eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip. The no-scheduling streak is undefeated.)
• eden-x4 — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip. All those extensions and still no scheduling — it's giving "all dressed up, nowhere to schedule.")
• nano — Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip. 🦋)
• nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-3000 — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• nano-x2 — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip. Two cores, zero schedulers, iconic.)
• nano-x4 — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip. Four cores, still no scheduling. The consistency?? Obsessed.)
• geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support. (And this one doesn't have the scheduling disclaimer, so don't get it twisted. Read carefully, bestie. 🫶)

---

• -mtune=cpu-type — okay so this one tunes everything applicable about the generated code to cpu-type, with two big exceptions: it does not touch the ABI, and it does not touch the set of available instructions. So like, picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler still does not generate any code that can't run on the default machine type — unless you also use a -march=cpu-type option. Example incoming, because this is the part people fumble: 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. Cute, right? ✨

The choices for cpu-type are the same as for -march. And on top of that, -mtune supports 2 extra choices for cpu-type:

• generic — produces code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you do not know exactly what CPU users of your application will have, then you should use this option.

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

And no, 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. Different jobs, bestie. 💅

• intel — produces code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But if you want your application performs better on both Haswell and Silvermont, then you should use this option.

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

And again: 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 as generic, same reasoning. 🌈

• -mcpu=cpu-type — a deprecated synonym for -mtune. That's it. That's the whole option. She's retired, let her rest. 🙈
• -mfpmath=unit — generates floating-point arithmetic for the selected unit unit. The choices for unit are:
  • 387 — use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. Lowkey a glow-up, highkey a risk. ⚠️

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. So like... use it with care. They literally said "with care." Listen to them. 😭

• -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. F in the chat for Darwin intel stans. 💀
• -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. That's the whole point of them, bestie.
• -m80387
• -mhard-float — generate output containing 80387 instructions for floating point. Hard float, she's doing the math herself. 💪
• -mno-80387
• -msoft-float — generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used. Sneaky. 🍓

• -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

• -mno-fancy-math-387 — some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is 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, it's a chain. Two conditions or nothing. 🧚
• -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 for memory, the classic trade, no cap.

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. This is the ABI-breaking kind of warning. Take it seriously, bestie. 🫶

• -m96bit-long-double
• -m128bit-long-double — these switches control the size of "long double" type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer, bestie 💅) prefer long double to be aligned to an 8- or 16-byte boundary. But in arrays or structures that conform to the ABI? Yeah, that's just not possible. So when you specify -m128bit-long-double, it aligns long double to a 16-byte boundary by padding the long double with an extra 32-bit zero. Iconic little hack, honestly.

In the x86-64 compiler, -m128bit-long-double is the default choice because its ABI specifies that long double is aligned on a 16-byte boundary.

Notice that neither of these options enables any extra precision over the x87 standard of 80 bits for a long double. It's alignment vibes only, no extra accuracy. 🫶

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. So like… be careful, fr fr. 😭

---

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes long double equivalent to the double type — and that's the default for the 32-bit Bionic C library. A size of 128 bits makes long double equivalent to the __float128 type — and that's the default for the 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. Same warning, same energy. 💀

• -malign-data=type — controls how GCC aligns variables. Supported values for type are compat, which uses an increased alignment value compatible with GCC 4.8 and earlier; abi, which uses the alignment value as specified by the psABI; and cacheline, which uses an 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 get placed in the large data section. This value must be the same across all objects linked into the binary, and it defaults to 65535.
• -mrtd — use a different function-calling convention, where functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. Efficiency slay. ✨

You can specify that an individual function is called with this calling sequence using the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

Warning: this calling convention is incompatible with the one normally used on Unix, so you 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. Not here though. Here it's chaos. 🙈)

• -mregparm=num — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function 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. No skipping, bestie. 😤

• -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. Same deal, same drama. 💅

• -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks, to match the ABI of the Sun Studio compilers up until version 12. Only use this option if you need to stay compatible with existing code produced by those previous compiler versions or older versions of GCC. Legacy support moment. 🧚
• -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. Not the cancellation being cute, that's the actual name. 😭

