Optimize Options - Using the GNU Compiler Collection (GCC) (original) (raw)

These options control various sorts of optimizations.

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 would expect from the source code.

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. Optimization levels -O and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a-time mode allows the compiler to use information 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.

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

The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.

-fno-default-inline

Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify-O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add `inline' in front of the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

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

-fforward-propagate

Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O2,-O3, -Os.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.

On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See Register Usage.

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

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

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

-fno-inline

Don't pay attention to the inline keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-finline-small-functions

Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

Enabled at level -O2.

-finline-functions

Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in 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 level -O3.

-finline-functions-called-once

Consider all static functions called once for inlining into their caller even if they are 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 if -funit-at-a-time is enabled.

-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 real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.

Enabled by default.

-finline-limit=n

By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this 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 as follows:

max-inline-insns-single

is set to n/2.

max-inline-insns-auto

is set to n/2.

See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.

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

Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.

-fkeep-inline-functions

In C, emit static functions that are declared inlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inline extension in GNU C89. In C++, emit any and all inline functions into the object file.

-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 if the variable was 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 non-automatic variable to have distinct location, so using this option will result 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-dependences edges will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This option is effective only with-fmodulo-sched enabled.

-fno-branch-count-reg

Do not use “decrement and branch” instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, 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.

The default is -fbranch-count-reg.

-fno-function-cse

Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

The default is -ffunction-cse

-fno-zero-initialized-in-bss

If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.

This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

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

-fmudflap -fmudflapth -fmudflapir

For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if-fmudflap is given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONSenvironment variable. See env MUDFLAP_OPTIONS=-help a.outfor its options.

Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

-fthread-jumps

Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

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

-fsplit-wide-types

When using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult.

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

-fcse-follow-jumps

In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with anelse clause, CSE will follow the jump when the condition tested is false.

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

-fcse-skip-blocks

This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which 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 has been performed.

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

-fgcse

Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime 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 will attempt to move loads which are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.

Enabled by default when gcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

Not enabled at any optimization level.

-fgcse-las

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

Not enabled at any optimization level.

-fgcse-after-reload

When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to cleanup redundant spilling.

-funsafe-loop-optimizations

If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping.

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

-fauto-inc-dec

Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.

-fdce

Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.

-fdse

Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.

-fif-conversion

Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by if-conversion2.

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

-fif-conversion2

Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

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

-fdelete-null-pointer-checks

Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null.

In some environments, this assumption is not true, and programs can safely dereference null pointers. Use-fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.

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

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

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

-foptimize-register-move

-fregmove

Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.

Note -fregmove and -foptimize-register-move are the same optimization.

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

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

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

-fschedule-insns

If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

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

-fschedule-insns2

Similar to -fschedule-insns, but 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

Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec

Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load

Allow 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

Allow 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.

-fsched-stalled-insns

-fsched-stalled-insns=n

Define 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 that no insns will be moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to-fsched-stalled-insns=1.

-fsched-stalled-insns-dep

-fsched-stalled-insns-dep=n

Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is 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. -fsched-stalled-insns-dep without a value is equivalent to-fsched-stalled-insns-dep=1.

-fsched2-use-superblocks

When scheduling after register allocation, do use superblock scheduling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on faster schedules. This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

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

-fsched2-use-traces

Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details on trace formation.

This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with-fschedule-insns2 or at -O2 or higher.

-fsee

Eliminate redundant sign extension instructions and move the non-redundant ones to optimal placement using lazy code motion (LCM).

-freschedule-modulo-scheduled-loops

The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.

-fcaller-saves

Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.

This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

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

-ftree-reassoc

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

-ftree-pre

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

-ftree-fre

Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.

-ftree-copy-prop

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

-ftree-salias

Perform structural alias analysis on trees. This flag is enabled by default at -O and higher.

-fipa-pure-const

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

-fipa-reference

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

-fipa-struct-reorg

Perform structure reorganization optimization, that change C-like structures layout in order to better utilize spatial locality. This transformation is affective for programs containing arrays of structures. Available in two compilation modes: profile-based (enabled with -fprofile-generate) or static (which uses built-in heuristics). Require -fipa-type-escapeto provide the safety of this transformation. It works only in whole program mode, so it requires -fwhole-program and -combine to be enabled. Structures considered `cold' by this transformation are not affected (see --param struct-reorg-cold-struct-ratio=value).

With this flag, the program debug info reflects a new structure layout.

-fipa-pta

Perform interprocedural pointer analysis.

-fipa-cp

Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions, but because this optimization can create multiple copies of functions, it may significantly increase code size.

-fipa-matrix-reorg

Perform matrix flattening and transposing. Matrix flattening tries to replace a m-dimensional matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of the matrix. The second optimization is matrix transposing that attemps to change the order of the matrix's dimensions in order to improve cache locality. Both optimizations need fwhole-program flag. Transposing is enabled only if profiling information is avaliable.

-ftree-sink

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

-ftree-ccp

Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.

-ftree-store-ccp

Perform sparse conditional constant propagation (CCP) on trees. This pass operates on both local scalar variables and memory stores and loads (global variables, structures, arrays, etc). This flag is enabled by default at -O2 and higher.

-ftree-dce

Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.

-ftree-dominator-opts

Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.

-ftree-dse

Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location which will later be overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher.

-ftree-ch

Perform 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

Perform loop optimizations on trees. This flag is enabled by default at -O and higher.

-ftree-loop-linear

Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.

-fcheck-data-deps

Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.

