Options That Control Optimization

[duchengyao]

-O -O1	-O2	-O3
-fauto-inc-dec	-falign-functions	-fgcse-after-reload
-fbranch-count-reg	-falign-jumps	-fipa-cp-clone
-fcombine-stack-adjustments	-falign-labels	-floop-interchange
-fcompare-elim	-falign-loops	-floop-unroll-and-jam
-fcprop-registers	-fcaller-saves	-fpeel-loops
-fdce	-fcode-hoisting	-fpredictive-commoning
-fdefer-pop	-fcrossjumping	-fsplit-loops
-fdelayed-branch	-fcse-follow-jumps	-fsplit-paths
-fdse	-fcse-skip-blocks	-ftree-loop-distribution
-fforward-propagate	-fdelete-null-pointer-checks	-ftree-partial-pre
-fguess-branch-probability	-fdevirtualize	-funswitch-loops
-fif-conversion	-fdevirtualize-speculatively	-fvect-cost-model=dynamic
-fif-conversion2	-fexpensive-optimizations	-fversion-loops-for-strides
-finline-functions-called-once	-ffinite-loops
-fipa-modref	-fgcse -fgcse-lm	-Os
-fipa-profile	-fhoist-adjacent-loads	-falign-functions
-fipa-pure-const	-finline-functions	-falign-jumps
-fipa-reference	-finline-small-functions	-falign-labels
-fipa-reference-addressable	-findirect-inlining	-falign-loops
-fmerge-constants	-fipa-bit-cp -fipa-cp	-fprefetch-loop-arrays
-fmove-loop-invariants	-fipa-icf -fipa-ra	-freorder-blocks-algorithm=stc
-fmove-loop-stores	-fipa-sra -fipa-vrp
-fomit-frame-pointer	-fisolate-erroneous-paths-dereference	-Og
-freorder-blocks	-flra-remat	-fbranch-count-reg
-fshrink-wrap	-foptimize-sibling-calls	-fdelayed-branch
-fshrink-wrap-separate	-foptimize-strlen	-fdse
-fsplit-wide-types	-fpartial-inlining	-fif-conversion
-fssa-backprop	-fpeephole2	-fif-conversion2
-fssa-phiopt	-freorder-blocks-algorithm=stc	-finline-functions-called-once
-ftree-bit-ccp	-freorder-blocks-and-partition	-fmove-loop-invariants
-ftree-ccp	-freorder-functions	-fmove-loop-stores
-ftree-ch	-frerun-cse-after-loop	-fssa-phiopt
-ftree-coalesce-vars	-fschedule-insns	-ftree-bit-ccp
-ftree-copy-prop	-fschedule-insns2	-ftree-dse
-ftree-dce	-fsched-interblock	-ftree-pta
-ftree-dominator-opts	-fsched-spec	-ftree-sra
-ftree-dse	-fstore-merging
-ftree-forwprop	-fstrict-aliasing
-ftree-fre	-fthread-jumps
-ftree-phiprop	-ftree-builtin-call-dce
-ftree-pta	-ftree-loop-vectorize
-ftree-scev-cprop	-ftree-pre
-ftree-sink	-ftree-slp-vectorize
-ftree-slsr	-ftree-switch-conversion
-ftree-sra	-ftree-tail-merge
-ftree-ter	-ftree-vrp
-funit-at-a-time	-fvect-cost-model=very-cheap

[duchengyao]

Specific Optimizations

Options	Descriptions
-O<number>	Set optimization level to <number>.
-Ofast	Optimize for speed disregarding exact standards compliance.
-Og	Optimize for debugging experience rather than speed or size.
-Os	Optimize for space rather than speed.
-faggressive-loop-optimizations	Aggressively optimize loops using language constraints.
-falign-functions	Align the start of functions.
-falign-jumps	Align labels which are only reached by jumping.
-falign-labels	Align all labels.
-falign-loops	Align the start of loops.
-fallocation-dce	Tell DCE to remove unused C++ allocations.
-fallow-store-data-races	Allow the compiler to introduce new data races on stores.
-fassociative-math	Allow optimization for floating-point arithmetic which may change the result of the operation due to rounding.
-fassume-phsa	Assume we are finalizing for phsa and its libhsail-rt. Enables additional phsa- specific optimizations (default).
