GCC(1)

💖 GCC(1) 💖 — the GNU project C and C++ compiler ✨

okay bestie, sit down, hydrate, because this is THE manual page for gcc, the GNU project C and C++ compiler, and it's a whole journey 🫶

---

🌸 NAME 🌸

gcc — GNU project C and C++ compiler 💅

---

🎀 SYNOPSIS 🎀

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

so like, only the most useful options are listed up there — the rest of them are down below, keep scrolling bestie 😌 and heads up: g++ accepts mostly the same options as gcc.

---

🦋 DESCRIPTION 🦋

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" let you stop this whole process at an intermediate stage — like hitting pause mid-glow-up 💅 for example, the -c option says do NOT run the linker. Then the output is just object files that the assembler spat out.

Other options get passed on to one or more stages of processing. Some options control the preprocessor, others control the compiler itself. And then there's a whole other crew of options controlling the assembler and the linker — most of those aren't documented here, because fr fr you rarely ever need them.

Most of the command-line options you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. And if a particular option's description doesn't mention a source language? Then you can use that option with all supported languages, periodt ✨

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. And when you compile C++ programs, you should invoke GCC as g++ instead. iykyk 👑

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. not me learning that the hard way 😭

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. yes queen, position matters 💅

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. iconic behavior honestly.

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 rise up.

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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, we're doing the whole list, no skipping, we're thorough like that 😤)

🔥 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, this is the warning flag menu and it's giving encyclopedia energy 💅. Every single one is here, no skips, no cap. Periodt.

• -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 🧪

bestie, this is the static analyzer corner of the group chat. It's lowkey detective mode 🔍.

• -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 🍎

bestie, these are the C and Objective-C-only warning flags. It's giving specialized 💅.

• -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 🐛

okay so debugging is where you get the tea ☕. These flags are the magnifying glass energy 🧐.

• -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

✨ Optimization Options ✨

okay bestie, this is the optimization menu and it's literally a whole mood. no descriptions in this part, just the names, but you know I got you. every single flag, no cap. 💅

• -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
• -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
• -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
• -fgcse
• -fgcse-after-reload
• -fgcse-las
• -fgcse-lm
• -fgraphite-identity
• -fgcse-sm
• -fhoist-adjacent-loads
• -fif-conversion
• -fif-conversion2
• -findirect-inlining
• -finline-functions
• -finline-functions-called-once
• -finline-limit=n
• -finline-small-functions
• -fipa-cp
• -fipa-cp-clone
• -fipa-bit-cp
• -fipa-vrp
• -fipa-pta
• -fipa-profile
• -fipa-pure-const
• -fipa-reference
• -fipa-reference-addressable
• -fipa-stack-alignment
• -fipa-icf
• -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
• -fisolate-erroneous-paths-dereference
• -fisolate-erroneous-paths-attribute
• -fivopts
• -fkeep-inline-functions
• -fkeep-static-functions
• -fkeep-static-consts
• -flimit-function-alignment
• -flive-range-shrinkage
• -floop-block
• -floop-interchange
• -floop-strip-mine
• -floop-unroll-and-jam
• -floop-nest-optimize
• -floop-parallelize-all
• -flra-remat
• -flto
• -flto-compression-level
• -flto-partition=alg
• -fmerge-all-constants
• -fmerge-constants
• -fmodulo-sched
• -fmodulo-sched-allow-regmoves
• -fmove-loop-invariants
• -fno-branch-count-reg
• -fno-defer-pop
• -fno-fp-int-builtin-inexact
• -fno-function-cse
• -fno-guess-branch-probability
• -fno-inline
• -fno-math-errno
• -fno-peephole
• -fno-peephole2
• -fno-printf-return-value
• -fno-sched-interblock
• -fno-sched-spec
• -fno-signed-zeros
• -fno-toplevel-reorder
• -fno-trapping-math
• -fno-zero-initialized-in-bss
• -fomit-frame-pointer
• -foptimize-sibling-calls
• -fpartial-inlining
• -fpeel-loops
• -fpredictive-commoning
• -fprefetch-loop-arrays
• -fprofile-correction
• -fprofile-use
• -fprofile-use=path
• -fprofile-partial-training
• -fprofile-values
• -fprofile-reorder-functions
• -freciprocal-math
• -free
• -frename-registers
• -freorder-blocks
• -freorder-blocks-algorithm=algorithm
• -freorder-blocks-and-partition
• -freorder-functions
• -frerun-cse-after-loop
• -freschedule-modulo-scheduled-loops
• -frounding-math
• -fsave-optimization-record
• -fsched2-use-superblocks
• -fsched-pressure
• -fsched-spec-load
• -fsched-spec-load-dangerous
• -fsched-stalled-insns-dep[=n]
• -fsched-stalled-insns[=n]
• -fsched-group-heuristic
• -fsched-critical-path-heuristic
• -fsched-spec-insn-heuristic
• -fsched-rank-heuristic
• -fsched-last-insn-heuristic
• -fsched-dep-count-heuristic
• -fschedule-fusion
• -fschedule-insns
• -fschedule-insns2
• -fsection-anchors
• -fselective-scheduling
• -fselective-scheduling2
• -fsel-sched-pipelining
• -fsel-sched-pipelining-outer-loops
• -fsemantic-interposition
• -fshrink-wrap
• -fshrink-wrap-separate
• -fsignaling-nans
• -fsingle-precision-constant
• -fsplit-ivs-in-unroller
• -fsplit-loops
• -fsplit-paths
• -fsplit-wide-types
• -fsplit-wide-types-early
• -fssa-backprop
• -fssa-phiopt
• -fstdarg-opt
• -fstore-merging
• -fstrict-aliasing
• -fthread-jumps
• -ftracer
• -ftree-bit-ccp
• -ftree-builtin-call-dce
• -ftree-ccp
• -ftree-ch
• -ftree-coalesce-vars
• -ftree-copy-prop
• -ftree-dce
• -ftree-dominator-opts
• -ftree-dse
• -ftree-forwprop
• -ftree-fre
• -fcode-hoisting
• -ftree-loop-if-convert
• -ftree-loop-im
• -ftree-phiprop
• -ftree-loop-distribution
• -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
• -funconstrained-commons
• -funit-at-a-time
• -funroll-all-loops
• -funroll-loops
• -funsafe-math-optimizations
• -funswitch-loops
• -fipa-ra
• -fvariable-expansion-in-unroller
• -fvect-cost-model
• -fvpt
• -fweb
• -fwhole-program
• -fwpa
• -fuse-linker-plugin
• --param name=value
• -O
• -O0
• -O1
• -O2
• -O3
• -Os
• -Ofast
• -Og

---

🎀 Program Instrumentation Options 🎀

bestie, this section is all about spying on your program (in a cute way, for debugging and coverage). here's the full list, no skips. 🫶

• -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]
• -fsanitize=style
• -fsanitize-recover
• -fsanitize-recover=style
• -fasan-shadow-offset=number
• -fsanitize-sections=s1,s2,...
• -fsanitize-undefined-trap-on-error
• -fbounds-check
• -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
• -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 is like the opening act before the main compiler show. here's every flag, periodt. ✨

• -Aquestion=answer
• -A-question[=answer]
• -C
• -CC
• -Dmacro[=defn]
• -dD
• -dI
• -dM
• -dN
• -dU
• -fdebug-cpp
• -fdirectives-only
• -fdollars-in-identifiers
• -fexec-charset=charset
• -fextended-identifiers
• -finput-charset=charset
• -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
• -H
• -imacros file
• -include file
• -M
• -MD
• -MF
• -MG
• -MM
• -MMD
• -MP
• -MQ
• -MT
• -no-integrated-cpp
• -P
• -pthread
• -remap
• -traditional
• -traditional-cpp
• -trigraphs
• -Umacro
• -undef
• -Wp,option
• -Xpreprocessor option

---

🦋 Assembler Options 🦋

just two, but they're iconic. sending these straight to the assembler. 💌

• -Wa,option
• -Xassembler option

---

💜 Linker Options 💜

linking time, bestie. this is where all your object files become besties and form an executable. here's the whole squad. 👑

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

---

🍓 Directory Options 🍓

where to find all the files and libs? this section is literally the map. no cap. 🗺️

• -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 ⭐

code gen is where the magic happens. these flags control how your code actually turns into machine instructions. it's giving low-level realness. 💀

• -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
• -fpic
• -fPIC
• -fpie
• -fPIE
• -fno-plt
• -fno-jump-tables
• -frecord-gcc-switches
• -freg-struct-return
• -fshort-enums
• -fshort-wchar
• -fverbose-asm
• -fpack-struct[=n]
• -fleading-underscore
• -ftls-model=model
• -fstack-reuse=reuse_level
• -ftrampolines
• -ftrapv
• -fwrapv
• -fvisibility=[default|internal|hidden|protected]
• -fstrict-volatile-bitfields
• -fsync-libcalls

---

🌈 Developer Options 🌈

for when you're literally a GCC dev or just curious. these are the deep cuts, bestie. iykyk. 🧚

• -dletters
• -dumpspecs
• -dumpmachine
• -dumpversion
• -dumpfullversion
• -fcallgraph-info[=su,da]
• -fchecking
• -fchecking=n
• -fdbg-cnt-list
• -fdbg-cnt=counter-value-list
• -fdisable-ipa-pass_name
• -fdisable-rtl-pass_name
• -fdisable-rtl-pass-name=range-list
• -fdisable-tree-pass_name
• -fdisable-tree-pass-name=range-list
• -fdump-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
• -fcompare-debug[=opts]
• -fcompare-debug-second
• -fenable-kind-pass
• -fenable-kind-pass=range-list
• -fira-verbose=n
• -flto-report
• -flto-report-wpa
• -fmem-report-wpa
• -fmem-report
• -fpre-ipa-mem-report
• -fpost-ipa-mem-report
• -fopt-info
• -fopt-info-options[=file]
• -fprofile-report
• -frandom-seed=string
• -fsched-verbose=n
• -fsel-sched-verbose
• -fsel-sched-dump-cfg
• -fsel-sched-pipelining-verbose
• -fstats
• -fstack-usage
• -ftime-report
• -ftime-report-details
• -fvar-tracking-assignments-toggle
• -gtoggle
• -print-file-name=library
• -print-libgcc-file-name
• -print-multi-directory
• -print-multi-lib
• -print-multi-os-directory
• -print-prog-name=program
• -print-search-dirs
• -Q
• -print-sysroot
• -print-sysroot-headers-suffix
• -save-temps
• -save-temps=cwd
• -save-temps=obj
• -time[=file]

---

👑 Machine-Dependent Options 👑

not all machines are the same, bestie. these flags change depending on the target. it's giving "it depends" energy. 💅

🎀 AArch64 Options 🎀

here's the AArch64 squad, slaying on ARM 64-bit. 💖

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

🌸 Adapteva Epiphany Options 🌸

okay bestie, welcome to the part of the manual where every CPU family gets its own lil closet of flags 💅 we're deep in the machine-dependent options now, and I am not about to skip a single one. First up, Adapteva Epiphany — here's the entire lineup, spelled exactly how the manual spells it:

• -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

not me double-checking -m1reg-reg like five times. yes. that's what it says. we copy it as-is, no editorializing, periodt.

---

🎀 AMD GCN Options 🎀

short and sweet, three of them, and they are:

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

🦋 ARC Options 🦋

okay this one is a whole meal, fr fr. every single one, no cap:

• -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

see mlra-priority-noncompact sitting there with no leading dash? yeah. that's literally what's printed. I'm not fixing it, you're not fixing it, we're all just living with it 💀

🌈 ARM Options 🌈

the ARM section. it's giving iconic, it's giving enormous. hold my boba:

• -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 ⭐

tiny microcontroller energy, adorable, still got a full list:

• -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

yes bestie, -Waddr-space-convert and -Wmisspelled-isr are in this list and they start with a capital W, not an m. that's not a typo, that's just how it is 🫶

👑 Blackfin Options 👑

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

💜 C6X Options 💜

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

🧚 CRIS Options 🧚

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

-sim and -sim2? no m. no W. just vibes. that's what the manual says and I am but a humble messenger 💅

🍓 CR16 Options 🍓

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

🌸 C-SKY Options 🌸

• -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 🎀

okay so this one is the long one. and listen — some of these literally appear twice in the original list. I'm keeping the duplicates because keeping every detail is the whole assignment, so don't come for me:

• -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

ngl -F, -gused and -gfull are just out here living their lowercase life while everything else has an underscore. main character behavior tbh ✨

🦋 DEC Alpha Options 🦋

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

💜 eBPF Options 💜

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

⭐ FR30 Options ⭐

• -msmall-model
• -mno-lsim

🌈 FT32 Options 🌈

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

🧚 FRV Options 🧚

another absolute unit of a list. here we go:

• -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

-mTLS and -mtls sitting right next to each other, differing only by ONE capital letter. the manual is testing us and honestly? we passed 😭

---

🌸 GNU/Linux Options 🌸

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

🍓 H8/300 Options 🍓

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

👑 HPPA Options 👑

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

-nolibdld, -static and -threads in here with no m prefix at all. they're just built different 💅

💜 IA-64 Options 💜

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

🌈 LM32 Options 🌈

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

🎀 M32R/D Options 🎀

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

🦋 M32C Options 🦋

• -mcpu=cpu
• -msim
• -memregs=number

⭐ M680x0 Options ⭐

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

note that -mbitfield has a -mno-bitfield friend AND a whole separate -mnobitfield spelling. three whole flags for one vibe. obsessed 💖

🧚 MCore Options 🧚

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

🌸 MeP Options 🌸

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

-ml, -mm, -ms, -mtf… the single-letter flags are out here thriving. tiny names, big presence 🫶

👑 MicroBlaze Options 👑

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

and that is NOT the end of the machine-dependent saga, bestie — we have not even gotten to the letters past M yet 😭 keep scrolling, the next batch is right there 💅

---

✨ MIPS Options ✨

bestie, MIPS is giving so many flags it’s not even funny. here’s the full list, no cap:

• -EL
• -EB
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mno-mips16
• -mflip-mips16
• -minterlink-compressed
• -mno-interlink-compressed
• -minterlink-mips16
• -mno-interlink-mips16
• -mabi=abi
• -mabicalls
• -mno-abicalls
• -mshared
• -mno-shared
• -mplt
• -mno-plt
• -mxgot
• -mno-xgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mno-odd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mno-dsp
• -mdspr2
• -mno-dspr2
• -mmcu
• -mmno-mcu
• -meva
• -mno-eva
• -mvirt
• -mno-virt
• -mxpa
• -mno-xpa
• -mcrc
• -mno-crc
• -mginv
• -mno-ginv
• -mmicromips
• -mno-micromips
• -mmsa
• -mno-msa
• -mloongson-mmi
• -mno-loongson-mmi
• -mloongson-ext
• -mno-loongson-ext
• -mloongson-ext2
• -mno-loongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mno-smartmips
• -mpaired-single
• -mno-paired-single
• -mdmx
• -mno-mdmx
• -mips3d
• -mno-mips3d
• -mmt
• -mno-mt
• -mllsc
• -mno-llsc
• -mlong64
• -mlong32
• -msym32
• -mno-sym32
• -Gnum
• -mlocal-sdata
• -mno-local-sdata
• -mextern-sdata
• -mno-extern-sdata
• -mgpopt
• -mno-gopt
• -membedded-data
• -mno-embedded-data
• -muninit-const-in-rodata
• -mno-uninit-const-in-rodata
• -mcode-readable=setting
• -msplit-addresses
• -mno-split-addresses
• -mexplicit-relocs
• -mno-explicit-relocs
• -mcheck-zero-division
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mload-store-pairs
• -mno-load-store-pairs
• -mmemcpy
• -mno-memcpy
• -mlong-calls
• -mno-long-calls
• -mmad
• -mno-mad
• -mimadd
• -mno-imadd
• -mfused-madd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mno-fix-24k
• -mfix-r4000
• -mno-fix-r4000
• -mfix-r4400
• -mno-fix-r4400
• -mfix-r5900
• -mno-fix-r5900
• -mfix-r10000
• -mno-fix-r10000
• -mfix-rm7000
• -mno-fix-rm7000
• -mfix-vr4120
• -mno-fix-vr4120
• -mfix-vr4130
• -mno-fix-vr4130
• -mfix-sb1
• -mno-fix-sb1
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mno-branch-likely
• -mcompact-branches=policy
• -mfp-exceptions
• -mno-fp-exceptions
• -mvr4130-align
• -mno-vr4130-align
• -msynci
• -mno-synci
• -mlxc1-sxc1
• -mno-lxc1-sxc1
• -mmadd4
• -mno-madd4
• -mrelax-pic-calls
• -mno-relax-pic-calls
• -mmcount-ra-address
• -mframe-header-opt
• -mno-frame-header-opt

---

🌙 MMIX Options 🌙

MMIX bestie, you get your own little set. slay:

• -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 ✨

these are lowkey specific but we keep every one:

• -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, periodt:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

🌸 MSP430 Options 🌸

MSP430 girlies, this is your moment:

• -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 💅

NDS32 said “I have options too,” and honestly? valid:

• -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 🌈

Nios II bestie, here’s your whole vibe:

• -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 💙

Nvidia PTX, keeping it cute and short:

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

---

🌿 OpenRISC Options 🌿

OpenRISC bestie, this is your flag bouquet:

• -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 💖

PDP-11 said “retro but make it fashion”:

• -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 🎀

picoChip, tiny but mighty:

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

---

👑 PowerPC Options 👑

bestie, this one’s literally just:

See RS/6000 and PowerPC Options.

periodt.

---

⭐ PRU Options ⭐

PRU bestie, here’s your set:

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

---

🦋 RISC-V Options 🦋

RISC-V is that girl, and these are her flags:

• -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 🌼

RL78 said “I’m small but I have range”:

• -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 💅

okay bestie, this is the big one. RS/6000 and PowerPC came prepared. no skipping:

• -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
• -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 🦋

RX bestie, this is your moment:

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

---

💙 S/390 and zSeries Options 💙

S/390 and zSeries, mainframe energy, let’s go:

• -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 🌸

Score bestie, short but iconic:

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

---

🎀 SH Options 🎀

SH said “I contain multitudes,” and honestly? facts:

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

---

☀️ Solaris 2 Options ☀️

Solaris 2 bestie, here’s your set:

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

---

💜 SPARC Options 💜

SPARC is giving range, bestie:

• -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 ⭐

System V, keeping it classic:

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

---

💚 TILE-Gx Options 💚

TILE-Gx bestie, here you go:

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

---

🩵 TILEPro Options 🩵

TILEPro, tiny list, big energy:

• -mcpu=cpu
• -m32

---

💖 V850 Options 💖

V850 bestie, this one’s stacked:

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

---

🌿 VAX Options 🌿

VAX, the retro icon. here’s the whole vibe:

• -mg
• -mgnu
• -munix

👽 Visium Options

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

---

🧊 VMS Options

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

---

🌊 VxWorks Options

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

---

💅 x86 Options

okay bestie, this one is A LOT, but we're gonna get through it together, no cap.

• -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.

---

🎀 Options Controlling the Kind of Output

so bestie, compiling isn't one single thing — it's a whole little pipeline, and the way that it goes is: up to four stages: preprocessing, compilation proper, assembly and linking, always in that exact order. GCC can preprocess and compile a bunch of files either into several assembler input files, or smush them into one assembler input file. Then every assembler input file makes an object file, and linking brings all the object files together — both the freshly compiled ones and the ones you passed in as input — into one executable file. ✨

For any given input file, the file name suffix is what decides which 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. Heads up though: you must link with the libobjc library to make an Objective-C program actually work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. And yes, you must link with the libobjc library to make an Objective-C++ program work. Also 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 (that's the default), or a 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. Careful: in .cxx the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f, file.for, file.ftn — Fixed form Fortran source code that should not be 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). These files are also called specs. 🧚
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). These 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 gets treated this way.

You can also just say it out loud which language it is 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. The 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). back to vibes-based defaults, we love to see it.

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

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

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

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

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

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

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. receipts for everything, love that for us.
• -### — 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.

---

okay that was a lot but you're doing amazing sweetie, keep scrolling, there's more 💖

• --help={class|[^]qualifier}[,...] — bestie this one PRINTS stuff to standard output, ok? It shows you a description of every command-line option the compiler understands that fits into all the classes and qualifiers you listed. Here are the classes you can pick from:
  • optimizers — shows you literally every optimization option the compiler supports. all of them. the whole squad. 🔥
  • warnings — shows all the options that control warning messages the compiler spits out.
  • target — shows target-specific options. But heads up, unlike the --target-help option, this does not show target-specific options of the linker and assembler, because those tools don't currently support the extended --help= syntax. ngl kind of a them problem.
  • params — shows the values that the --param option recognizes.
  • language — shows 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 gotta select the common class instead.
  • common — shows the options that are common to every language. the shared faves ✨

And here are the supported qualifiers:

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

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

--help=target,undocumented

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

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers. fr fr, don't do it.

Combining several classes is possible, though it usually restricts the output so much that there's literally 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. periodt. 👑

If the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= displays changes. Instead of describing the displayed options, it gives you 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

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

💅 iconic detective work honestly.

• --version — displays the version number and copyrights of the GCC you just invoked. the receipts, basically. 🧾
• -pass-exit-codes — normally the gcc program exits with the code 1 if any phase of the compiler returns a non-success return code. But if you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase that returned an error indication. Also, the C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. good to know, bestie.
• -pipe — uses pipes instead of temporary files for communication between the various stages of compilation. Lowkey 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. 💅
• -specs=file — processes file after the compiler reads in the standard specs file, in order to override the defaults that the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. You can specify more than one -specs=file on the command line, and they get 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, so the invocation of cc1 becomes gdb --args cc1 .... cute little inception moment 🌀

• -ffile-prefix-map=old=new — when compiling files that live in directory old, record any references to them in the compilation result as if the files lived 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. slay, cross-language queen.
• -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. 🦋
• @file — reads command-line options from file. The options read are inserted in place of the original @file option. If file doesn't exist, or can't be read, then the option gets treated literally, and is 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. recursion bestie, we love to see it. 🔁

---

💖 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 important part bestie — 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.

---

🌸 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.

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. noted. 📝

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

So like, the compiler is not a one-standard stan, bestie 💅 — she can accept several base standards, like c90 or c++98, plus the GNU dialects of those standards, like gnu90 or gnu++98. When you pick a base standard, she accepts every program that follows 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 just don't have a meaning in ISO C90, like omitting the middle term of a ?: expression. iconic, honestly ✨

But flip it — when you specify a GNU dialect of a standard, all the features the compiler supports get enabled, even when those features change the meaning of the base standard 😳. As a result, some strict-conforming programs may get rejected. The particular standard you picked 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. the way she keeps track of all that… 🎀

A value for this option must be provided, no vibes-based guessing allowed. Possible values are:

• c90, c89, iso9899:1990 — support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code. ✨
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <http://gcc.gnu.org/c99status.html> for more info. The names c9x and iso9899:199x are deprecated. 🫠
• c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. 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. 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. 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 — output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

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

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

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

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

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on 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: #pragma acc in C/C++ and !$acc in Fortran 💖. When you pass -fopenacc, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. Heads up though: this option implies -pthread, so it's only supported on targets that actually have support for -pthread. No -pthread, no vibes ✨.
• -fopenacc-dim=geom — sets the default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in order gang, worker, and vector 🫶. Any size can be omitted, and then it just uses a target-specific default value. Lowkey flexible of it.
• -fopenmp — enables handling of OpenMP directives: #pragma omp in C/C++ and !$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 — same deal as above — it's only supported on targets that have support for -pthread. Also, -fopenmp implies -fopenmp-simd. It's giving "package deal" 💅.
• -fopenmp-simd — enables handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored, periodt. So it's SIMD only, the rest can sit down 🙈.
• -fgnu-tm — when -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 🔥. Now listen: this is an experimental feature whose 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. Not everyone gets an invite to this party.

For more information on GCC's support for transactional memory, (that's where the original trails off, bestie, it just... stops ✨).

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

• -fms-extensions — accepts some non-standard constructs used in Microsoft header files. Very "we tolerate your ex" energy 🎀.

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

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

Some cases of unnamed fields in structures and unions are only accepted with this option. And 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 — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++. Sorry C++ girlies, not today 💀.
• -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. Um, this option should not be used for new code. It's a legacy thing, iykyk 🙈.
• -funsigned-char — let the type char be unsigned, like unsigned char 🌸.

Each 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. No cap.

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

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. Main character with a costume change, fr.

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

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. So many ways to say the same thing, iconic 💅.

• -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. Consistency slay 🌈.

• -fsso-struct=endianness — set the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Do not mix and match, bestie, it will not go well 😭.

---

🎀 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. It's giving "living document", fr fr.

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

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

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

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

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

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

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

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

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

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

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

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

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

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. Coordinated, no cap ✨.

• -fno-access-control — turn off all access checking. This switch is mainly useful for working around bugs in the access control code. It's giving "the rules are a suggestion" 💀.
• -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;

okay so bestie, the glow-up to the types of UTF-8 string and character literals? it introduces incompatibilities with ISO C++11 and later standards. 😭 Like, this lil guy right here is well-formed under ISO C++11, but the literal second you add -fchar8_t it becomes ill-formed. no cap:

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 — vibe check the pointer, bestie ✨ Before touching the storage that operator new handed you, make sure that pointer isn't null. Normally you don't need this at all, 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 every other case, when operator new has a non-empty exception specification, running out of memory gets signalled by throwing std::bad_alloc instead. See also new (nothrow). 🫶
• -fconcepts
• -fconcepts-ts — Okay so below -std=c++2a, -fconcepts turns on support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015). 📜 And with -std=c++2a and above, Concepts are part of the language standard, so -fconcepts defaults to on. BUT — and this is the tea — the standard's 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 with -fconcepts-ts. 💅
• *-fconstexpr-depth=n* — Sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need a limit, otherwise the compiler would just spiral into endless recursion during constant expression evaluation forever. 😵‍💫 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 gets cached to n. It's a heuristic, so it's trading off compilation speed (when the cache avoids repeated calculations 🏎️) against memory consumption (when the cache grows huge from highly recursive evaluations 🎈). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. 🔬
• *-fconstexpr-loop-limit=n* — Sets the max number of iterations for a loop in C++14 constexpr functions to n. Again, a limit is needed to detect infinite loops during constant expression evaluation. 🌀 The default is 262144 (1<<18).
• *-fconstexpr-ops-limit=n* — Sets the 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 way too long. ⏳ The default is 33554432 (1<<25).
• -fcoroutines — Turns on support for the C++ coroutines extension (experimental). 🧪 experimental, bestie, you've been warned
• -fno-elide-constructors — So the C++ standard allows an implementation to skip 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 it may be useful for reducing code size in production builds, much like defining NDEBUG. Heads up though: this does not give user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. 💀
• -fextern-tls-init
• -fno-extern-tls-init — The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. 🔄 When the use and definition of the variable are in the same translation unit, this overhead can be optimized away — but when the use is in a different translation unit there's significant overhead even if the variable doesn't actually need dynamic initialization. So 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. periodt.
• -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, otherwise you're getting undefined symbols at link time. link errors are not the vibe. 😭
• -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, don't emit out-of-line copies of inline functions controlled by #pragma implementation. ⚠️ This causes linker errors if these functions aren't inlined everywhere they're called. so like, yikes.
• -fms-extensions — Disables 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 — Enables 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 — Enables 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 — Disables 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 — Treats 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 — Don't treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. ✂️
• -fno-optional-diags — Disables diagnostics that the standard says a compiler doesn't need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. 🤫
• -fpermissive — Downgrades some diagnostics about nonconformant code from errors to warnings. 📉 So using -fpermissive allows some nonconforming code to compile. (the compiler is being nice to you, don't push it 💅)
• -fno-pretty-templates — So 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. 🔍 And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
• -fno-rtti — Disables 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. fr fr, keep your RTI settings consistent bestie. 😭

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

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

as introduced in C++14. It's giving very useful energy for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. And the flag -Wsized-deallocation warns about places that might want to add a definition. 👑

• -fstrict-enums — lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). ✨ Heads up though: this assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. So like, don't do that.
• -fstrong-eval-order — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. The flavor -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17. Ordering? We love her. 💅
• -ftemplate-backtrace-limit=n — sets the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.
• -ftemplate-depth=n — sets the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. 😭 the stack said no.
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. Lowkey a size-queen move.
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit". No cap, check your libc first. 🫶
• -fno-use-cxa-get-exception-ptr — don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available. Trade-offs, babe. 💀
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥 iconic behavior honestly.

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 is provided by the linker. By default, G++ uses weak symbols if they are available.

This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 🚨 bestie do NOT.

• -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 information about ABI tags.
• -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++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. Parentheses are the moment, fr fr. 🎀

• -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. She's private and unusable, the way that nobody can even touch her 😭
• -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. 🚨 unsafe, bestie!
• -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.

undefined behavior, babes. literally do not. 🫠

• -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>

• -Wno-narrowing (C++ and Objective-C++ only) — okay so bestie, for C++11 and later standards, narrowing conversions get diagnosed by default, because the standard literally says so, no cap. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing shuts the whole diagnostic up. ⚠️ Heads up though: this does not change the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts. It's giving "silenced but not forgiven."

With -Wnarrowing in C++98, you'll get a warning when a narrowing conversion prohibited by C++11 happens inside { }, like:

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. She's messy like that. 💅
• -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. Lowkey a glow-up that breaks stuff.

For 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's 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. She's an icon, she's a legend, and she's gone. 💀
• -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. Not the vibe. 🙈

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. Slay, modernize bestie. 💅
• -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) — bestie this one is so specific. It yells at you when overload resolution picks a promotion from an unsigned or enumerated type to a signed type instead of a conversion to an unsigned type of the same size. Fun lore drop: previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. So like… the old vibe is gone, the standard said what it said. 💅
• -Wtemplates (C++ and Objective-C++ only) — warns whenever a primary template declaration shows up. Some coding rules straight-up ban templates, and this is the enforcer for that rule. BUT: the warning is inactive inside a system header file, such as the STL, so you can still use the STL like a normal person. You may also instantiate or specialize templates — those are fine, no cap. ✨
• -Wmismatched-tags (C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations where the class-key doesn't 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*);

Okay and listen — it is not wrong to declare a class with the class-key struct, like the example above shows. -Wmismatched-tags is just here to help you achieve a consistent style of class declarations. In code 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 is the vibe-check for that rule. The warning is inactive inside a system header file, such as the STL, so the STL is still yours to use. You may also define classes that indirectly use multiple inheritance — indirect is fine, direct is the problem. 😌
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Same deal as above: some coding rules disallow multiple inheritance and this can be used to enforce that rule. Inactive inside a system header file, such as the STL, so the STL stays usable. You may also define classes that indirectly use virtual inheritance. 🌸
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. And bestie, this one is genuinely dangerous: 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. So only silence it if you actually did the work. Periodt.
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this enforces that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL. You may also use using directives and qualified names — those don't trip it. ✨
• -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. Not even once. 🙈
• -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
  • 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. Yes bestie, 0 is not nullptr, we been knew. 👑
• -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. That's it, that's the whole vibe. ✨
• -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. So silencing it is a choice you're making with your whole chest. 😭
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons after in-class function definitions. The little ; that didn't need to be there. We see it. 💅
• -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. ... stays with the base, it does not get passed down. 🫶
• -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. So like, you know what you're doing. We hope. 👀

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. Do it in that order bestie, don't skip steps. 💜
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. Say it with your chest: override. ✨
• -Wuseless-cast (C++ and Objective-C++ only) — warns when an expression is casted to its own type. (int)x where x is already int? Caught. It's giving nothing. 💀
• -Wno-conversion-null <span style="color:pink">(C++ and Objective-C++ only)</span> — bestie, this one just says: don't warn when someone converts between NULL and non-pointer types. And heads up, -Wconversion-null is on by default, so the warnings are coming unless you shut them off. 💅

---

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

<span style="color:red">NOTE:</span> this manual does not describe the Objective-C and Objective-C++ languages themselves. That's on you, bestie. 😭

Okay so this section is all about the command-line options that only hit different for Objective-C and Objective-C++ programs. You can still use most of the language-independent GNU compiler options too, no worries. Like, say you wanna compile a file called some_class.m, you'd do:

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

In this little example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs — the other options work with any language GCC supports. 🫶

Also, quick note: since Objective-C is literally an extension of C, your Objective-C compilations can also use options specific to the C front-end (like -Wtraditional). And similarly, Objective-C++ compilations can use C++-specific options (like -Wabi). It's giving "everything is connected." 🦋

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

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string you write with the @"..." syntax. The default class name is NXConstantString if you're on the GNU runtime, and NSConstantString if you're on the NeXT runtime (more on that below). Oh and if -fconstant-cfstrings is also present, it overrides the -fconstant-string-class setting and makes those @"..." literals get laid out as constant CoreFoundation strings instead. 💖
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. Slay, GNU.
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. And the macro __NEXT_RUNTIME__ gets predefined if (and only if) you use this option. <span style="color:cyan">if and only if, bestie, they're being precise</span> ✨
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit make sure the receiver is not nil. This lets the runtime use more efficient entry points. Heads up: 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. Currently this option is only supported for the NeXT runtime. In that case:
  • Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
  • Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
  • Version 2 is the modern (64-bit) ABI.