• -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. Bridging the generational divide, we love to see it. 🌈
• -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 an 8 byte boundary. Since x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with 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. Yeah. All of them. Every single one. 😭

• -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 these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. So calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🫶

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

---

Okay so here comes the big one — the extended instruction set squad. 🎀 Every single one of these is a switch that enables the use of instructions from a specific extended instruction set:

-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
-mavx512bf16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-mavx512vpopcntdq
-mavx512vp2intersect
-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, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or CLDEMOTE extended instruction sets. 💅

Each one has a corresponding -mno- option to disable use of these instructions. So like, -msse turns it on, -mno-sse turns it off. Symmetry, periodt. ✨

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. 👑

okay bestie, we're still in the x86 option garden and there's so much to water 💅

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.

These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.

---

• -mdump-tune-features — this one tells GCC to dump the names of the x86 performance tuning features and default settings. The names it spits out can then be used in -mtune-ctrl=feature-list. iykyk 🎀
• -mtune-ctrl=feature-list — this is your fine grain control knob for x86 code generation features. feature-list is a comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^, otherwise it is turned off. Honestly? -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. not for casual use, bestie 😭
• -mno-default — instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. everything off, no crumbs left ✨
• -mcld — this one tells GCC to emit a "cld" instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld" instructions can be suppressed with the -mno-cld compiler option in this case.
• -mvzeroupper — instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper" intrinsics. the way it just cleans up after itself 🧚
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. small but mighty energy 💅
• -mprefer-vector-width=opt — instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.
  • none — No extra limitations applied to GCC other than defined by the selected platform.
  • 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 — enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. atomic slay ✨
• -msahf — enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the "LAHF" and "SAHF" instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize "fmod", "drem", and "remainder" built-in functions; see Other Builtins for details. history lesson fr fr 📚
• -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 built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the "crc32" machine instruction.
• -mrecip — enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and "RSQRTPS") with an additional Newton-Raphson step to increase precision instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). precision loss alert, bestie 😭

Note that GCC implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the above option combination), and doesn't need -mrecip.

Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

• -mrecip=opt — controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — Enable all estimate instructions.
  • default — Enable the default instructions, equivalent to -mrecip.
  • none — Disable all estimate instructions, equivalent to -mno-recip.
  • div — Enable the approximation for scalar division.
  • vec-div — Enable the approximation for vectorized division.
  • sqrt — Enable the approximation for scalar square root.
  • vec-sqrt — Enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. the ! is doing so much work here 💅

• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.

GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "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 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. that's a whole lot of function names, icon behavior ✨

• -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 — force 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. indirect but make it precise 🎯
• -mmanual-endbr — insert 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 — due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. trade-off queen 💅
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. gnu2 is giving efficient but high-maintenance 🦋
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. faster but chonkier, pick your struggle 😭
• -mthreads — support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. thread-safe and tidy 🧚
• -mms-bitfields, -mno-ms-bitfields — enable/disable bit-field layout compatible with the native Microsoft Windows compiler.

Okay bestie, plot twist incoming: if packed gets used on a structure, or if bit-fields show up in it, it is entirely possible that the Microsoft ABI lays the thing out differently than the way GCC normally does. And like… this matters. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler — whether that's over a function call or as raw data sitting in a file — because then you might have to be able to access either format. 💀

This option is on by default for Microsoft Windows targets. You can also control this behavior locally by using variable or type attributes — for the receipts, go see x86 Variable Attributes and x86 Type Attributes. 🫶

The Microsoft structure layout algorithm is, ngl, fairly simple — except for the bit-field packing, which is where it gets spicy. The padding and alignment of structure members, and whether a bit-field is allowed to 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 has the lowest memory address and the last member has 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 gets allocated an offset such 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 and the zero-length bit-field situation 👀

MSVC interprets zero-length bit-fields in the following ways:

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.

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.