-fasynchronous-unwind-tables	Generate unwind tables that are exact at each instruction boundary.
-fauto-inc-dec	Generate auto-inc/dec instructions.
-fbit-tests	Use bit tests for sufficiently large switch statements.
-fbranch-count-reg	Replace add, compare, branch with branch on count register.
-fbranch-probabilities	Use profiling information for branch probabilities.
-fcaller-saves	Save registers around function calls.
-fcode-hoisting	Enable code hoisting.
-fcombine-stack-adjustments	Looks for opportunities to reduce stack adjustments and stack references.
-fcompare-elim	Perform comparison elimination after register allocation has finished.
-fconserve-stack	Do not perform optimizations increasing noticeably stack usage.
-fcprop-registers	Perform a register copy-propagation optimization pass.
-fcrossjumping	Perform cross-jumping optimization.
-fcse-follow-jumps	When running CSE, follow jumps to their targets.
-fcx-fortran-rules	Complex multiplication and division follow Fortran rules.
-fcx-limited-range	Omit range reduction step when performing complex division.
-fdce	Use the RTL dead code elimination pass.
-fdefer-pop	Defer popping functions args from stack until later.
-fdelayed-branch	Attempt to fill delay slots of branch instructions.
-fdelete-dead-exceptions	Delete dead instructions that may throw exceptions.
-fdelete-null-pointer-checks	Delete useless null pointer checks.
-fdevirtualize	Try to convert virtual calls to direct ones.
-fdevirtualize-speculatively	Perform speculative devirtualization.
-fdse	Use the RTL dead store elimination pass.
-fearly-inlining	Perform early inlining.
-fexceptions	Enable exception handling.
-fexcess-precision=[fast | standard]	Specify handling of excess floating-point precision.
-fexpensive-optimizations	Perform a number of minor, expensive optimizations.
-ffast-math	This option lacks documentation.
-ffinite-loops	Assume that loops with an exit will terminate and not loop indefinitely.
-ffinite-math-only	Assume no NaNs or infinities are generated.
-ffloat-store	Don't allocate floats and doubles in extended-precision registers.
-fforward-propagate	Perform a forward propagation pass on RTL.
-ffp-contract=[off | on | fast]	Perform floating-point expression contraction.
-ffp-int-builtin-inexact	Allow built-in functions ceil, floor, round, trunc to raise "inexact" exceptions.
-ffunction-cse	Allow function addresses to be held in registers.
-fgcse	Perform global common subexpression elimination.
-fgcse-after-reload	Perform global common subexpression elimination after register allocation has finished.
-fgcse-las	Perform redundant load after store elimination in global common subexpression elimination.
-fgcse-lm	Perform enhanced load motion during global common subexpression elimination.
-fgcse-sm	Perform store motion after global common subexpression elimination.
-fgraphite	Enable in and out of Graphite representation.
-fgraphite-identity	Enable Graphite Identity transformation.
-fguess-branch-probability	Enable guessing of branch probabilities.
-fhandle-exceptions	Same as -fexceptions. Uses of this option are diagnosed.
-fhoist-adjacent-loads	Enable hoisting adjacent loads to encourage generating conditional move instructions.
-fif-conversion	Perform conversion of conditional jumps to branchless equivalents.
-fif-conversion2	Perform conversion of conditional jumps to conditional execution.
-findirect-inlining	Perform indirect inlining.
-finline	Enable inlining of function declared "inline", disabling disables all inlining.
-finline-atomics	Inline __atomic operations when a lock free instruction sequence is available.
-finline-functions	Integrate functions not declared "inline" into their callers when profitable.
-finline-functions-called-once	Integrate functions only required by their single caller.
-finline-small-functions	Integrate functions into their callers when code size is known not to grow.
-fipa-bit-cp	Perform interprocedural bitwise constant propagation.
-fipa-cp	Perform interprocedural constant propagation.
-fipa-cp-clone	Perform cloning to make Interprocedural constant propagation stronger.
-fipa-icf	Perform Identical Code Folding for functions and read-only variables.
-fipa-icf-functions	Perform Identical Code Folding for functions.
-fipa-icf-variables	Perform Identical Code Folding for variables.
-fipa-modref	Perform interprocedural modref analysis.
-fipa-profile	Perform interprocedural profile propagation.