If you specify nothing, 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, it synthesizes a special - (id) .cxx_construct instance method which runs those non-trivial default constructors on any such instance variables, in order, and then returns self. Same energy for destructors: 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 those default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way only operate on instance variables declared in the current Objective-C class, not the ones inherited from superclasses. It's the Objective-C runtime's job to invoke all such methods across an object's inheritance hierarchy. The - (id) .cxx_construct methods get invoked by the runtime right after a new object instance is allocated; the - (void) .cxx_destruct methods get invoked right 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. <span style="color:peach">read that again before you go feral</span> 🍓

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. Zoom zoom. 🔥
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. You need this option 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 one is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. different vibes, different runtime.
• -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 you can turn it off with -fno-objc-nilcheck. Class methods and super calls are never checked for nil this way, no matter what this flag is set to. And currently this flag does nothing at all 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 you use this option with the Objective-C or Objective-C++ compiler, any Objective-C syntax not recognized by GCC 4.0 gets rejected. Super useful if you need to make sure your Objective-C code can compile 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 let dyld(1) load it in at run time instead. This gets used with the Fix-and-Continue debugging mode, where the object file in question can be recompiled and dynamically reloaded while the program is running, no restart needed. Currently, Fix-and-Continue functionality is only available 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 class name 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 that behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it lets individual class implementations be modified during program execution. And btw, the GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. periodt. 💅
• -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 those variable shadowing issues. <span style="color:mint">no more name drama</span> 🧚
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option, so that instance variables declared outside the scope of any access modifier directives default to that specified visibility. 👑
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. cute little receipt. 🎀
• -Wassign-intercept <span style="color:pink">(Objective-C and Objective-C++ only)</span> — warn whenever an Objective-C assignment is being intercepted by the garbage collector. 🫶
• -Wno-property-assign-default <span style="color:pink">(Objective-C and Objective-C++ only)</span> — do not warn if a property for an Objective-C object has no assign semantics specified. silence is golden.
• -Wno-protocol <span style="color:pink">(Objective-C and Objective-C++ only)</span> — okay so if a class is declared to implement a protocol, a warning gets issued for every method in the protocol that isn't 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 count as implemented, and no warning is issued for them. inherited counts, bestie 💖
• -Wselector <span style="color:pink">(Objective-C and Objective-C++ only)</span> — warn if multiple methods of different types for the same selector are found during compilation. The check happens 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. And because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation isn't reached — for example because an error is found during compilation, or because the -fsyntax-only option is being used. <span style="color:peach">iykyk, timing matters</span> 🍓
• -Wstrict-selector-match <span style="color:pink">(Objective-C and Objective-C++ only)</span> — warn if multiple methods with differing argument and/or return types are found for a given selector when you're 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 <span style="color:hotpink">(Objective-C and Objective-C++ only)</span> — bestie this one yells at you if a @selector(...) expression shows up referring to a selector that was never declared. And when do we call a selector "undeclared"? When no method with that name got declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. The way this one works: it always performs its checks the second a @selector(...) expression is found, whereas -Wselector only does its checks in the final stage of compilation. It also enforces that coding style convention that methods and selectors must be declared before being used. ✨
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if any. 🫶

---

🌈 Options to Control Diagnostic Messages Formatting 🌈

Okay so traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (like its width, ...). Which, rude, honestly. 💀 You can use the options below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information gets reported. Just know that some language front ends may not honor these options. Not everyone got the memo, iykyk.

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

Heads up — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas. So those two are just out here living their own life. 🙈

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. It tells the diagnostic messages reporter to emit source location information once; that is, if the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. 🎀
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. It tells the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line. Every line. Every single one. 😭
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler has been configured, and it can be any of the above 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 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 are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. 💅

The default GCC_COLORS is

error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01: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. 👑

• "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=, such as the identifiers of individual events and lines indicating interprocedural calls and returns.
• "range1=" — SGR substring for first additional range.
• "range2=" — SGR substring for second additional range.
• "locus=" — SGR substring for location information, file: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] — use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed a URL for 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 not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below. 👀

The default depends on how the compiler has been configured. It can be any of the above 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.

However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: if GCC_URLS is set to empty or no, do not 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 is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep. 🔔

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

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

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

This option suppresses the printing of these 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 https://cwe.mitre.org/index.html CWE identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata. 🕵️
• -fno-diagnostics-show-line-numbers — by default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin. 🔢
• -fdiagnostics-minimum-margin-width=width — this option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. ✨
• -fdiagnostics-parseable-fixits — emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string "fix-it:". For example:

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

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:

00000000011111111112222222222
12345678901234567890123456789
  gtk_widget_showall (dlg);
  ^^^^^^^^^^^^^^^^^^
  gtk_widget_show_all

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

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

---

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

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

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

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

• -fdiagnostics-show-template-tree — in the C++ frontend, when printing diagnostics showing mismatching template types, such as:

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

the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:

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

The parts that differ are highlighted with color ("double" and "float" in this case). cute right?? ✨

• -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. they're besties fr fr

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

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

• none means to not print diagnostic paths.
• separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:

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

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

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

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

the little arrows?? the nesting?? it's giving ✨family tree✨ honestly

• -fdiagnostics-show-path-depths — this option provides additional information when printing control-flow paths associated with a diagnostic.

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

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. not really a you-and-me feature bestie, but go off 💜

• -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 — select a different format for printing diagnostics. FORMAT is text or json. The default is text.

The json format consists of 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. don't come for me about the pretty-printing, it's for YOU 😭

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. 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 🎀

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. 👀

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

• -fsyntax-only — Check the code for syntax errors, but don't do anything beyond that. Just a lil vibe check on your syntax, bestie. ✨
• *-fmax-errors=n* — Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
• -w — Inhibit all warning messages. (Sshh. Silence. 🤫)
• -Werror — Make all warnings into errors. No more "just a warning" energy, it's giving hard fail. 💀
• -Werror= — Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

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

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. 🙈

• -Wfatal-errors — This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. Like, it's done. It's over. It's not doing this today. 💅

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. ✨

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. 👑

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.

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

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

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

Some 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. 😭

Quick lil sidebar before we get into the flags, bestie:

A feature that reports every single failure to conform to ISO C might sound cute in some instances, but fr it would need considerable extra work and would be pretty different from -Wpedantic. We lowkey don't have plans to support such a feature in the near future. 💀

Okay so when the standard you picked with -std is a GNU extended dialect of C — like gnu90 or gnu99 — there's a corresponding base standard, aka the version of ISO C that the GNU extended dialect is built on top of. Warnings from -Wpedantic get given where the base standard requires them. And like, it wouldn't even make sense to only warn about features that aren't in the specified GNU C dialect, because by definition the GNU dialects of C include all the features the compiler supports with the given option, so there'd be nothing left to warn about. 🦋

---

✨ -pedantic-errors ✨

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic — in some cases where there's undefined behavior at compile-time, and in some other cases that don't even prevent compilation of programs that are valid according to the standard. 🙈

Heads up: this is not equivalent to -Werror=pedantic, because there are errors enabled by this option that aren't enabled by the latter, and vice versa. 💅

---

✨ -Wall ✨

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

-Wall turns on the following warning flags:

-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare
-Wbool-operation -Wc++11-compat  -Wc++14-compat -Wcatch-value (C++
and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare
(in C/ObjC; this is on by default in C++) -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 me needing to tell you: some warning flags are not implied by -Wall. Some of them warn about constructions that users generally don't consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. 💅

---

✨ -Wextra ✨

This enables some extra warning flags that aren't enabled by -Wall. (Fun fact: this option used to be called -W. The older name is still supported, but the newer name is more descriptive. ✨)

-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) ✨

Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. 😭

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

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

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

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

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

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

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

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

This was fixed in -fabi-version=3.

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

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

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

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

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there's 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 ✨

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

---

✨ -Wno-coverage-mismatch ✨

Warn if feedback profiles don't match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error.

Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 💀

---

✨ -Wno-cpp ✨

(C, Objective-C, C++, Objective-C++ and Fortran only) Suppress warning messages emitted by "#warning" directives. 🤫

• -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — okay bestie, this one gives a warning when a value of type float is implicitly promoted to double. why is that a thing? because CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation. 😭

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

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

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

• -Wduplicate-decl-specifier (C and Objective-C only) — warn if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. periodt.
• -Wformat, -Wformat=n — check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

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

• -Wformat=1, -Wformat — Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. iykyk.
• -Wformat=2 — Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. glow up!

• -Wno-format-contains-nul — if -Wformat is specified, do not warn about format strings that contain NUL bytes. no cap.
• -Wno-format-extra-args — if -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored.

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

• -Wformat-overflow, -Wformat-overflow=level — warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. yikes.
  • -Wformat-overflow, -Wformat-overflow=1 — Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.

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

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

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

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

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

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

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

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

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

• -Wnull-dereference — okay so this is your lil guardian angel 🕵️♀️ and what she does is yell at you when the compiler detects paths that trigger erroneous or undefined behavior because you're dereferencing a null pointer. Big yikes energy. Important caveat bestie: this option is only active when -fdelete-null-pointer-checks is active, and that one is enabled by optimizations in most targets. Also the precision of the warnings depends on the optimization options used. So like, the vibes are not guaranteed. 💀
• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warns you about uninitialized variables that are initialized with themselves (a true "I'm my own source of truth" moment, and not in a main-character way 😭). Heads up: this option can only be used together with the -Wuninitialized option.

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

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

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

• -Wno-implicit-int (C and Objective-C only) — this one's the switch that controls warnings when a declaration does not specify a type. (Yes queen, what are you even declaring? 👀) 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 when a function is used before being declared. The classic "hi who are you, you weren't introduced" situation 🙈. This warning is enabled by default in C99 and later dialects of C, and also by -Wall. And plot twist: the warning is made into an error by -pedantic-errors.
• -Wimplicit (C and Objective-C only) — literally the same as -Wimplicit-int and -Wimplicit-function-declaration combined. Two-for-one deal ✨. This warning is enabled by -Wall.
• -Wimplicit-fallthrough — so -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3, and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. Slay, symmetry.
• -Wimplicit-fallthrough=n — warns when a switch case falls through. (You know, when case 2 just casually stumbles into case 3 without a break like it lives there now 🫶.) For example:

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

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

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

Since there are occasions where a switch case fall through is desirable (sometimes the fallthrough is the moment, no cap), GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:

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

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

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches .*falls?[ \t-]*thr(ough|u).* regular expression.
• -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 such return types always receive a warning even without this option. Rude of them, honestly.

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. (Bestie, main has ONE job and we need it to look right 💅.) 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 does not use braces, followed by an unguarded statement with the same indentation. The visual gaslighting is real 😭.

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 is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. One of them understood the assignment 🎀.

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. Like, bestie, that folder is not there 💀.
• -Wno-missing-profile — okay so bestie, this one is the vibe check for your feedback profiles 💅 It controls warnings for when those profiles are missing while you're using -fprofile-use. It catches those situations where you add a brand-new function or a whole new file in between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. And in those cases?? The profile feedback data files just... don't have any profile feedback info for the freshly added function or file. 😭 Also, if your profile count data (.gcda) files get deleted, GCC can't use any profile feedback information. In all these cases, warnings get issued to tell you that a profile generation step is due. Ignoring that warning can leave you with poorly optimized code. You can use -Wno-missing-profile to silence it, but this is not recommended and should only be done when non-existent profile data is justified. fr fr, don't just mute it because it's annoying.
• -Wmultistatement-macros — warns about unsafe multiple statement macros that look like they're guarded by a clause like if, else, for, switch, or while, but where only the first statement is actually guarded after the macro expands. The way that this bites you is iconic in the worst way:

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

That will increment y unconditionally, not just when c holds. 😳 The usual fix is wrapping the macro in a do-while loop:

#define DOIT do { x++; y++; } while (0)
if (c)
  DOIT;

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

• -Wparentheses — warns if parentheses are omitted in certain contexts, like when there's an assignment in a spot where a truth value is expected, or when operators are nested whose precedence people keep getting confused about. 🙈

It also warns if a comparison like x<=y<=z shows up; that's equivalent to (x<=y ? 1 : 0) <= z, which is a completely different interpretation from ordinary mathematical notation. not me doing math notation in C 💀

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

For C++ it also warns about some cases of unnecessary parentheses in declarations, which can hint at an attempted function call instead of a declaration:

{
  // Declares a local variable called mymutex.
  std::unique_lock<std::mutex> (mymutex);
  // User meant std::unique_lock<std::mutex> lock (mymutex);
}

This warning is enabled by -Wall.

• -Wsequence-point — warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. buckle up bestie, this one's a whole essay ✨

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 — warns whenever a function is defined with a return type that defaults to int. Also warns 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, it warns 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 — warns 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 — warns 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 — warns whenever a switch statement does not have a default case. 💜
• -Wswitch-enum — warns 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's 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) — warns 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 — warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). ✨
• -Wunused-but-set-variable — bestie, this one yells at you whenever a local variable gets assigned to but is otherwise just... sitting there, unused (declaration doesn't count, that's not a use, periodt). Enabled by -Wall. 💅

Wanna silence it? Slap the "unused" attribute on it.

It's also enabled by -Wunused, which is itself enabled by -Wall. it's giving stacked. ✨

• -Wunused-function — warns whenever a static function is declared but never defined, OR a non-inline static function is just... never used. 😭 Enabled by -Wall.
• -Wunused-label — warns whenever a label is declared but not used. Enabled by -Wall.

Suppress it with the "unused" attribute. 💖

• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef that's locally defined inside a function goes unused. Enabled by -Wall. the way that typedef just sat there... 🙈
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration.

Silence it with the "unused" attribute. 🌸

• -Wno-unused-result — do not warn if a caller of a function marked with attribute "warn_unused_result" doesn't use its return value. The default is -Wunused-result. 👑
• -Wunused-variable — warns whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. Enabled by -Wall.

Suppress it with 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++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of "#define"s.

Suppress it with the "unused" attribute. 💜

• -Wunused-const-variable=1 — this is the warning level that's enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. so it's the chill level, lowkey.
• -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested, since in C++ this isn't an error and in C it might be harder to clean up all the headers included. highkey extra, iykyk. 🔥

• -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 contains no side effects. For example, an expression such as "x[i,j]" causes a warning, while "x[(void)i,j]" does not. ngl that's a glow-up right there. ✨

Enabled by -Wall.

• -Wunused — all the above -Wunused options combined. the whole squad. 🫶

So like, to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (heads up: -Wall implies -Wunused), or separately specify -Wunused-parameter. no cap.

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

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

These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used. it's giving circumstantial. 😭

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. 💀

• -Wno-invalid-memory-model — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:

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

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

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

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

These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here's one example of how this can happen:

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

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

This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem, because "longjmp" cannot in fact be called at the place that would cause a problem. the compiler is doing its best and we love that for it. 💅

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

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warns when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option. so -Wunknown-pragmas said "we're going there" and -Wall did not. 😤
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.
• -Wno-prio-ctor-dtor — do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allows you to assign a priority to the constructor/destructor to control its order of execution before "main" is called or after it returns. The priority values must be greater than 100, as the compiler reserves priority values between 0–100 for the implementation. those numbers are taken, bestie, hands off. 🚫
• -Wstrict-aliasing — this option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It's included in -Wall. It's equivalent to -Wstrict-aliasing=3. 🔍
• -Wstrict-aliasing=n — same deal: only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only. it's giving chaotic oversharer. 💀

Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.

Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types. the balanced queen we deserve. 👑

• -Wstrict-overflow
• -Wstrict-overflow=n — this option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level. 🔥

Okay so the vibe here is: an optimization that assumes signed overflow doesn't happen is completely safe when the variables involved genuinely never overflow. Which means this warning can absolutely give you a false positive — yelling at code that was never a problem. 🙈 To keep the drama focused on stuff that actually matters, there are several warning levels. And no cap: 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 runs at all.

• -Wstrict-overflow=1 — Warns about the cases that are both questionable and easy to dodge. Like the compiler simplifying x + 1 > x down to 1. This level of -Wstrict-overflow is turned on by -Wall; the higher levels are not, bestie, you have to ask for them explicitly. 💅
• -Wstrict-overflow=2 — Also warns about other cases where a comparison gets folded into a constant. Example: abs (x) >= 0. This one 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? Same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — Also warns about other cases where a comparison is simplified. Example: x + 1 > 1 gets simplified to x > 0.
• -Wstrict-overflow=4 — Also warns about other simplifications that the cases above don't cover. Example: (x * 10) / 5 gets simplified to x * 2.
• -Wstrict-overflow=5 — Also warns 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 only gets reported at the highest warning level, because this simplification touches so many comparisons that this level will hand you a truly enormous pile of false positives. 💀

---

-Wstring-compare

Warns about calls to strcmp and strncmp whose result is already determined to be either zero or non-zero in equality tests — because one argument's length is greater than the size of the array the other argument lives in (or the bound, in the strncmp case). Calls like this could totally be mistakes. For example, the strcmp call below gets diagnosed because its result is necessarily non-zero no matter what's actually inside 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, -Wstringop-overflow=type — Warns about 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 to perform when figuring out the destination's size. That argument only means something for functions that operate on character arrays — it does nothing for raw memory functions like memcpy, which always use Object Size type-0. The option also warns for calls that specify a size bigger than the largest possible object, or at most SIZE_MAX / 2 bytes. It gives its best results with optimization enabled, 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, it only warns about a subset of the buffer overflows that the corresponding overflow checking built-ins would detect. For example, the option 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 work out 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 that are accessed through pointers, unless the size of the largest surrounding object is known. When the destination could be one of several objects, it assumes it's the largest one of them. On Linux systems, with optimization enabled at this setting, the option warns for the exact 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 work out 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 about excessive writes to those same members of unknown objects referenced by pointers, since those could point to arrays with an unknown number of elements.
• -Wstringop-overflow=3 — Uses type-two Object Size Checking to work out the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that can produce warnings for code that's actually safe. 😬
• -Wstringop-overflow=4 — Uses type-three Object Size Checking to work out 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 can produce warnings for perfectly benign code.

---

-Wno-stringop-truncation

Don't warn about calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that might either truncate the copied string or leave the destination untouched.

In the example below, the strncat call specifies a bound that's less than the length of the source string. So the copy of the source gets truncated, and the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound.

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

Another one: the strncpy call below ends up copying to d just the characters before the terminating NUL, without appending that NUL at the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, so the call gets diagnosed. If your result isn't expected to be NUL-terminated, call memcpy instead to avoid the warning. 💡

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

And here's another: this strncpy call uses the size of the destination buffer as the bound. If the source string's length is equal to or greater than that size, the copy's result won't be NUL-terminated. So yes, 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 hold a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to shut this warning up. But heads up: arrays like that are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuse of those arrays, GCC issues warnings unless it can prove the use is safe.

---

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

Warns about 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 — Warns about functions that might be candidates for the attributes pure, const, noreturn or malloc. The compiler only warns for functions that are visible in other compilation units, or (for pure and const) when it can't prove the function returns normally. A function returns normally if it doesn't contain an 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 — Warns about function pointers that might be candidates for format attributes. Note these are only possible candidates, not guaranteed ones. GCC guesses that function pointers with format attributes used in assignment, initialization, parameter passing or return statements should have a matching format attribute in the resulting type. That is: the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function, respectively, should also have a format attribute to avoid the warning.

GCC also side-eyes function definitions that might be candidates for format attributes. And bestie, again — these are only possible candidates, okay? GCC is literally just guessing that format attributes might be appropriate for any function that calls something like vprintf or vscanf, but that's not always the case, and some functions where format attributes would totally fit may just fly under the radar. It's giving vibes, not verdicts. 🙈

• -Wsuggest-attribute=cold — Warns you about functions that might be candidates for the cold attribute. This is based on static detection, so it generally only calls out functions that always lead to a call to another cold function — think wrappers around C++ throw, or fatal-error-reporting functions that lead to abort. 💀
• -Walloc-zero — Warns about calls to allocation functions decorated with the alloc_size attribute that specify zero bytes, including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Here's the tea: because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc it's been deprecated), relying on it may result in subtle portability bugs and should be avoided. fr fr, just don't.
• *-Walloc-size-larger-than=byte-size* — Warns about calls to functions decorated with the alloc_size attribute that try to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than.
• -Wno-alloc-size-larger-than — Disables -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.
• -Walloca — This option warns on all uses of alloca in the source. every single one. 👑
• *-Walloca-larger-than=byte-size* — This one warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate limiting 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. sneaky. 🙈

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 — Disables -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. vibe check: your index is not it.
  • -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. ⚠️ yes it's noisy
• -Wattribute-alias=n</n>
• -Wno-attribute-alias — Warns 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 — Warns about a 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 — Warns 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.) bestie, why are you doing math on a bool 😭

This warning is enabled by -Wall.

• -Wduplicated-branches — Warns 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 warns for conditional operators:

int i = x ? *p : *p;

• -Wduplicated-cond — Warns 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 — Warns 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 — Warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because 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 — okay so this one is like, please don't yell at me about integer division by zero happening at compile time. That's the whole vibe. But floating-point division by zero? nah, that's fine actually, because dividing a float by zero is a totally legit way to get yourself an infinity or a NaN. So no warning there, bestie. 💅
• -Wsystem-headers — this one tells GCC to spill the tea about warnings hiding in system header files. Normally those get suppressed because the assumption is they're usually not real problems and they just make the compiler output unreadable. Turning this on makes GCC emit warnings from system headers like they were in your own code. ✨ But heads up: using -Wall with this option still does not warn about unknown pragmas in system headers — for that you also gotta use -Wunknown-pragmas. no cap.
• -Wtautological-compare — warn if a self-comparison always evaluates to true or false. It catches mistakes like:

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

It also warns about bitwise comparisons that always evaluate to true or false, like:

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

which will always be false. This warning is enabled by -Wall. 🎀

• -Wtrampolines — warn about trampolines generated for pointers to nested functions. So like, a trampoline is a small piece of data or code created at run time on the stack when you take the address of a nested function, and it's used to call that nested function indirectly. For some targets it's made up of data only, so no special treatment needed. But for most targets, it's made up of code, which means the stack has to be made executable for the program to work properly. 🦋
• -Wfloat-equal — warn if floating-point values are used in equality comparisons. The idea here is that sometimes it's convenient (for the programmer, bestie) to treat floating-point values as approximations to infinitely precise real numbers. If you're doing that, you need to compute (by analyzing the code or some other way) the maximum or likely maximum error the computation introduces, and allow for it when doing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check whether the two values have ranges that overlap — and that's done with relational operators. So equality comparisons are probably mistaken. 😭
• -Wtraditional (C and Objective-C only) — warn about certain constructs that behave differently in traditional vs ISO C. Also warns about ISO C constructs with no traditional C equivalent, and/or problematic constructs you should avoid. Here's the whole list:
  • Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement happens inside string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives didn't exist. Traditional preprocessors only considered a line a directive if the # appeared in column 1. So -Wtraditional warns about directives that traditional C understands but ignores because the # isn't the first character on the line. It also suggests you hide directives like #pragma that traditional C doesn't understand by indenting them. Some traditional implementations don't 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 that these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Using these macros in user code might normally lead to spurious warnings, but 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. Some traditional C compilers don't accept this construct.
  • 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's different from what would happen to the same argument without 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 it's 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, which you know from C++, was introduced with ISO C99 and is allowed in GCC by default. 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 also includes all instances of local shadowing. That means -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 (on top of -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional, not a bug or typo, like in this 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 a byte-size of SIZE_MAX or more, or by -Wno-larger-than. 💅
• -Wno-larger-than — disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger. 🎀
• -Wframe-larger-than=byte-size — warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you don't get a warning. On top of that, 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 a 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 is watching your stack like a hawk 👀 It warns you if a function's stack usage might exceed byte-size. Heads up tho: the computation it does to figure out the stack usage is conservative, so it's giving cautious energy, not exact. And anything you allocate via alloca, variable-length arrays, or related constructs gets counted by the compiler when it decides whether or not to warn you. No sneaky stacks allowed 🫶

The message matches what -fstack-usage spits out, so here's the vibe check on all three flavours:

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

warning: stack usage is 1120 bytes

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

warning: stack usage might be 1648 bytes

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

warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default, no cap. And you can silence the warnings this option controls either by specifying byte-size of SIZE_MAX or more, or by using -Wno-stack-usage. Easy ✨

• -Wno-stack-usage — turns off all the -Wstack-usage= warnings. It's literally the same thing as -Wstack-usage=SIZE_MAX or larger. Same energy, different spelling 💅
• -Wunsafe-loop-optimizations — warns you when a loop can't be optimized because the compiler isn't allowed to assume anything about the bounds of the loop indices. Like, it's standing at the door going "I can't go in there, I don't know what's inside 😭". And with -funsafe-loop-optimizations, it warns if the compiler does go ahead and make those assumptions. Chaos either way fr fr
• -Wno-pedantic-ms-format (MinGW targets only) — when you use this together with -Wformat and -pedantic without GNU extensions, it disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime. Windows-specific behavior, Windows-specific silence 🪟
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. Why? Because GNU C assigns these types a size of 1 for convenience in calculations with void * pointers and pointers to functions. Iconic little hack. In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.
• -Wno-pointer-compare — do not warn if a pointer is compared with a zero character constant. Because bestie, that usually means the pointer was meant to be dereferenced 🙈 Like this:

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

Note that the code above is invalid in C++11. So if you're on C++11, that's not just a warning situation, that's a whole problem.

This warning is enabled by default.

• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type. But it does not warn for constant expressions. For example, it warns if an unsigned variable is compared against zero with < or >=. (Like girl, an unsigned can't be negative, what are we doing 💀) This warning is also enabled by -Wextra.
• -Wabsolute-value (C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function exists. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is fabs. Ma'am that is not the function you're looking for ✨ It also warns when the argument in a call to such a function has an unsigned type. You can suppress this with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. Nested-comment jumpscare 😭 This warning is enabled by -Wall.
• -Wtrigraphs — warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines. This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. Combo move 🎀
• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Such identifiers get replaced with zero. Surprise zero, the worst kind of zero.
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that are unused. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it gets redefined or undefined. Built-in macros, macros defined on the command line, and macros defined in include files are not warned about. So your main file macros better be pulling their weight 💅

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

#if defined the_macro_causing_the_warning
#endif

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

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

The second and third FOO should be in comments, bestie. They're just vibing out there in the wild 🙈 This warning is on by default.

• -Wbad-function-cast (C and Objective-C only) — warns when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. That's not a glow-up, that's a type crime 🔥
• -Wc90-c99-compat (C and Objective-C only) — warns about features not present in ISO C90 but present in ISO C99. For instance: variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. Fancy little escape hatch ✨
• -Wc99-c11-compat (C and Objective-C only) — warns about features not present in ISO C99 but present in ISO C11. For instance: anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc11-c2x-compat (C and Objective-C only) — warns about features not present 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. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from void * to a pointer to non-void type. Living in both worlds is hard, okay 😭
• -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall.
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is 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. This warning is enabled by -Wall. Standard-by-standard glow-up, one era at a time 👑
• -Wcast-qual — warns whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *. Girl why are we stripping the const off, that's its whole personality 💅

It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:

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

• -Wcast-align — warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. Alignment is real, bestie, respect the boundaries ✨
• -Wcast-align=strict — same idea, but stricter energy: warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine. No exceptions, no mercy 🔥
• -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.
• -Wwrite-strings — when compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find, at compile time, code that can try to write into a string constant — but only if you've been very careful about using const in declarations and prototypes. Otherwise, it's just a nuisance. This is why we did not make -Wall request these warnings. Fair, honestly 🫶

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

• -Wclobbered — warns for variables that might be changed by longjmp or vfork. Sneaky little mutation gremlins 👀 This warning is also enabled by -Wextra.
• -Wconversion — bestie, this one yells at you for implicit conversions that could literally change your value. We're talking:
  • real ↔ integer conversions, like abs (x) when x is double
  • signed ↔ unsigned conversions, like unsigned ui = -1
  • conversions to smaller types, like sqrtf (M_PI)

It stays quiet for explicit casts like abs ((int) x) and ui = (unsigned) -1, and also when the conversion doesn't actually change the value, like in abs (2.0). If the signed/unsigned warnings are too much for you, -Wno-sign-conversion turns those off.

For C++, it also warns about confusing overload resolution for user-defined conversions, plus conversions that never use a type conversion operator — conversions to void, the same type, a base class or a reference to them. And heads up: in C++, the signed/unsigned conversion warnings are off by default unless you explicitly turn on -Wsign-conversion.

Oh and one more thing: warnings about conversion from arithmetic on a small type back to that type only show up with -Warith-conversion. ✨

• -Wdangling-else — warns you about the classic "wait, which if does this else belong to??" situation. Iconic confusion, honestly. Example:

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

So in C/C++, every else branch belongs to the innermost possible if — which here is if (b). That's usually not what the programmer meant, and the indentation in that example is exactly the vibe of someone who got got. When this confusion is possible, GCC throws a warning if you passed this flag. Want it gone? Add explicit braces around the innermost if so there's zero way the else can belong to the enclosing if. Like this:

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

This warning is enabled by -Wparentheses. 🫶

• -Wdate-time — warns whenever the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they might stop you from getting bit-wise-identical reproducible compilations. Reproducibility queen behavior only. 🎀
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. Also enabled by -Wextra.
• -Wno-endif-labels — stops the warning about stray tokens after #else and #endif.
• -Wenum-compare — warns when you compare values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed, and the warning is on by default there. In C, it's enabled by -Wall.
• -Wenum-conversion (C, Objective-C only) — warns when a value of an enumerated type gets implicitly converted to a different enumerated type. Enabled by -Wextra.
• -Wjump-misses-init (C, Objective-C only) — warns if a goto or a switch jumps forward across a variable's initialization, or jumps backward to a label after the variable was initialized. It only cares about variables initialized at declaration, okay? And it's only supported for C and Objective-C — in C++, this kind of branch is an error no matter what.

-Wjump-misses-init is included in -Wc++-compat, and you can turn it off with -Wno-jump-misses-init. 💅

• -Wsign-compare — warns when comparing signed and unsigned values could give a wrong result because the signed value gets converted to unsigned. In C++, it's also enabled by -Wall. In C, it's also enabled by -Wextra.
• -Wsign-conversion — warns about implicit conversions that might flip the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast shuts it up. In C, this option is also enabled by -Wconversion.
• -Wfloat-conversion — warns about implicit conversions that reduce the precision of a real value. That includes real → integer, and higher-precision real → lower-precision real. Also enabled by -Wconversion. 😭 (rip your precision)
• -Wno-scalar-storage-order — don't warn on suspicious constructs involving reverse scalar storage order.
• -Wsizeof-pointer-div — warns about suspicious divisions of two sizeof expressions that divide pointer size by element size. That's the classic way to compute an array size, but it does not work right with pointers. So it warns about e.g. sizeof (ptr) / sizeof (ptr[0]) when ptr isn't an array, it's a pointer. Enabled by -Wall. 🕵️
• -Wsizeof-pointer-memaccess — warns about suspicious length parameters to certain string and memory built-in functions when the argument uses sizeof. It fires for example on memset (ptr, 0, sizeof (ptr)); when ptr is not an array but a pointer — and it even suggests a possible fix — or on memcpy (&foo, ptr, sizeof (&foo));.

It also warns about calls to bounded string copy functions like strncat or strncpy that use a sizeof expression of the source array as the bound. For example, in this function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake, so the call gets diagnosed:

void make_file (const char *name)
{
  char path[PATH_MAX];
  strncpy (path, name, sizeof path - 1);
  strncat (path, ".text", sizeof ".text");
  ...
}

-Wsizeof-pointer-memaccess is enabled by -Wall.

• -Wno-sizeof-array-argument — don't warn when sizeof is applied to a parameter declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
• -Wmemset-elt-size — warns about suspicious calls to the memset built-in when 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 means somebody forgot to multiply by the element size. Enabled by -Wall. 🍓
• -Wmemset-transposed-args — warns about suspicious memset calls where the second argument isn't zero and the third one is zero. For example, memset (buf, sizeof buf, 0) gets diagnosed because memset (buf, 0, sizeof buf) was clearly what you meant. The diagnostic only fires if the third argument is a literal zero — if it's an expression folded to zero, or a cast of zero to some type, it's way less likely the args were accidentally swapped, so no warning. Enabled by -Wall.
• -Waddress — warns about suspicious uses of memory addresses. That includes using the address of a function in a conditional expression, like void func(void); if (func), and comparisons against the memory address of a string literal, like if (x == "abc"). These usually scream programmer error: the address of a function always evaluates to true, so using it in a conditional usually means you forgot the parentheses in a function call; and comparing against string literals gives unspecified behavior and isn't portable in C, so it usually means you wanted strcmp. Enabled by -Wall. 💀
• -Wno-address-of-packed-member — don't warn when the address of a packed member of a struct or union is taken, which usually gives you an unaligned pointer value. This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. That includes using logical operators in contexts where a bit-wise operator is probably expected. It also warns when both operands of a logical operator are the same:

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

• -Wlogical-not-parentheses — warns about logical not used on the left hand side operand of a comparison. It doesn't warn if the right operand is considered a boolean expression. Its whole purpose is catching suspicious code like this:

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

You can silence it by wrapping the LHS in parentheses:

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

This warning is enabled by -Wall. ✨

• -Waggregate-return — warns if any function that returns structures or unions gets defined or called. (In languages where you can return an array, that also gets you a warning.)
• -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.
• -Wno-attributes — don't warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes applied to variables, etc. Note that this does not stop errors for incorrect use of supported attributes.
• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects. 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 so the prototypes of built-in functions are in scope.

For example, the call to memset below gets diagnosed 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 gets diagnosed too.

extern void* memset ();
void f (void *d)
{
  memset (d, '\0', 32);
}

---

• -Wno-builtin-macro-redefined — bestie, do NOT yell at me if certain built-in macros get redefined. This one shushes warnings for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__". 💅
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is totally allowed with no warning if it's preceded by a declaration that specifies the argument types. okay so it's giving "at least somebody told me the types" energy. 🫶)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like "static" are not the first things in a declaration. This warning is also enabled by -Wextra. 👑
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there is a previous prototype. A definition using () is not considered an old-style definition in C2X mode, because it is equivalent to (void) in that case, but is considered an old-style definition for older standards. the way that standards keep changing… 💀
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:

        void foo(bar) { }

This warning is also enabled by -Wextra.