---

✨ -mno-align-stringops ✨

Do not align the destination of inlined string operations. This switch reduces code size and improves performance in the case where the destination is already aligned — but GCC doesn't know about it. 🙈

🎀 -minline-all-stringops 🎀

By default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but it may improve performance of code that depends on fast memcpy and memset for short lengths. This option also enables inline expansion of strlen for all pointer alignments. 🦋

🌸 -minline-stringops-dynamically 🌸

For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks.

⭐ -mstringop-strategy=alg ⭐

Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:

• rep_byte
• rep_4byte
• rep_8byte — expand using i386 rep prefix of the specified size.

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• byte_loop
• loop
• unrolled_loop — expand into an inline loop.

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• libcall — always use a library call.

💖 -mmemcpy-strategy=strategy 💖

Override the internal decision heuristic that decides if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. 🔥

💅 -mmemset-strategy=strategy 💅

The option is similar to -mmemcpy-strategy= except that it's here to control __builtin_memset expansion.

🧚 -momit-leaf-frame-pointer 🧚

Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. 😭

🌈 -mtls-direct-seg-refs / -mno-tls-direct-seg-refs 🌈

Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to 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 the name of the __fentry__ symbol called at function entry for -pg -mfentry functions.

🌸 -mfentry-section=name 🌸

Set the name of the 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 the 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 doesn'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. slay. 💅

🌈 -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 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.

okay so with that second choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset are the ones that say which segment register (%fs or %gs) is the base register we read the canary from, and how far the offset is from that base register. and the default for those two? whatever the relevant ABI says, bestie. not our call. 💅

• -mgeneral-regs-only — makes code that ONLY touches the general-purpose registers. like, no floating-point, no vector, no mask, no bound registers allowed at the party. 🚫
• -mindirect-branch=choice — converts indirect call and jump according to choice. the default is keep, which keeps indirect call and jump completely unmodified (respect the original, periodt). thunk converts indirect call and jump to call and return thunk. thunk-inline converts them to an inlined call and return thunk. thunk-extern converts them to an external call and return thunk that lives in a separate object file. and you can control this per-function using the function attribute "indirect_branch". 🎀

ngl, -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern, because the thunk function might not even be reachable in the large code model. 😭

also -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check. slay. ✨

• -mfunction-return=choice — converts function return according to choice. default is keep (leaves function return unmodified, iconic). thunk converts function return to call and return thunk. thunk-inline converts it to an inlined call and return thunk. thunk-extern converts it to an external call and return thunk provided in a separate object file. per-function control? use the function attribute "function_return". 🦋

-mindirect-return=thunk-extern is compatible with -fcf-protection=branch, because the external thunk can be made to enable control-flow check. ✅

and again: -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 to go via register. that's it, that's the whole vibe. 🫶
• -mharden-sls=choice — generates code to mitigate against straight line speculation (SLS) with choice. default is none, which disables all SLS hardening. return enables SLS hardening for function returns. indirect-jmp enables SLS hardening for indirect jumps. all enables all SLS hardening. pick your fighter. 🔥
• -mindirect-branch-cs-prefix — adds a CS prefix to call and jmp to indirect thunk with the branch target sitting in r8-r15 registers, so that the call and jmp instruction length is 6 bytes, which lets them be replaced at run-time with lfence; call *%r8-r15 or lfence; jmp *%r8-r15. 💫

---

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.