-fipa-pta	Perform interprocedural points-to analysis.
-fipa-pure-const	Discover pure and const functions.
-fipa-ra	Use caller save register across calls if possible.
-fipa-reference	Discover read-only and non addressable static variables.
-fipa-reference-addressable	Discover read-only, write-only and non-addressable static variables.
-fipa-sra	Perform interprocedural reduction of aggregates.
-fipa-stack-alignment	Reduce stack alignment on call sites if possible.
-fipa-vrp	Perform IPA Value Range Propagation.
-fira-algorithm=[CB | priority]	Set the used IRA algorithm.
-fira-hoist-pressure	Use IRA based register pressure calculation in RTL hoist optimizations.
-fira-loop-pressure	Use IRA based register pressure calculation in RTL loop optimizations.
-fira-region=[one | all | mixed]	Set regions for IRA.
-fira-share-save-slots	Share slots for saving different hard registers.
-fira-share-spill-slots	Share stack slots for spilled pseudo-registers.
-fisolate-erroneous-paths-attribute	Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap.
-fisolate-erroneous-paths-dereference	Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap.
-fivopts	Optimize induction variables on trees.
-fjump-tables	Use jump tables for sufficiently large switch statements.
-fkeep-gc-roots-live	This option lacks documentation.
-flifetime-dse=<0,2>	Tell DSE that the storage for a C++ object is dead when the constructor starts and when the destructor finishes.
-flimit-function-alignment	This option lacks documentation.
-flive-patching=	[inline-only-static | inline-clone] Control IPA optimizations to provide a safe compilation for live-patching. At the same time, provides multiple-level control on the enabled IPA optimizations.
-flive-range-shrinkage	Relief of register pressure through live range shrinkage.
-floop-interchange	Enable loop interchange on trees.
-floop-nest-optimize	Enable the loop nest optimizer.
-floop-parallelize-all	Mark all loops as parallel.
-floop-unroll-and-jam	Perform unroll-and-jam on loops.
-flra-remat	Do CFG-sensitive rematerialization in LRA.
-fmath-errno	Set errno after built-in math functions.
-fmodulo-sched	Perform SMS based modulo scheduling before the first scheduling pass.
-fmodulo-sched-allow-regmoves	Perform SMS based modulo scheduling with register moves allowed.
-fmove-loop-invariants	Move loop invariant computations out of loops.
-fnon-call-exceptions	Support synchronous non-call exceptions.
-fnothrow-opt	Treat a throw() exception specification as noexcept to improve code size.
-fomit-frame-pointer	When possible do not generate stack frames.
-fopt-info	Enable all optimization info dumps on stderr.
-foptimize-sibling-calls	Optimize sibling and tail recursive calls.
-foptimize-strlen	Enable string length optimizations on trees.
-fpack-struct	Pack structure members together without holes.
-fpack-struct=<number>	Set initial maximum structure member alignment.
-fpartial-inlining	Perform partial inlining.
-fpatchable-function-entry=	Insert NOP instructions at each function entry.
-fpeel-loops	Perform loop peeling.
-fpeephole	Enable machine specific peephole optimizations.
-fpeephole2	Enable an RTL peephole pass before sched2.
-fplt	Use PLT for PIC calls (-fno-plt: load the address from GOT at call site).
-fpredictive-commoning	Run predictive commoning optimization.
-fprefetch-loop-arrays	Generate prefetch instructions, if available, for arrays in loops.
-fprintf-return-value	Treat known sprintf return values as constants.
-fprofile-partial-training	Do not assume that functions never executed during the train run are cold.
-fprofile-reorder-functions	Enable function reordering that improves code placement.
-freciprocal-math	Same as -fassociative-math for expressions which include division.
-free	Turn on Redundant Extensions Elimination pass.
-freg-struct-return	Return small aggregates in registers.
-frename-registers	Perform a register renaming optimization pass.
-freorder-blocks	Reorder basic blocks to improve code placement.
-freorder-blocks-algorithm=	[simple | stc] Set the used basic block reordering algorithm.
-freorder-blocks-and-partition	Reorder basic blocks and partition into hot and cold sections.
-freorder-functions	Reorder functions to improve code placement.
-frerun-cse-after-loop	Add a common subexpression elimination pass after loop optimizations.
