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The compiler performs optimization based on the knowledge it has of theprogram. Compiling multiple files at once to a single output file mode allowsthe compiler to use information gained from all of the files when compilingeach of them.
Most optimizations are completely disabled at -O0 or if an-O level is not set on the command line, even if individualoptimization flags are specified. Similarly, -Og suppressesmany optimization passes.
Depending on the target and how GCC was configured, a slightly differentset of optimizations may be enabled at each -O level thanthose listed here. You can invoke GCC with -Q --help=optimizersto find out the exact set of optimizations that are enabled at each level.See Options Controlling the Kind of Output, for examples.
Optimize even more. GCC performs nearly all supported optimizationsthat do not involve a space-speed tradeoff.As compared to -O, this option increases both compilation timeand the performance of the generated code.
Disregard strict standards compliance. -Ofast enables all-O3 optimizations. It also enables optimizations that are notvalid for all standard-compliant programs.It turns on -ffast-math, -fallow-store-data-racesand the Fortran-specific -fstack-arrays, unless-fmax-stack-var-size is specified, and -fno-protect-parens.It turns off -fsemantic-interposition.
Optimize debugging experience. -Og should be the optimizationlevel of choice for the standard edit-compile-debug cycle, offeringa reasonable level of optimization while maintaining fast compilationand a good debugging experience. It is a better choice than -O0for producing debuggable code because some compiler passesthat collect debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them haveno effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:
Optimize aggressively for size rather than speed. This may increasethe number of instructions executed if those instructions requirefewer bytes to encode. -Oz behaves similarly to -Osincluding enabling most -O2 optimizations.
For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default;this allows the compiler to let arguments accumulate on the stack for severalfunction calls and pop them all at once.
Perform a forward propagation pass on RTL. The pass tries to combine twoinstructions and checks if the result can be simplified. If loop unrollingis active, two passes are performed and the second is scheduled afterloop unrolling.
-ffp-contract=off disables floating-point expression contraction.-ffp-contract=fast enables floating-point expression contractionsuch as forming of fused multiply-add operations if the target hasnative support for them.-ffp-contract=on enables floating-point expression contractionif allowed by the language standard. This is implemented for C and C++,where it enables contraction within one expression, but not acrossdifferent statements.
In some circumstances, it enables the compiler to generate code thattakes advantage of known alignment and length multipliers, but even thenit may be less efficient than optimized runtime implementations, andgrow code size so much that even a less performant but sharedimplementation runs faster due to better use of code caches. Thisoption is disabled by default.
Integrate functions into their callers when their body is smaller than expectedfunction call code (so overall size of program gets smaller). The compilerheuristically decides which functions are simple enough to be worth integratingin this way. This inlining applies to all functions, even those not declaredinline.
Inline also indirect calls that are discovered to be known at compiletime thanks to previous inlining. This option has any effect onlywhen inlining itself is turned on by the -finline-functionsor -finline-small-functions options.
Consider all static functions called once for inlining into theircaller even if they are not marked inline. If a call to a givenfunction is integrated, then the function is not output as assembler codein its own right.
Inline functions marked by always_inline and functions whose body seemssmaller than the function call overhead early before doing-fprofile-generate instrumentation and real inlining pass. Doing somakes profiling significantly cheaper and usually inlining faster on programshaving large chains of nested wrapper functions.
In C, emit static functions that are declared inlineinto the object file, even if the function has been inlined into allof its callers. This switch does not affect functions using theextern inline extension in GNU C90. In C++, emit any and allinline functions into the object file.
GCC enables this option by default. If you want to force the compiler tocheck if a variable is referenced, regardless of whether or notoptimization is turned on, use the -fno-keep-static-consts option.
This option implies -fmerge-constants. In addition to-fmerge-constants this considers e.g. even constant initializedarrays or initialized constant variables with integral or floating-pointtypes. Languages like C or C++ require each variable, including multipleinstances of the same variable in recursive calls, to have distinct locations,so using this option results in non-conformingbehavior.