• -m32
• -m64
• -mx32
• -m16
• -miamcu — generate code for a 16-bit, 32-bit or 64-bit environment. let me break it down bestie:
  • -m32 sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
  • -m64 sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. on Darwin, -m64 also turns off the -fno-pic and -mdynamic-no-pic options. special little exception, we love her. 🌸
  • -mx32 sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
  • -m16 is the same as -m32, except it outputs the .code16gcc assembly directive at the beginning of the assembly output so the binary can run in 16-bit mode. cute throwback honestly. 💜
  • -miamcu generates code that conforms to Intel MCU psABI. it requires the -m32 option to be turned on, no exceptions. 👑
• -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's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, which means it can be used for temporary data without adjusting the stack pointer. -mno-red-zone disables this red zone entirely. bye zone 🚫
• -mcmodel=small — 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. the main character. ⭐
• -mcmodel=kernel — 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. non-negotiable fr fr. 🌈
• -mcmodel=medium — 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 get put into large data or BSS sections and can live above 2GB. programs can be statically or dynamically linked. 🍓
• -mcmodel=large — code for the large model. this model makes no assumptions about addresses and sizes of sections. free spirit energy. 🧚
• -maddress-mode=long — code for long address mode. only supported for 64-bit and x32 environments, and it's the default address mode for 64-bit environments. ✨
• -maddress-mode=short — code for short address mode. only supported for 32-bit and x32 environments, and it's the default address mode for 32-bit and x32 environments. 💖

---

✨ x86 Windows Options ✨

These additional options are available for Microsoft Windows targets:

• -mconsole — specifies that a console application is to be generated, by telling the linker to set the PE header subsystem type required for console applications. available for Cygwin and MinGW targets, and it's enabled by default on those targets. 🎀
• -mdll — available for Cygwin and MinGW targets. specifies that a DLL — a dynamic link library — is to be generated, which enables the selection of the required runtime startup object and entry point. ✨
• -mnop-fun-dllimport — available for Cygwin and MinGW targets. specifies that the "dllimport" attribute should be ignored. simple as that. 🫶
• -mthread — available for MinGW targets. specifies that MinGW-specific thread support is to be used. 🧵
• -municode — available for MinGW-w64 targets. it causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 💜
• -mwin32 — available for Cygwin and MinGW targets. specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code. 🤔
• -mwindows — available for Cygwin and MinGW targets. specifies that a GUI application is to be generated by telling the linker to set the PE header subsystem type appropriately. pretty window vibes only. 🌸
• -fno-set-stack-executable — available for MinGW targets. 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, because there the User32 API — which is what's used to set executable privileges — isn't available. 💀
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. specifies that relocated-data in the read-only section is put into the .data section. this is necessary for older runtimes that don't support modification of .rdata sections for pseudo-relocation. 🙈
• -mpe-aligned-commons — available for Cygwin and MinGW targets. specifies that the GNU extension to the PE file format which permits the correct alignment of COMMON variables should be used when generating code. it's 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. that's the whole list, she's short and sweet. 🎀

---

🦋 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. 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 isn't 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 don't round the intermediate result, so they produce results with more bits of precision than the IEEE standard specifies. 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</p> / -mno-text-section-literals 💖

Okay bestie, these two are literally the literal-pool babysitters, and they decide where your literals live. The default is -mno-text-section-literals, and what that does is drop the literals into their own separate section in the output file. Why is that kinda iconic? Because it means the literal pool can be parked in a data RAM/ROM, and the linker can smoosh together literal pools from different object files to yeet the redundant literals and make your code size smaller. Two-for-one glow-up ✨

Flip it with -mtext-section-literals and now the literals get interspersed in the text section, so they stay as close as physically possible to the references that need them. This may be necessary for large assembly files. And ngl the placement is very much on purpose: literals for each function get placed right before that function. 👑

---

-mauto-litpools</p> / -mno-auto-litpools 🦋

Same energy, different mechanic — these also control the treatment of literal pools. Default here is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used.

But with -mauto-litpools, the literals get interspersed in the text section by the assembler itself. And the compiler is doing something very specific here: it does not produce explicit .literal directives, and instead it loads literals into registers with MOVI instructions rather than L32R, which lets the assembler do relaxation and place literals wherever necessary. The whole point? It lets the assembler create several literal pools per function and assemble very big functions — which, heads up, may not be possible with -mtext-section-literals. 💅

---

-mtarget-align</p> / -mno-target-align 🎀

Turn this on and GCC tells the assembler to automatically align instructions, so you reduce branch penalties — at the cost of a little code density. Trade-offs, we love a trade-off. 🙈