-freschedule-modulo-scheduled-loops	Enable/Disable the traditional scheduling in loops that already passed modulo scheduling.
-frounding-math	Disable optimizations that assume default FP rounding behavior.
-frtti	Generate run time type descriptor information.
-fsave-optimization-record	Write a SRCFILE.opt-record.json file detailing what optimizations were performed.
-fsched-critical-path-heuristic	Enable the critical path heuristic in the scheduler.
-fsched-dep-count-heuristic	Enable the dependent count heuristic in the scheduler.
-fsched-group-heuristic	Enable the group heuristic in the scheduler.
-fsched-interblock	Enable scheduling across basic blocks.
-fsched-last-insn-heuristic	Enable the last instruction heuristic in the scheduler.
-fsched-pressure	Enable register pressure sensitive insn scheduling.
-fsched-rank-heuristic	Enable the rank heuristic in the scheduler.
-fsched-spec	Allow speculative motion of non-loads.
-fsched-spec-insn-heuristic	Enable the speculative instruction heuristic in the scheduler.
-fsched-spec-load	Allow speculative motion of some loads.
-fsched-spec-load-dangerous	Allow speculative motion of more loads.
-fsched-stalled-insns	Allow premature scheduling of queued insns.
-fsched-stalled-insns-dep	Set dependence distance checking in premature scheduling of queued insns.
-fsched-stalled-insns-dep=<number>	Set dependence distance checking in premature scheduling of queued insns.
-fsched-stalled-insns=<number>	Set number of queued insns that can be prematurely scheduled.
-fsched2-use-superblocks	If scheduling post reload, do superblock scheduling.
-fschedule-fusion	Perform a target dependent instruction fusion optimization pass.
-fschedule-insns	Reschedule instructions before register allocation.
-fschedule-insns2	Reschedule instructions after register allocation.
-fsection-anchors	Access data in the same section from shared anchor points.
-fsel-sched-pipelining	Perform software pipelining of inner loops during selective scheduling.
-fsel-sched-pipelining-outer-loops	Perform software pipelining of outer loops during selective scheduling.
-fsel-sched-reschedule-pipelined	Reschedule pipelined regions without pipelining.
-fselective-scheduling	Schedule instructions using selective scheduling algorithm.
-fselective-scheduling2	Run selective scheduling after reload.
-fshort-enums	Use the narrowest integer type possible for enumeration types.
-fshort-wchar	Force the underlying type for "wchar_t" to be "unsigned short".
-fshrink-wrap	Emit function prologues only before parts of the function that need it, rather than at the top of the function.
-fshrink-wrap-separate	Shrink-wrap parts of the prologue and epilogue separately.
-fsignaling-nans	Disable optimizations observable by IEEE signaling NaNs.
-fsigned-zeros	Disable floating point optimizations that ignore the IEEE signedness of zero.
-fsimd-cost-model=	[unlimited | dynamic | cheap | very-cheap] Specifies the vectorization cost model for code marked with a simd directive.
-fsingle-precision-constant	Convert floating point constants to single precision constants.
-fsplit-ivs-in-unroller	Split lifetimes of induction variables when loops are unrolled.
-fsplit-loops	Perform loop splitting.
-fsplit-paths	Split paths leading to loop backedges.
-fsplit-wide-types	Split wide types into independent registers.
-fsplit-wide-types-early	Split wide types into independent registers earlier.
-fssa-backprop	Enable backward propagation of use properties at the SSA level.
-fssa-phiopt	Optimize conditional patterns using SSA PHI nodes.
-fstack-check=	[no | generic | specific] Insert stack checking code into the program.
-fstack-clash-protection	Insert code to probe each page of stack space as it is allocated to protect from stack-clash style attacks.
-fstack-protector	Use propolice as a stack protection method.
-fstack-protector-all	Use a stack protection method for every function.
-fstack-protector-explicit	Use stack protection method only for functions with the stack_protect attribute.
-fstack-protector-strong	Use a smart stack protection method for certain functions.
-fstack-reuse=	[all | named_vars | none] Set stack reuse level for local variables.
-fstdarg-opt	Optimize amount of stdarg registers saved to stack at start of function.
-fstore-merging	Merge adjacent stores.
-fstrict-aliasing	Assume strict aliasing rules apply.
-fstrict-enums	Assume that values of enumeration type are always within the minimum range of that type.