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++. ✨
• -Wmissing-declarations — warns if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 🌸
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. For example, the following code causes such a warning, because "x.h" is implicitly zero:

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

This option does not warn about designated initializers, so the following modification does not trigger a warning:

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

In C this option does not warn about the universal zero initializer { 0 }:

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

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

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

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. slay 🎀

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code. lowkey a red flag in ur source fr fr 🙈
• -Wnormalized=[none|id|nfc|nfkc] — okay so in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning. 👑

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default. 🧚

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. and bestie that is not the vibe 💀

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🌈

• -Wno-attribute-warning — do not warn about usage of functions declared with "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 -Werror flag. 💖
• -Wno-deprecated — do not warn about usage of deprecated features.
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.
• -Wno-overflow — do not warn about compile-time overflow in constant expressions.
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default. (yes the name says "no" and the description says it's on by default, that's just how it is bestie, don't come for me 🫶)
• -Wopenmp-simd — warns if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. 🔥
• -Woverride-init (C and Objective-C only) — warns if an initialized field without side effects is overridden when using designated initializers.

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

• -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. 🙈
• -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 little benefit. For instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" does not itself have the packed attribute:

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

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

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

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

• -Wpacked-not-aligned (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 will be issued on "struct S", like, "warning: alignment 1 of 'struct S' is less than 8", in this code:

        struct __attribute__ ((aligned (8))) S8 { char a[8]; };
        struct __attribute__ ((packed)) S {
          struct S8 s8;
        };

This warning is enabled by -Wall. ⭐

• -Wpadded — warns if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. the way ur struct is just wasting bytes… rearrange ur life bestie 🎀
• -Wredundant-decls — warns if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. not me saying the same thing twice 🫶
• -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. For example, the call to the "strcpy" function below attempts to truncate the string by replacing its initial characters with the last four. However, 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 works best at -O2 and above. It is included in -Wall. 🔥

• -Wnested-externs (C and Objective-C only) — warns if an "extern" declaration is encountered within a function. 💀
• -Winline — bestie, this one yells at you when a function you swore was inline straight-up cannot be inlined. Plot twist tho: even with this flag on, the compiler stays silent about failures to inline functions that live in system headers. Not our problem, not our warning. 💅

The compiler is out here using a whole bunch of heuristics to decide whether to inline something or not. Like, it's literally weighing the size of the function being inlined against how much inlining has already gone down in the current function. So the tiniest little nothing change in your source can make -Winline warnings pop up or ghost you entirely. It's giving flaky. 😭

• -Wint-in-bool-context — warns when you're using integer values in places where a boolean was clearly the vibe. Example: conditional expressions (?:) that use non-boolean integer constants in a boolean context, like if (a <= b ? 2 : 3). Or left shifting signed integers in boolean context, like for (a = 0; 1 << a; a++);. Same energy for all kinds of multiplications regardless of data type. This warning is enabled by -Wall.
• -Wno-int-to-pointer-cast — suppresses warnings from casting an integer of a different size to a pointer type. Heads up: in C++, casting to a pointer type of smaller size is an error, not a warning. Wint-to-pointer-cast is enabled by default.
• -Wno-pointer-to-int-cast (C and Objective-C only) — suppresses warnings from casting from a pointer to an integer type of a different size. 🫶
• -Winvalid-pch — warns if a precompiled header is found chilling in the search path but cannot actually be used. Awkward. 🙈
• -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 silence the messages, 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.
• -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning, iykyk. 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. -Wno-vla prevents the -Wpedantic warning of the variable-length array. 🦋
• -Wvla-larger-than=byte-size — when this option is used, 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. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays. 🌸

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

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

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

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger. Bye bestie. 👋
• -Wvolatile-register-var — warns if a register variable is declared volatile. Real talk: the volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. Like, it's not mad at you, it's just tired. 😭
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
• -Wstack-protector — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. Protect ya neck. 🔥
• -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 — this option enables an static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them. 👑

Heads up bestie: this analysis is much more expensive than other GCC warnings. Your build is gonna take a minute. 💀

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. So like, vibe check your build first. 🫶

• -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.

• -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. Not the double-dip bestie. 😭

• -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). 🚨 Not the password in the log file, bestie!!

• -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). Rude.

• -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. The classic. 🧚

• -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.

This diagnostic is like, hey bestie that pointer is literally NULL and you're just dereferencing it?? 💀 It warns you about paths through the code where a value you know is NULL gets dereferenced. Rude of the code, honestly.

• -Wno-analyzer-stale-setjmp-buffer — okay this one needs -fanalyzer to even be a thing (that's what enables it), and you use -Wno-analyzer-stale-setjmp-buffer to turn it back off. It flags paths through the code where longjmp is called to rewind to a jmp_buf that belongs to a setjmp call in a function that has already returned. Here's the tea 🫖: when setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. So trying to rewind to it via longjmp would reference a stack frame that no longer exists, and that likely leads to a crash — or worse. 😭
• -Wno-analyzer-tainted-array-index — this one is extra, it needs both -fanalyzer and -fanalyzer-checker=taint to be enabled; use -Wno-analyzer-tainted-array-index to disable it. It warns for paths through the code where a value that could be under an attacker's control 🕵️ gets used as the index of an array access without being sanitized. Not the vibe. Not the vibe at all.
• -Wno-analyzer-unsafe-call-within-signal-handler — requires -fanalyzer (that's the enabler); -Wno-analyzer-unsafe-call-within-signal-handler disables it. It flags paths through the code where a function known to be async-signal-unsafe (like fprintf) gets called from a signal handler. 🚨
• -Wno-analyzer-use-after-free — requires -fanalyzer, which enables it; -Wno-analyzer-use-after-free disables it. It warns for paths through the code where a pointer is used after free is called on it. the way that memory is just... gone 💅
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — requires -fanalyzer, which enables it; -Wno-analyzer-use-of-pointer-in-stale-stack-frame disables it. It warns for paths through the code where a pointer is dereferenced that points to a variable in a stale stack frame. 🫠

---

Pertinent parameters for controlling the exploration 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.

The following options control the analyzer. 🧠✨

• -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 enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value). efficiency queen 👑
• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. Some checkers are disabled by default (even with -fanalyzer), such as the taint checker that implements -Wanalyzer-tainted-array-index, and this option is required to enable them.
• -fanalyzer-fine-grained — this option is intended for analyzer developers. 🛠️ Internally the analyzer builds an exploded graph that combines control flow graphs with data flow information. By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. one statement, one edge, no sharing ✨
• -fanalyzer-show-duplicate-count — this option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. the receipts of how many were silenced 💅
• -fno-analyzer-state-merge — this option is intended for analyzer developers. By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its exploded graph. With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.
• -fno-analyzer-state-purge — this option is intended for analyzer developers. By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis. With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.
• -fanalyzer-transitivity — this option enables transitivity of constraints within the analyzer. 🔗
• -fanalyzer-verbose-edges — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.
• -fanalyzer-verbose-state-changes — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
• -fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that are emitted for analyzer diagnostics. The level can be one of:
  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. both of them. the whole saga. 🍿
  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 — dump internal details about what the analyzer is doing to file.analyzer.txt. This option is overridden by -fdump-analyzer-stderr. 📄
• -fdump-analyzer-stderr — dump internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.
• -fdump-analyzer-callgraph — dump a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. 🌐
• -fdump-analyzer-exploded-graph — dump 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 — emit diagnostics showing where nodes in the exploded graph are in relation to the program source.
• -fdump-analyzer-exploded-nodes-2 — dump a textual representation of the exploded graph to file.eg.txt.
• -fdump-analyzer-exploded-nodes-3 — dump 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. 😭
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dump a representation of the supergraph suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot. 🧹
• -fdump-analyzer-supergraph — dump 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 information 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 trick. 💅

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

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

• -g — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below). 🎀
• -ggdb — produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. GDB girlies rise 🫶
• -gdwarf
• -gdwarf-version — produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 4. DWARF Version 5 is only experimental. 🧪 Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables. Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. iykyk 💜
• -gstabs — makes debug info in stabs format (if ur target even supports it), no GDB extensions. This is the format DBX uses on most BSD systems. okay but listen — on MIPS, Alpha and System V Release 4 systems this option spits out stabs debug output that DBX straight up does not understand 💀. And on System V Release 4 systems it needs the GNU assembler.
• -gstabs+ — same stabs format (if supported) but with GNU extensions that only the GNU debugger (GDB) gets. heads up bestie: using these extensions is likely to make other debuggers crash or refuse to read the program. not a vibe.
• -gxcoff — debug info in XCOFF format (if supported). This is the format the DBX debugger uses on IBM RS/6000 systems. 🎀
• -gxcoff+ — XCOFF format (if supported) but with GNU extensions understood only by GDB. fr be careful: using these extensions is likely to make other debuggers crash or refuse to read the program, AND it may cause assemblers other than the GNU assembler (GAS) to fail with an error. 😭
• -gvms — debug info in Alpha/VMS debug format (if supported). This is the format DEBUG uses on Alpha/VMS systems. 🦋
• -glevel, -ggdblevel, -gstabslevel, -gxcofflevel, -gvmslevel — ask for debug info AND use level to say how much info. The default level is 2. ✨
  • Level 0 produces no debug info at all. so -g0 negates -g. periodt.
  • Level 1 produces minimal info — enough for making backtraces in parts of the program u don't plan to debug. It includes descriptions of functions and external variables, and line number tables, but no info about local variables.
  • Level 3 includes extra info, like all the macro definitions present in the program. Some debuggers support macro expansion when u use -g3. 💅
  • If u use multiple -g options, with or without level numbers, the last such option is the one that's effective. (last one wins, always.)
  • -gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF.
• -fno-eliminate-unused-debug-symbols — by default, no debug info is produced for symbols that aren't actually used. Use this option if u want debug info for all symbols. 🫶
• -femit-class-debug-always — instead of emitting debugging info for a C++ class in only one object file, emit it in all object files using the class. only use this with debuggers that can't handle the way GCC normally emits debugging info for classes, because using this option increases the size of debugging info by as much as a factor of two. 💀
• -fno-merge-debug-strings — tells the linker to not merge together strings in the debugging info that are identical in different object files. Merging isn't supported by all assemblers or linkers. Merging decreases the size of the debug info in the output file at the cost of increasing link processing time. Merging is enabled by default. 🌸
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging info describing them as if the files resided in directory new instead. This can replace a build-time path with an install-time path in the debug info. It can also change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map. 🌈
• -fvar-tracking — run the variable tracking pass. It computes where variables are stored at each position in code. Better debugging info is then generated (if the debugging info format supports this info). 💖

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

• -fvar-tracking-assignments — annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the whole compilation all the way to the end, to try to improve debug info while optimizing. Using -gdwarf-4 is recommended along with it. ⭐

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

• -gsplit-dwarf — separate as much DWARF debugging info as possible into a separate output file with the extension .dwo. This option lets the build system avoid linking files with debug info. To be useful, this option requires a debugger capable of reading .dwo files. 🧚
• -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. 🔥
• -gpubnames — generate DWARF .debug_pubnames and .debug_pubtypes sections.
• -ggnu-pubnames — generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.
• -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It's more efficient to put them in a separate comdat section 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. 🙈
• -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 are 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 — disallow using extensions of 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.
• -gno-strict-dwarf — allow using extensions of a later DWARF standard version than the one selected with -gdwarf-version. (yep, this is the flip side 🦋)
• -gas-loc-support — tell the compiler that the assembler supports .loc directives. It may then use them for the assembler to generate DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. ✨

This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.

• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.
• -gas-locview-support — tell the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives.

This option will be enabled by default if, at GCC configure time, the assembler was found to support them.

• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column info into DWARF debugging info, rather than just file and line. This option is enabled by default. 💅
• -gstatement-frontiers, -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. This is 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 — augment variable location lists with progressive view numbers implied from the line number table. This enables debug info consumers to 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 cause the compiler to 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 they can be consumed by debug info consumers that aren't aware of these augmentations — but they won't derive any benefit from them either. 😭

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 isn't enabled, that will most certainly lead to silently mismatching location views. 💀

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, and is only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists using it. 🙈

• -ginternal-reset-location-views, -gno-internal-reset-location-views — okay so this one tries to figure out which location views can just be yeeted from the location view lists ✨. The catch? It needs the compiler to have super accurate insn length estimates, and bestie, that is not always the case 😭. And if you're using an assembler that doesn't support location view lists, it can silently generate wrong view lists. The GNU assembler will call that out as a "view number mismatch" 💀. This is only turned on for ports that define a reliable estimation function, so like, don't worry about it if yours doesn't.
• -ginline-points, -gno-inline-points — 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 into debug info. Fun fact: you can enable this independently of location views — but then the view numbers won't be output. It can only be enabled along with statement frontiers tho, and it's only on by default if location views are enabled. iykyk 💅
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported 🌸. Don't pass a type? The default depends on what your assembler and linker can actually do. type can be one of none (no compression, we're keeping it raw), zlib (zlib compression in ELF gABI format), or zlib-gnu (zlib compression in traditional GNU format). If the linker can't write compressed debug sections, the option gets rejected, periodt 🙈. Otherwise, if the assembler doesn't support them, -gz is silently ignored while producing object files.
• -femit-struct-debug-baseonly — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the struct is defined 👑

This option substantially shrinks your debugging info size, but at significant potential loss of type info to the debugger 😭. Check -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-reduced — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined — unless the struct is a template or defined in a system header 💖.

This one significantly reduces debug info size, with some potential loss of type info to the debugger. See -femit-struct-debug-baseonly for a more aggressive option, and -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-detailed[=spec-list] — lets you specify exactly which struct-like types the compiler generates debug info for 🎀. The whole point is reducing duplicate struct debug info between different object files in the same program.

This is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and it serves for most needs, ngl.

A specification has the syntax:

[dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the spec to structs that are 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 happen through pointers to structs — like, when using an incomplete struct is valid, that use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:) ⭐. Generic structs are a bit complicated to explain, fr fr. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed doesn't implement them yet.

The third word specifies the source files for those structs the compiler should emit debug info for 💜. The values none and any have the normal meaning. The value base means the base of the name of the file where the type declaration appears has to match the base of the name of the main compilation file. In practice, that means when you're compiling foo.c, debug info gets generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application, bestie 🔥.

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

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler-generated ".eh_frame" section instead of using GAS ".cfi_*" directives 💫.
• -fno-eliminate-unused-debug-types — so normally, when producing DWARF output, GCC avoids making debug symbol output for types that are nowhere used in the source file being compiled 🙈. But sometimes it's useful to have GCC emit debugging info for all types declared in a compilation unit, whether or not they're actually used in it — like if you wanna cast a value to a type in the debugger that isn't used in your program (but is declared). More often tho? This just results in a significant amount of wasted space 😭.

---

✨ Options That Control Optimization ✨

These options control various sorts of optimizations, 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. Iconic, honestly.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program 🫶.

The compiler performs optimization based on the knowledge it has of the program 👑. Compiling multiple files at once to a single output file mode lets the compiler use info gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag — only optimizations that have a flag are listed in this section, periodt.

Most optimizations are completely disabled at -O0 or if an -O level isn't 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 the ones listed here 🌸. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations enabled at each level. Slay.

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

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

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-profile
-fipa-pure-const -fipa-reference -fipa-reference-addressable
-fmerge-constants -fmove-loop-invariants -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time

• -O2 — Optimize even more 🔥. GCC performs nearly all supported optimizations that don't involve a space-speed tradeoff. Compared to -O, this option increases both compilation time and the performance of the generated code.

-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 👀.

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, bestie.
• -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 making the code smaller. 💖

• -Ofast — bestie, this one literally says "rules are fake" 😌. It disregards strict standards compliance. -Ofast enables all -O3 optimizations, and on top of that it turns on optimizations that are not valid for all standard-compliant programs. It flips on -ffast-math, -fallow-store-data-races, and the Fortran-specific -fstack-arrays (unless you specified -fmax-stack-var-size), plus -fno-protect-parens.
• -Og — optimize the debugging experience ✨. -Og should be your go-to optimization level 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 pick than -O0 for producing debuggable code, because some compiler passes that collect debug info are just... disabled at -O0. 🙈

Same energy as -O0, -Og completely disables a bunch of optimization passes, so the individual options controlling them have no effect. Otherwise, -Og enables all -O1 optimization flags except for the ones that might interfere 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

Okay so real talk: if you use multiple -O options, with or without level numbers, the last one is the one that's actually effective. No cap. 👑

---

Options of the form -fflag are the machine-independent flags, iykyk. 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. 💅

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

---

• -fno-defer-pop — for machines that have to pop arguments after a function call, always pop the arguments the moment each function returns. At levels -O1 and higher, -fdefer-pop is the default; that lets the compiler let arguments pile up on the stack across several function calls and then pop them all at once. 🫶
• -fforward-propagate — perform a forward propagation pass on RTL. This pass tries to combine two instructions and checks whether the result can be simplified. If loop unrolling is active, two passes get 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 — okay so:
  • -ffp-contract=off disables floating-point expression contraction.
  • -ffp-contract=fast enables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them.
  • -ffp-contract=on enables floating-point expression contraction if the language standard allows it. This is currently not implemented and treated equal to -ffp-contract=off. 😭

The default is -ffp-contract=fast.

• -fomit-frame-pointer — omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up, and restore the frame pointer; on many targets it also 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 does not guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions. 🙈

Enabled by default at -O and higher.

• -foptimize-sibling-calls — optimize sibling and tail recursive calls.

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

• -foptimize-strlen — optimize various standard C string functions (like strlen, strchr, or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. 🔥

Enabled at levels -O2, -O3.

• -fno-inline — do not expand any functions inline, apart from 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 — integrate functions into their callers when their body is smaller than the expected function call code (so the overall size of the program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even the ones not declared inline. ✨

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

• -findirect-inlining — inline indirect calls too, when they're 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 — consider all functions for inlining, even if they're not 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 — consider all static functions called once for inlining into their caller, even if they're not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right. 🌸

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

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

Enabled by default.

• -fipa-sra — perform 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. It in no way represents 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, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file.
• -fkeep-static-functions — emit static functions into the object file, even if the function is never used. 🙈
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check whether a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option. ✨

• -fmerge-constants — attempt to merge identical constants (string constants and floating-point constants) across compilation units. 🫶

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

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

• -fmerge-all-constants — attempt to merge identical constants and identical variables. 🔥

This option implies -fmerge-constants. In addition to -fmerge-constants, this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. ⚠️

• -fmodulo-sched — perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. 🌀
• -fmodulo-sched-allow-regmoves — perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-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 — disable the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-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. 💅

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• -fno-function-cse — okay so this one says: do NOT put function addresses in registers, bestie. Every single instruction that calls a constant function has to spell out that function's address explicitly, right there, in the open. 🫶

It gives you less efficient code ngl, but some weird little hacks that mess with the assembler output can get confused by the optimizations that happen when this option isn't used. So it's a vibe-preservation thing. 😭

The default is -ffunction-cse.

• -fno-zero-initialized-in-bss — if your target supports a BSS section, GCC by default yeets variables that are initialized to zero into BSS. Saves space in the resulting code, iconic behavior honestly. ✨

This option turns that off, because some programs explicitly rely on variables going to the data section — like, so the resulting executable can find the beginning of that section and/or make assumptions based on that. They built their whole personality around it. 🙈

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

• -fthread-jumps — does optimizations that check whether a jump branches to a location where another comparison subsumed by the first is found. 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 occupies multiple registers, like long long on a 32-bit system, this splits the registers apart and allocates them independently. Normally it generates better code for those types, but it may make debugging more difficult. Trade-offs, always. 💀

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

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

This is the default on some targets.

• -fcse-follow-jumps — in common subexpression elimination (CSE), scan through jump instructions when the target of the jump isn't reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

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

• -fcse-skip-blocks — 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 causes CSE to follow the jump around the body of the if.

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

• -frerun-cse-after-loop — re-run common subexpression elimination after loop optimizations are performed. Round two, bestie. 🔥

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

• -fgcse — performs a global common subexpression elimination pass. This pass also does global constant and copy propagation. It's giving overachiever. 👑

Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

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

• -fgcse-lm — when -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. Literally rearranging the whole situation. ✨

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. The glow-up is real. 💖

Not enabled at any optimization level.

• -fgcse-las — when -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

Not enabled at any optimization level.

• -fgcse-after-reload — when -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling. It's the cleanup crew. 🧹

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

• -faggressive-loop-optimizations — this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by, for example, causing signed integer overflows or out-of-bound array accesses. 🚨 (yes that's a real assumption, don't be weird about it)

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. So like, don't assume, okay? 🙈
• -fcrossjumping — performs 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 — performs dead code elimination (DCE) on RTL. Enabled by default at -O and higher. RIP to the code that didn't make it. 🪦
• -fdse — performs 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. With this option, the base and complete variants are changed to be thunks that call a common implementation. Deduplication slay. 💅

Enabled by -Os.

• -fdelete-null-pointer-checks — assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior. 👀

This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled.

Passes that use the dataflow information are enabled independently at different optimization levels.

• -fdevirtualize — attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
• -fdevirtualize-speculatively — attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form. It's giving "we tried, and honestly? we'd do it again". 🌈
• -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. Big data, big cost. 💀
• -fexpensive-optimizations — perform a number of minor optimizations that are relatively expensive. The name says it all, bestie.

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

• -free — attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.

Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.

• -fno-lifetime-dse — in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
• -flive-range-shrinkage — attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. Tiny register set, big dreams. 🎀
• -fira-algorithm=algorithm — picks the coloring algorithm for the integrated register allocator, bestie. algorithm can be priority (that's Chow's priority coloring) or CB (that's Chaitin-Briggs coloring). Now here's the tea: CB isn't implemented for every architecture, but for the targets that do support it, it's the default because it generates better code. Periodt. 💅
• -fira-region=region — chooses the regions used by the integrated register allocator. region has to be one of these three, okay?
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set. ✨
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and it's enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — use all functions as a single region. This typically results in the smallest code size, and it's enabled by default for -Os or -O0.
• -fira-hoist-pressure — makes IRA evaluate register pressure in the code hoisting pass so it can decide whether to hoist expressions. Usually this gives you smaller code, but lowkey it can slow the compiler down. 🙈

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 this generates faster and smaller code on machines with large register files (>= 32 registers), but again, it can slow the compiler down. No cap.

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

• -fno-ira-share-save-slots — turns off sharing of stack slots used for saving call-used hard registers living through a call. Every single hard register gets its own separate stack slot, and as a result, function stack frames are bigger. It's giving "more space, less sharing." 💀
• -fno-ira-share-spill-slots — turns off sharing of stack slots allocated for pseudo-registers. Each pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result, function stack frames are larger. Yeah. Same energy.
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Iconic behavior fr. 🔥

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

• -fdelayed-branch — if the target machine supports it, this attempts to reorder instructions to exploit instruction slots available after delayed branch instructions. Slay.

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

• -fschedule-insns — if the target machine supports it, this attempts to reorder instructions to eliminate execution stalls due to required data being unavailable. It helps machines that have 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. 🦋

Enabled at levels -O2, -O3.

• -fschedule-insns2 — same vibe as -fschedule-insns, but it requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. ✨

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

• -fno-sched-interblock — 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 option 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. Big brain move.
• -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. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. The name literally tells you it's dangerous, bestie. Respect the label. 😭
• -fsched-stalled-insns and -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. -fno-sched-stalled-insns means no insns get moved prematurely, -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. And -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1. 🌸
• -fsched-stalled-insns-dep and -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) are examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. And -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1. 💜
• -fsched2-use-superblocks — when scheduling after register allocation, this uses superblock scheduling. That allows motion across basic block boundaries, resulting in faster schedules. Heads up though: this option is experimental, because 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. This heuristic favors the instruction that belongs to a schedule group. It's 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. This heuristic 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. This heuristic 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. This heuristic 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. This heuristic 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. This heuristic 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 — so modulo scheduling is performed before traditional scheduling. And if a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior. 🎀
• -fselective-scheduling — schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
• -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.
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, this also pipelines 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 is that 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. And while this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it is expensive in the terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak. 🧚
• -fshrink-wrap — emits function prologues only before the parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher. Main character energy for your prologue. ⭐
• -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. 💖

-fcaller-saves

okay so this one lets the compiler stash values in registers that function calls would normally clobber — and to make that work, it emits extra instructions to save and restore those registers around the calls. 💾 It only does this when it actually looks like it'll produce better code, so it's not out here wasting your cycles for fun.

On some machines this is always on by default — usually the ones that have literally no call-preserved registers to fall back on. iykyk.

Enabled at levels -O2, -O3, -Os. ✨

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-fcombine-stack-adjustments

Tracks stack adjustments (the pushes and pops) plus stack memory references, and then goes looking for ways to combine them. Little glow-up for your stack traffic. 💅

Enabled by default at -O1 and higher.

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-fipa-ra

Uses caller save registers for allocation if no called function actually touches those registers. When that's true, there's no need to save and restore them around calls at all — chef's kiss. 🫶

But bestie, there's a catch: 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 it gets 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. 😭

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-fconserve-stack

Attempts to minimize stack usage. The compiler will try to use less stack space even if that makes the program slower — it's giving "be frugal, queen." 🌸

This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.

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-ftree-reassoc

Performs reassociation on trees. 🌳 This flag is enabled by default at -O and higher.

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-fcode-hoisting

Performs code hoisting. What that means: it tries to move the evaluation of expressions that get executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps code speed too. ✨

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

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-ftree-pre

Performs partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3. 🔥

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-ftree-partial-pre

Makes partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.

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-ftree-forwprop

Performs forward propagation on trees. This flag is enabled by default at -O and higher. 💖

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-ftree-fre

Performs full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. So this analysis is faster than PRE, though it exposes fewer redundancies. Tradeoffs, bestie. ⚖️

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

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-ftree-phiprop

Performs hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher. 🦋

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-fhoist-adjacent-loads

Speculatively hoists loads from both branches of an if-then-else — but only if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Very specific, very slay. 💅

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

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-ftree-copy-prop

Performs copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher. ✨

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-fipa-pure-const

Discovers which functions are pure or constant. Enabled by default at -O and higher.

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-fipa-reference

Discovers which static variables do not escape the compilation unit. Enabled by default at -O and higher. 🎀

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-fipa-reference-addressable

Discovers read-only, write-only and non-addressable static variables. Enabled by default at -O and higher. 🌸

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-fipa-stack-alignment

Reduces stack alignment on call sites if possible. Enabled by default. 💖

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-fipa-pta

Performs interprocedural pointer analysis plus interprocedural modification and reference analysis. Heads up 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.

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-fipa-profile

Performs interprocedural profile propagation. Functions called only from cold functions get 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.

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-fipa-cp

Performs interprocedural constant propagation. This optimization analyzes the program to figure out when values passed to functions are constants, and then optimizes accordingly. If your app passes constants to functions, this can substantially increase performance. 🚀

This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile.

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-fipa-cp-clone

Performs function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when an externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size — see --param ipa-cp-unit-growth=value. 😬

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

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-fipa-bit-cp

When enabled, performs 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. 🔗

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-fipa-vrp

When enabled, performs interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled. ✨

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-fipa-icf

Performs Identical Code Folding for functions and read-only variables. This optimization reduces code size, and it 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 the 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.

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-flive-patching=level

Controls 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, that latter one is called an impacted function of the former. And bestie, 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 enables 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 enables 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

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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). 🚫

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-fisolate-erroneous-paths-dereference

Detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. 🪤

This flag is enabled by default at -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled.

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-fisolate-erroneous-paths-attribute

Detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. 🪤

This is not currently enabled, but it may be enabled by -O2 in the future. 🔮

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-ftree-sink

Performs forward store motion on trees. This flag is enabled by default at -O and higher. 🌈

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-ftree-bit-ccp

Performs sparse conditional bit constant propagation on trees, and propagates pointer alignment information. This pass only operates on local scalar variables, and it's enabled by default at -O1 and higher — except for -Og. It requires that -ftree-ccp is enabled. 🔗

---

-ftree-ccp

Performs sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables, and it's enabled by default at -O and higher. ✨

---

-fssa-backprop

Propagates information about uses of a value up the definition chain, in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. 🪄

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, and it's on by default at -O1 and higher... except -Og, obviously, because -Og said "not today" 💅
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array ✨ enabled by default at -O2 and higher.
• -ftree-tail-merge — hunts for identical code sequences, and when it finds them, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. On by default at -O2 and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter. 🎀
• -ftree-dce — performs dead code elimination (DCE) on trees. Enabled by default at -O and higher. bye bye useless code 💀
• -ftree-builtin-call-dce — performs conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is not also specified. not me being conditional about it 🙈
• -ffinite-loops — assumes 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). This flag is enabled by default at -O and higher. it's giving "clean up your room" but for IR 🧹
• -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 that case the earlier store can be deleted. This flag is 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. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size. ⚠️
• -ftree-loop-optimize — performs loop optimizations on trees. This flag is 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. the glow-up round trip 💫
• -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. Parallelizes all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops. not it skipping the vibe check on profitability 😭
• -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. This is 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. This flag is 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. swapping the loop order so your cache stops crying 🔥

• -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. glow-up for your loop counters 💅
• *-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. This flag is 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. This flag is enabled by default at -O1 and higher, except for -Og. your structs said "not today, memory" 🙈
• -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. This is 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. This is 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. This is enabled by default at -O and higher. turning multiplication into something cheaper, we love a budget queen 👑
• -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 — does basic block vectorization on trees 🧚 like it's taking your straight-line code and making it do the whole squad's work at once. It's on by default at -O3, and also gets switched on by -ftree-vectorize, -fprofile-use, and -fauto-profile. no cap ✨
• -fvect-cost-model=model — swap out the cost model that vectorization uses to decide if it's even worth it 💅 model has to be one of unlimited, dynamic, or cheap. Here's the tea on each:
  • unlimited — the vectorized code-path is just assumed to be profitable. no questions, full send 🫶
  • dynamic — a runtime check guards the vectorized code-path, so it only kicks in for iteration counts that will likely run faster than the original scalar loop. it's giving "let me check first, bestie" 🔥
  • cheap — straight-up disables vectorization of loops where doing it would be cost prohibitive, like when you'd need runtime checks for data dependence or alignment. otherwise? identical to dynamic.

The default cost model depends on your other optimization flags, and it's either dynamic or cheap 🎀

• -fsimd-cost-model=model — same energy as above but specifically for vectorizing loops marked with the OpenMP simd directive ✨ model should be one of unlimited, dynamic, cheap. Every value of model means exactly what it means in -fvect-cost-model, and by default the cost model you set with -fvect-cost-model is the one that gets used 🌸
• -ftree-vrp — does Value Range Propagation on trees 🌈. It's like the constant propagation pass but instead of propagating values, it propagates ranges of values. Which means the optimizers get to yeet unnecessary range checks — array bound checks, null pointer checks, all that noise 🗑️. On by default at -O2 and higher. Oh and heads up: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled 💀
• -fsplit-paths — splits the paths that lead to loop backedges 🦋. This can level up dead code elimination and common subexpression elimination. Enabled by default at -O3 and above ✨
• -fsplit-ivs-in-unroller — lets values of induction variables in later iterations of the unrolled loop be expressed using the value from the first iteration 🎀. This breaks those long dependency chains, which makes the scheduling passes way more efficient. periodt.

Ngl, a combo of -fweb and CSE is often enough to get the same effect. However, that's not reliable when the loop body is more complicated than a single basic block 😭. It also straight up does not work at all on some architectures because of restrictions in the CSE pass. rip.

This optimization is enabled by default. 💖

• -fvariable-expansion-in-unroller — with this one, the compiler makes multiple copies of some local variables while unrolling a loop, which can give you superior code 👑.

Enabled by default for PowerPC targets, but disabled by default everywhere else 🙈

• -fpartial-inlining — inlines parts of functions 🧩. Important: this option only does anything when inlining itself is already turned on by -finline-functions or -finline-small-functions. iykyk.

Enabled at levels -O2, -O3, -Os ✨

• -fpredictive-commoning — does predictive commoning optimization, aka reusing computations (especially memory loads and stores) that already happened in previous iterations of loops ♻️🔥.

Enabled at level -O3. It's also switched on by -fprofile-use and -fauto-profile 💅

• -fprefetch-loop-arrays — if your target machine supports it, this generates instructions to prefetch memory, which improves performance of loops that access large arrays 📚✨.

Careful bestie — this option may generate better or worse code. Results are highly dependent on the structure of loops within the source code 😭 so it's a vibe check every time.

Disabled at level -Os 💀

• -fno-printf-return-value — says do not substitute constants for the known return value of formatted output functions like sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf) 🙈. That transformation is what 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 determining the exact return value possible 🔮. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer, because the return value is guaranteed to be at most 8. slay 💅

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

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

• -fno-peephole and -fno-peephole2 — disable any machine-specific peephole optimizations 👀. The difference between -fno-peephole and -fno-peephole2 is just how they're implemented in the compiler — some targets use one, some use the other, and a few use both 🌈.

-fpeephole is enabled by default. -fpeephole2 is enabled at levels -O2, -O3, -Os 💖

• -fno-guess-branch-probability — tells GCC not to guess branch probabilities using heuristics 🙅‍♀️.

So here's the deal: GCC uses heuristics to guess branch probabilities when they aren't provided by profiling feedback (-fprofile-arcs). Those 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 get 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 an expression with the __builtin_expect_with_probability built-in function 👑

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

• -freorder-blocks — reorders basic blocks in the compiled function so there are fewer taken branches and better code locality 🚦💫.

Enabled at levels -O, -O2, -O3, -Os ✨

• -freorder-blocks-algorithm=algorithm — picks the algorithm used for basic block reordering 🎛️. The algorithm argument can be:
  • simple — does not increase code size (except sometimes from secondary effects like alignment) 🍓
  • stc — the "software trace cache" algorithm, which tries to put all often-executed code together, minimizing the number of branches executed by making extra copies of code 🔥

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

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

This optimization is automatically turned off when exception handling or unwind tables are present (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 😭. Also, when -fsplit-stack is used, this option is not enabled by default (to avoid linker errors) — but you can enable it 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 🌈. It's implemented using special subsections: .text.hot for the most frequently executed functions and .text.unlikely for the unlikely-executed ones 👑. The reordering is done by the linker, so your object file format must support named sections and the linker has to place them in a reasonable way. no pressure or anything 💀

Heads up: this option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes ✍️.