-ftree-loop-im

Perform loop invariant motion on trees. This pass moves only invariants that would be 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

Create a canonical counter for number of iterations in the loop for that determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.

-fivopts

Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.

-ftree-parallelize-loops=n

Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-ftree-sra

Perform 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 -O and higher.

-ftree-copyrename

Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher.

-ftree-ter

Perform 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-vectorize

Perform loop vectorization on trees. This flag is enabled by default at-O3.

-ftree-vect-loop-version

Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled.

-fvect-cost-model

Enable cost model for vectorization.

-ftree-vrp

Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

-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. This option makes code larger, and may or may not make it run faster.

-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,

-fsplit-ivs-in-unroller

Enables expressing of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.

Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller

With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.

-fpredictive-commoning

Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops.

This option is enabled at level -O3.

-fprefetch-loop-arrays

If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.

Disabled at level -Os.

-fno-peephole

-fno-peephole2

Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

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

-fno-guess-branch-probability

Do not guess branch probabilities using heuristics.

GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by `__builtin_expect', then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the `__builtin_expect' info into account. The interactions between the heuristics and `__builtin_expect' can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of `__builtin_expect' are easier to understand.

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

-freorder-blocks

Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

Enabled at levels -O2, -O3.

-freorder-blocks-and-partition

In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.

This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

-freorder-functions

Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections .text.hot for most frequently executed functions and.text.unlikely for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

Also profile feedback must be available in to make this option effective. See-fprofile-arcs for details.

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

-fstrict-aliasing

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

Pay special attention to code like this:

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

The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with-fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. See Structures unions enumerations and bit-fields implementation. However, this code might not:

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

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

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

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

-fstrict-overflow

Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it will not happen. This permits various optimizations. For example, the compiler will assume that an expression like i + 10 > i will always be true for signed i. This assumption is only valid if signed overflow is undefined, as the expression is false if i + 10 overflows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually involve overflow.

This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the compiler to conclude that p + u > p is always true for a pointer p and unsigned integeru. This assumption is only valid because pointer wraparound is undefined, as the expression is false if p + u overflows using twos complement arithmetic.

See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it wraps. When-fwrapv is used, there is no difference between-fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with-fwrapv, but not otherwise.

The -fstrict-overflow option is enabled at levels-O2, -O3, -Os.

-falign-functions

-falign-functions=n

Align the start of functions to the next power-of-two greater thann, skipping up to n bytes. For instance,-falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.

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

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

Enabled at levels -O2, -O3.

-falign-labels

-falign-labels=n

Align all branch targets to a power-of-two boundary, skipping up ton bytes like -falign-functions. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.

-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.

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

If n is not specified or is zero, use a machine-dependent default which is very likely to be `1', meaning no alignment.

Enabled at levels -O2, -O3.

-falign-loops

-falign-loops=n

Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

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

Enabled at levels -O2, -O3.

-falign-jumps

-falign-jumps=n

Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to nbytes like -falign-functions. In this case, no dummy operations need be executed.

-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.

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

Enabled at levels -O2, -O3.

-funit-at-a-time

Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory (in general). There are some compatibility issues with unit-at-a-time mode:

• enabling unit-at-a-time mode may change the order in which functions, variables, and top-level asm statements are emitted, and will likely break code relying on some particular ordering. The majority of such top-level asm statements, though, can be replaced by section attributes. Thefno-toplevel-reorder option may be used to keep the ordering used in the input file, at the cost of some optimizations.
• unit-at-a-time mode removes unreferenced static variables and functions. This may result in undefined references when an asm statement refers directly to variables or functions that are otherwise unused. In that case either the variable/function shall be listed as an operand of the asm statement operand or, in the case of top-level asm statements the attribute usedshall be used on the declaration.
• Static functions now can use non-standard passing conventions that may break asm statements calling functions directly. Again, attribute used will prevent this behavior.

As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC.

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

-fno-toplevel-reorder

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

-fweb

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

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents whole program being compiled. All public functions and variables with the exception of mainand those merged by attribute externally_visible become static functions and in a affect gets more aggressively optimized by interprocedural optimizers. While this option is equivalent to proper use of static keyword for programs consisting of single file, in combination with option--combine this flag can be used to compile most of smaller scale C programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.

This option is not supported for Fortran programs.

-fcprop-registers

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

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

-fprofile-generate

Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program.

The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.

-fprofile-use

Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

The following options are enabled: -fbranch-probabilities, -fvpt,-funroll-loops, -fpeel-loops, -ftracer

By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using-Wcoverage-mismatch. Note this may result in poorly optimized code.

The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.

-ffloat-store

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

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations,-ffinite-math-only, -fno-rounding-math,-fno-signaling-nans and -fcx-limited-range.

This option causes the preprocessor macro __FAST_MATH__ to be defined.

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

The default is -fmath-errno.

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

-funsafe-math-optimizations

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

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

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

-fassociative-math

Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on a code which 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.

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 / ycan 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 which 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 which 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.

-frtl-abstract-sequences

It is a size optimization method. This option is to find identical sequences of code, which can be turned into pseudo-procedures and then replace all occurrences with calls to the newly created subroutine. It is kind of an opposite of -finline-functions. This optimization runs at RTL level.

-fsignaling-nans

Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

The default is -fno-signaling-nans.

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

-fsingle-precision-constant

Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

-fcx-limited-range

When enabled, this option states that a range reduction step is not needed when performing complex division. The default is-fno-cx-limited-range, but is enabled by -ffast-math.

This option controls the default setting of the ISO C99CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.

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.