Here's the tea: the assembler tries to widen density instructions to align branch targets and the instructions that follow call instructions. But if there aren't enough preceding safe density instructions to align a target, then nope, no widening happens. Sorry bestie, the math said no. 😭

The default is -mtarget-align. And important caveat, don't skip this one: these options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler always aligns — either by widening density instructions or by inserting NOP instructions. Periodt. 👑

---

-mlongcalls</p> / -mno-longcalls 🔥

When this is enabled, GCC tells the assembler to translate direct calls into indirect calls — unless it can figure out that the target of a direct call is within the range allowed by the call instruction. This translation typically happens for calls to functions in other source files. Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. Very much a glow-up in disguise. ✨

The default is -mno-longcalls. You should use this option in programs where the call target can potentially be out of range.

Heads up bestie: this option is implemented in the assembler, not the compiler. So the assembly code GCC generates still shows direct call instructions — you gotta look at the disassembled object code to see the actual instructions. 🔍

And one more thing, no cap: note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. So it's a little extra, but that's just how it is. 💜

---

zSeries Options 🌈

These are listed under

🌈 ENVIRONMENT 🌈

okay bestie, this section is all about the environment variables that change how GCC does its thing. Some of them work by handing GCC directories or prefixes to look in when it's hunting down different kinds of files, and some of them just tell GCC other stuff about the compilation environment. both are important 💖

Quick heads up though: you can also point GCC at places to search using options like -B, -I and -L. And the priority order is very much a thing, so listen up:

• options like -B, -I and -L take precedence over anything you set with environment variables
• environment variables take precedence over the places GCC's own configuration specifies

so it's options first, then env vars, then the built-in config. the way that's literally a whole hierarchy 👑

---

💫 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL 💫

These four control how GCC uses localization info, which is the whole reason GCC can vibe with different national conventions. GCC peeks at the locale categories LC_CTYPE and LC_MESSAGES — if it's been configured to do that, which is a big if, so no cap. Those locale categories can be set to literally any value your installation supports. A classic example value is en_GB.UTF-8, which is English in the United Kingdom encoded in UTF-8. so specific, so correct 🇬🇧

• LC_CTYPE — this one specifies character classification. GCC uses it to figure out where the character boundaries are inside a string. Why does that matter? Because some multibyte encodings contain quote and escape characters that would otherwise get read as the end of a string or as an escape. chaos avoided 😌
• LC_MESSAGES — this one specifies the language to use for diagnostic messages. So your errors can yell at you in your language of choice 💅

Now the override rules, pay attention:

• if LC_ALL is set, it overrides the value of LC_CTYPE and LC_MESSAGES — it said what it said
• otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable
• and if none of these variables are set, GCC defaults to traditional C English behavior

---

🧊 TMPDIR 🧊

If TMPDIR is set, it tells GCC which directory to use for temporary files. And why does GCC need temp files at all? Because it holds the output of one stage of compilation that's about to become the input to the next stage. The example given: the output of the preprocessor, which is the input to the compiler proper. pipeline behavior fr fr 🫶

---

🔍 GCC_COMPARE_DEBUG 🔍

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. Nearly! Not exactly! Go read the docs for that option if you want the full picture, bestie 📖

---

🦋 GCC_EXEC_PREFIX 🦋

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms that the compiler executes. Important detail: no slash gets added when this prefix is combined with the name of a subprogram — but you can specify a prefix that ends with a slash yourself if you want to. your call, your slay ✨

If GCC_EXEC_PREFIX is not set, GCC tries to figure out an appropriate prefix on its own, based on the pathname it was invoked with. resourceful queen

And if GCC can't find the subprogram using the prefix you gave it? 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. iykyk.