Enabled at levels -O2, -O3, -Os 🎀

• -fstrict-aliasing — 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. Specifically: an object of one type is assumed to never live at the same address as an object of a different type, unless the types are almost the same. Like, an unsigned int can alias an int, but it can not alias a void* or a double 🙅‍♀️. A character type, though? A character type may alias any other type 💅

Pay extra 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;
}

Reading from a different union member than the one most recently written to (this is called "type-punning") is super common 🍓. And even with -fstrict-aliasing, type-punning is allowed — as long as 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, accessing by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior 😱 — even if the cast uses a union type, like this:

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

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os 🔥

• -falign-functions, -falign-functions=n, -falign-functions=n:m, -falign-functions=n:m:n2, -falign-functions=n:m:n2:m2 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes 🎯. This guarantees 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, because examples are everything:

• -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 🍓
• -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less 💅

The second pair of n2:m2 values lets you set up a secondary alignment, bestie: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can happen by skipping 6 bytes or fewer, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or fewer. If m2 isn't specified, it defaults to n2. 🌸

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 they mean functions are not aligned.

If n isn't specified or is zero, it uses a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. ✨

---

• -flimit-function-alignment — turn this on and 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. 💅

---

🌈 the -falign-labels family

• -falign-labels
• -falign-labels=n
• -falign-labels=n:m
• -falign-labels=n:m:n2
• -falign-labels=n:m:n2:m2 — align all branch targets to a power-of-two boundary. 🎯

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

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead. 👑

If n isn't specified or is zero, it uses 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. 💖

---

🌀 the -falign-loops family

• -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. 🔁

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 isn't specified or is zero, it uses a machine-dependent default.

Enabled at levels -O2, -O3. 🌟

---

🦋 the -falign-jumps family

• -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 isn't specified or is zero, it uses 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. 😭

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 assigns each web its own 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? Yeah, those get passed straight through to the link stage, and their setting matches the compile-step one at function granularity. ✨ Lowkey important note: this only matters for diagnostics that get emitted during optimization. And another one — code transforms like inlining can literally lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time. 😭

When you need to pass options to the assembler via -Wa or -Xassembler, bestie, make sure you either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. No cap. You can alternatively also specify assembler options at LTO link time, so there's that 🫶

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

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

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

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

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

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

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

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. ✨

If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. 💖

Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin). 👑

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

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

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 🌸

You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. 🦋

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

---

✨ OPTIONS ✨ (continued)

• -flto-partition=alg — bestie, this picks the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create a new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only. 🙈 The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase. 💅
• -flto-compression-level=n — this one 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 is not given, a default balanced compression setting is used. 🎀
• -fuse-linker-plugin — enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. 🔥

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

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). ✨

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

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

Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib). 🌸

The default is -fno-fat-lto-objects on targets with linker plugin support. 💅

• -fcompare-elim — after register allocation and post-register allocation instruction splitting, this identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, it eliminates the explicit comparison operation. 🦋

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

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

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

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

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

This option is enabled by -fauto-profile. 💖

• -fprofile-partial-training — with -fprofile-use, all portions of programs not executed during the train run are optimized agressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, program may contain functions specific for a given hardware and trianing may not cover all hardware configurations 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 is not representative, but also leads to significantly bigger code. 🙈
• -fprofile-use
• -fprofile-use=path — enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available: 👑

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Bestie, plot twist: before you can even use this option, you have to generate profiling information first. No profile data, no party. 💅

By default, GCC throws an error if the feedback profiles don't match the source code. You can turn that error into a warning with -Wno-error=coverage-mismatch — but heads up, that may give you poorly optimized code. 😭 Also, by default GCC throws a warning if the feedback profiles straight up don't exist (peep -Wmissing-profile for that one).

If path is specified, GCC goes looking at path to find the profile feedback data files. See -fprofile-dir. 🗂️

---

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

Enable sampling-based feedback-directed optimizations, plus all of these optimizations — many of which are honestly only worth it when profile feedback is 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

p<span>ath</span> is the name of a file containing AutoFDO profile information. If you leave it out, it defaults to fbdata.afdo in the current directory. 📁

To make an AutoFDO profile data file, you have to run your program with the perf utility on a supported GNU/Linux target system. More info here: https://perf.wiki.kernel.org/

E.g.

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

Then use the create_gcov tool to convert that raw profile data into a format GCC can actually use. You must also hand this tool the unstripped binary for your program. See https://github.com/google/autofdo.

E.g.

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

---

The next batch of options controls how the compiler behaves around floating-point arithmetic. These options are all about trading speed against correctness, and all of them have to be specifically enabled — no sneaky defaults turning them on for you. ✨

🌸 -ffloat-store 🌸

Don't store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is pulled from a register or from memory.

This option prevents that undesirable excess precision on machines like the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same deal on 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 gives you further control over excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types.

By default, -fexcess-precision=fast is in effect. What that means: operations may be carried out in a wider precision than the types specified in the source, if that would result in faster code, and it's unpredictable when rounding to the types specified in the source code takes place. 🙈

When compiling C, if -fexcess-precision=standard is specified, then excess precision follows the rules specified in ISO C99. In particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used.

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

💖 -ffast-math 💖

Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.

This option causes the preprocessor macro __FAST_MATH__ to be defined. 👑

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

🎀 -fno-math-errno 🎀

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

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. So there's literally no reason for the compiler to consider the possibility that it might — and -fno-math-errno is the default there. 🍎

🌈 -funsafe-math-optimizations 🌈

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

This option is not turned on by any -O option, 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. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

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

⭐ -fassociative-math ⭐

Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result.

NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required.

This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

🍓 -freciprocal-math 🍓

Allow the reciprocal of a value to be used instead of dividing by the value, if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 😬

The default is -fno-reciprocal-math.

🧚 -ffinite-math-only 🧚

Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

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

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

💜 -fno-signed-zeros 💜

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

The default is -fsigned-zeros.

🫶 -fno-trapping-math 🫶

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

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

The default is -ftrapping-math.

👑 -frounding-math 👑

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

The default is -fno-rounding-math.

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

• -fsignaling-nans — okay so this one tells GCC to compile your code assuming that IEEE signaling NaNs might throw user-visible traps while floating-point ops are happening. Turning this on disables optimizations that could change how many exceptions are visible with signaling NaNs, so it's the cautious-girlie option. This option implies -ftrapping-math. 💅

It also makes the preprocessor macro __SUPPORT_SNAN__ get defined, which is lowkey a whole thing.

The default is -fno-signaling-nans.

Heads up bestie: this option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. So like... it's giving "trying her best," not "legally binding promise." 😭

---

• -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. ISO C99 and C11 are totally fine with these functions raising inexact, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C2X, says nope, they can't do that. ✨

The default is -ffp-int-builtin-inexact, which allows the exception to be raised — unless C2X or a later C standard is selected. And this option does nothing unless -ftrapping-math is in effect. fr fr, it's powerless without its partner.

Even if you use -fno-fp-int-builtin-inexact, if the functions generate a call to a library function then the inexact exception may still be raised if the library implementation doesn't follow TS 18661. 🙈

---

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. No silent glow-up to double, periodt.

---

• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there's no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but it gets enabled by -ffast-math. 🦋

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless — and this is the iconic part — the option applies to all languages. She does not discriminate. 👑

---

• -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, with an attempt to rescue the situation in that case.

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

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code. 💀 (yes, literally broken code, proceed with the vibes of a girl who reads the warnings)

• -fbranch-probabilities — after running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The 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 switching up the fit between takes. 🎀

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they'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. She's not guessing anymore, she knows. 💅

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

• -fprofile-values — if combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered. 🍓

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

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

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order. Neat and tidy, she loves a lineup. ✨

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions. 🔍

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. iykyk.

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". 😭 sobbing but worth it for the speed sometimes.

Enabled by default with -funroll-loops.

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

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 isn't always better" but make it compiler flags. 💅

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

• -funroll-all-loops — unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. 💀 -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there'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). Like a glow-up peel but for loops. 🧖‍♀️

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, main character energy. 👑

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). She said "get out of the loop, girl." 🚪

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. Every function gets her own lil room. 🎀

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. So like... it costs you, bestie. Know the price. 💅

• -fstdarg-opt — optimize the prologue of variadic argument functions with respect to usage of those arguments.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found. 👀

• -fsection-anchors — try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. She's the anchor friend, holding everyone together. 🫶

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];
        }

(and yes, that's literally not valid C, don't come for her, it's pseudocode 💀)

Not all targets support this option.

• --param name=value — bestie, this is the secret dial room 🔧✨ GCC has a whole bunch of internal constants that quietly decide how much optimizing it does — like, it literally won't inline a function if that function has more instructions than some magic number. --param lets you poke at those constants from the command line instead of just vibing with the defaults.

Heads up though: the actual parameter names and what their values mean are tied to the compiler's internals, and they can change without notice in future releases. So like, don't build your whole personality on them. 💀

Wanna know the minimal, maximal and default value of a parameter? You can get all of that with the --help=param -Q options. Iconic, honestly. 🎀

In each case, the value is an integer. And these are the name choices recognized for all targets:

• predictable-branch-outcome — when a branch is predicted to be taken with a probability lower than this threshold (in percent), it gets 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 is the max number of instructions in a block that should even be considered for if-conversion. And ngl, the compiler also uses other heuristics to decide whether if-conversion is likely to be profitable, so this isn't the whole story.
• 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 swap them for conditionally executed instructions. These two give the max permissible cost for the sequence that if-conversion would generate, 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. And the compiler will try to give you a reasonable default for this parameter using the BRANCH_COST target macro. 🫶
• max-crossjump-edges — the max 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. Bump this up and you get more aggressive optimization, but compile time goes up too, with probably only a small improvement in executable size. Trade-offs, bestie. 🙈
• min-crossjump-insns — the minimum number of instructions that have to match at the end of two blocks before cross-jumping is even performed on them. This value gets ignored when all the instructions in the block being cross-jumped from are matched.
• max-grow-copy-bb-insns — the max code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.
• max-goto-duplication-insns — the max number of instructions to duplicate into a block that jumps to a computed goto. To avoid O(N^2) behavior across a bunch of passes, GCC factors computed gotos early in the compilation process and unfactors them as late as possible. Only computed jumps at the end of a basic block with no more than max-goto-duplication-insns get unfactored.
• max-delay-slot-insn-search — the max number of instructions to consider when hunting for an instruction to fill a delay slot. If more than this arbitrary number of instructions get searched, the time savings from filling the delay slot are minimal, so it just stops searching. Higher values = more aggressive optimization, more compilation time, and probably only a small improvement in execution time. 😭
• max-delay-slot-live-search — when trying to fill delay slots, this is the max number of instructions to consider while searching for a block with valid live register information. Cranking this arbitrarily chosen value up means more aggressive optimization and more 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 max amount of memory that can be allocated in order to do the global common subexpression elimination optimization. If more memory than specified is required, the optimization simply doesn't happen. ✨
• max-gcse-insertion-ratio — if the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.
• max-pending-list-length — the max number of pending dependencies scheduling allows before it flushes the current state and starts over. Large functions with few branches or calls can create excessively large lists, which needlessly consume memory and resources. Not the vibe. 🍓
• max-modulo-backtrack-attempts — the max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time, so like... be careful. 💀
• max-inline-insns-single — several parameters control the tree inliner used in GCC, and this number sets the max number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++).
• max-inline-insns-auto — when you use -finline-functions (which is included in -O3), a lot of functions that would otherwise never be considered for inlining get investigated. For those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto).
• max-inline-insns-small — this is the bound applied to calls which are considered relevant with -finline-small-functions.
• max-inline-insns-size — this is the bound applied to calls which are optimized for size. A little bit of growth may be desirable to anticipate optimization opportunities exposed by inlining. 👑
• uninlined-function-insns — the number of instructions accounted by the inliner for function overhead, like function prologue and epilogue.
• uninlined-function-time — extra time accounted by the inliner for function overhead, like the 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, and inline-insns-auto when inline heuristics hint that inlining is very profitable (like, it'll enable later optimizations). Slay.
• uninlined-thunk-insns, uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time, but applied to function thunks.
• inline-min-speedup — when the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. Periodt. 💅
• large-function-insns — the limit specifying really large functions. For functions larger than this limit after inlining, inlining gets 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 the maximal growth of a large function caused by inlining, in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size.
• large-unit-insns — the limit specifying a large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of the unit is 300% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth.
• inline-unit-growth — specifies the maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.
• ipa-cp-unit-growth — specifies the maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size.
• large-stack-frame — the limit specifying large stack frames. While inlining, the algorithm tries not to grow past this limit too much.
• large-stack-frame-growth — specifies the maximal growth of large stack frames caused by inlining, in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size.
• max-inline-insns-recursive, max-inline-insns-recursive-auto — specifies the max number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. --param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining only happens 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. 🌀

okay so real talk — when a function is declared inline, --param max-inline-recursive-depth is the one running the show 💅 but for functions that are not declared inline, recursive inlining only happens when -finline-functions (which, bestie, is included in -O3) is enabled — and when that's the case, it's --param max-inline-recursive-depth-auto that applies instead. no cap.

• min-inline-recursive-probability — recursive inlining is only worth it for functions that have deep recursion on average, and it can legitimately hurt functions with a tiny recursion depth, because it bloats the prologue size or makes the function body more complex for the other optimizers to deal with 😭. When profile feedback is available (peep -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 only to call expressions whose probability exceeds the given threshold (in percents).
• early-inlining-insns — specify the growth that the early inliner is allowed to make. Effectively it cranks up the amount of inlining for code that has a large abstraction penalty. slay.
• max-early-inliner-iterations — limit of iterations of the early inliner. Basically it bounds how many nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining, so don't stress 🫶
• comdat-sharing-probability — probability (in percent) that C++ inline functions with comdat visibility get shared across multiple compilation units.
• profile-func-internal-id — a parameter controlling whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. iykyk ✨
• min-vect-loop-bound — the minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.
• gcse-cost-distance-ratio — scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. Currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions.
• gcse-unrestricted-cost — cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. Currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances 🔥
• max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in the hoisting algorithm. The value of 0 does not limit the search, but may 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 limit compilation time in tree tail merging.
• store-merging-allow-unaligned — allow the store merging pass to introduce unaligned stores if it is legal to do so 👑
• max-stores-to-merge — the maximum number of stores to attempt to merge into wider stores in the store merging pass.
• max-unrolled-insns — the maximum number of instructions a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
• max-average-unrolled-insns — the maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
• max-unroll-times — the maximum number of unrollings of a single loop.
• max-peeled-insns — the maximum number of instructions a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
• max-peel-times — the maximum number of peelings of a single loop.
• max-peel-branches — the maximum number of branches on the hot path through the peeled sequence.
• max-completely-peeled-insns — the maximum number of insns of a completely peeled loop.
• max-completely-peel-times — the maximum number of iterations of a loop to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling.
• max-unswitch-insns — the maximum number of insns of an unswitched loop.
• max-unswitch-level — the maximum number of branches unswitched in a single loop.
• lim-expensive — the minimum cost of an expensive expression in the loop invariant motion.
• min-loop-cond-split-prob — when FDO profile information is available, min-loop-cond-split-prob specifies the minimum threshold for probability of a semi-invariant condition statement to trigger loop split.
• iv-consider-all-candidates-bound — bound on the number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones get considered, to avoid quadratic time complexity.
• iv-max-considered-uses — the induction variable optimizations give up on loops that contain more induction variable uses.
• iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, always try to remove unnecessary 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 — bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
• scev-max-expr-complexity — bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.
• max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with simd pragma.
• vect-max-version-for-alignment-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
• vect-max-version-for-alias-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit. periodt.
• max-iterations-to-track — the maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
• hot-bb-count-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 is considered 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 to which the execution count of a basic block must be part of in order 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 is 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 the basic block to be considered unlikely executed. The default is 20, which means a basic block is considered unlikely executed if it is 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 in cases where a function contains a single loop with a known bound and another loop with an unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one 🥶
• builtin-expect-probability — this sets how likely an expression is to have the value you specified. It takes a percentage (so 0 ... 100), that's it, that's the whole input ✨
• builtin-string-cmp-inline-length — the max length of a constant string for a builtin string cmp call that's eligible for inlining.
• align-threshold — picks what fraction of the max execution frequency of a basic block in a function you need before that basic block gets aligned 💅
• align-loop-iterations — a loop that's expected to iterate at least this many times gets aligned.
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value caps superblock formation once a given percentage of executed instructions is covered. It's there to limit unnecessary code size expansion, bestie. The tracer-dynamic-coverage-feedback parameter only kicks in when profile feedback is available — because real profiles (as opposed to statically estimated ones) are much less balanced, which lets the threshold be a larger value.
• tracer-max-code-growth — stops tail duplication once code growth hits the given percentage. Honestly this is a pretty artificial limit, since most of the duplicates get eliminated later in cross jumping anyway, so you can set it way higher than the code growth you actually want 🫶
• tracer-min-branch-ratio — stops reverse growth when the reverse probability of the best edge drops below this threshold (in percent).
• tracer-min-branch-probability, tracer-min-branch-probability-feedback — stops forward growth if the best edge's probability is lower than this threshold. Same deal as tracer-dynamic-coverage, two parameters: tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability for compilation without. The value for compilation with profile feedback has to be more conservative (higher) to actually make the tracer effective.
• stack-clash-protection-guard-size — specify the size of the OS-provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the OS-provided guard will leave your code vulnerable to stack clash style attacks 😭 so like, be careful.
• stack-clash-protection-probe-interval — stack clash protection probes stack space as it's allocated. This param controls the max distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but again — a value larger than the OS-provided guard leaves code vulnerable to stack clash style attacks. Same warning, same energy.
• max-cse-path-length — the max number of basic blocks on a path that CSE considers.
• max-cse-insns — the max number of instructions CSE processes before flushing.
• ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation, iconic behaviour honestly. 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 can't calculate RAM on a particular platform, the lower bound of 30% is used. And setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity — extremely slow, but it can be useful for debugging 💀
• ggc-min-heapsize — the minimum size of the garbage collector's heap before it even bothers to collect garbage. The first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and it has no effect on code generation. The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS are not exceeded — but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC can't calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. And setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. Same slow-but-debuggy energy as above 🐢
• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. Increasing the value means more aggressive optimization, which bumps up compilation time with probably slightly better performance ✨
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. Increasing it means more aggressive optimization, so compilation time goes up with probably slightly better performance.
• max-sched-ready-insns — the max number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Increasing this means more thorough searches, which makes compilation time increase with probably, like, little benefit 🙈
• max-sched-region-blocks — the max number of blocks in a region to be considered for interblock scheduling.
• max-pipeline-region-blocks — the max number of blocks in a region to be considered for pipelining in the selective scheduler.
• max-sched-region-insns — the max number of insns in a region to be considered for interblock scheduling.
• max-pipeline-region-insns — the max 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 max number of iterations through CFG to extend regions. A value of 0 disables region extensions, periodt.
• max-sched-insn-conflict-delay — the max 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 — the minimal distance (in CPU cycles) between a store and a load targeting the same memory locations.
• selsched-max-lookahead — the max size of the lookahead window of selective scheduling. It's basically a depth of search for available instructions, bestie.
• selsched-max-sched-times — the max number of times an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.
• selsched-insns-to-rename — the max number of best instructions in the ready list that get considered for renaming in the selective scheduler.
• sms-min-sc — the minimum value of stage count that the swing modulo scheduler generates.
• max-last-value-rtl — the max size, measured as number of RTLs, that can be recorded in an expression in combiner for a pseudo register as the last known value of that register.
• max-combine-insns — the max number of instructions the RTL combiner tries to combine.
• integer-share-limit — small integer constants can share a data structure, which reduces the compiler's memory usage and increases its speed 🏎️. This sets the max 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's "4", so more functions end up 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 — the max number of statements allowed in a block that needs to be duplicated when threading jumps.
• max-fields-for-field-sensitive — the max number of fields in a structure treated in a field sensitive manner during pointer analysis.
• prefetch-latency — an estimate of the average number of instructions that get executed before a prefetch finishes. The distance prefetched ahead is proportional to this constant. Heads up: increasing this number may also lead to fewer streams being prefetched (see simultaneous-prefetches).
• simultaneous-prefetches — the max number of prefetches that can run at the same time.
• l1-cache-line-size — the size of a cache line in the L1 data cache, in bytes.
• l1-cache-size — the size of the L1 data cache, in kilobytes.
• l2-cache-size — the size of the 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 that the stride is non-constant may make it hard to predict when there's a clear benefit to issuing these hints. Set it to 1 if prefetch hints should be issued for non-constant strides. Set it 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 — the 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 🌸

...and the vibe check on that whole thing is: this setting is genuinely useful for processors that have hardware prefetchers, because otherwise the hardware prefetchers and the software prefetchers start beefing with each other 💀. So if your hardware prefetchers have a maximum stride they can handle, that's the number you wanna drop here — it makes the software prefetchers way more useful. ✨

A value of -1 means we don't have a threshold at all, so prefetch hints can be issued for literally any constant stride. 🦋

Real talk though bestie: this setting is only useful for strides that are known and constant. That's the whole vibe. Don't try to make it fetch something chaotic.

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loop-interchange-max-num-stmts

The maximum number of stmts in a loop that's allowed to be interchanged. That's it, that's the rule. 🎀

loop-interchange-stride-ratio

The minimum ratio between the stride of two loops for interchange to actually be profitable — like, worth the effort. If it's below this, the compiler's not interested. 💅

min-insn-to-prefetch-ratio

The minimum ratio between the number of instructions and the number of prefetches needed to enable prefetching in a loop. Basically the bar you gotta clear. 🔥

prefetch-min-insn-to-mem-ratio

The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop. Same energy, different ratio. ⭐

use-canonical-types

Whether the compiler should use the "canonical" type system. Should ALWAYS be 1 — that's the one that uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However (and this is the important bit 😭), if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. Emergency brake, use only when needed.

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. It just says no. 🙅‍♀️

max-partial-antic-length

Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. And here's the drama 🍓: for some sorts of source code, the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. Yeah. All of it. 😭

This parameter puts a limit on the length of the sets that are computed, which prevents that runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length — so like, if you enjoy chaos, that's there for you. 🙈

rpo-vn-max-loop-depth

Maximum loop depth that is value-numbered optimistically. When the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically, and the remaining ones are not. Very "some of you get invited to the party" of it. 👑

sccvn-max-alias-queries-per-access

Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit, the search is aborted and the load or store is not considered redundant. 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. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. Popular loops only, sorry. ✨

ira-max-conflict-table-size

Even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still require excessive amounts of memory for huge functions. So: 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. Speed over quality, a trade-off arc. 💀

ira-loop-reserved-regs

IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. Default of the parameter is the best found from numerous experiments — so like, they tested. 🧚

lra-inheritance-ebb-probability-cutoff

LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. 👑

loop-invariant-max-bbs-in-loop

Loop invariant motion can be very expensive — both in compilation time and in the amount of needed compile-time memory — with very large loops. So loops with more basic blocks than this parameter simply won't have loop invariant motion optimization performed on them. Not invited. 🎀

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 not 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 is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited. 😭

max-vartrack-expr-depth

Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. Set it too low and value expressions that are available and could be represented in debug information may end up not being used; set it higher and the compiler may find more complex debug expressions, but compile time and memory use may grow. Classic trade-off behavior. 💅

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. Bye bestie 👋

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. Reserved seating energy. 🎀

ipa-sra-ptr-growth-factor

IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. Only then. It's giving boundaries. ✨

ipa-sra-max-replacements

Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the maximum number of replacements of a formal parameter. Two-for-one info, slay. 💅

sra-max-scalarization-size-Ospeed

sra-max-scalarization-size-Osize

The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. Speed vs. size, the eternal showdown. ⭐

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. Tracking, but make it bounded. 🦋

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 zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. Yes, that's the definition, write it down. 📝

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. Customizable strips, iconic. 🎀

ipa-cp-value-list-size

IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. 👑

ipa-cp-eval-threshold

IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. It's grading itself and only accepting the A's. 💅

ipa-cp-max-recursive-depth

Maximum depth of recursive cloning for self-recursive function. How deep does the recursion go? This deep. 🌀

ipa-cp-min-recursive-probability

Recursive cloning only when the probability of call being executed exceeds the parameter. Gotta be likely, or it's not happening. ✨

ipa-cp-recursion-penalty

Percentage penalty the recursive functions will receive when they are evaluated for cloning. Recursion tax, bestie. 😭

ipa-cp-single-call-penalty

Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. One call? Penalty. That's just how it is. 💀

ipa-max-agg-items

IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter. 🌈

• ipa-cp-loop-hint-bonus — When IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of that candidate. 💅
• ipa-max-aa-steps — During its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. But it doesn't want to spend too much time analyzing huge functions, so after examining ipa-max-aa-steps statements that modify memory, it gives up and considers all memory clobbered. 😭
• ipa-max-switch-predicate-bounds — Maximal number of boundary endpoints of case ranges of a switch statement. For a switch that exceeds this limit, IPA-CP will not construct the cloning cost predicate (the thing used to estimate cloning benefit) for the default case of the switch statement. 🚫
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter to estimate benefit for cloning upon a certain constant value. But if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, the expression is treated as a complicated one and is not handled by IPA analysis. 💀
• lto-partitions — Specify the desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. 👑
• lto-min-partition — Size of minimal partition for WHOPR (in estimated instructions). This prevents the expense of splitting very small programs into too many partitions. 🧚
• lto-max-partition — Size of max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. Meant to be used only with balanced partitioning. ⚖️
• lto-max-streaming-parallelism — Maximal number of parallel processes used for LTO streaming. 🌊
• cxx-max-namespaces-for-diagnostic-help — The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. 🔍
• sink-frequency-threshold — The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands, as those are even more profitable to sink. 💸
• max-stores-to-sink — The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. 🛑
• case-values-threshold — The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. 🎯
• jump-table-max-growth-ratio-for-size — The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used 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). The parameter is used when optimizing for speed. 🏎️
• tree-reassoc-width — Set the maximum number of instructions executed in parallel in a reassociated tree. This parameter overrides target dependent heuristics used by default if it has a non-zero value. 💥
• sched-pressure-algorithm — Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.c in the GCC sources for more details. The default choice depends on the target. 🤷
• max-slsr-cand-scan — Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. 🔎
• asan-globals — Enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using the -fsanitize=address option. To disable global objects protection, use --param asan-globals=0. 🛡️
• asan-stack — Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using -fsanitize=address. To disable stack protection, use --param asan-stack=0. 📚
• asan-instrument-reads — Enable buffer overflow detection for memory reads. This kind of protection is enabled by default when using -fsanitize=address. To disable memory reads protection, use --param asan-instrument-reads=0. 👀
• asan-instrument-writes — Enable buffer overflow detection for memory writes. This kind of protection is enabled by default when using -fsanitize=address. To disable memory writes protection, use --param asan-instrument-writes=0. ✍️
• asan-memintrin — Enable detection for built-in functions. This kind of protection is enabled by default when using -fsanitize=address. To disable built-in functions protection, use --param asan-memintrin=0. 🧰
• asan-use-after-return — Enable detection of use-after-return. This kind of protection is enabled by default when using the -fsanitize=address option. To disable it, use --param asan-use-after-return=0. 🕵️

Note: By default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.

• asan-instrumentation-with-call-threshold — If 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 omp schedule for loops parallelized by parloops. 🍰
• parloops-schedule — Schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime). 🗓️
• parloops-min-per-thread — The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. Note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. ✌️
• max-ssa-name-query-depth — Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain. 🌀
• hsa-gen-debug-stores — Enable emission of special debug stores within HSA kernels which are then read and reported by the libgomp plugin. Generation of these stores is disabled by default; use --param hsa-gen-debug-stores=1 to enable it. 🐞
• max-speculative-devirt-maydefs — The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. 🕵️‍♀️
• max-vrp-switch-assertions — The maximum number of assertions to add along the default edge of a switch statement during VRP. 📝
• unroll-jam-min-percent — The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. 💰
• unroll-jam-max-unroll — The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. 🔄
• max-rtl-if-conversion-unpredictable-cost — Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. 🎲
• max-variable-expansions-in-unroller — If -fvariable-expansion-in-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 entry BB of the function) to make partial inlining happen. 🎯
• max-tracked-strlens — Maximum number of strings for which the strlen optimization pass will track string lengths. 🧵
• gcse-after-reload-partial-fraction — The threshold ratio for performing partial redundancy elimination after reload. ♻️
• gcse-after-reload-critical-fraction — The threshold ratio of critical edges execution count that permit performing redundancy elimination after reload. 🔁
• max-loop-header-insns — The maximum number of insns in loop header duplicated by the copy loop headers pass. 🧬
• vect-epilogues-nomask — Enable loop epilogue vectorization using smaller vector size. 🧩
• slp-max-insns-in-bb — Maximum number of instructions in basic block to be considered for SLP vectorization. 📦
• avoid-fma-max-bits — Maximum number of bits for which we avoid creating FMAs. 🚫
• sms-loop-average-count-threshold — the threshold on the average loop count that the swing modulo scheduler is willing to look at.
• sms-dfa-history — how many cycles the swing modulo scheduler peeks at when it's checking conflicts using DFA.
• max-inline-insns-recursive-auto — the max number of instructions a non-inline function is allowed to balloon 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 that the swing modulo scheduler uses when scheduling a loop.
• lra-max-considered-reload-pseudos — the max number of reload pseudos that get considered while spilling a non-reload pseudo.
• max-pow-sqrt-depth — the maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.
• max-dse-active-local-stores — the 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, and 0 means unlimited.
• graphite-max-arrays-per-scop — maximum number of arrays per scop.
• max-vartrack-reverse-op-size — max size of the loc list for which reverse ops should be added.
• tracer-dynamic-coverage-feedback — the percentage of a 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 — the scale factor to apply to the number of statements in a threading path when comparing against the number of (scaled) blocks.
• fsm-maximum-phi-arguments — the maximum number of arguments a PHI may have before the FSM threader will not try to thread through its block.
• uninit-control-dep-attempts — the maximum number of nested calls to search for control dependencies during uninitialized variable analysis.
• sra-max-scalarization-size-Osize — the maximum size, in storage units, of an aggregate that should be considered for scalarization when compiling for size.
• fsm-scale-path-blocks — the scale factor to apply to the number of blocks in a threading path when comparing against the number of (scaled) statements.
• sched-autopref-queue-depth — a hardware autoprefetcher scheduler model control flag. It's the number of lookahead cycles the model looks into; at ' ' only enable the instruction sorting heuristic.
• loop-versioning-max-inner-insns — the maximum number of instructions 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 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 when determining a property of a variable, like its value. This caps the number of iterations or recursive calls GCC performs when optimizing certain statements, or when figuring out their validity before it goes and issues 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 it terminates 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 show up in a call stack within the analyzer, before it terminates analysis of a call that would recurse deeper.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that makes it count as hot. This 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, plus 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 bestie, this whole section is about GCC sneaking extra code into the stuff it generates so you can watch it run. ✨

GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, one reason you'd do this is to collect profiling statistics so you can find program hot spots, do code coverage analysis, or run profile-guided optimizations. 💖

Another class of program instrumentation is adding run-time checking to catch programming errors — think invalid pointer dereferences or out-of-bounds array accesses — plus deliberately hostile attacks like stack smashing or C++ vtable hijacking. 😤 Not today, attacker.

There's also a general hook that can be used to implement other forms of tracing or function-level instrumentation, for debug or program analysis purposes. Very main character of it.

• -p, -pg — generate extra code to write profile information suitable for the analysis program prof (that's for -p) or gprof (that's 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. That 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 one is explicitly specified and it is not the final executable — otherwise it's 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. It's a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details. 👑
  • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
  • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations occur with different working directories.
  • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  • Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting. 🧚
  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  • For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

okay so with -fprofile-arcs, for every single function in your program GCC builds a program flow graph, and then it finds a spanning tree for that graph. The way this works: only the arcs that are not on the spanning tree actually need to be instrumented — so the compiler sprinkles in code that counts how many times those arcs get executed. ✨ And when an arc happens to be the only exit or the only entrance to a block, the instrumentation code can just vibe inside that block. Otherwise? A whole new basic block has to be created just to hold the instrumentation code. no cap. 💅

---

• -ftest-coverage — this one produces a notes file that the gcov code-coverage utility can use to show off your program's coverage. Each source file gets its own note file, and it's called auxname.gcno. Bestie, if you wanna know what auxname even means, scroll up to the -fprofile-arcs option above — it explains auxname and walks you through generating test coverage data. 🫶 Also important: your coverage data matches your source files way more closely if you don't optimize. fr fr.
• -fprofile-abs-path — automatically converts relative source file names into absolute path names inside the .gcno files. Why do we care? Because it lets gcov find the correct sources in projects where the compilations happen with different working directories. iconic behavior honestly. 🌸
• -fprofile-dir=path — sets the directory to search for the profile data files to path. Heads up though: this option only affects the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs, and the data used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths are allowed. By default, GCC uses the current directory as path, so the profile data file just shows up in the same directory as the object file. And to prevent the file name from clashing, if the object file name isn't an absolute path, we mangle the absolute path of the sourcename.gcda file and use that as the file name of a .gcda file. There's a similar option too: -fprofile-note. 👑

When an executable is run in a massive parallel environment, it's recommended to save the profile to different folders. You can do that with variables in path that get exported during run-time:

• %p — process ID. 🆔
• %q{VAR} — value of environment variable VAR.