-fstrict-volatile-bitfields	Force bitfield accesses to match their type width.
-fthread-jumps	Perform jump threading optimizations.
-fno-threadsafe-statics	Do not generate thread-safe code for initializing local statics.
-ftoplevel-reorder	Reorder top level functions, variables, and asms.
-ftracer	Perform superblock formation via tail duplication.
-ftrapping-math	Assume floating-point operations can trap.
-ftrapv	Trap for signed overflow in addition, subtraction and multiplication.
-ftree-bit-ccp	Enable SSA-BIT-CCP optimization on trees.
-ftree-builtin-call-dce	Enable conditional dead code elimination for builtin calls.
-ftree-ccp	Enable SSA-CCP optimization on trees.
-ftree-ch	Enable loop header copying on trees.
-ftree-coalesce-vars	Enable SSA coalescing of user variables.
-ftree-copy-prop	Enable copy propagation on trees.
-ftree-cselim	Transform condition stores into unconditional ones.
-ftree-dce	Enable SSA dead code elimination optimization on trees.
-ftree-dominator-opts	Enable dominator optimizations.
-ftree-dse	Enable dead store elimination.
-ftree-forwprop	Enable forward propagation on trees.
-ftree-fre	Enable Full Redundancy Elimination (FRE) on trees.
-ftree-loop-distribute-patterns	Enable loop distribution for patterns transformed into a library call.
-ftree-loop-distribution	Enable loop distribution on trees.
-ftree-loop-if-convert	Convert conditional jumps in innermost loops to branchless equivalents.
-ftree-loop-im	Enable loop invariant motion on trees.
-ftree-loop-ivcanon	Create canonical induction variables in loops.
-ftree-loop-optimize	Enable loop optimizations on tree level.
-ftree-loop-vectorize	Enable loop vectorization on trees.
-ftree-lrs	Perform live range splitting during the SSA->normal pass.
-ftree-parallelize-loops=<number>	Enable automatic parallelization of loops.
-ftree-partial-pre	In SSA-PRE optimization on trees, enable partial-partial redundancy elimination.
-ftree-phiprop	Enable hoisting loads from conditional pointers.
-ftree-pre	Enable SSA-PRE optimization on trees.
-ftree-pta	Perform function-local points-to analysis on trees.
-ftree-reassoc	Enable reassociation on tree level.
-ftree-scev-cprop	Enable copy propagation of scalar-evolution information.
-ftree-sink	Enable SSA code sinking on trees.
-ftree-slp-vectorize	Enable basic block vectorization (SLP) on trees.
-ftree-slsr	Perform straight-line strength reduction.
-ftree-sra	Perform scalar replacement of aggregates.
-ftree-switch-conversion	Perform conversions of switch initializations.
-ftree-tail-merge	Enable tail merging on trees.
-ftree-ter	Replace temporary expressions in the SSA->normal pass.
-ftree-vectorize	Enable vectorization on trees.
-ftree-vrp	Perform Value Range Propagation on trees.
-funconstrained-commons	Assume common declarations may be overridden with ones with a larger trailing array.
-funroll-all-loops	Perform loop unrolling for all loops.
-funroll-completely-grow-size	This option lacks documentation.
-funroll-loops	Perform loop unrolling when iteration count is known.
-funsafe-math-optimizations	Allow math optimizations that may violate IEEE or ISO standards.
-funswitch-loops	Perform loop unswitching.
-funwind-tables	Just generate unwind tables for exception handling.
-fvar-tracking	Perform variable tracking.
-fvar-tracking-assignments	Perform variable tracking by annotating assignments.
-fvar-tracking-assignments-toggle	Toggle -fvar-tracking-assignments.
-fvar-tracking-uninit	Perform variable tracking and also tag variables that are uninitialized.
-fvariable-expansion-in-unroller	Apply variable expansion when loops are unrolled.
-fvect-cost-model=	[unlimited | dynamic | cheap | very-cheap] Specifies the cost model for vectorization.
-fversion-loops-for-strides	Version loops based on whether indices have a stride of one.
-fvpt	Use expression value profiles in optimizations.
-fweb	Construct webs and split unrelated uses of single variable.
-fwrapv	Assume signed arithmetic overflow wraps around.
-fwrapv-pointer	Assume pointer overflow wraps around.