Other prefixes specified with -B take precedence over this prefix. priority again, she's everywhere

This prefix also gets used for finding files like crt0.o that are used for linking. and then there's the unusual part:

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. 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.

like okayyy, that's a whole trick 🎩 the way it swaps prefixes around

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📚 COMPILER_PATH 📚

The value of COMPILER_PATH is a colon-separated list of directories, a lot like PATH. GCC tries the directories you listed there when it's searching for subprograms — but only if it couldn't find those subprograms using GCC_EXEC_PREFIX first. second choice but still iconic

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🏛️ LIBRARY_PATH 🏛️

Same energy: the value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When GCC is configured as a native compiler, it tries the directories you list here when searching for special linker files — again, only if it can't find them using GCC_EXEC_PREFIX. backup plan energy

Also, linking using GCC uses these directories when searching for ordinary libraries for the -l option. BUT — directories specified with -L come first. always. periodt. 👑

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🌸 LANG (yes, again, different job) 🌸

This variable is used to pass locale information to the compiler. One way that info gets used is to determine the character set to use when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, these are the recognized values for LANG:

• C-JIS — recognize JIS characters 🇯🇵
• C-SJIS — recognize SJIS characters
• C-EUCJP — recognize EUCJP characters

And if LANG is not defined, or if it's set to some other value entirely, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters. fallback behavior, we love her

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okay so some additional environment variables affect the behavior of the preprocessor. here we go 🎀

🧚 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH 🧚

Each of these variables holds a list of directories separated by a special character, much like PATH, and that's where GCC looks for header files. That special character is "PATH_SEPARATOR", and it's target-dependent — it gets determined at GCC build time. For Microsoft Windows-based targets it's a semicolon, and for almost literally every other target it's a colon. semicolon vs colon, the eternal drama

• CPATH — specifies a list of directories to be searched as if they'd been given with -I, but they get searched after any paths given with -I options on the command line. And this environment variable is used no matter which language is being preprocessed. universal queen 🌍
• The remaining environment variables apply only when preprocessing the particular language indicated. Each one specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

Now here's a lil gotcha in all these variables: an empty element tells the compiler to search its current working directory. And empty elements can appear at the beginning or at the end of a path. For example, if the value of CPATH is ":/special/include", that has the same effect as -I. -I/special/include. sneaky but iconic 💅

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📝 DEPENDENCIES_OUTPUT 📝

If this variable is set, its value specifies how to output dependencies for Make, based on the non-system header files processed by the compiler. system header files are ignored in the dependency output. not invited to the party, sorry 🚫

The value of DEPENDENCIES_OUTPUT can be:

• just a file name — in which case the Make rules get written to that file, and GCC guesses the target name from the source file name
• or the form file target — in which case the rules get written to file file using target as the target name

In other words (and this is the clean way to think about it), this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. combo move ✨

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☀️ SUNPRO_DEPENDENCIES ☀️

This variable is the same as DEPENDENCIES_OUTPUT (see above) — except system header files are not ignored, so it implies -M rather than -MM. However! The dependence on the main input file is omitted. tiny but crucial difference, bestie

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⏰ SOURCE_DATE_EPOCH ⏰

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible. reproducible builds supremacy 🌈

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. It's identical to the output of @command{date +%s} on GNU/Linux and other systems that support the %s extension in the "date" command. epoch girlies rise

And the value should be a known timestamp — like the last modification time of the source or the package — and it should be set by the build process. don't just make one up, please 🙈

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💀 BUGS 💀

For instructions on how to report bugs, go look at <file:///usr/share/doc/gcc-10/README.Bugs>. they want your bug reports, bestie, hand them over 🫶

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🎀 FOOTNOTES 🎀

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. And on multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. So like, be careful. But also: supplying them in cases where they aren't necessary is innocuous. so worst case is fine, bestie 💅

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👀 SEE ALSO 👀

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. a whole friend group of manuals 🌸

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🫶 AUTHOR 🫶

See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. so many legends, cannot list them all here

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👑 COPYRIGHT 👑

Copyright (c) 1988-2020 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. legal but make it cute

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

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and that's a wrap on the whole entire page bestie 🎉 we did it, all 107 parts, no skips, no shortcuts this lil manual is gcc-10, dated 2023-07-07, and it lives in section GCC(1) of your friendly neighborhood man pages. go compile something iconic ✨💖