• -fprofile-generate
• -fprofile-generate=path — enables the options that are usually used for instrumenting an application so it produces a profile that's useful for later recompilation with profile-feedback-based optimization. Bestie, you must use -fprofile-generate both when compiling and when linking your program, okay? Both times. 💀

The following options get 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 when you want to optimize the program based on all that collected profile info, you use -fprofile-use. 💖

• -fprofile-note=path — if path is specified, GCC saves the .gcno file into the path location. But watch out: if you combine this option with multiple source files, the .gcno file will be overwritten. 😭
• -fprofile-prefix-path=path — this one can be used together 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 contains files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary is different 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 those setups, you use -fprofile-prefix-path=path with path pointing to the base directory of the build, and it strips the irrelevant part of the path and keeps all file names relative to the main build directory. 🧚
• -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 doesn't properly join all its threads (or creates a detached thread), the profile file can still be corrupted. 💀

Using prefer-atomic gets transformed either into atomic, when it's supported by a target, or into single otherwise. And the GCC driver automatically selects prefer-atomic when -pthread is present in the command line. ✨

• -fprofile-filter-files=regex — instruments only the functions from files where the names match any regular expression (separated by a semi-colon).

For example, -fprofile-filter-files=main.c;module.*.c will instrument only main.c and all C files starting with 'module'. 🎀

• -fprofile-exclude-files=regex — instruments only the functions from files where the names do not match all the regular expressions (separated by a semi-colon).

For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are located in the /usr/ folder. 🚫

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild the program with the same outcome, which is useful for, like, 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's not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run but also before the fork function is invoked.

Also note that it's pretty common for execution counts of some part of programs to depend on stuff like the length of temporary file names or memory space randomization (which can affect hash-table collision rate). Such non-reproducible parts of programs can be annotated with the no_instrument_function function attribute. And gcov-dump with -l can be used to dump the gathered data and verify that it's actually reproducible. 💅

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible regardless of the order the data is streamed into the gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (like with make -j). It does reduce the quality of the gathered data though — especially of indirect call profiling. 🙈

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector. Memory access instructions get instrumented so it can detect out-of-bounds and use-after-free bugs. This option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When it's set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. And bestie, this option cannot be combined with -fsanitize=thread. 😤
• -fsanitize=kernel-address — enables AddressSanitizer for the Linux kernel. See https://github.com/google/kasan/wiki for more details. 🐧
• -fsanitize=pointer-compare — instruments comparison operations (<, <=, >, >=) with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. And it cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects the invalid operation only when both pointers are non-null. 🔍
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. And it cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects the invalid operation only when both pointers are non-null. 🔍
• -fsanitize=thread — enables ThreadSanitizer, a fast data race detector. Memory access instructions get instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. And nope — this option cannot be combined with -fsanitize=address or -fsanitize=leak. 💀

Also note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions). 🌸

• -fsanitize=leak — enables LeakSanitizer, a memory leak detector. This option only matters for linking of executables, and the executable gets linked against a library that overrides malloc and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. And this option cannot be combined with -fsanitize=thread. 💧
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations get instrumented so undefined behavior is detected 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 counts as undefined differs slightly between C and C++, and also 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 — okay so if the second arg of a shift is actually in range, this checks that the result of the shift isn't undefined 🌀. Heads up bestie: what counts as "undefined" here is lowkey different between C and C++, and also between ISO C90 and C99, etc. etc. — it's giving "it depends" 💅.
• -fsanitize=integer-divide-by-zero — catches integer division by zero, and also the iconic INT_MIN / -1 division 💀. Not me dividing by zero, fr fr.
• -fsanitize=unreachable — with this on, the compiler turns the __builtin_unreachable call into a diagnostics message call instead 📢. And like… when you actually reach that __builtin_unreachable call, the behavior is undefined. So yeah, it's a ✨vibe✨ but not a safe one.
• -fsanitize=vla-bound — tells the compiler to check that the size of a variable length array is positive 🔢. Positive size or bust.
• -fsanitize=null — pointer checking time 🫶. Specifically: an app built with this on will throw an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) gets bound to a NULL pointer, or if a method gets invoked on an object pointed to by a NULL pointer. NULL said not today 🚫.
• -fsanitize=return — return statement checking, bestie 🎀. Programs built with this 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. Sorry C girlies 😭.
• -fsanitize=signed-integer-overflow — signed integer overflow checking, periodt 🔥. We check that the result of +, *, and both unary and binary - doesn't overflow in signed arithmetics. But lowkey — integer promotion rules must be taken into account. So the following is not an overflow, okay?

signed char a = SCHAR_MAX;
a++;

Like the promotion said "let her cook" 🧑‍🍳.

• -fsanitize=bounds — instruments 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. So, partial coverage, we move.
• -fsanitize=bounds-strict — the ✨strict✨ version of array bounds instrumentation 👑. Most out-of-bounds accesses get detected, including flexible array members and flexible array member-like arrays. Still not instrumented though: initializers of variables with static storage. She does more, but not everything 💅.
• -fsanitize=alignment — checks pointer alignment 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 queen stays aligned ✨.
• -fsanitize=object-size — instruments memory references using the __builtin_object_size function 📏. Various out-of-bounds pointer accesses get detected. It's giving "how big even is that" 😭.
• -fsanitize=float-divide-by-zero — detects floating-point division by zero 🍓. And unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can legitimately be how you obtain infinities and NaNs. So she's special like that, no cap.
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking 🔄. We check that the result of the conversion doesn't overflow. And again, 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, bestie.
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values aren't being passed to arguments marked as requiring a non-null value by the nonnull function attribute 🚫. The attribute said non-null, so act like it.
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect returning null values from such functions 🙅. You promised non-null, you return non-null, it's called integrity.
• -fsanitize=bool — instruments loads from bool 💜. If a value other than 0/1 gets loaded, a run-time error is issued. Binary or nothing, fr.
• -fsanitize=enum — instruments loads from an enum type 🌸. If a value outside the range of values for the enum type gets loaded, a run-time error is issued. Stay in your enum lane.
• -fsanitize=vptr — instruments C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type 🧚. C++ girlies, this one's for you.
• -fsanitize=pointer-overflow — instruments pointer arithmetics 🌀. If the pointer arithmetics overflows, a run-time error is issued. Pointers doing too much, caught in 4K.
• -fsanitize=builtin — instruments 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 💀.

---

And okay, real talk about the difference here: while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives you a diagnostic message instead 📢. This currently works only for the C family of languages. Periodt.

• -fno-sanitize=all — disables all previously enabled sanitizers 🧹. And just so you know: -fsanitize=all is not allowed, because some sanitizers literally cannot be used together 💥. So don't try it, bestie.
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks 🕶️. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. Experimental queen.
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections 🌈. Note that si may contain wildcards. Pick your sections, girl.
• -fsanitize-recover[=opts] — okay this one controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts 💅. Enabling this for a sanitizer component makes it attempt to keep running the program as if no error happened. That means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 😭. The -fno-sanitize-recover= option can 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 🧪. Both -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

And like, even if recovery mode is turned on the compiler side, it also needs to be enabled on the runtime library side, otherwise the failures are still fatal 💀. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable 🌸.

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 — enables 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 — instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine 🪤. The advantage of this? The libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. Fancy and self-sufficient, we love 💖.
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation 🎯. Inserts a call to __sanitizer_cov_trace_pc into every basic block.
• -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation 🔍. Inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, plus __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. Whew, that was a mouthful, but every single one matters 🫶.
• -fcf-protection=[full|branch|return|none|check] — enables code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (like indirect function call, function return, indirect jump) are valid 🔐. This prevents diverting the flow of control to an unexpected target. It's intended to protect against threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Security queen stays safe 💅.

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 🚫.

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. Noted, bestie 📝.

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. Bit math, but make it cute 🌸.

okay so you can ALSO use the "nocf_check" attribute to flag exactly which functions and calls should be skipped from instrumentation 💅 like you're literally hand-picking who gets the vibe check.

Currently the x86 GNU/Linux target ships an implementation based on Intel Control-flow Enforcement Technology (CET), and it works for i686 processor or newer 🧚

⚠️ NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found. so bestie if you didn't ask for it, it's still showing up 😭

---

• -fstack-protector — emits extra code to check for buffer overflows, like stack smashing attacks 💀 the way it works is by adding a guard variable to functions with vulnerable objects. this includes functions that call "alloca", plus functions with buffers larger than or equal to 8 bytes. the guards get initialized when a function is entered and then checked when the function exits 🫶 and if a guard check fails? an error message is printed and the program exits, periodt. only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count, so don't come for them ✨
• -fstack-protector-all — like -fstack-protector except that all functions are protected 👑 no favorites, everyone gets a guard.
• -fstack-protector-strong — like -fstack-protector but it includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses 🌸 only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count. same rules, more coverage 💖
• -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the "stack_protect" attribute. invite-only energy fr fr.
• -fstack-check — generates code to verify that you do not 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 is only one stack.

Note that this switch does not actually cause checking to be done — the operating system or the language runtime must do that. the switch causes generation of code to ensure that they see the stack being extended 👀

You can additionally specify a string parameter:

• no means no checking
• generic means force the use of old-style checking
• specific means use the best checking method and is equivalent to bare -fstack-check

Old-style checking is a generic mechanism that requires no specific target support in the compiler, but bestie, it comes with 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 though. to protect against those you want -fstack-clash-protection.

• -fstack-clash-protection — generates code to prevent stack clash style attacks 🛡️ when this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. thus, it prevents allocations from jumping over any stack guard page provided by the operating system. no sneaking past, no cap.

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. it's giving "on by default, sorry not sorry" 💅

• -fstack-limit-register=reg, -fstack-limit-symbol=sym, -fno-stack-limit — generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol 📏 if a larger stack is required, a signal is raised at run time. for most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions 🎀

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the "no_stack_limit" function attribute ✨

• -fsplit-stack — 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, as it is no longer necessary to calculate a good stack size to use for each thread 👑 this is currently only implemented for the x86 targets running GNU/Linux.

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😬 if compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later 🌟

• -fvtable-verify=[std|preinit|none] — this option is only available when compiling C++ code 💜 it turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten 🔒 if an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. no second chances, it's done.

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers 🧚 the options std and preinit control the timing of when these data structures are built. in both cases the data structures are built before execution reaches "main". using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.

If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std 📋 the hierarchy is real.

• -fvtv-debug — when used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called 🐛 this flag also causes the compiler to log information about which vtable pointers it finds for each class. this information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined, or the current working directory otherwise 🌸

⚠️ Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. learn from my mistakes bestie 😭

• -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. two logs, two chances to forget 🙈

• -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.) living dangerously fr fr 💅

Okay so bestie, here's the tea: a function can be given the attribute no_instrument_function, and when that happens, this instrumentation just... does not get done. 💅

And the way that's actually useful is for the profiling functions listed above, for high-priority interrupt routines, and for literally any function from which the profiling functions can't safely be called (maybe signal handlers — like, if the profiling routines generate output or allocate memory, that's a whole situation). 😭

• -finstrument-functions-exclude-file-list=file,file,... — this sets the list of functions that are excluded from instrumentation (peep the description of -finstrument-functions for the lore). If the file that contains a function definition matches with one of file, then that function is not instrumented. And the match is done on substrings: if the file parameter is a substring of the file name, it's considered to be a match.

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. 🫶

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).

• -finstrument-functions-exclude-function-list=sym,sym,... — this one is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring of the function name, it's considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.
• -fpatchable-function-entry=N[,M] — generate 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. 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.

For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M, are additionally collected in the "__patchable_function_entries" section of the resulting binary.

Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If "N=0", no pad location is recorded.

The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue. 🔥

---

🌸 Options Controlling the Preprocessor 🌸

These options control the C preprocessor, which gets run on each C source file before the actual compilation happens. Slay.

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. Noted.

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.

• -D name — predefine name as a macro, with definition 1.
• -D name=definition — the contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters.

If you're 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. Shells are dramatic like that. 🙈

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options.

• -U name — cancel any previous definition of name, either built in or provided with a -D option.
• -include file — process file as if "#include "file"" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the "#include "..."" search chain as normal.

If multiple -include options are given, the files are included in the order they appear on the command line.

• -imacros file — exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. It's giving efficiency. ✨

All files specified by -imacros are processed before all files specified by -include.

• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
• -pthread — define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
• -M — instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

• -MM — like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output.

• -MF file — when used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout.

• -MG — in conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles.

• -MP — this option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match.

This is typical output:

test.o: test.c test.h

test.h:

• -MT target — change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.

An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

• -MQ target — same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process.

• -MMD — like -MD except mention only user header files, not system header files. 🦋
• -fpreprocessed — bestie, this one is for when your file has already been preprocessed and you're like, we been knew. It suppresses macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. It still recognizes and removes comments though, so you can pass a file that was preprocessed with -C to the compiler without any problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. Periodt.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives, but do not expand macros. ✨

The option's behavior depends on the -E and -fpreprocessed options, so let's do a vibe check on all three situations:

• With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. Also, the -dD option is implicitly enabled.
• With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.
• With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.

• -fdollars-in-identifiers — accept $ in identifiers. Iconic, honestly.
• -fextended-identifiers — accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization.
• *-fmax-include-depth=depth* — sets 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 appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
• -ftrack-macro-expansion[=level] — tracks the locations of tokens across macro expansions. This lets the compiler emit a diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Heads up though: using this option makes the preprocessor and the compiler consume more memory. 😭

The level parameter can be used to choose the level of precision of token location tracking, thus decreasing the memory consumption if necessary:

• Value 0 of level de-activates this option.
• Value 1 tracks 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. This can be used to change an absolute path to a relative path by using . for new, which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map.
• *-fexec-charset=charset* — set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's iconv library routine.
• *-fwide-exec-charset=charset* — set the wide execution character set, used for wide string and character constants. The default is 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 does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine.
• -fpch-deps — when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header is listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used.
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename", in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. 💅

• -fworking-directory — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.
• *-A predicate*=answer — make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. ✨
• *-A -predicate*=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

Be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
• -traditional
• -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. 🧚

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.

• -trigraphs — support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline.

The nine trigraphs and their replacements are

Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
Replacement:      [    ]    {    }    #    \    ^    |    ~

By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.

• -remap — enable special code to work around file systems which only permit very short file names, such as MS-DOS.
• -H — print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...!. 👑
• -dletters — says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 💀
  • -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, then -dM is interpreted as a synonym for -fdump-rtl-mach. yeah. iconic little plot twist, iykyk.

• -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — like -dD, but emit only the macro names, not their expansions.
• -dI — output #include directives in addition to the result of preprocessing.
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.

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• -fdebug-cpp — this option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

When used from GCC without -E, this option has no effect. 💀

• -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.

bestie this is the "i know a shortcut" of compiler flags and it is lowkey a trap 🙈

• -Xpreprocessor option — pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.

• -no-integrated-cpp — perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.

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🧚 Passing Options to the Assembler 🧚

You can pass options to the assembler. periodt.

• -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.

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💅 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. like, literally no thoughts head empty if there's no linking happening.

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
• -c, -S, -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments.
• -flinker-output=type — this option controls 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.

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.

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.

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.

• -fuse-ld=bfd — use the bfd linker instead of the default linker.
• -fuse-ld=gold — use the gold linker instead of the default linker.
• -fuse-ld=lld — use the LLVM lld linker instead of the default linker.
• -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker.
• -llibrary, -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.)

The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.

order matters bestie, it's giving "the way that the linker is just reading left to right and not being psychic" 😭

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ unless options preventing their inclusion are used as well. This typically removes -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available.
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. 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.

you're out here raw-dogging the link step fr fr. no safety net, no vibes, just you and the linker 🌸

okay so real talk bestie: in most cases you need libgcc.a even when you're actively trying to dodge every other standard library. Translation? When you pass -nostdlib or -nodefaultlibs, you should usually also pass -lgcc. Why? Because otherwise you're left with unresolved references to GCC's own internal library subroutines, and that's not the vibe. (A cute lil example of one of those internal subroutines is "__main", which exists to make sure your C++ constructors actually get called.) 💅

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• -e entry / --entry=entry — says "hey, the program's entry point is entry". The linker is the one interpreting that argument, and the GNU linker is chill with 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 pass the exact same set of options you used for compilation (-fpie, -fPIE, or model suboptions) when you hand this linker option over. No cap. ✨
• -no-pie — don't produce a dynamically linked position independent executable. Simple as that. 🌸
• -static-pie — produce a static position independent executable on targets that support it. It's like a static executable but the glow-up is that it can be loaded at any address with no dynamic linker needed. Same deal as -pie: 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. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Heads up: on some targets this option also sets flags for the preprocessor, so use it consistently for both compilation and linking, okay? 🔥
• -r — produce a relocatable object as output. Also known as partial linking, iykyk. 🍓
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This tells the linker to add all symbols, not only the used ones, to the dynamic symbol table. You need this for some uses of "dlopen" or to allow obtaining backtraces from within a program. 👑
• -s — remove all symbol table and relocation information from the executable. Stripped, clean, iconic. 💅
• -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 where an app should use the shared libgcc instead of the static version. The most common one is when the app wants 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.

So 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. periodt.

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. rip, don't do that.

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. 🦋

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🌈 Options for Directory Search 🌈

These options specify directories to search for header files, for libraries and for parts of the compiler. so basically: where do we look for your stuff.

• -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 search order, memorise it bestie, it WILL come up.

You can totally use -I to override a system header file and swap in your own version, since those directories get searched before the standard system header directories. But — and bestie, listen — do not use this option to add directories that hold vendor-supplied system header files. That's what -isystem is for, okay? Different tool, different job. 💅

The -isystem and -idirafter options also mark the directory as a system directory, so it gets the exact same special treatment that the standard system directories get. Same energy, same VIP pass. 🎀

And here's a lil plot twist: if a standard system include directory, or a directory specified with -isystem, is also specified with -I, then the -I option is ignored. The directory is still searched, bestie, don't panic — but as a system directory, at its normal position in the system include chain. Why? Because this makes sure GCC's whole procedure for fixing buggy system headers, plus the ordering for the #include_next directive, don't get inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options instead. Periodt.

• -I- — Split the include path. 🚨 This option has been deprecated. Please use -iquote instead for -I directories before the -I-, and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". And there is no way to override this effect of -I-. Like, none. It's giving irreversible. 😭

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. Don't forget it, babe, it matters. 🦋
• -iwithprefix dir, -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. Two sisters, two different seats in the include-search auditorium. 👑
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. iykyk. 💜
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. Cute lil nesting situation. 🎀
• -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. You're on your own now, bestie. 🌸
• -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.) Selective exclusion, we love to see it. ✨
• -iplugindir= dir — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. Heads up though: this option is not meant to be used by the user, but only passed by the driver. So like, not your business, but now you know. 🙈
• -L dir — Add directory dir to the list of directories to be searched for -l. Simple. Iconic. Does exactly what it says. 💅
• -B prefix — This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. Big responsibility, honestly.

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. Fallback chain, gorgeous. 🦋

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. Multitasking king. 👑

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. No backup plan after that, bestie. 😭

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. Kludge is literally the word they used, I'm just the messenger. 💀

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. Leave it messy, leave it raw. 🎀
• --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. Relocating the whole vibe. ✨

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. Turning off the extra lil garnish. 🌸

---

✨ Options for Code Generation Conventions ✨

These machine-independent options control the interface conventions used in code generation. Very "the rules of how we speak to each other" energy. 🫶

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. Do the math, babe. 💅

• -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. Sneaky lil last-one-standing rules. 👑
• -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. Same chaotic override drama, opposite winner. 💜
• -fwrapv-pointer — bestie this one is about pointer math going feral 💅 it tells the compiler to just assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. Cute, right? But heads up — this flag disables some optimizations which assume pointer overflow is invalid. So like, you're trading speed for predictability, no cap. 🦋
• -fstrict-overflow — okay so this one implies -fno-wrapv and -fno-wrapv-pointer, and when it's negated it implies -fwrapv and -fwrapv-pointer. The way that one option just drags two others along with it… it's giving main character energy 👑
• -fexceptions — turn ON exception handling, slay ✨ It generates extra code that's needed to propagate exceptions. For some targets, this means GCC generates frame unwind information for all functions, which can produce significant data size overhead — though it does not affect execution, so breathe, babe. If you don't specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that don't normally require it. But — you may need to enable it when compiling C code that needs to interoperate properly with exception handlers written in C++. 🙈 You may also wish to disable it if you're compiling older C++ programs that don't use exception handling. Iconic either way fr fr.
• -fnon-call-exceptions — generate code that lets trapping instructions throw exceptions. 🎀 Lowkey important: this requires platform-specific runtime support that does not exist everywhere. And more — it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". no cap, don't try it 💀
• -fdelete-dead-exceptions — the vibe here: instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. 💅 This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. so it's a whole layered situation 🦋
• -funwind-tables — similar to -fexceptions, except it just generates any needed static data, but does not affect the generated code in any other way. 🌸 You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. So basically, let the language handle it, bestie 🫶
• -fasynchronous-unwind-tables — generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). ⭐ precise at every single boundary, obsessed
• -fno-gnu-unique — okay this one is a whole saga, sit down. 🍓 On systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different "RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. 😭 But this causes "dlclose" to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique. there, crisis averted 💅
• -fpcc-struct-return — return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). 🌈

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. 💀

• -freg-struct-return — return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. ⚡

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. 👑

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. 💀

• -fshort-enums — allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. 🎀 Specifically, the "enum" type is equivalent to the smallest integer type that has enough room. tiny and efficient, we love her ✨

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. ikyk, they're different things, don't @ me

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. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. ⚠️ It is mainly useful to enable legacy code to link without errors. grandma code deserves love too 💜

• -fno-ident — ignore the "#ident" directive. Simple. Clean. periodt 💅
• -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. seriously bestie, hands off this one 🙈
• -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. 📝 This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. 👑
• -fpic — bestie, this one generates position-independent code (PIC) 💖 which is the vibe you want for a shared library, if the target machine supports it. What does that even mean? It means all your constant addresses get accessed through a global offset table (GOT) instead of being hardcoded. The dynamic loader resolves the GOT entries when the program starts — and no, the dynamic loader is not part of GCC, it's part of the operating system, okurrr. 🫶

Now, plot twist: if the GOT size for your linked executable goes over a machine-specific max, the linker is gonna throw an error at you saying -fpic does not work. In that case, just recompile with -fPIC instead, slay. ✨ (The maximums are 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. The x86 has no such limit, main character behavior honestly. 💅)

Position-independent code requires 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, periodt.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.

• -fPIC — if it's supported for the target machine, this emits position-independent code suitable for dynamic linking and it avoids any limit on the size of the global offset table. 🔥 This option actually makes a difference on AArch64, m68k, PowerPC and SPARC.

Position-independent code requires special support, so it works only on certain machines, ngl.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.

• -fpie
• -fPIE — these are basically -fpic and -fPIC's cousins 🎀 but the generated position-independent code can only be linked into executables. Usually you use these to compile code that'll be linked using the -pie GCC option.

-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for -fPIE.

• -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. ✨ This gives you more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures like 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives the compiler more register allocation freedom. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time. 🦋

Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific external functions.

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

• -fno-jump-tables — do not use jump tables for switch statements, even where it would be more efficient than other code generation strategies. This option is useful in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed. 💜
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer or in some other fixed role). 👑

reg must be the name of a register. The register names accepted are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-used-reg — treat the register named reg as an allocable register that gets clobbered by function calls. It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way do not save and restore the register reg. 💅

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. 🌸

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 😭

And a different sort of disaster results from the use of this flag for a register in which function values may be returned. 💀

This flag does not have a negative form, because it specifies a three-way choice.

• -fpack-struct[=n] — without a value specified, pack all structure members together without holes. When a value n is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location). 🧚

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

• -fleading-underscore — this option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code. 🌈

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

• -ftls-model=model — alter the thread-local storage model to be used. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. ✨

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

• -ftrampolines — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like, for example, HP-PA or IA-64 — do nothing. 🦋

A trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. 👀

-fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. ⚠️

• -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. 💖

Despite the nomenclature, default always means public — i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public. 💅

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 🔥

For those adding visibility support to existing code, you may find "#pragma GCC visibility" of use. This works by you enclosing the declarations you wish to set visibility for with (for example) "#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden so as to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, "operator new" and "operator delete" must always be of default visibility. 👑

Be aware that headers from outside your project — in particular system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. You may need to explicitly say "#pragma GCC visibility push(default)" before including any such headers. 🙈

"extern" declarations do NOT get touched by -fvisibility, so bestie, a whole lotta code can be recompiled with -fvisibility=hidden and literally nothing needs changing. 💅 BUT — and this is the plot twist — it means calls to "extern" functions with no explicit visibility go through the PLT, so it's actually way more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden. iykyk. 🫶

Also real talk: -fvisibility does affect C++ vague linkage entities. Which means, for instance, an exception class that gets thrown between DSOs must be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. no cap. 😭

Wanna see the whole vibe, the benefits, and how to actually use all this? There's an overview at <http://gcc.gnu.org/wiki/Visibility>. go bestie. ✨

• -fstrict-volatile-bitfields — use this one if accesses to volatile bit-fields (or other structure fields, even 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. Like, picture targets with memory-mapped peripheral registers that 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 these targets) to force GCC to use 16-bit accesses instead of, maybe, a much chiller-looking-but-wrong 32-bit access. 💀

If this option is disabled, the compiler just uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers totally unrelated to the one being updated. 🙈

In some cases, like when the "packed" attribute is applied to a structure field, it may not be possible to access the field with a single read or write that's correctly aligned for the target machine. When that happens, GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 🫡

Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members. fr fr.

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 of this option is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. periodt.

---

🛠️ GCC Developer Options 🛠️

Okay so this section is for the devs — command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. It's giving: 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. You should rarely need any of these for ordinary compilation and linking tasks, bestie. 💅

Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. ✨

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it's the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. 📁

• -fcallgraph-info, -fcallgraph-info=MARKERS — makes the compiler output callgraph information for the program, on a per-object-file basis. The info is generated in the common VCG format. It can be decorated with extra per-node and/or per-edge info if a list of comma-separated markers is additionally specified. When the "su" marker is specified, the callgraph gets decorated with stack usage information — it's equivalent to -fstack-usage. When the "da" marker is specified, the callgraph is decorated with info 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:

• -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 — these 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 — these 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 — these 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 — these 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 — these enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — these 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 — 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 — these 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 — these 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 everything right after virtual registers get turned into hard registers. glow-up complete ✨
• -fdump-rtl-web — dumps after live range splitting. bestie's ranges are getting split, and we're watching 💅
• -fdump-rtl-regclass
• -fdump-rtl-subregs_of_mode_init
• -fdump-rtl-subregs_of_mode_finish
• -fdump-rtl-dfinit
• -fdump-rtl-dfinish — okay so these dumps are defined, they exist, they're real… but they always produce empty files. it's giving ghost town 👻 not one single byte of tea.
• -da
• -fdump-rtl-all — produces all the dumps listed above. the whole collection. the full album, no skips 🔥
• -dA — annotates the assembler output with miscellaneous debugging info. little notes everywhere, like margin doodles in your notes app 🎀
• -dD — dumps all macro definitions at the end of preprocessing, in addition to normal output. so you get your usual plus the macro lore 📝
• -dH — produces a core dump whenever an error occurs. yeah. it just… fully crashes on purpose when something goes wrong 💀
• -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. receipts for every instruction, no cap 🧾
• -dP — dumps the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation, so you get both, bestie.
• -dx — just generates RTL for a function instead of actually compiling it. usually used with -fdump-rtl-expand. it's a "look, don't touch" situation 🙈

---

🦋 the debug-dump girlies 🦋

• -fdump-debug — dumps debugging information generated during the debug generation phase.
• -fdump-earlydebug — dumps debugging information generated during the early debug generation phase. yes there are two phases, and yes they both get a dump 💖
• -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. basically: same vibes, different addresses, now comparable 🫶
• -freport-bug — collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs. so when the compiler fully has a meltdown, at least you get receipts to send 📸
• -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. clean comparison, slay ✨
• -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. no numbers on the little chain links 🔗

---

🌈 IPA dump section, we're here 🌈

• -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 is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. the following dumps are possible:
  • all — enables all inter-procedural analysis dumps. all of them. the entire buffet 🍓
  • cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions.
  • inline — dump after function inlining.

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. both sides of the story, periodt 💅

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

---

💜 language-specific dumps 💜

• -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. the following switch values are accepted:
  • all — enables all language-specific dumps.
  • class — dumps class hierarchy information. virtual table information is emitted unless 'slim' is specified. this option is applicable to C++ only.
  • raw — dumps 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. a little guest list for your passes 👑
• -fdump-statistics-option — enables and controls dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .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. numbers girlies rise up 📊

---

🌸 the big one: tree dumps 🌸

• -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; the ones that aren't meaningful are ignored. the following options are available:
  • address — prints 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 dumps 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, prints the RTL in slim (condensed) form instead of the default LISP-like representation.
  • raw — prints a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.
  • details — enables more detailed dumps (not honored by every dump option). also includes information from the optimization passes.
  • stats — enables dumping various statistics about the pass (not honored by every dump option).
  • blocks — enables showing basic block boundaries (disabled in raw dumps).
  • graph — for each of the other indicated dump files (-fdump-rtl-pass), dumps 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 — enables showing virtual operands for every statement.
  • lineno — enables showing line numbers for statements.
  • uid — enables showing the unique ID ("DECL_UID") for each variable.
  • verbose — enables showing the tree dump for each statement.
  • eh — enables showing the EH region number holding each statement.
  • scev — enables showing scalar evolution analysis details.
  • optimized — enables showing optimization information (only available in certain passes).
  • missed — enables showing missed optimization information (only available in certain passes).
  • note — enables other detailed optimization information (only available in certain passes).
  • all — turns on all options, except raw, slim, verbose and lineno.
  • optall — turns on all optimization options, i.e., optimized, missed, and note.

to determine what tree dumps are available or find the dump for a pass of interest, follow the steps below. it's a whole treasure hunt, bestie 🗺️

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. for example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. for example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. note that the numeric codes are not stable and may change from one version of GCC to another. ⚠️

---

🎀 optimization dump vibes 🎀

• -fopt-info
• -fopt-info-options
• -fopt-info-options=filename — controls optimization dumps from various optimization passes. if the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.

the options can be divided into three groups:

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 as they are non-overlapping. however, in case of any conflicts, the later options override the earlier options on the command line. last one wins, always 💅

the following options control which kinds of messages should be emitted:

• optimized — prints information when an optimization is successfully applied. it is up to a pass to decide which information is relevant. for example, the vectorizer passes print the source location of loops which are successfully vectorized.
• missed — prints information about missed optimizations. individual passes control which information to include in the output.

• note — spills the tea ☕ on optimizations: certain transformations, plus more detailed messages about the decisions being made and stuff like that.
• all — the full rundown, bestie. Detailed optimization info about everything — optimized, missed, and note, all of it.

The following option controls the dump verbosity:

• internals — by default you only get the "high-level" messages, the ✨public-facing✨ ones. This one unlocks the extra-detailed messages, which are lowkey only gonna be interesting to GCC devs themselves. iykyk.

---

One or more of the following option keywords can describe a whole group of optimizations:

• ipa — turn on dumps from all interprocedural optimizations 🫶
• loop — turn on dumps from all loop optimizations 🔁
• inline — turn on dumps from all inlining optimizations 🌸
• omp — turn on dumps from all OMP (Offloading and Multi Processing) optimizations ⚡
• vec — turn on dumps from all vectorization optimizations 🦋
• optall — turn on dumps from all optimizations. It's a superset of every optimization group listed above, no cap 👑

---

If options is omitted, it defaults to optimized-optall, which literally means: dump messages about successful optimizations from all the passes, while skipping the messages that count as "internals".

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 even though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames show up, then all but the first such option are just ignored 🙈.

Heads up bestie: the output filename gets overwritten in the case of multiple translation units. If you want a combined output from multiple translation units, use stderr instead 💅.

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs the missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints info about missed optimization opportunities from the 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, periodt.

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. So only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities.

---

-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 inside that compressed JSON file is subject to change. Don't build your whole life around it, babe.

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 came from actual profile data, or was just an estimate — which lets consumers prioritize messages by code hotness 🔥
• the function name of the code being optimized, where applicable
• the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can tell the copies apart
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location
• the GCC pass that emitted the message, and
• the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. Nested, like, drama within drama 🫠.

---

-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 higher you crank it, the more it yaps 💀.

---

-fenable-kind-pass

-fdisable-kind-pass=range-list

This is a 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. This is dev-only territory, bestie — not your everyday vibe.

• -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. So like, extra careful mode, but it can change what your code compiles to 👀.

---

-frandom-seed=string 🎲

This option provides a seed that GCC uses in place of random numbers when generating certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files. Slay, deterministic king 👑.

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, bestie. Fr fr.

---

-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. It's giving "keepsakes, but for compilers" 🎀.

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.

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 😭.

---

-save-temps=obj 🌸

Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.

For example:

gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar

creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and dir2/yfoobar.o.

---

• -time[=file] — okay bestie, this one is the stopwatch moment ⏱️. It reports the CPU time that each subprocess in the compilation sequence eats up. For C source files, that means the compiler proper and the assembler (plus the linker if you're actually linking).