Perform more aggressive SMS-based modulo scheduling with register movesallowed. By setting this flag certain anti-dependences edges aredeleted, which triggers the generation of reg-moves based on thelife-range analysis. This option is effective only with-fmodulo-sched enabled.
Perform optimizations that check to see if a jump branches to alocation where another comparison subsumed by the first is found. Ifso, the first branch is redirected to either the destination of thesecond branch or a point immediately following it, depending on whetherthe condition is known to be true or false.
When using a type that occupies multiple registers, such as longlong on a 32-bit system, split the registers apart and allocate themindependently. This normally generates better code for those types,but may make debugging more difficult.
In common subexpression elimination (CSE), scan through jump instructionswhen the target of the jump is not reached by any other path. Forexample, when CSE encounters an if statement with anelse clause, CSE follows the jump when the conditiontested is false.
This is similar to -fcse-follow-jumps, but causes CSE tofollow jumps that conditionally skip over blocks. When CSEencounters a simple if statement with no else clause,-fcse-skip-blocks causes CSE to follow the jump around thebody of the if.
Note: When compiling a program using computed gotos, a GCCextension, you may get better run-time performance if you disablethe global common subexpression elimination pass by adding-fno-gcse to the command line.
When -fgcse-lm is enabled, global common subexpression eliminationattempts to move loads that are only killed by stores into themselves. Thisallows a loop containing a load/store sequence to be changed to a load outsidethe loop, and a copy/store within the loop.
When -fgcse-sm is enabled, a store motion pass is run afterglobal common subexpression elimination. This pass attempts to movestores out of loops. When used in conjunction with -fgcse-lm,loops containing a load/store sequence can be changed to a load beforethe loop and a store after the loop.
This option tells the loop optimizer to use language constraints toderive bounds for the number of iterations of a loop. This assumes thatloop code does not invoke undefined behavior by for example causing signedinteger overflows or out-of-bound array accesses. The bounds for thenumber of iterations of a loop are used to guide loop unrolling and peelingand loop exit test optimizations.This option is enabled by default.
This option tells the compiler that variables declared in common blocks(e.g. Fortran) may later be overridden with longer trailing arrays. Thisprevents certain optimizations that depend on knowing the array bounds.
Combine increments or decrements of addresses with memory accesses.This pass is always skipped on architectures that do not haveinstructions to support this. Enabled by default at -O1 andhigher on architectures that support this.
Attempt to transform conditional jumps into branch-less equivalents. Thisincludes use of conditional moves, min, max, set flags and abs instructions, andsome tricks doable by standard arithmetics. The use of conditional executionon chips where it is available is controlled by -fif-conversion2.
The C++ ABI requires multiple entry points for constructors anddestructors: one for a base subobject, one for a complete object, andone for a virtual destructor that calls operator delete afterwards.For a hierarchy with virtual bases, the base and complete variants areclones, which means two copies of the function. With this option, thebase and complete variants are changed to be thunks that call a commonimplementation.
Assume that programs cannot safely dereference null pointers, and thatno code or data element resides at address zero.This option enables simple constantfolding optimizations at all optimization levels. In addition, otheroptimization passes in GCC use this flag to control global dataflowanalyses that eliminate useless checks for null pointers; these assumethat a memory access to address zero always results in a trap, sothat if a pointer is checked after it has already been dereferenced,it cannot be null.
Attempt to convert calls to virtual functions to direct calls. Thisis done both within a procedure and interprocedurally as part ofindirect inlining (-findirect-inlining) and interprocedural constantpropagation (-fipa-cp).Enabled at levels -O2, -O3, -Os.
Attempt to convert calls to virtual functions to speculative direct calls.Based on the analysis of the type inheritance graph, determine for a given callthe set of likely targets. If the set is small, preferably of size 1, changethe call into a conditional deciding between direct and indirect calls. Thespeculative calls enable more optimizations, such as inlining. When they seemuseless after further optimization, they are converted back into original form.
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