If you don't give it an output file, the output is giving:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", aka the time spent executing the program itself. The second number is "system time", the time spent executing operating system routines on behalf of the program. Both of those are in seconds, no cap. 🕰️

Now if you do specify an output file, the output gets appended to the named file and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" move up in front of the program name, and the options passed to the program get displayed, so later you can actually tell what file was being compiled and with which options. obsessed with the receipts 🧾

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If you leave out the optional argument (or if file is .), the dump file's name is determined by appending .gkd to the compilation output file name. 💾
• -fcompare-debug[=opts] — if no error occurs during compilation, this 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. Double-checking queen 👑✨

If you leave out the equal sign, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's used for opts; otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). And if you only want a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. the drama of it all 💅

• -fcompare-debug-second — this one is implicitly passed to the compiler for the second compilation that -fcompare-debug asks for, 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 it doesn't overwrite the ones generated by the first. 🙈

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. niche but valid

• -gtoggle — turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. This is mainly intended to be used with -fcompare-debug. 🔀
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. Flip it, reverse it ✨
• -Q — makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. the play-by-play commentary we deserve 📢
• -ftime-report — makes the compiler print some statistics about the time consumed by each pass when it finishes. ⏳
• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass. 🔍
• -fira-verbose=n — 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. 🎚️
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version, bestie, it's not consistent like that. 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. 🧠
• -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, we love a glow-up arc 🦋
• -fprofile-report — makes the compiler print some statistics about consistency of the (estimated) profile and the effect of individual passes. 📊
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if it's explicitly specified and it isn't an executable; otherwise it's the basename of the source file. An entry is made up of three fields:
  • The name of the function.
  • A number of bytes.
  • One or more qualifiers: "static", "dynamic", "bounded".

The qualifier "static" means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier "dynamic" means the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it isn't present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 📚 study hall vibes

• -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 niche, so specific 💅
• -fdbg-cnt-list — prints the name and the counter upper bound for all debug counters. 📋
• -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 set 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. For dbg_cnt(tail_call) true is returned for the first 10 invocations. 🎯

• -print-file-name=library — prints the full absolute name of the library file library that would be used when linking — and doesn't do anything else. With this option, GCC doesn't compile or link anything; it just prints the file name. a one-job icon 📁
• -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. 🗺️
• -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`

chef's kiss, honestly 👨‍🍳💋

• -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 /. the trailing slash is not optional bestie, don't play ⚠️

• -print-sysroot — prints the target sysroot directory that's used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. 🤐
• -print-sysroot-headers-suffix — 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 compiler's target machine (think i686-pc-linux-gnu) and then dips, like, it does literally nothing else 💅
• -dumpversion — prints the compiler version (like 3.0, 6.3.0 or 7) and does nothing else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). iykyk 🎀
• -dumpfullversion — prints the full compiler version and nothing else. The output is always three numbers separated by dots: major, minor and patchlevel version. no crumbs, just facts ✨
• -dumpspecs — prints the compiler's built-in specs and nothing else. (This is used when GCC itself is being built.) lowkey niche but she's iconic 🛠️

---

🦋 Machine-Dependent Options 🦋

Every target machine that GCC supports can have its own lil set of options, bestie — like, to let you compile for a particular processor variant or ABI, or to control optimizations that are specific to that machine. By convention, the names of machine-specific options start with -m. 🌸

Some configurations of the compiler also support extra target-specific options, usually for compatibility with other compilers on the same platform. slay, options for days 💖

⭐ AArch64 Options ⭐

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. Permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. like, no mixing, fr fr 😭

• -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. bestie behaves, assembler stays wild 🙈
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -mcmodel=tiny — generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. main character energy 👑
• -mcmodel=large — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC. > ⚠️ heads up bestie, that incompatibility is real, don't do it
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior is the default.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or sysreg for a canary in an appropriate system register.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. 🐧

• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or sysreg for a canary in an appropriate system register.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. 🐧

💀 not me telling you this exact same entry appears twice in the original — so I'm keeping both, no erasing, periodt

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-size=size — specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. she's doing patchwork 💅
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. accuracy said goodbye 😭
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_copy" to permit a more efficient code sequence to be generated.
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default. ✨

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they enable by default:

arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a
armv8.4-a : Armv8.4-A : armv8.3-a, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : 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. right-most wins, that's the rule 🌈

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.

• -mtune=name — specify the name of the target processor for which GCC should tune the performance of the code. Permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-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. big.LITTLE besties, both vibes at once 🫶

ok so the value neoverse-512tvb means GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, you're telling GCC: "tune me for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle, and the same amount of SVE arithmetic instructions per cycle (that's 2 for 256-bit SVE, 4 for 128-bit SVE)." It's more general than tuning for one specific core like Neoverse V1, but it's highkey more specific than the default tuning we talk about below. 💅

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. This option does literally nothing if the compiler can't recognize the host system's processor. 🙈

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. No +feature glow-ups allowed, bestie.

---

• -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. It's giving "internal tools only", fr fr.

• -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.

💀 Heads up bestie: 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.

---

🎀 -march and -mcpu Feature Modifiers 🎀

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

• crc — Enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• sve — Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
• rdma — Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
• fp16 — Enable FP16 extension. This also enables floating-point instructions.
• fp16fml — Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• rcpc — Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
• dotprod — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
• aes — Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
• sha2 — Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
• sha3 — Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• sm4 — Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• profile — Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.
• rng — Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
• memtag — Enable the Armv8.5-a Memory Tagging Extensions. 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 implies aes, sha2, and simd, which implies fp. And conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. Like a little domino chain of vibes 💅

---

👑 Adapteva Epiphany Options 👑

These -m options are defined for Adapteva Epiphany, bestie:

• -mhalf-reg-file — Don't allocate any register in the range "r32"..."r63". That's the whole point: it lets your code run on hardware variants that just don't have those registers. Slay, flexible.
• -mprefer-short-insn-regs — Preferentially allocate registers that allow short instruction generation. It can bump up the instruction count though, so this might either reduce or increase your overall code size. It's giving chaos, honestly. 🙈
• -mbranch-cost=num — Set the cost of branches to roughly num "simple" instructions. Heads up: this cost is only a heuristic and is not guaranteed to give you consistent results across releases. Lowkey vibes-based, no cap.
• -mcmove — Enable the generation of conditional moves. Iconic.
• -mnops=num — Emit num NOPs before every other generated instruction. Just NOPs, NOPs, NOPs. Periodt.
• -mno-soft-cmpsf — For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. 😭 The default is -msoft-cmpsf, which uses slower but IEEE-compliant software comparisons. So: speed or correctness, 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, ngl.

Also note that this option changes the ABI. Compiling a program with a different stack offset than the libraries were compiled with generally does not work. 🚩

This option can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it's recommended to configure 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 — If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing an (otherwise direct) call. This is the default. 👑
• -mshort-calls — If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
• -msmall16 — Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, for which -mlong-calls semantics are in effect.
• -mfp-mode=mode — Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. ✨

mode can be set to one of the following values:

• caller — Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
• truncate — This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller.

• -mno-split-lohi
• -mno-postinc
• -mno-postmodify — Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
• -mnovect-double — Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
• -max-vect-align=num — The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. 🚩
• -msplit-vecmove-early — Split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 💀 Theory said slay, reality said no.
• -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. Let's goooo.

• -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.

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🦋 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.
  • 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 is 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. 💅
• -mswap — bestie, this one generates swap instructions. that's it, that's the whole vibe 💅
• -matomic — turns on the locked load/store conditional extension so atomic memory built-in functions actually work. heads up tho: it's not available for ARC 6xx or ARC EM cores, so don't come crying to me 🙈
• -mdiv-rem — enables div and rem instructions for ARCv2 cores. slay ✨
• -mcode-density — enables code density instructions for ARC EM. and for ARC HS? it's on by default, no cap 👑
• -mll64 — enables double load/store operations for ARC HS cores 🫶
• *-mtp-regno=regno* — specify the thread pointer register number, that's the whole job 💜
• *-mmpy-option=multo* — compile ARCv2 code with a multiplier design option. you can pass multo as either a string or a numeric value, and wlh1 is the default. here are the recognized values, hold on to your wig:
  • 0, none — no multiplier available. nothing. zero. the void 💀
  • 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.

this option is only available for ARCv2 cores. iykyk 🌸

• *-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. supported values for fpu are giving:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. the single-precision floating-point extension is also enabled. not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. single-precision floating-point is also enabled. only available for ARC EM.
  • fpuda_div — same double-precision assist energy, but the single-precision floating-point, square-root, and divide extensions are also enabled. only available for ARC EM.
  • fpuda_fma — double-precision assist again, plus single-precision floating-point and fused multiply and add hardware extensions are also enabled. only available for ARC EM.
  • fpuda_all — double-precision assist, and all single-precision floating-point hardware extensions are also enabled. only available for ARC EM.
  • fpus_div — enables support for single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables support for double-precision floating-point, square-root and divide hardware extensions. this option includes option fpus_div. not available for ARC EM.
  • fpus_fma — enables support for single-precision floating-point and fused multiply and add hardware extensions.
  • fpud_fma — enables support for double-precision floating-point and fused multiply and add hardware extensions. this option includes option fpus_fma. not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions.
  • 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 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 on entry to a fast interrupt. and what's a fast interrupt? it's an interrupt with the highest priority level P0. these save only the PC and STATUS32 registers, to avoid memory transactions during interrupt entry and exit sequences. use this when you're rocking fast interrupts in an ARC V2 family processor. permitted values are 4, 8, 16, and 32 💖
• *-mlpc-width=width* — specify the width of the lp_count register. valid width values are 8, 16, 20, 24, 28 and 32 bits. default width is fixed at 32 bits. if the width is less than 32, the compiler won't try to transform loops in your program to use the zero-delay loop mechanism unless it knows the lp_count register can hold the required loop-counter value. depending on the width you pick, the compiler and run-time library might still keep using the loop mechanism for various needs. also 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. this option defines the __ARC_RF16__ preprocessor macro ⭐
• -mbranch-index — enable use of bi or bih instructions to implement jump tables. iconic behavior fr fr 🔥

---

the following options are passed through to the assembler, and also define preprocessor macro symbols.

• -mdsp-packa — passed down to the assembler to enable the DSP Pack A extensions. also sets the preprocessor symbol __Xdsp_packa. this option is deprecated.
• -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension. also sets the preprocessor symbol __Xdvbf. this option is deprecated.
• -mlock — passed down to the assembler to enable the locked load/store conditional extension. also sets the preprocessor symbol __Xlock.
• -mmac-d16 — passed down to the assembler. also sets the preprocessor symbol __Xxmac_d16. this option is deprecated.
• -mmac-24 — passed down to the assembler. also sets the preprocessor symbol __Xxmac_24. this option is deprecated.
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. also sets the preprocessor symbol __Xrtsc. this option is deprecated.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. also sets the preprocessor symbol __Xswape.
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. also sets the preprocessor symbol __Xtelephony. this option is deprecated.
• -mxy — passed down to the assembler to enable the XY memory extension. also sets the preprocessor symbol __Xxy.

---

the following options control how the assembly code is annotated:

• -misize — annotate assembler instructions with estimated addresses.
• -mannotate-align — explain what alignment considerations led to the decision to make an instruction short or long. like, it tells you why 🦋

---

the following options are passed through to the linker:

• -marclinux — passed through to the linker, to specify use of the arclinux emulation. this one 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. enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested 💅

---

the following options control the semantics of generated code:

• -mlong-calls — generate calls as register indirect calls, which gives you access to the full 32-bit address range. main character energy honestly 🌈
• -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 you can use the 25-bit range instead of the 21-bit range with conditional branch-and-link. this is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• *-G *num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. the default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations 🍓
• -mno-sdata — do not generate sdata references. this is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache — use ordinarily cached memory accesses for volatile references. this is the default.
• -mno-volatile-cache — enable cache bypass for volatile references 🧚

---

the following options fine tune code generation:

• -malign-call — do alignment optimizations for call instructions.
• -mauto-modify-reg — enable the use of pre/post modify with register displacement.
• -mbbit-peephole — enable bbit peephole2.
• -mno-brcc — disables a target-specific pass in arc_reorg to generate compare-and-branch (brcc) instructions. ngl it has no effect on generation of these instructions driven by the combiner pass 🙈
• -mcase-vector-pcrel — use PC-relative switch case tables so the case table can get shortened. This is the default for -Os. 💅
• -mcompact-casesi — enables the compact casesi pattern. Default for -Os, and only available on ARCv1 cores. Heads up bestie: this option is deprecated.
• -mno-cond-exec — turns OFF the ARCompact-specific pass that generates conditional execution instructions. 😤

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 kinda lacking. So the ARC port kept a whole special pass around that hunts for more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling are already done. This pass generally (but not always, ngl) 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 using -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. This option is deprecated. 💀
• -mindexed-loads — enables the use of indexed loads. This can be problematic because some optimizers then assume indexed stores exist, which... they don't. 🙈
• -mlra — enables Local Register Allocation. Still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra). 🧪
• -mlra-priority-none — don't indicate any priority for target registers.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15.
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15.
• -mmillicode — when optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by calling a special function in libgcc; that's what a millicode call is. But these calls can cause performance issues and/or linking issues when you're linking in a nonstandard way, so 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, so don't try it elsewhere.
• -mmixed-code — tweaks register allocation to help 16-bit instruction generation. Generally this decreases the average instruction size while increasing the instruction count. it's giving "smaller but more" ✨
• -mq-class — this option is deprecated. Enables q instruction alternatives. This is the default for -Os.
• -mRcq — enables Rcq constraint handling. Most short code generation depends on this. This is the default.
• -mRcw — enables Rcw constraint handling. Most ccfsm condexec mostly depends on this. This is the default.
• -msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1.
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding anything implied by -mcpu=. 🌈

Supported values for cpu are:

• ARC600 — tune for ARC600 CPU.
• ARC601 — tune for ARC601 CPU.
• ARC700 — tune for ARC700 CPU with standard multiplier block.
• ARC700-xmac — tune for ARC700 CPU with XMAC block.
• ARC725D — tune for ARC725D CPU.
• ARC750D — tune for ARC750D CPU.

• -mmultcost=num — cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
• -munalign-prob-threshold=probability — set the probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without a filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability of the branch being taken is below probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.

---

The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release. like... they're on borrowed time, bestie 🕰️

• -margonaut — obsolete FPX.
• -mbig-endian, -EB — compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build arceb-elf32 and arceb-linux-uclibc targets, for which big endian is the default.
• -mlittle-endian, -EL — compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build arc-elf32 and arc-linux-uclibc targets, for which little endian is the default.
• -mbarrel_shifter — replaced by -mbarrel-shifter.
• -mdpfp_compact — replaced by -mdpfp-compact.
• -mdpfp_fast — replaced by -mdpfp-fast.
• -mdsp_packa — replaced by -mdsp-packa.
• -mEA — replaced by -mea.
• -mmac_24 — replaced by -mmac-24.
• -mmac_d16 — replaced by -mmac-d16.
• -mspfp_compact — replaced by -mspfp-compact.
• -mspfp_fast — replaced by -mspfp-fast.
• -mtune=cpu — values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
• -multcost=num — replaced by -mmultcost.

---

🌸 ARM Options 🌸

These -m options are defined for the ARM port:

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
• -mapcs-frame — generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this isn't strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated. 💀
• -mapcs — this is a synonym for -mapcs-frame and is deprecated.
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. 🤷
• -mno-sched-prolog — prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. This means all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions. ✨

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. no mixing and matching, bestie 😭

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations.
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
• -mbe8, -mbe32 — when linking a big-endian image, select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 🫠
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option.

Permissible names are: 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. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. 👑

The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.

Okay so for example, -march=armv7-a+simd+nofp+vfpv4 is literally equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it. 💅

Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. It’s giving “same name, different font,” no cap. ✨

The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions. Periodt.

• armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
  • +fp — The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
  • +nofp — Disable the floating-point instructions.
• armv7 — The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
  • +fp — The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
  • +nofp — Disable the floating-point instructions.
• armv7-a
  • +mp — The multiprocessing extension.
  • +sec — The security extension.
  • +fp — The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
  • +simd — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can be used as aliases for this extension.
  • +vfpv3 — The VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-fp16 — The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — The VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — The VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon-fp16 — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +neon-vfpv4 — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
  • +nosimd — Disable the Advanced SIMD instructions (does not disable floating point).
  • +nofp — Disable the floating-point and Advanced SIMD instructions.
• armv7ve — The extended version of the ARMv7-A architecture with support for virtualization.
  • +fp — The VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can be used as an alias for this extension.
  • +simd — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
  • +vfpv3-d16 — The VFPv3 floating-point instructions, with 16 double-precision registers.
  • +vfpv3 — The VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-fp16 — The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — The VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — The VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension +neon-vfpv3 can be used as an alias for this extension.
  • +neon-fp16 — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +nosimd — Disable the Advanced SIMD instructions (does not disable floating point).
  • +nofp — Disable the floating-point and Advanced SIMD instructions.
• armv8-a
  • +crc — The Cyclic Redundancy Check (CRC) instructions.
  • +simd — The ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions.
  • +nocrypto — Disable the cryptographic instructions.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
• armv8.1-a
  • +simd — The ARMv8.1-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +nocrypto — Disable the cryptographic instructions.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
• armv8.2-a, armv8.3-a
  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +fp16fml — The half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.
  • +simd — The ARMv8.1-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +dotprod — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
• armv8.4-a
  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
  • +simd — The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +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 — Disable the cryptographic extension.
  • +nofp — Disable 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.
• armv8.6-a
  • +fp16 — the half-precision floating-point data processing instructions, bestie 💅 This one also switches on the Advanced SIMD and floating-point instructions, plus the Dot Product extension and the half-precision floating-point fmla extension. It's giving bundle deal ✨
  • +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension.
  • +crypto — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +nocrypto — disable the cryptographic extension. bye 👋
  • +nofp — disable 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.
• armv7-r
  • +fp.sp — the single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias for this extension.
  • +fp — the VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
  • +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +nofp — disable the floating-point extension.
  • +idiv — the ARM-state integer division instructions.
  • +noidiv — disable the ARM-state integer division extension.
• armv7e-m
  • +fp — the single-precision VFPv4 floating-point instructions.
  • +fpv5 — the single-precision FPv5 floating-point instructions.
  • +fp.dp — the single- and double-precision FPv5 floating-point instructions.
  • +nofp — disable the floating-point extensions.
• armv8.1-m.main
  • +dsp — the DSP instructions.
  • +mve — the M-Profile Vector Extension (MVE) integer instructions.
  • +mve.fp — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
• armv8-m.main
  • +dsp — the DSP instructions.
  • +nodsp — disable the DSP extension.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
• armv8-r
  • +crc — the Cyclic Redundancy Check (CRC) instructions.
  • +fp.sp — the single-precision FPv5 floating-point instructions.
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions.
  • +nocrypto — disable the cryptographic instructions.
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.

Okay so real talk, -march=native makes the compiler auto-detect the architecture of the build computer. But at present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful, the option has no effect — it just shrugs and does nothing 😭 no cap.

---

✨ -mtune=name ✨

This one 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. 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 🫶 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 specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. Heads up though — the effects of this option may change in future GCC versions as CPU models come and go 👑

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. they're just there for the vibe, not the tuning 💀

-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. same energy as before, fr fr 🙈

---

✨ -mcpu=name[+extension...] ✨

This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so if you're double-dipping, -march/-mtune win, periodt 💅

Many of the supported CPUs implement optional architectural extensions. Where this is so 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. 👀

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 — disable 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 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 — bestie, this tells the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. 💅
• -mfpu=name — okay so this specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. 🦋

The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. ✨

• -mfp16-format=name — specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. 💖
• -mstructure-size-boundary=n — the sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it. 👑

Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. 💀

This option is deprecated. 🙈

• -mabort-on-noreturn — generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return. 🫶
• -mlong-calls, -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction. 🌈

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls. 🔥

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. ⭐

• -msingle-pic-base — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. 💜
• -mpic-register=reg — specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10. 🎀
• -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default. 🌸
• -mpoke-function-name — write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

                        t0
                            .ascii "arm_poke_function_name", 0
                            .align
                        t1
                            .word 0xff000000 + (t1 - t0)
                        arm_poke_function_name
                            mov     ip, sp
                            stmfd   sp!, {fp, ip, lr, pc}
                            sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). 🧚

• -mthumb, -marm — select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option. 👑

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. 💅

• -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. 🦋
• -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. 🔥
• -mcaller-super-interworking — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. 💜
• -mtp=name — specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to __aeabi_read_tp, cp15, which fetches the thread pointer from cp15 directly (supported in the arm6k architecture), and auto, which uses the best available method for the selected processor. The default setting is auto. 🎀
• -mtls-dialect=dialect — specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. 🌈
• -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. ⭐
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. 💅
• -munaligned-access, -mno-unaligned-access — enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time. 🫶

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ✨

• -mneon-for-64bits — > This option is deprecated and has no effect. 😭
• -mslow-flash-data — assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default. It conflicts with -mword-relocations. 💀
• -masm-syntax-unified — okay so this one is like, "assume inline assembler is using unified asm syntax." The default is currently off, which implies divided syntax. Heads up bestie: this option has no impact on Thumb2. However — and this is the tea — this may change in future releases of GCC. Divided syntax should be considered deprecated. 💀
• -mrestrict-it — restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This one is on by default for ARMv8-A Thumb mode. ✨
• -mprint-tune-info — prints CPU tuning information as a comment in the assembler file. Lowkey this is only for regression testing of the compiler and is not intended for ordinary use in compiling code. Disabled by default. 🙈
• -mverbose-cost-dump — enables verbose cost model dumping in the debug dump files. It's provided for use in debugging the compiler. iykyk 🧚
• -mpure-code — do not allow constant data to be placed in code sections. Also, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". This option is only available when generating non-pic code for M-profile targets. 🎀
• -mcmse — generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which can be found at https://developer.arm.com/documentation/ecm0359818/latest/. 🔥
• -mfix-cmse-cve-2021-35465 — mitigate against a potential security issue with the VLLDM instruction in some M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option -mcpu= is used with cortex-m33, cortex-m35p or cortex-m55. The option -mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation. 🛡️
• -mfdpic, -mno-fdpic — select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and the user should explicitly provide the PIC/PIE-related options as needed. 💅

Note that static linking is not supported because it would still involve the dynamic linker when the program self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options. 😭

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs. 🌈

---

🌸 AVR Options 🌸

These options are defined for AVR implementations, bestie. Buckle up, there's a lot. 🦋

• -mmcu=mcu — specify Atmel AVR instruction set architectures (ISA) or MCU type. The default for this option is avr2.

GCC supports the following AVR devices and ISAs:

• avr2 — "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "Classic" devices with up to 8 KiB of program memory and with the MOVW instruction. mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.
• avr3 — "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355.
• avr31 — "Classic" devices with 128 KiB of program memory. mcu = atmega103, at43usb320.
• avr35 — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction. mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.
• avr4 — "Enhanced" devices with up to 8 KiB of program memory. mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
• 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 option has only an effect on reduced Tiny devices like ATtiny40. See also the absdata AVR Variable Attributes, variable attribute. ✨
• -maccumulate-args — accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack, like calls to printf-like functions. 💖

• -mbranch-cost=cost — set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0. 🌟
• -mcall-prologues — function prologues/epilogues get expanded as calls to the appropriate subroutines. Code size goes down. ✨
• -mdouble=bits, -mlong-double=bits — set the size (in bits) of the double or long double type, respectively. bits can be 32 or 64. Whether a specific bits value is even allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the default values of the options depend on those too. iykyk
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the no_gccisr function attribute from AVR Function Attributes. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683). slay
• -mint8 — assume int is an 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. > Heads up bestie: this option does not conform to the C standards, but it results in smaller code size. ⚠️
• -mmain-is-OS_task — do not save registers in main. The effect is the same like attaching attribute OS_task from AVR Function Attributes to main. It is activated per default if optimization is on. main character energy fr
• -mn-flash=num — assume that the flash memory has a size of num times 64 KiB. math girlie
• -mno-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller. don't say I didn't warn you
• -mrelax — try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below. periodt

• -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT. read modify write, we love a multitasker
• -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. compiler internals, not your business bestie

• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.

This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not. compiler lore

• -mstrict-X — use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

adiw r26, const   ; X += const
ld   <Rn>, X        ; <Rn> = *X
sbiw r26, const   ; X -= const

hardware said no emulation today

• -mtiny-stack — only change the lower 8 bits of the stack pointer. tiny stack, tiny vibes
• -mfract-convert-truncate — allow to use truncation instead of rounding towards zero for fractional fixed-point types. math but make it chaotic
• -nodevicelib — don't link against AVR-LibC's device specific library lib<mcu>.a. not today, device lib
• -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 that 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. DIY era

• -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. address space drama
• -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default. spelling matters bestie

✨ "EIND" and Devices with More Than 128 Ki Bytes of Flash ✨

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

• The compiler never sets EIND. respectfully, not its job
• 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. sneaky
• 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. set it and forget it? no, actually don't forget it
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. trampolines but make it embedded
• 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. > Bestie, turn on relaxation or risk cryptic linker tears.
• 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. custom linker script era
• 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/). team effort
• 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. early bird gets the EIND

• 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. two conditions, one slay

• 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. gs modifier everywhere
• 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. no raw address jumps, bestie

🌈 Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers 🌈

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as high part of the address: The X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing.

• The startup code initializes the RAMP special function registers with zero. zero it out, bestie
• If a AVR Named Address Spaces named address space that isn't generic or __flash is used, then RAMPZ gets set as needed before the operation. ✨
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to get an operation done, then 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. So if you use inline assembler to read from locations outside the 16-bit address range and you change one of the RAMP registers, you must reset it to zero after the access. No cap. 😭

🦋 AVR Built-in Macros 🦋

GCC defines several built-in macros so that your code can test for the presence or absence of features. Almost any of the built-in macros below are deduced from device capabilities, which means they're 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__ — a built-in macro that resolves to a decimal number identifying 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, and it reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, and -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros' names follow the scheme __AVR_Device__, where Device is the device name as it appears in 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 isn't actually 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 isn't actually 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, which performs 16-bit register-register moves.
• __AVR_HAVE_LPMX__ — the device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MUL__ — the device has a hardware multiplier. 🔥
• __AVR_HAVE_JMP_CALL__ — the device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
• __AVR_HAVE_EIJMP_EICALL__, __AVR_3_BYTE_PC__ — the device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. It also means 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 an 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. 😭 The 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 than if they were addressed by an instruction accessing 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 it is defined, the address space is linear and there's 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's 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 isn't used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in, if -msim is not given.

Without this option, bf532 is used as the processor by default.

Heads up bestie: support 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 the 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. 💅
• -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 — bestie, this makes code that does not assume ID-based shared libraries are in play. And plot twist: this is the default 💅
• -mleaf-id-shared-library — generates code that supports shared libs the library-ID way, but lowkey assumes this library or executable won't link against any other ID shared libraries. Why do we care? Because that assumption lets the compiler use faster code for jumps and calls 🔥
• -mno-leaf-id-shared-library — the opposite vibe: do NOT assume the code being compiled won't link against ID shared libraries. The price? Slower code gets generated for jump and call insns 😭
• -mshared-library-id=n — tells it the identification number of the ID-based shared library being compiled. A value of 0 generates more compact code; any other value forces that number to be allocated to the current library, but ngl it's no more space- or time-efficient than just omitting this option entirely 🙈
• -msep-data — generates code where the data segment can live in a totally different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management, by eliminating relocations against the text section ✨
• -mno-sep-data — generates code that assumes the data segment follows the text segment. This is the default, periodt.
• -mlong-calls, -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register, and then doing a subroutine call on that register. You need this switch 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, and specifying -mno-long-calls restores the default behavior. Also heads up: these switches have no effect on how the compiler generates code to handle function calls via function pointers 👑
• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance ⚡
• -minline-plt — enable inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors. This option 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 isn't used, the single-core application programming model is used.

• -mcorea — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore 🫶
• -mcoreb — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro "__BFIN_COREB" is defined. When this option is used, "coreb_main" should be used instead of "main". This option can only be used in conjunction with -mmulticore.
• -msdram — build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM before loading the application 🌸
• -micplb — assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

---

✨ C6X Options ✨

• -march=name — this specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default 💖
• -msim — choose startup files and linker script suitable for the simulator.
• -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const.
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them 👑
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data into the .const section.

---

🌈 CRIS Options 🌈

These options are defined specifically for the CRIS ports, bestie 💅

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes 🚨
• -metrax4, -metrax100 — these are synonyms for -march=v3 and -march=v8 respectively.
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is active by default.
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file 🔍
• -mcc-init — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (and the no- versions) arrange (or eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options 🫶
• -m32-bit, -m16-bit, -m8-bit — similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code.

⚠️ Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. Like, it will just silently do the thing. Be careful, bestie 😭

• -mno-gotplt, -mgotplt — with -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt.
• -melf — legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
• -mlinux — legacy no-op option only recognized with the cris-axis-linux-gnu target.
• -sim — this option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively ✨
• -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

---

🦋 CR16 Options 🦋

These options are defined specifically for the CR16 ports.

• -mmac — enable the use of multiply-accumulate instructions. Disabled by default.
• -mcr16cplus, -mcr16c — generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default 💅
• -msim — links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.
• -mint32 — choose integer type as 32-bit wide.
• -mbit-ops — generates "sbit"/"cbit" instructions for bit manipulations 🔥
• -mdata-model=model — choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model 🚫

---

🧚 C-SKY Options 🧚

GCC supports these options when compiling for C-SKY V2 processors, no cap.

• -march=arch — specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810 ⭐
• -mcpu=cpu — bestie, this is where you tell GCC which C-SKY target processor you're repping 🎀. The valid values for cpu are:

ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t,
ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht,
ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1,
ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1,
ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft,
ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv.

like that's a whole alphabet of chips, periodt 💅

• -mbig-endian, -EB, -mlittle-endian, -EL — pick whether you want big- or little-endian code. The default is little-endian ✨.
• -mhard-float, -msoft-float — choose between hardware or software floating-point implementations. Default is soft float 🦋.
• -mdouble-float, -mno-double-float — when -mhard-float is in effect, this turns on generation of double-precision float instructions. That's the default except when you're compiling for CK803 💖.
• -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, turn on generation of frecipd, fsqrtd, and fdivd instructions. Default except when compiling for CK803 🌸.
• -mfpu=fpu — choose your floating-point processor, bestie. This one can only be used with -mhard-float. The values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu) ✨.
• -melrw, -mno-elrw — enable the extended lrw instruction. This defaults to on for CK801 and off otherwise 🎀.
• -mistack, -mno-istack — enable interrupt stack instructions; default is off. 🫶 Lowkey important: 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 💀

okay yes, -mcp and -mcache have the exact same description in the original, that's not me being lazy, that's just the lore 🙈

• -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 you can use 16-bit instructions. This is ignored for CK801, where this is just 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. 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 is the default and it's required for correct code generation on CK801 and CK802, and 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. 🚨 Careful bestie: using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. That's giving "do not pass go" energy ngl 💀

---

🍎 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 that 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 ⭐

bestie listen. these next ones? not even our business fr. GCC literally just takes them and yeets them straight over to the Darwin linker, and the Darwin linker's own man page does all the explaining. We are merely the messenger here 🫶

• -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. (go read that one, she's the one with the lore 💅)

---

🌈 DEC Alpha Options 🌈

ok so these -m options are the ones defined specifically for the DEC Alpha implementations. Alpha girlies, this one's for you 💜

• -mno-soft-float, -msoft-float — use (or don't use, hence the no) 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.

heads up bestie: Alpha implementations without floating-point operations are required to have floating-point registers. that's just how she is ✨

• -mfp-reg, -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers, and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. main character kernel behavior honestly 👑

• -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.
• -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. it's giving self-made icon 💅

• -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 (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 has zero instruction set extensions. that's it, that's the whole vibe 💅
• ev56, 21164a — schedules like an EV5 but supports the BWX extension.
• pca56, 21164pc, 21164PC — schedules like an EV5 and supports BWX and MAX. two extensions, she's eating 🍓
• ev6, 21264 — schedules like an EV6 and supports BWX, FIX, and MAX.
• ev67, 21264a — schedules like an EV6 and supports BWX, CIX, FIX, and MAX. the whole friend group fr fr 👑

Native toolchains also support the value native, which selects the best architecture option for the host processor. heads up bestie: -mcpu=native has no effect if GCC doesn't recognize the processor. she simply doesn't know her. 😭

• -mtune=cpu_type — sets only the instruction scheduling parameters for machine type cpu_type. the instruction set itself? untouched. not changed. we're just tuning the vibe, not the whole personality. ✨

Native toolchains also support the value native, which picks the best architecture option for the host processor. and again — -mtune=native has no effect if GCC doesn't recognize the processor. 🙈

• -mmemory-latency=time — sets how many cycles the scheduler should assume a typical memory reference takes, as seen by the application. ngl this number is super dependent on the memory access patterns your app uses and the size of the external cache on the machine. so like… it's giving "it depends" 💀

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. quick note bestie: L3 is only valid for EV5. 👀

---

✨ eBPF Options ✨

• -mframe-limit=bytes — sets the hard limit for frame sizes, in bytes. currently the value you can specify has to be less than or equal to 32767. defaults to whatever limit the version of the Linux kernel you're targeting imposes. no cap.
• -mkernel=version — says what the minimum version of the kernel is that will run the compiled program. GCC uses this to figure out which instructions to use, which 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 lot of versions but okay girl we're keeping up 💪
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. and bestie… this is the default. 🎀
• -mxbpf — generate code for an expanded version of BPF, one that relaxes some of the restrictions the BPF architecture imposes:
  • <Save and restore callee-saved registers at function entry and> exit, respectively.

---

✨ FR30 Options ✨

These options are defined specifically for the FR30 port.

• -msmall-model — use the small address space model. this can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. so like, know your data first bestie 😭
• -mno-lsim — assume that runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.

---

✨ FT32 Options ✨

These options are defined specifically for the FT32 port.

• -msim — specifies that the program will be run on the simulator. this makes an alternate runtime startup and library get linked in. > WARNING 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. 💀
• -mlra — enable Local Register Allocation. this is still experimental for FT32, so by default the compiler uses standard reload. she's in her testing era 🧪
• -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".
• -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. with no PIC/PIE-related options at all, 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. and with a bfin-elf target, this option implies -msim. 🦋
• -minline-plt — enable inlining of PLT entries in function calls to functions that aren't known to bind locally. it has no effect without -mfdpic. it's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — assume a large TLS segment when generating thread-local code.
• -mtls — do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — enable the use of "GPREL" relocations in the FDPIC ABI for data that's 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's not, -mno-gprel-ro can be used to disable it.
• -multilib-library-pic — link with the (library, not FD) pic libraries. it's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. bestie you should never have to use it explicitly. 🙈
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. this option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. this lets those functions be placed anywhere within the 32-bit address space. ✨
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. this option only has an effect when VLIW packing is enabled. it doesn't create new packets; it merely adds NOPs to existing ones. no new drama, just filler 💅
• -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, don't get attached. 💀

• -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's enabled by default. ✨
• -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code.
• -mtomcat-stats — cause gas to print out tomcat statistics. 📊
• -mcpu=cpu — select the processor type for which to generate code. possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. pick your fighter 🫶

---

✨ GNU/Linux Options ✨

okay bestie, these -m options are literally for GNU/Linux targets:

• -mglibc — Use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — Use 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 compatible with Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined.

• -tno-android-cc — Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

---

✨ H8/300 Options ✨

These -m options are defined for the H8/300 implementations:

• -mrelax — Shorten some address references at link time, when possible; uses the linker option -relax.
• -mh — Generate code for the H8/300H.
• -ms — Generate code for the H8S.
• -mn — Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
• -ms2600 — Generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — Extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — Extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — Make "int" data 32 bits by default.
• -malign-300 — On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

---

🌈 HPPA Options 🌈

These -m options are defined for the HPPA family of computers:

• -march=architecture-type — Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around.
• -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
• -mcaller-copies — The caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
• -mjump-in-delay — This option is ignored and provided for compatibility purposes only.
• -mdisable-fpregs — Prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler aborts.
• -mdisable-indexing — Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
• -mno-space-regs — Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

Such code is suitable for level 0 PA systems and kernels.

• -mfast-indirect-calls — Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.

This option does not work in the presence of shared libraries or nested functions.

• -mfixed-range=register-range — Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• -mlong-load-store — Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
• -mportable-runtime — Use the portable calling conventions proposed by HP for ELF systems.
• -mgas — Enable the use of assembler directives only GAS understands.
• -mschedule=cpu-type — Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700, 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.
• -mlinker-opt — Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
• -msoft-float — Generate output containing library calls for floating point.

Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• -msio — Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
• -mgnu-ld — Use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mhp-ld — Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mlong-calls — Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

• -munix=unix-std — Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.

Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.

• -nolibdld — Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — okay so here's the HP-UX tea 🫖: the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. And there is no archive version of libdld.sl. So the moment you pass -static, some special link options are needed to resolve that dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl whenever -static is specified. Which means… the resulting binary ends up being dynamic. 💀 And on the 64-bit port, the linkers generate dynamic binaries by default in any case anyway. If you want to stop the GCC driver from adding these link options, that's what -nolibdld is for.

• -threads — adds support for multithreading with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker. Teamwork 💅

---

✨ IA-64 Options ✨

These are the -m options defined for the Intel IA-64 architecture. Buckle up bestie, there's a lot 🎀

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX.
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. This is the default.
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. This is the default.
• -mno-pic — generate code that does not use a global pointer register. 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. Useful when compiling kernel code.
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. 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 does not always do so. Not me promising results I can't guarantee 😭
• -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. 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.

Okay real talk bestie: you must not use this option when generating programs that will run on real hardware. You must provide your own runtime library for whatever I/O functions are needed. No shortcuts 💀

• -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. Consistency is the vibe check ✨

---

💜 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 — bestie, this is the ✨vibe✨ where every single object is assumed to live in the lower 16MB of memory, so its address can just be loaded with the ld24 instruction, and every subroutine is assumed to be reachable with the bl instruction. This is the default. The addressability of a particular object can be set with the model attribute. 💖
• -mmodel=medium — now we're assuming objects could be anywhere in the 32-bit address space (so the compiler generates seth/add3 instructions to load their addresses), but all subroutines are still assumed reachable with the bl instruction. 🦋
• -mmodel=large — objects may be anywhere in the 32-bit address space (compiler generates seth/add3 instructions to load their addresses), and subroutines may not be reachable with the bl instruction. So the compiler generates the much slower seth/add3/jl instruction sequence instead. 💀
• -msdata=none — disables use of the small data area entirely. Variables get put into one of .data, .bss, or .rodata (unless the section attribute has been specified). This is the default. Just so you know: the small data area consists of the sections .sdata and .sbss, and objects may be explicitly put in the small data area with the section attribute using one of those sections. 🌸
• -msdata=sdata — puts small global and static data in the small data area, but does not generate special code to reference them. 🎀
• -msdata=use — puts small global and static data in the small data area, and generates special instructions to reference them. slay. 💅
• -G num — puts global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. Heads up: the -msdata option must be set to one of sdata or use for this option to have any effect at all. 🫶

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 display some statistics that might help in debugging programs. it's giving detective energy 🔍
• -malign-loops — aligns all loops to a 32-byte boundary. 📐
• -mno-align-loops — do not enforce a 32-byte alignment for loops. This is the default. 🙈
• -missue-rate=number — issue number instructions per cycle. number can only be 1 or 2. ⭐
• -mbranch-cost=number — number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies. 👑
• -mflush-trap=number — specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive. 🌈
• -mno-flush-trap — specifies that the cache cannot be flushed by using a trap. 🚫
• -mflush-func=name — specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available. ✨
• -mno-flush-func — indicates that there is no OS function for flushing the cache. 😭

---

💖 M680x0 Options 💖

These are the -m options defined for M680x0 and ColdFire processors, bestie. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below. 🌸

• -march=arch — generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac. 🦋

GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above. 👑

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture. no cap 💅

• -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families: 💖
  • Family 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
  • Family 5206 : 5202 5204 5206
  • Family 5206e : 5206e
  • Family 5208 : 5207 5208
  • Family 5211a : 5210a 5211a
  • Family 5213 : 5211 5212 5213
  • Family 5216 : 5214 5216
  • Family 52235 : 52230 52231 52232 52233 52234 52235
  • Family 5225 : 5224 5225
  • Family 52259 : 52252 52254 52255 52256 52258 52259
  • Family 5235 : 5232 5233 5234 5235 523x
  • Family 5249 : 5249
  • Family 5250 : 5250
  • Family 5271 : 5270 5271
  • Family 5272 : 5272
  • Family 5275 : 5274 5275
  • Family 5282 : 5280 5281 5282 528x
  • Family 53017 : 53011 53012 53013 53014 53015 53016 53017
  • Family 5307 : 5307
  • Family 5329 : 5327 5328 5329 532x
  • Family 5373 : 5372 5373 537x
  • Family 5407 : 5407
  • Family 5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚫

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above. ✨

• -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e. 🌈

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively. 🎀

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range. 👑

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above. 💜

• -m68000
• -mc68000

Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000. ✨

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. 🦋

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010. 🌸
• -m68020
• -mc68020

Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020. 💖

• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030. ⭐
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040. 🎀

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions. 🫶

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060. 💅

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions. 😭

• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32. 🔥

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. 👑

• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option. 🙈

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206. ✨

• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e. 💜
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x. 🌸
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307. 🦋
• -m5407 — generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407. ⭐
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option. 💖
• -m68020-40 — generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040. 🎀

The option is equivalent to -march=68020 -mtune=68020-40. ✨

• -m68020-60 — generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060. 🌈

The option is equivalent to -march=68020 -mtune=68020-60. 💅

• -mhard-float
• -m68881

Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets. 👑💜

• -msoft-float — do NOT generate floating-point instructions, use library calls instead. This is the default for 68000, 68010, and 68832 targets, and it's also the default for ColdFire devices that have no FPU. no FPU? no problem bestie 💅
• -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 gets 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.

GCC defines the macro __mcfhwdiv__ when this option is enabled. manifesting ✨

• -mshort — treat type int as 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. little guys 💜
• -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. Heads up: 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. bit-fields are eating fr 🎀
• -mrtd — use a different function-calling convention, where functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there. efficiency queen 👑

⚠️ This calling convention is incompatible with the one normally used on Unix, so you can't use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) not the vibe, bestie 😭

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

The default is -mno-rtd.

• -malign-int, -mno-align-int — control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory. trade-offs are so real 🍓

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. proceed with caution fr fr 💀

• -mpcrel — use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors. yes the manpage just casually drops this one on you 🦋
• -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system.
• -msep-data — generate code that lets the data segment live 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. living separately, thriving individually ✨
• -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. sharing is caring 🫶
• -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. zero is the moment, the rest is just extra 💜
• -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. 4 whole instructions?? the horror 😭

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. very few fr 💅

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. deep lore incoming 🦋

• -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. we don't need it bestie, trust 💅
• -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. segfault jumpscare 💀 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. the everything bagel ✨
• -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. 128 bytes, that's it, that's the whole budget 💜
• -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. they're a package deal, no solo acts 🎀
• -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. fancy ✨
• -mdc — causes constant variables to be placed in the .near section.
• -mdiv — enables the div and divu instructions.
• -meb — generate big-endian code.
• -mel — generate little-endian code.
• -mio-volatile — tells the compiler that any variable marked with the io attribute is to be considered volatile. volatile, the drama queen of keywords 💅
• -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. anti-everything-bagel 😭
• -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. tiny but mighty 🦋
• -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. it's giving "for the homies" 🫶
• -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 — bestie this one makes every single function default to the .far section ✨. Skip it and they all chill in .near instead. That's the whole vibe. 💅
• -mtiny=n — any variable that's n bytes or smaller gets yeeted into the .tiny section 🌸. These cuties use the $gp base register. The default for this option is 4, but heads up — there's a 65536-byte limit on the .tiny section, so don't get too comfy in there 🫶.

🦋 MicroBlaze Options 🦋

• -msoft-float — use software emulation for floating point. This is the default, no cap 💜.
• -mhard-float — use actual hardware floating-point instructions. Fancy 🔥.
• -mmemcpy — do not optimize block moves, just use memcpy. Let the library girlies handle it ✨.
• -mno-clearbss — okay so this one is deprecated 💀. Use -fno-zero-initialized-in-bss instead, periodt.
• -mcpu=cpu-type — use the features of, and schedule code for, the CPU you name. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code 🌈. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a. Iconic little naming scheme honestly 👑
• -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. Slay 💅.
• -mxl-pattern-compare — use pattern compare instructions ✨.
• -msmall-divides — use table lookup optimization for small signed integer divisions. Lowkey efficient 🔥.
• -mxl-stack-check — deprecated alarm 🚨! Use -fstack-check instead, bestie.
• -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, iykyk) 🔄.
• -mxl-mode-app-model — select application model app-model. Valid models are:
  • executable — normal executable (the default), and it uses the startup code crt0.o ✨.
  • -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This 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 one uses the startup file crt1.o and sets the start address of the program to 0x800 💅.
  • bootstrap — for applications that get loaded using a bootloader. This model uses the startup file crt2.o, which does not contain a processor reset vector handler. Perfect for handing control on a processor reset over to the bootloader instead of the application 🌈.
  • novectors — for applications that don't need any of the MicroBlaze vectors. This can be handy for applications running inside a monitoring application. This model uses crt3.o as a startup file 🧚.

Also: option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model 💀.

---

👑 MIPS Options 👑

• -EB — generate big-endian code 👑.
• -EL — generate little-endian code. This is the default for mips*el-*-* configurations 🎀.
• -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, 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 at 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 (do not generate) MIPS16 code 🌈. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE ✨.

MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes 🎀.

• -mflip-mips16 — generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code 💀. Testing girlies only.
• -minterlink-compressed, -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa 🫶.

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed 🌸.

• -minterlink-mips16, -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility 💜.
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI ✨.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead 🎀.

For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html> 🔗.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64 💅. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors 🫶.

The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair 🌸. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved ✨.

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg) 👑. The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both 💀. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes 🔥.

• -mabicalls, -mno-abicalls — generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems 🎀.
• -mshared, -mno-shared — generate (or, with the no- version, do not generate) code that's fully position-independent, so it can get linked into shared libraries. Bestie, this option only does anything to -mabicalls.

All -mabicalls code has traditionally been position-independent, no matter what you pass for stuff like -fPIC and -fpic. But plot twist ✨ — as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. You get that mode with -mno-shared.

-mno-shared is highkey needy: it depends on binutils 2.16 or higher, and it generates objects that only the GNU linker can link. It does not touch the ABI of the final executable tho — it only affects the ABI of relocatable objects. And using -mno-shared generally makes executables both smaller and quicker. slay 💅

-mshared is the default.

• -mplt, -mno-plt — assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. Again, this only hits -mno-shared -mabicalls. For the n64 ABI, it does nothing at all 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 how big the global offset table can be.

GCC normally loads values from the GOT with a single instruction. Efficient, yes 🫶 — but it only works if the GOT is smaller than about 64k. Anything bigger and the linker starts screaming at you with an error like:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens, recompile your code with -mxgot. It works with very large GOTs, but the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. trade-off queen 👑

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent code.

---

• -mgp32 — assume 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 era 🔮
• -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 it also 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 control how the special not-a-number (NaN) IEEE 754 floating-point data gets treated 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. iconic behavior honestly ✨

• -mnan=2008, -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. yes bestie, they literally just flipped the bit 🌸

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. package deal 🎀
• -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 ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller.

• -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=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 so this bit is still deep in MIPS land and it's a lot, but it's literally just rules about small data, GP-relative stuff, and every single CPU errata workaround under the sun. Stay with me bestie, it's all here 💖

---

If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. And if Var is common? You must link the application with a high-enough -G setting. No skipping steps, bestie 🫶

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. Iconic problem-solving energy ✨

• -mgpopt, -mno-gpopt — Use (or don't use, bestie) GP-relative accesses for symbols that are known to be in a small data section; peep -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations, periodt.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

Heads up: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. It's giving cascade 💅

• -membedded-data, -mno-embedded-data — Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. Slow and steady, but it slays on memory 🌸
• -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:
  • -mcode-readable=yes — Instructions may freely access executable sections. This is the default setting. Live your truth ✨
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
  • -mcode-readable=no — Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
• -msplit-addresses, -mno-split-addresses — Enable (disable) use of the %hi() and %lo() assembler relocation operators. Ngl this option has been superseded by -mexplicit-relocs but it's retained for backwards compatibility. Legacy icon behavior 💜
• -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 (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. Choose wisely bestie 😭

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.

• -mload-store-pairs, -mno-load-store-pairs — Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. It's giving efficiency 🔥
• -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. Big ask, fr fr.

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 the 74k catching strays 💀
• -mfused-madd, -mno-fused-madd — Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is -mfused-madd.

On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them. No preprocessing, we're raw-dogging the assembler 🙈
• -mfix-24k, -mno-fix-24k — Work around the 24K E48 (lost data on stores during refill) errata. Just so you know, the workarounds are implemented by the assembler rather than by GCC.
• -mfix-r4000, -mno-fix-r4000 — Work around certain R4000 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
  • A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
  • An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
• -mfix-r4400, -mno-fix-r4400 — Work around certain R4400 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
• -mfix-r10000, -mno-fix-r10000 — Work around certain R10000 errata:
  • "ll"/"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. Sneaky smart, lowkey 👑
• -mfix-sb1, -mno-fix-sb1 — Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2" floating-point errata.)
• -mr10k-cache-barrier=setting — Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. Rude, honestly 😭

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.

This is a real correctness thing, bestie — those speculatively-executed side effects are not a vibe. If you're on R10K and this applies to you, read the processor manual, fr fr. 🚨

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 literally your job, not GCC's 💅

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. so if you're out here hand-writing asm, no cap, you gotta play by the same rules 🙈

The option has three forms:

• -mr10k-cache-barrier=load-store — inserts 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 — inserts a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disables 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, and one of them is literally just 3. iconic behavior tbh ✨

• 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 means "we vibes-based it", fr fr
• -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, they said it themselves 🫠

• -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. it's giving "I'll take the delay slot if it's free, otherwise whatever" 😌

• -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 >>> one pipe, do the math 💅

• -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 thicc, 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 bestie, that's the scary one 😭

• -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. they handled it themselves, main character energy fr 🦋

• -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. two ways to change where you came back to. unhinged. obsessed. 👑

• -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 everywhere, even at -O3. it simply does not want to be perceived 🙈

• -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: yes the processor is literally named MMIX, slay ⭐

• -mlibfuncs
• -mno-libfuncs

Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. no matter the size bestie, we don't discriminate 🫶

• -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 entirely different neighborhoods for your registers 💜

• -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. named after Knuth himself, we have no choice but to stan 👑

• -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 it can only hold so many friends 😭 range 0 to 255, that's it

• -msingle-exit
• -mno-single-exit

Force (do not force) generated code to have a single exit point in each function.

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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. four choices, pick your fighter 💅
• -mreturn-pointer-on-d0 — when a function returns a pointer, it returns it in both a0 and d0. Otherwise the pointer only comes back in a0, and the second you try to call one of those functions without a prototype… errors. 🫠 Fun fact: this option is on by default, so to turn it off you go -mno-return-pointer-on-d0.
• -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 "bestie pls run a relaxation optimization pass" so it can shorten branches, calls and absolute memory addresses. ✨ It only does anything when you use it on the command line for the final link step.

This option makes symbolic debugging impossible. 💀 not a drill.

• -mliw — lets the compiler generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. It also defines the preprocessor macro __LIW__.
• -mno-liw — do not let the compiler generate Long Instruction Word instructions. This one defines the preprocessor macro __NO_LIW__.
• -msetlb — lets the compiler generate the SETLB and Lcc instructions if the target is the AM33 or later. Default behavior. Defines the preprocessor macro __SETLB__.
• -mno-setlb — do not generate SETLB or Lcc instructions. Defines the preprocessor macro __NO_SETLB__.

---

🦋 Moxie Options 🦋

• -meb — generate big-endian code. This is the default for moxie-*-* configurations.
• -mel — generate little-endian code.
• -mmul.x — generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
• -mno-crt0 — do not link in the C run-time initialization object file. (yes, again. iconic of it.) 🎀

---

🍓 MSP430 Options 🍓

These options are defined for the MSP430:

• -masm-hex — forces assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. 💅
• -mmcu= — selects the MCU to target. This is 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's what gets selected — otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

On top of all that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and causes the linker to search for a script called xxx.ld.

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 those that have been hard-coded. 🌈

GCC searches for the devices.csv file using the following 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 — define 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 will also be registered as an include path, and a 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 the user has 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 a 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 — this option enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default.
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated — the -mmcu= option is what you should be using to select the ISA. 🫶
• -msim — link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the -mmcu= option.
• -mlarge — use large-model addressing (20-bit pointers, 32-bit size_t).
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
• -mrelax — this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link.
• mhwmult= — describes the type of hardware multiply supported by the target. Accepted values are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs, 32bit for the 16/32-bit multiply supported by later MCUs, and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given — this tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified, or if the MCU name is not recognized, then no hardware multiply support is assumed. auto is the default setting.

Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher, however, the hardware multiplier is invoked inline. This makes for bigger, but faster code. 🔥

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code.

• -minrt — enable the use of a minimum runtime environment — no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
• -mtiny-printf — enable 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.

Bestie, this one's got caveats, so listen up. 👑

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= — this option passes on a request to the assembler to enable the fixes for the named silicon errata.
• -msilicon-errata-warn= — this option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.
• -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 ⭐

• -G num — bestie, this one puts all your global and static objects that are num bytes or smaller into the small data or BSS sections instead of the regular data or BSS sections. The default for num is 8. ✨
• -mgpopt=option
• -mgpopt
• -mno-gpopt — these make GCC generate (or not generate) GP-relative accesses. Here's the vibe check on which option names it actually knows about:
  • none — do not generate GP-relative accesses. simple as that 💅
  • local — generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. It also uses GP-relative addressing for objects you explicitly placed in a small data section via a "section" attribute.
  • global — same as local, but it also generates GP-relative accesses for small data objects that are external, weak, or common. 🚨 If you use this, you MUST make sure every part of your program (libraries included, no exceptions) is compiled with the same -G setting.
  • data — generate 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.
  • all — generate 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.
• -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. periodt.

You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. In that case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. 😭

• -mgprel-sec=regexp — this specifies extra section names that can be accessed via GP-relative addressing. It's most useful alongside "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.

Heads up: 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 recognizes.

• -mr0rel-sec=regexp — this one specifies names of sections that can be accessed via a 16-bit offset from "r0" — meaning, in the low 32K or high 32K of the 32-bit address space. Most useful in conjunction with "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.

Unlike 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.

• -mel
• -meb — generate little-endian (the default) or big-endian (experimental 👀) code, respectively.
• -march=arch — this names the target Nios II architecture. GCC uses that name to figure out what kind of instructions it's allowed to 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 access bypasses the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.
• -mno-fast-sw-div
• -mfast-sw-div — do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3 and above.
• -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 instructions are optional extensions to the R2 architecture, the default is not to emit them.
• -mcustom-insn=N
• -mno-custom-insn — each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. For example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call. 🎀

The following values of insn are supported. Except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

• fadds, fsubs, fdivs, fmuls — binary arithmetic operations.
• fnegs — unary negation.
• fabss — unary absolute value.
• fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes — comparison operations.
• fmins, fmaxs — floating-point minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrts — unary square root operation.
• fcoss, fsins, ftans, fatans, fexps, flogs — floating-point trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

• faddd, fsubd, fdivd, fmuld — binary arithmetic operations.
• fnegd — unary negation.
• fabsd — unary absolute value.
• fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned — comparison operations.
• fmind, fmaxd — double-precision minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrtd — unary square root operation.
• fcosd, fsind, ftand, fatand, fexpd, flogd — double-precision trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Conversions:

• fextsd — conversion from single precision to double precision.
• ftruncds — conversion from double precision to single precision.
• fixsi, fixsu, fixdi, fixdu — conversion from floating point to signed or unsigned integer types, with truncation towards zero.
• round — conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the "__builtin_lroundf" function when -fno-math-errno is used.
• floatis, floatus, floatid, floatud — conversion from signed or unsigned integer types to floating-point types.

In addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. 🦋

• 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.

Ngl, you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas. 💜

• -mcustom-fpu-cfg=name — this enables a predefined, named set of custom instruction encodings (peep -mcustom-insn above). Currently, the following sets are defined:

-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant

-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. 🔥

And bestie, 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 — links with the HAL BSP 💫. This one suppresses linking with the C runtime startup and termination code that GCC usually hands you, and it's typically used together with -msys-crt0= to say where the alternate startup code from the HAL BSP lives.
• -msmallc — link with a limited version of the C library, -lsmallc, instead of Newlib. smol library, big vibes 🎀.
• *-msys-crt0=startfile* — 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 provides the low-level system calls the C library needs, e.g. "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. pick your lane 🦋.
• *-misa=ISA-string* — generate code for the specified PTX ISA (e.g. sm_35). ISA strings must be lower-case. Valid ISA strings include sm_30 and sm_35. The default ISA is sm_30.
• -mmainkernel — link in code for a __main kernel. This is for stand-alone execution instead of offloading 👑.
• -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 the 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 all threads in each warp execute, while keeping memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions 🌈. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position "tid.y" with the current lane index to compute the master lane index 🧚.
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant. two options in a trench coat, iconic 💜.

---

🌸 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's for compatibility purposes only 🙈. It used to select linker and preprocessor options for use with newlib.
• -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, 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, 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 is 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 are 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 are generated to store the immediate to a register first.

---

🎀 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 show up, 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 target an AE type that has the necessary hardware support. This option disables those warnings.

---

💜 PowerPC Options 💜

These are listed under

---

🔥 PRU Options 🔥

These command-line options are defined for the 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, how expensive is a jump, bestie? that many instructions. 💸
• -mplt, -mno-plt — when you're generating PIC code, this is your yes/no on letting it use PLTs. Ignored for non-PIC, so don't stress if that's not your situation. The default is -mplt. ✨
• -mabi=ABI-string — ok this one is a whole moment, sit with me. 🫶 You're specifying the integer and floating-point calling convention. ABI-string has two parts: the size of integer types, and the registers used for floating-point types. So for example, -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (which implicitly defines int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast that with -march=rv64ifd -mabi=lp64f, which still lets the compiler 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, where no floating-point arguments get passed in registers at all. 😤

The default for this argument is system dependent, and if you want a specific calling convention you should just say so explicitly, no cap. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are literally 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. 💀 And there's also the ilp32e ABI, which can only be used with the rv32e architecture. That ABI is not well specified at present, and is subject to change — so, fair warning, it's giving "we'll see". 🦋

• -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 the given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case, bestie, don't test me on this. Examples include rv64i, rv32g, rv32e, and rv32imaf. 🌸
• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture name. The 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. It's 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. it's giving "small but still fast", iconic. 💅

• -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. like, all of them. every single one. don't come crying to me when it breaks. 😭

• -msmall-data-limit=n — put global and static data smaller than n bytes into a special section (on some targets). 🍓
• -msave-restore, -mno-save-restore — do, or don't, use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues. ⚡
• -mstrict-align, -mno-strict-align — do not, or do, generate unaligned memory accesses. The default is set depending on whether the processor we're optimizing for supports fast unaligned access or not. 🧚
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model. 🌈
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked. 🌟
• -mexplicit-relocs, -mno-exlicit-relocs — use, or do not use, assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. 🙈
• -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. The 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. So 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're 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, since 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're using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above. 👑

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. 🌸

The -mmfcrf option lets GCC spit out the move-from-condition-register-field instruction that lives on the POWER4 processor and any other chip that supports the PowerPC V2.01 architecture. ✨ The -mpopcntb option lets GCC generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option lets GCC generate the popcount instruction implemented on the POWER7 processor and other processors that support 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 that support the PowerPC V2.03 architecture. The -mcmpb option lets GCC generate the compare-bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. And the -mhard-dfp option lets GCC generate the decimal floating-point instructions implemented on some POWER processors. 🎀

The -mpowerpc64 option lets GCC generate the extra 64-bit instructions you get in the full PowerPC64 architecture, and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

• -mcpu=cpu_type — Sets 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, 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 the cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor. 🙈

The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others.

The -mcpu options automatically enable or disable the following options:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mpower8-vector -mquad-memory  -mquad-memory-atomic  -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma

Which particular options get 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. So 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 — Sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage, the way -mcpu=cpu_type does. 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 — Generates PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — Generates 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 — Generates 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 match array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform.

• -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 keep) 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 — okay so this one flips the 64-bit AIX ABI and calling convention on or off, bestie: that means 64-bit pointers, a 64-bit long type, and the whole infrastructure needed to support them. The way that -maix64 works, it implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 💅
• -mxl-compat, -mno-xl-compat — this produces code that conforms more closely to IBM XL compiler semantics when using the AIX-compatible ABI. It passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It 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. It uses XL symbol names for long double support routines. ✨

Okay, storytime, because this is important: the AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. 😭

• -mpe — this supports the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible. periodt, they cannot sit together. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI. 🦋

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. so don't try it there, bestie.

• -msoft-float, -mhard-float — generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. 💖
• -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode. 🌈
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. lowkey cursed, be careful. 😭
• -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. vibe check: Power6 gets the special treatment. ✨
• -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. iykyk, it got a glow-up. 💅
• -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. string-search slay. 🍓
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. 🌸

For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. the way that one option changes the whole size, obsessed. 👑

• -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. ngl, this is a trust issue. 🦋
• -mrelocatable, -mno-relocatable — generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary. ✨
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. flexible friendship energy. 🫶
• -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. little-endian bestie representation. 🌸
• -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. big-endian main character energy. 👑
• -mdynamic-no-pic — on Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. 💀
• -msingle-pic-base — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. so the runtime has to handle it, not you. ✨
• -mprioritize-restricted-insns=priority — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. 🎀
• -msched-costly-dep=dependence_type — this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
  • no — no dependence is costly. 🙈
  • all — all dependences are costly. 💀
  • true_store_to_load — a true dependence from store to load is costly. 🔥
  • store_to_load — any dependence from store to load is costly. 🌈
  • number — any dependence for which the latency is greater than or equal to number is costly. ⭐
• -minsert-sched-nops=scheme — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
  • no — don't insert NOPs. 🚫
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping. 🎀
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping. ✨
  • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group. 💅
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix. 👑
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. 🫶
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options. 💜
• -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system. 🌸
• -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system. 🐧
• -mcall-freebsd — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system. 😈
• -mcall-netbsd — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system. 🌈
• -mcall-openbsd — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system. 🐡
• -mtraceback=traceback_type — select the type of traceback table. Valid values for traceback_type are full, part, and no. pick your fighter. ✨
• -maix-struct-return — return all structures in memory (as specified by the AIX ABI). 💾
• -msvr4-struct-return — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI). 🎀
• -mabi=abi-type — the ABI glow-up switch ✨. It extends your current ABI with one specific extension, or yoinks that extension back out if you're over it. The valid values are altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and, for AIX specifically, vec-extabi and vec-default.
• -mabi=ibmlongdouble — flips the current ABI to use IBM extended-precision long double. ngl bestie, this is not likely to work if your system defaults to IEEE extended-precision long double. And if you change the long double type away from IEEE extended-precision, the compiler will hit you with a warning unless you pass -Wno-psabi. Requires -mlong-double-128 to be enabled.
• -mabi=ieeelongdouble — flips the current ABI to use IEEE extended-precision long double. Same vibe check as above: not likely to work if your system defaults to IBM extended-precision long double. Change the long double type away from IBM extended-precision and the compiler issues a warning unless you use -Wno-psabi. Requires -mlong-double-128 to be enabled.
• -mabi=elfv1 — swaps the current ABI over to the ELFv1 ABI. That's the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI needs special system support and is likely to fail in spectacular ways 💀.
• -mabi=elfv2 — swaps the current ABI over to the ELFv2 ABI. That's the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI needs special system support and is likely to fail in spectacular ways 💀.
• -mgnu-attribute, -mno-gnu-attribute — emit .gnu_attribute assembly directives (or don't) to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values. 🎀
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear that bit.
• -msim — on embedded PowerPC systems, assume the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations. 🦋
• -mmvme — on embedded PowerPC systems, assume 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. (iconic name fr 🌸)
• -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Picking -meabi means the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Picking -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and the -msdata option only uses "r13" to point to a single small data area. -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, put small initialized "const" global and static data in the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which sits adjacent to .sdata. Heads up: -msdata=eabi is incompatible with -mrelocatable. It also sets -memb.
• -msdata=sysv — on System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, adjacent to .sdata. Also 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 it exactly like -msdata=sysv.
• -msdata=data — on System V.4 and embedded PowerPC systems, put small global data in the .sdata section. Put small uninitialized global data in the .sbss section. 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, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.
• -mreadonly-in-sdata — put 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 — generate non-looping inline code for all block compares (think calls to "memcmp" or structure compares) that are less than or equal to num bytes. If num is 0, all inline expansion (both non-loop and loop) of block compare is disabled. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — generate 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 get compared before "memcmp" is called to compare the rest of the block. The default value is target-specific.
• -mstring-compare-inline-limit=num — compare 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, put 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. All modules should be compiled with the same -G num value.
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms.
• -mlongcall, -mno-longcall — by default, assume that all calls are far away, so a longer and more expensive calling sequence is required. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't 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 generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's planned 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 a bl callee if the PPC bl instruction reaches the callee directly; otherwise the linker generates bl L42 to call the branch island. The branch island gets appended to the body of the calling function; 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 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, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. -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 a function call with its argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. 💅
• -mrecip, -mno-recip — okay so this one lets you use the reciprocal estimate and reciprocal square root estimate instructions, and then it stacks additional Newton-Raphson steps on top to bump the precision back up, instead of doing a whole actual divide — or a square root and a divide — for floating-point arguments. You should use the -ffast-math option when you're using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Now, heads up: even though the throughput of this whole sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can get decreased by up to 2 ulp — like, the inverse of 1.0 equals 0.99999994 — for reciprocal square roots. 💅
• -mrecip=opt — this is the one that controls which reciprocal estimate instructions are allowed to show up. opt is a comma-separated list of options, and any of them can be prefixed with a "!" to invert it:
  • all — enable all estimate instructions. all of them. no notes. 👑
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the reciprocal approximation instructions for both single and double precision.
  • divf — enable the single-precision reciprocal approximation instructions.
  • divd — enable the double-precision reciprocal approximation instructions.
  • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations. it's giving very "everything but the girl" energy 🎀

• -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions hand you higher-precision estimates than the PowerPC ABI mandates. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. Also fyi 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 — this specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. And the MASS libraries must be specified at link time. (yeah that's a lot of names, drink some water bestie 💜)
• -mfriz, -mno-friz — generate (or don't generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Lowkey important caveat: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer. 🫶
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (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, TOC value to be loaded in register r2, and static chain value to be loaded in register r11. The -mpointers-to-nested-functions is on by default. And real talk: 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. so like. don't. 💀
• -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 is not saved in the prologue, it is saved just before the call through the pointer. The -mno-save-toc-indirect option is the default.
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

Okay so story time: 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.

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 canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default with GNU libc version 2.4 or later).

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. canary said "protect me" and honestly? valid 🐤

• -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. it's a package deal, no a la carte here ✨
• -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:

• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is why the default is -m32bit-doubles. the hardware said "32 bits or nothing" and we respect it 💅
• -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 unsafe math behind your back 😭

• -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200 series hardware and the specific RX610 CPU. The default 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. code stays out of it, this is a data-only situation 🌸
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX's registers (usually r13) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.

Note, common variables (variables that have not been initialized) and constants are not placed into the small data area as they are assigned to other sections in the output executable.

The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. it's giving "you gotta test it yourself, bestie" energy 🧪

• -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 is compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default. 🦋
• -mmax-constant-size=N — bestie, this one sets the maximum size, in bytes, of a constant that's allowed to show up as an operand inside an RX instruction. And yeah, the RX instruction set does let constants up to 4 bytes long ride along in instructions, but a longer value means a longer instruction — so sometimes it's lowkey beneficial to cap how big those constants get. Constants that are too big get yeeted into a constant pool instead and referenced via register indirection. N can be anywhere from 0 to 4. A value of 0 (that's the default) or 4 means constants of any size are allowed, no cap, periodt. 💅
• -mrelax — turns on linker relaxation. It's a whole process where the linker tries to shrink your program by finding shorter versions of various instructions. It's giving glow-up energy. Disabled by default, bestie. ✨
• -mint-register=N — tells GCC how many registers to reserve for fast interrupt handler functions. N lives between 0 and 4. A value of 1 means register "r13" is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through "r10". A value of 0 — the default, obviously — doesn't reserve any registers at all. 🎀
• -msave-acc-in-interrupts — says interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, like when it does 64-bit multiplications. The default is to ignore the accumulator, because ignoring it makes the interrupt handlers faster. Speed queen behavior. 🔥
• -mpid — -mno-pid — enables the 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. This lets the location of constant data be figured out at run time without needing the executable to be relocated — which is a whole benefit for embedded applications with tight memory constraints. Data that can be modified is not affected by this option. 🦋

Heads up though: using this feature reserves a register, usually "r13", for the constant data base address. That can result in slower and/or larger code, especially in complicated functions. Not the vibe you want in a hot loop, ngl.

The actual register chosen to hold the constant data base address depends on whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register "r13" and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. So it's totally possible for the small data area register to be "r8" if both -mint-register=4 and -mpid are specified on the command line. Wild, right? 😭

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

• -mno-warn-multiple-fast-interrupts — -mwarn-multiple-fast-interrupts — prevents GCC from issuing a warning message if it finds more than one fast interrupt handler while it's compiling a file. The default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt. One. Singular. That's it. 💀
• -mallow-string-insns — -mno-allow-string-insns — enables or disables the use of the string manipulation instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL" "SWHILE" and also the "RMPA" instruction. These instructions may prefetch data, which is not safe to do if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more information.)

The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space. So like, that's on you bestie, GCC can't read your mind. 🙈

When the instructions are enabled GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol "__RX_DISALLOW_STRING_INSNS__".

• -mjsr — -mno-jsr — use only (or not only) "JSR" instructions to access functions. This option can be used when code size exceeds the range of "BSR" instructions. Note that -mno-jsr does not mean to not use "JSR" but instead means that any type of branch may be used. Sneaky naming, fr fr. 👑

Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the "interrupt" function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options.

---

🌸 S/390 and zSeries Options 🌸

These are the -m options defined for the S/390 and zSeries architecture. Let's get into it. 💖

• -mhard-float — -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. ✨
• -mhard-dfp — -mno-hard-dfp — use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 💅
• -mlong-double-64 — -mlong-double-128 — these switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the "double" type. This is the default. 🎀
• -mbackchain — -mno-backchain — store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools 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 the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. Efficiency queen behavior, honestly. 👑

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. 64k is giving so last season. 💀
• -m64 — -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64. Pick your fighter. ⭐
• -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. Iconic. 💜
• -mzvector, -mno-zvector — okay so -mzvector turns on the vector language extensions AND the builtins that ride on the instructions from the vector extension facility that IBM dropped with the z13 machine generation. It lets you use vector as a keyword to define vector type variables and arguments 🦋. But bestie, listen: 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. No exceptions, it's giving strict-mode energy. 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. And -mzvector is disabled by default. Periodt.
• -mmvcle, -mno-mvcle — generate (or don't generate) code using the mvcle instruction to do block moves. When you say -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) extra debug information while compiling. Default is to not print debug information, so if you want the tea you gotta ask for it 🙈.
• *-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. Heads up though: -march=native has no effect if GCC does not recognize the processor 😭.

• -mtune=cpu-type — tune to cpu-type everything applicable about the generated code, *except for the ABI and the set of available instructions. The list of *cpu-type values is the same as for -march. The default is the value used for -march 🌸.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS 💀.
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (does not change) the default branch targets enabled by -mtpf-trace so they 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 you're 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 🫠. It's 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. This is generally a bad idea with a limited stack size, ngl 🚩.
• -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, bestie, it goes down 📉). 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 debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation ✨. The given values have to be *exact powers of 2, and *stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. 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, bestie:

• -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, okay so buckle up because there are a lot of them:

• -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 — inlines the code that invalidates instruction cache entries after nested function trampolines get set up. Important vibe check tho: this option does nothing if -musermode is in effect and the code generation option you picked (like -m4) doesn't let you use the icbi instruction. And if your code generation option doesn't allow icbi and -musermode isn't in effect, then the inlined code goes and pokes the instruction cache address array directly with an associative write 💀 which not only needs privileged mode at run time, it also fails if the cache line got mapped via the TLB and then became unmapped. iykyk.
• -misize — dumps instruction size and location right into the assembly code. ✨
• -mpadstruct — bestie, this one is deprecated. It pads structures out to multiples of 4 bytes, which is just not compatible with the SH ABI. 🙈
• -matomic-model=model — sets the model for atomic operations plus extra parameters, all as a comma separated list. For the deets on the atomic built-in functions, go peek at __atomic Builtins. Here's everything that's supported:
  • none — disables compiler-generated atomic sequences and just emits library calls for atomic ops 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: these generated 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 is on by default when the target is sh*-*-linux* and it's SH3* or SH4*. And when the target is SH4A, it also partially uses the hardware atomic instructions movli.l and movco.l to make more efficient code — unless you specify strict.
  • soft-tcb — generates software atomic sequences that use a variable in the thread control block. It's a variation on the gUSA sequences that also works on SH1* and SH2* targets. Same deal as before: needs extra support from the system's interrupt/exception handling code, and 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 only works when the program runs in privileged mode and it's only suitable for single-core systems. Good news: no extra support needed from the system's interrupt/exception handling code. This is the 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 it's suitable for multi-core systems. Since those hardware instructions only support 32-bit atomic variable access, 8 or 16 bit variable access gets emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if it runs on an SH4A system. And no extra support from the interrupt/exception handling code is required for this model. 🫶
  • gbr-offset= — this parameter says what the offset in bytes is of the variable in the thread control block structure that the generated atomic sequences should use, but only when the soft-tcb model has been selected. For any other model this parameter is just ignored, no biggie. The value you give must be an integer multiple of four and sit in the range 0-1020.
  • strict — this parameter stops you from mixing multiple atomic models, even the compatible ones, and forces the compiler to generate atomic sequences of the specified model only. no mix-and-match energy. 💅
• -mtas — generates the tas.b opcode for __atomic_test_and_set. But listen: depending on your exact hardware and software config, this can tank overall performance because of the operand cache line flushes that tas.b implies. And on multi-core SH4A processors you gotta use tas.b with caution — it can cause data corruption for certain cache configurations. 😭
• -mprefergot — when generating position-independent code, this makes function calls go through 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 wouldn't 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 at all — because there's literally no user mode to speak of. 💀
• -multcost=number — sets the cost to assume for a multiply instruction.
• -mdiv=strategy — sets the division strategy used for integer division operations. strategy can be any one of these:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to do the thing. Division by zero calculates an unspecified result and doesn't trap. This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that does 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 just defaults to call-div1.
  • call-table — calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for bigger divisors. Division by zero calculates an unspecified result and doesn't trap. This is the default for SH4. If you specify this for targets without dynamic shift instructions, it defaults to call-div1.

And when no division strategy has been specified at all, the default strategy gets picked based on the current target. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls. 🌸

• -maccumulate-outgoing-args — reserves space once for outgoing arguments in the function prologue instead of around every single call. Generally a win for performance and size. It's also needed for unwinding, so the stack frame doesn't go changing 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, and the compiler still expects the exact same sets of input/output/clobbered registers as if this option weren't there. no sneaky changes. 💅
• -mfixed-range=register-range — generates code treating the given register range as fixed registers. A fixed register is one the register allocator can't use. Super useful when compiling kernel code. A register range is written as two registers separated by a dash, and you can specify multiple register ranges separated by commas.
• -mbranch-cost=num — assumes num is the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code when it can. If you don't specify it, the value gets selected depending on the processor type you're compiling for.
• -mzdcbranch, -mno-zdcbranch — assume (don't assume) that the 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, and you can explicitly turn it off with -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if no suitable instruction can be found. By default this option is disabled. You can enable it to work around hardware bugs like the ones found in the original SH7055.
• -mfused-madd, -mno-fused-madd — generate code that uses (doesn't use) the floating-point multiply and accumulate instructions. These instructions get 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 option -mfsca 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 option -mfsrra 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 that aren't 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 — okay so this one's whole job is shushing the linker's "relocations remain against allocatable but non-writable sections" error message. 🙈 But bestie, listen — the relocations it needs trigger copy-on-write, and then your shared object isn't actually shared across processes at all. Which is giving fake sharing energy. So instead of using -mimpure-text, you should compile all your source code with -fpic or -fPIC. Periodt. 💅

These switches are supported in addition to the above on Solaris 2:

• -pthreads — literally just a synonym for -pthread. Same thing, different name, iconic. ✨

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🌞 SPARC Options 🌞

These -m options are supported on the SPARC:

• -mno-app-regs, -mapp-regs — specify -mapp-regs and GCC generates output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Just like global register 1, each of global registers 2 through 4 gets treated as an allocable register that gets clobbered by function calls. This is the default 💚

Now, 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. Yes, seriously. 🫶

• -mflat, -mno-flat — with -mflat, the compiler does not generate save/restore instructions and instead uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and get 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. 😭

Also -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. No skipping steps 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 — and this is the big 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. 👑
• -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. Nostalgia era, I guess. 😭
• -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. Two different jobs, bestie, don't mix them up. 🦋

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. The stacking, the layering, we love to see it. 🫶
• -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. It's giving combo deal. 🔥
• -mvis4, -mno-vis4 — bestie, -mvis4 means GCC writes code that eats up version 4.0 of the UltraSPARC Visual Instruction Set extensions 💅. Default is -mvis4 when you're targeting a cpu that supports this stuff, like niagara-7 and later ✨. And the way that -mvis4 also turns on -mvis3, -mvis2 and -mvis? Obsessed, it's giving package deal 🎀
• -mvis4b, -mno-vis4b — with -mvis4b, GCC flexes version 4.0 of the UltraSPARC VIS extensions plus the extra VIS instructions that showed up in the Oracle SPARC Architecture 2017 👑. Default is -mvis4b for cpus that support it, like m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis — yes queen, all of them 🌈
• -mcbcond, -mno-cbcond — -mcbcond lets GCC use the UltraSPARC Compare-and-Branch-on-Condition instructions 🔥. Default is -mcbcond on a CPU that supports them, like Niagara-4 and later.
• -mfmaf, -mno-fmaf — -mfmaf = GCC takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions 💖. Default is -mfmaf when the CPU supports them, like Niagara-3 and later.
• -mfsmuld, -mno-fsmuld — -mfsmuld unlocks the Floating-point Multiply Single to Double (FsMULd) instruction 🦋. Default is -mfsmuld when targeting a CPU with architecture versions V8 or V9 and an FPU — except -mcpu=leon, which said no thanks 💀
• -mpopc, -mno-popc — -mpopc lets GCC use the UltraSPARC Population Count instruction ✨. Default is -mpopc on a CPU with that instruction, like Niagara-2 and later.
• -msubxc, -mno-subxc — -msubxc enables the UltraSPARC Subtract-Extended-with-Carry instruction 💜. Default is -msubxc when the CPU supports it, like Niagara-7 and later.
• -mfix-at697f — turns on the documented workaround for the single erratum of the Atmel AT697F processor (which is erratum #13 of the AT697E processor, iykyk) 🙈
• -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.

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And these -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments 💅

• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mcmodel=which — set the code model to one of 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 📖

• -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes 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 🫶

• -G — create a shared object. It is recommended that -symbolic or -shared be used instead.
• -Qy — identify the versions of each tool used by the compiler, in a .ident assembler directive in the output 👑
• -Qn — refrain from adding .ident directives to the output file (this is the default).
• -YP,dirs — search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option.

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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 🙈 (she's just there for the vibes)

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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. Periodt 💅

• -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 — turn hardware floating-point instruction generation off (or back on 🫶). Lowkey only matters when the target arch is V850E2V3 or higher, bestie. If hardware FP instructions are being generated, the C preprocessor symbol __FPU_OK__ gets defined; otherwise __NO_FPU__ is defined. That's the whole vibe check 💅
• -mloop — enables use of the e3v5 LOOP instruction. It is not on by default when the e3v5 architecture is selected, because using it is still experimental 🙈 (so like, it's giving "we're testing this, don't @ me").
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This one is the default. With this ABI version, the following rules apply:
  • Integer sized structures and unions are returned via a memory pointer rather than a register.
  • Large structures and unions (more than 8 bytes in size) are passed by value.
  • Functions are aligned to 16-bit boundaries.
  • The -m8byte-align command-line option is supported.
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported 💀

When this 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 — enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4 bytes. When -m8byte-align is in effect, the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined ✨
• -mbig-switch — generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table 🫠
• -mapp-regs — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default 👑
• -mno-app-regs — this option causes r2 and r5 to be treated as fixed 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.

-msoft-float changes the calling convention in the output file, so 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 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.

---

💜 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, 64-bit extensions plus 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, and it's serving MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support. ✨
• haswell — Intel Haswell CPU, 64-bit extensions, walking in with 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, 64-bit extensions, and bestie it has 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, bringing MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support. 🌸
• bonnell — Intel Bonnell CPU, 64-bit extensions, keeping it simple with MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. 🦋
• silvermont — Intel Silvermont CPU with 64-bit extensions, and it's giving MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support. 💜
• goldmont — Intel Goldmont CPU, 64-bit extensions, and the lineup is 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, no cap it has MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support. 👑
• tremont — Intel Tremont CPU, 64-bit extensions, and the way it carries 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 is iconic. 🎀
• knl — Intel Knight's Landing CPU with 64-bit extensions, serving MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support. 🌈
• knm — Intel Knights Mill CPU, 64-bit extensions, and it's lowkey stacked with 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, and it ate with MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support. 🔥
• cannonlake — Intel Cannonlake Server CPU, 64-bit extensions, and it has 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, and bestie it's main character energy with 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, 64-bit extensions, and it's serving 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, and it's giving 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, 64-bit extensions, holding 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, and omg it's stacked: 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. 👑
• 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, rocking MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip, bestie, so don't expect it to plan ahead 💅)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (Again: no scheduling implemented for this chip.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (And yes — no scheduling implemented for this chip. the way VIA said "we'll handle the timing ourselves" 😭)
• samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• eden-x4 — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)
• nano — Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-3000 — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• nano-x2 — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• nano-x4 — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support. (this one has no "no scheduling" disclaimer, iconic behavior 💖)

---

• -mtune=cpu-type — tunes everything applicable about the generated code to cpu-type, except the ABI and the set of available instructions. So like… it picks the vibe of the scheduling for that chip, but the compiler does not generate any code that can't run on the default machine type — unless you also pass a -march=cpu-type option. Fr fr, here's the tea: if GCC is configured for i686-pc-linux-gnu, then -mtune=pentium4 generates code tuned for Pentium 4 that still runs on i686 machines. ✨

The choices for cpu-type are the same as for -march. Plus, -mtune supports 2 extra choices for cpu-type:

• generic — produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU your code will run on, you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you have no idea what CPU your users will have? Yeah, use this one, bestie.

As new processors get deployed in the marketplace, the behavior of this option will change. So if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect whatever processors were most common at the time that version of GCC was released. (it's giving "moving target", and that's on purpose 🎀)

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. Meanwhile -mtune indicates the processor (or, in this case, a whole collection of processors) for which the code is optimized. got it? 💅

• intel — produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU your code will run on, use the corresponding -mtune or -march option instead of -mtune=intel. But if you want your application to perform better on both Haswell and Silvermont, then use this option.

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Upgrade GCC and code generation controlled by this option shifts to reflect the most current Intel processors at the time that version of GCC was released. 🦋

There is no -march=intel option, because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. -mtune, on the other hand, indicates the processor (or collection of processors) for which the code is optimized.

• -mcpu=cpu-type — a deprecated synonym for -mtune. 😭 (use -mtune, she's the main character now)
• -mfpmath=unit — generate floating-point arithmetic for the selected 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, which gives you slightly different results compared to most other chips. See -ffloat-store for a more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, so 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 avoids the numerical instability problems of 387 code — but it may break some existing code that expects temporaries to be 80 bits. ⚠️

This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default 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 it doubles the execution resources too. Use this option with care, because it's still experimental — the GCC register allocator does not model separate functional units well, which results in unstable performance. 🙈

• -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. 💀
• -mieee-fp
• -mno-ieee-fp — control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered.
• -m80387
• -mhard-float — generate output containing 80387 instructions for floating point.
• -mno-80387
• -msoft-float — generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 🫶

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

• -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be 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. iykyk 🌸
• -malign-double
• -mno-align-double — control whether GCC aligns "double", "long double" and "long long" variables on a two-word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory.

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32, and are not binary compatible with structures in code compiled without that switch. 💀

• -m96bit-long-double
• -m128bit-long-double — these switches control the size of the "long double" type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer) are obsessed with long double sitting pretty on an 8- or 16-byte boundary. But in arrays or structures that conform to the ABI, that literally can't happen. So if 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. 🎀

In the x86-64 compiler, -m128bit-long-double is the default choice, because its ABI says long double gets aligned on a 16-byte boundary.

Real talk though: neither of these options gives you any extra precision over the x87 standard of 80 bits for a long double. It's giving alignment, not accuracy. 💅

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, and it modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. No cap.

---

• -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 the long double type equivalent to the double type — and that's the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type — and that's the default for 64-bit Bionic C library.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. 🫶

• -malign-data=type — controls how GCC aligns variables. Supported values for type are: compat uses increased alignment value compatible with GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased alignment value to match the cache line size. compat is the default.
• -mlarge-data-threshold=threshold — when -mcmodel=medium is specified, data objects larger than threshold 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. ✨

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 bestie. 💀)

• -mregparm=num — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function 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. All of them. Periodt.

• -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm.

Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 🌈

• -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 🦋
• -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. 💀

• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. 💅
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 😭

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🎀

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

---

✨ The Big Instruction-Set Menu ✨

• -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. 🔥

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 so if you want SSE/SSE2 instructions to just materialize out of your floating-point code automatically (instead of the dusty old 387 instructions), go peek at -mfpmath=sse. 💅

GCC depresses SSEx instructions the second you use -mavx. It doesn't just leave them lying around — it generates brand new AVX instructions, or an AVX equivalent, for every SSEx instruction that's needed. Glow up. ✨

These options let GCC flex these extended instructions in generated code, even without -mfpmath=sse. But bestie, listen: apps that do run-time CPU detection have to compile separate files for each supported architecture, using the appropriate flags. And specifically — the file holding the CPU detection code should be compiled without these options. No cap. 🫶

---

• -mdump-tune-features — tells GCC to dump out the names of the x86 performance tuning features plus their default settings. Those names? They're exactly what you feed into -mtune-ctrl=feature-list. 🎀
• -mtune-ctrl=feature-list — for fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also -mdump-tune-features. When you specify one, the feature gets turned on — unless it's preceded with ^, in which case it's turned off. Heads up though: -mtune-ctrl=feature-list is meant for GCC developers. Using it may lead to code paths that aren't covered by testing and can potentially result in compiler ICEs or runtime errors. 💀
• -mno-default — instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — makes GCC emit a "cld" instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to choose between autoincrement or autodecrement mode. While the ABI says the DF flag should be cleared on function entry, some operating systems violate this spec by not clearing the DF flag in their exception dispatchers. So the exception handler can get 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. In that case you can suppress generation of "cld" instructions with the -mno-cld compiler option.
• -mvzeroupper — makes GCC emit a "vzeroupper" instruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty and also to remove unnecessary "zeroupper" intrinsics. 🧚
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform.
  • none — no extra limitations applied to GCC other than what the selected platform already defines.
  • 128 — prefer 128-bit vector width for instructions.
  • 256 — prefer 256-bit vector width for instructions.
  • 512 — prefer 512-bit vector width for instructions.
• -mcx16 — lets GCC generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. So useful for atomic updates of data structures bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. However — for __atomic Builtins operating on 128-bit integers, a library call is always used. Periodt.
• -msahf — enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step showed up in December 2005, lacked the "LAHF" and "SAHF" instructions that AMD64 supports. Those are load and store instructions, respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize "fmod", "drem", and "remainder" built-in functions; see Other Builtins for details. 🦋
• -mmovbe — 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 extra Newton-Raphson step to bump up 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).

Also 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.

And also note that GCC emits the above sequence with an 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. 🎀

• -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 the corresponding function type when -mveclibabi=svml is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used.

• -mabi=name — generate code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes "ms_abi" and "sysv_abi". 🌈
• -mforce-indirect-call — 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.
• -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 — because of 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, which gives you fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores, so function size goes down at the cost of a few extra instructions. 🫶
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. 👑
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback? A notable increase in code size. This switch implies -mno-push-args. 💀
• -mthreads — support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. ✨
• -mms-bitfields, -mno-ms-bitfields — enable/disable bit-field layout compatible with the native Microsoft Windows compiler. 💅

okay so bestie, we're deep in x86 territory now and it's getting spicy 🔥 so buckle up because there's structure layout lore and like twenty more -m flags, and yes, every single one of them matters 💅

---

If packed gets used on a struct, or if bit-fields are in play, the Microsoft ABI might lay that structure out completely differently than the way GCC normally does it. And specifically when you're moving packed data between functions compiled with GCC and functions compiled with the native Microsoft compiler — whether that's through a function call or as data sitting in a file — you may legitimately need to access either format. iykyk 🫶

This option is on by default for Microsoft Windows targets. You can also control this behavior locally with variable attributes or type attributes. For more info, go peek at x86 Variable Attributes and x86 Type Attributes. ✨

The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing, which is where things get feral. The padding and alignment of structure members, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order they were declared: the first member has the lowest memory address, and the last member has the highest. 🎀
2. Every data object has an alignment requirement. For everything except structures, unions, and arrays, that alignment requirement is either the size of the object or the current packing size (whichever one you set with the aligned attribute or the pack pragma) — whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement among its members. And every object gets allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields get packed into the same 1-, 2-, or 4-byte allocation unit, but only if the integral types are the same size <em>and</em> the next bit-field actually fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields. ✨

MSVC interprets zero-length bit-fields like this:

1. If a zero-length bit-field gets inserted between two bit-fields that would normally be coalesced, then 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 you removed that zero-length bit-field, t1's size would be 4 bytes. yeah. 💀

2. If a zero-length bit-field gets inserted after a bit-field foo, and the alignment of the zero-length bit-field is greater than that of 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, and therefore doesn't affect the size of the structure either.

Taking this into account, it's 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. slay 🌸

---

• -mno-align-stringops — do not align the destination of inlined string operations. This switch shrinks your code size and boosts performance when the destination is already aligned but GCC just… doesn't know that yet. 🙈
• -minline-all-stringops — by default GCC only inlines string operations when the destination is known to be aligned to at least a 4-byte boundary. This flag enables more inlining and grows your code size, but it may improve performance for code that relies on fast memcpy and memset for short lengths. It also enables inline expansion of strlen for all pointer alignments. 💅
• -minline-stringops-dynamically — for string operations of unknown size, use run-time checks: inline code for small blocks, and a library call for large blocks. best of both worlds fr fr ✨
• -mstringop-strategy=alg — override the internal decision heuristic for which algorithm to use when inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte — expand using the i386 rep prefix of the specified size.
  • byte_loop
  • loop
  • unrolled_loop — expand into an inline loop.
  • libcall — always use a library call. 📞
• -mmemcpy-strategy=strategy — override the internal decision heuristic that decides whether __builtin_memcpy should be inlined, and which 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, and max_size specifies the max byte size for which inline algorithm alg is allowed. For the last triplet, max_size must be -1. The max_size values 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 for everything after. 🎀
• -mmemset-strategy=strategy — same energy as -mmemcpy-strategy= except this one controls __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 frees up an extra register in leaf functions. Heads up: the -fomit-leaf-frame-pointer option 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 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 a VEX prefix. The -mavx option 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 containing 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. 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 *type*s 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 that gets called at function entry for -pg -mfentry functions. ⭐
• -mfentry-section=name — set the name of the section used to record -mrecord-mcount calls (default is __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 the dividend and the divisor are within the range 0 to 255, an 8-bit unsigned integer divide is used instead of a 32-bit/64-bit integer divide. 🔥
• -mavx256-split-unaligned-load / -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store. ✨
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (that one's the default). This option only has an effect when -fstack-protector or -fstack-protector-all is specified. 👑

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 💅

• -mgeneral-regs-only — generates code that uses only the general-purpose registers, bestie. It stops the compiler from using floating-point, vector, mask and bound registers. No cap. 🎀
• -mindirect-branch=choice — converts indirect call and jump with choice. The default is keep, which keeps indirect call and jump unmodified. thunk converts indirect call and jump to call and return thunk. thunk-inline converts indirect call and jump to inlined call and return thunk. thunk-extern converts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute indirect_branch.

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. 😭

Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check. ✨

• -mfunction-return=choice — converts function return with choice. The default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to inlined call and return thunk. thunk-extern converts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute function_return.

Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.

Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model. 💀

• -mindirect-branch-register — force indirect call and jump via register. Periodt. 💅
• -mharden-sls=choice — generates code to mitigate against straight line speculation (SLS) with choice. The 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. 🦋
• -mindirect-branch-cs-prefix — adds CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so that the call and jmp instruction length is 6 bytes to allow them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. ⭐

---

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.

• -m32
• -m64
• -mx32
• -m16
• -miamcu — generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.

The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options. 🌈

The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.

The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.

The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on. 👑

• -mno-red-zone — do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🍓
• -mcmodel=small — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model. 💖
• -mcmodel=kernel — generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. 🧚
• -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
• -mcmodel=large — generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
• -maddress-mode=long — generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
• -maddress-mode=short — generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments. ✨

---

🌸 x86 Windows Options 🌸

These additional options are available for Microsoft Windows targets:

• -mconsole — this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. 💻
• -mdll — this option is available for Cygwin and MinGW targets. It specifies that a DLL—a dynamic link library—is to be generated, enabling the selection of the required runtime startup object and entry point. 📦
• -mnop-fun-dllimport — this option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored.
• -mthread — this option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used. 🧵
• -municode — this option is available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.
• -mwin32 — this option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
• -mwindows — this option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 🪟
• -fno-set-stack-executable — this option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available.
• -fwritable-relocated-rdata — this option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the .data section. This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation.
• -mpe-aligned-commons — this option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. 🌈

See also under x86 Options for standard options. 👀

---

🌸 Xstormy16 Options 🌸

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. 🎮

---

🌸 Xtensa Options 🌸

These options are supported for Xtensa targets:

• -mconst16
• -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. The CONST16 instruction is currently not a standard option from Tensilica. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available. 💫
• -mfused-madd
• -mno-fused-madd — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. 🔥
• -mserialize-volatile
• -mno-serialize-volatile — when this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions. ⚡
• -mforce-no-pic — for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. 🐧
• -mtext-section-literals, -mno-text-section-literals — bestie, these two control how we treat literal pools ✨ The default is -mno-text-section-literals, which chucks the literals into their own separate section in the output file. Why is that cute? Because it lets the literal pool live in a data RAM/ROM, and it also lets the linker merge literal pools from different object files together — bye bye redundant literals, hiiii smaller code 🎀 With -mtext-section-literals, the literals get interspersed in the text section so they stay as close as they possibly can to the things that reference them. That might be straight-up necessary for big assembly files. And where do the literals for each function go? Right before that function. iconic placement honestly 💅
• -mauto-litpools, -mno-auto-litpools — okay so, again with the literal pools 🌸 default is -mno-auto-litpools, which puts literals in a separate section in the output file unless -mtext-section-literals is in play. With -mauto-litpools, the literals get interspersed in the text section by the assembler instead. The compiler does not produce explicit ".literal" directives, and it loads literals into registers with "MOVI" instructions instead of "L32R" — all so the assembler can do its relaxation thing and place literals wherever they need to go. This lets the assembler create several literal pools per function and assemble very big functions, which lowkey might not be possible with -mtext-section-literals 👑
• -mtarget-align, -mno-target-align — turn this on and GCC tells the assembler to auto-align instructions, trading a lil code density for fewer branch penalties 💖 The assembler tries to widen density instructions so branch targets line up, plus the instructions that come after call instructions. If there aren't enough preceding safe density instructions to align a target, then no widening happens, periodt. Default is -mtarget-align. Oh and heads up — these options don't touch auto-aligned instructions like "LOOP", because the assembler always aligns those, either by widening density instructions or by inserting NOP instructions. No exceptions, no cap 🦋
• -mlongcalls, -mno-longcalls — enable this and 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 the call instruction allows 📞 This translation typically kicks in for calls to functions in other source files. Specifically, the assembler turns a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. Default is -mno-longcalls. Use this in programs where the call target could potentially be out of range. Now the important tea: this option is implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions — go peek at the disassembled object code to see the actual instructions 👀 And note that the assembler uses an indirect call for every cross-file call, not just the ones that are actually out of range. extra for no reason but okay 💀

---

🌈 zSeries Options 🌈

These are listed under

---

🌈 ENVIRONMENT 🌈

okay bestie, this section is all about the env vars that make GCC do its thing. some of them are like "where do I look for files?" and some are like "how should I compile?" 💖

Also note: you can totally specify search places with options like -B, -I, and -L. Those win over env vars, and env vars win over whatever GCC's configuration says. Precedence, periodt. 👑

---

✨ LANG, LC_CTYPE, LC_MESSAGES, LC_ALL ✨

These four control localization, so GCC can vibe with different national conventions. GCC checks locale categories LC_CTYPE and LC_MESSAGES if it was configured to do so. You can set them to anything your install supports. A typical value is en_GB.UTF-8 for English in the UK encoded in UTF-8. 🇬🇧

LC_CTYPE is all about character classification. GCC uses it to figure out character boundaries in a string; this matters for some multibyte encodings that have quote and escape characters that would otherwise be read as a string end or escape. 😭

LC_MESSAGES tells GCC what language to use in diagnostic messages.

If LC_ALL is set, it overrides LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES default to the value of LANG. If none of these are set, GCC defaults to traditional C English behavior. 💅

---

TMPDIR

If TMPDIR is set, it says where to put temporary files. GCC uses temp files to hold the output of one compilation stage that's gonna be input to the next stage — like, the preprocessor's output is the compiler proper's input. 🔄

---

GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is almost the same as passing -fcompare-debug to the compiler driver. Check that option's docs for more. 👀

---

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it's a prefix used in the names of subprograms that the compiler executes. No slash gets added when this prefix combines with a subprogram name, but you can specify a prefix that ends with a slash if you want. ✨

If GCC_EXEC_PREFIX isn't set, GCC tries to figure out a good prefix from the pathname it was invoked with.

If GCC can't find the subprogram using that prefix, it looks in the usual places.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script. 🛠️

Other prefixes given with -B take precedence over this prefix.

This prefix is also used to find files like crt0.o that are used for linking.

Also, the prefix is used in a weird way when finding header search directories. For each standard directory whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to make an alternate directory name. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix, then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. 🤯

---

COMPILER_PATH

COMPILER_PATH is a colon-separated list of directories, kinda like PATH. GCC tries these when searching for subprograms, if it can't find them using GCC_EXEC_PREFIX. 🔍

---

LIBRARY_PATH

LIBRARY_PATH is also a colon-separated list of directories, much like PATH. When GCC is configured as a native compiler, it tries these when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking with GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). 📚

---

LANG

This variable passes locale info to the compiler. One way it's used: to determine the character set for parsing character literals, string literals, and comments in C and C++. When the compiler is configured to allow multibyte characters, these LANG values are recognized:

• C-JIS — Recognize JIS characters.
• C-SJIS — Recognize SJIS characters.
• C-EUCJP — Recognize EUCJP characters.

If LANG isn't defined, or has some other value, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters. 🌸

---

Some extra env vars affect the preprocessor's behavior.

CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, where GCC looks for header files. The special character, "PATH_SEPARATOR", is target-dependent and set when GCC is built. For Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon. 🪄

CPATH is a list of directories searched as if specified with -I, but after any paths given with -I options on the command line. This env var is used no matter which language is being preprocessed.

The remaining env vars apply only when preprocessing the particular language indicated. Each one is a list of directories searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element tells the compiler to search its current working directory. Empty elements can be at the beginning or end of a path. For instance, if CPATH is :/special/include, that's the same as -I. -I/special/include. 💡

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DEPENDENCIES_OUTPUT

If this variable is set, its value says how to output dependencies for Make based on the non-system header files the compiler processes. System header files are ignored in the dependency output. 🚫

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules go to that file, and GCC guesses the target name from the source file name. Or the value can be in the form file target, in which case rules go to file file using target as the target name.

In other words, this env var is equivalent to combining -MM and -MF, with an optional -MT switch too. 🎯

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SUNPRO_DEPENDENCIES

Same as DEPENDENCIES_OUTPUT (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. 💀

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SOURCE_DATE_EPOCH

If this variable is set, its value is a UNIX timestamp used instead of the current date and time in the "__DATE__" and "__TIME__" macros, so embedded timestamps become reproducible. ⏳

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00, represented in ASCII; identical to the output of @command{date +%s} on GNU/Linux and other systems that support the %s extension in the "date" command.

The value should be a known timestamp, like the last modification time of the source or package, and it should be set by the build process. 🕰️

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💀 BUGS 💀

For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. 🐛

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📝 FOOTNOTES 📝

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. 😌

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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. 🌈

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👑 AUTHOR 👑

See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. 🫶

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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.

(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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okay bestie that's the whole vibe. this iconic page is from gcc-10, dated 2023-07-07. slay, you made it to the end. 💅✨