This flag is enabled by default at -O3. Perform forward propagation on trees. This flag is enabled by default at -O1 and higher. 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. Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O1 and higher. Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Perform copy propagation on trees. This pass eliminates unnecessary copy operations. Discover which functions are pure or constant.

Enabled by default at -O1 and higher. Discover which static variables do not escape the compilation unit. Discover read-only, write-only and non-addressable static variables. Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. Perform interprocedural profile propagation.

The functions called only from cold functions are marked as cold. Also functions executed once such as cold , noreturn , static constructors or destructors are identified. Cold functions and loop less parts of functions executed once are then optimized for size. This optimization analyzes the side effects of functions memory locations that are modified or referenced and enables better optimization across the function call boundary.

Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions.

This flag is enabled by default at -O2 , -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile. Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile.

It requires that -fipa-cp is enabled. When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. Perform Identical Code Folding for functions and read-only variables.

The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled. If a function is patched, its impacted functions should be patched too. 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. Only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining.

Only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well. When -flive-patching is specified without any value, the default value is inline-clone. Note that -flive-patching is not supported with link-time optimization -flto. Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer.

Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled. This is not currently enabled, but may be enabled by -O2 in the future. Perform forward store motion on trees. Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information.

This pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled. Perform sparse conditional constant propagation CCP on trees. This pass only operates on local scalar variables and is enabled by default at -O1 and higher.

Propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at -O1 and higher. Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at -O1 and higher, except for -Og. Perform conversion of simple initializations in a switch to initializations from a scalar array. Look for identical code sequences.

When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.

Perform dead code elimination DCE on trees. Perform 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. Assume that a loop with an exit will eventually take the exit and not loop indefinitely. This allows the compiler to remove loops that otherwise have no side-effects, not considering eventual endless looping as such.

This also performs jump threading to reduce jumps to jumps. Perform dead store elimination DSE on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads.

In this case the earlier store can be deleted. Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. It is not enabled for -Os , since it usually increases code size. Perform loop optimizations on trees. 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.

Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops. Enable 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. Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.

While transforming the program out of the SSA representation, attempt 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.

Attempt 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. Perform 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. Perform 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. and the initialization loop is transformed into a call to memset zero. Perform loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. Apply 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. Perform 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. Create 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.

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

Parallelize loops, i. 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. by memory bandwidth. This option implies -pthread , and thus is only supported on targets that have support for -pthread.

Perform function-local points-to analysis on trees. This flag is enabled by default at -O1 and higher, except for -Og. Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. Perform 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. 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 -O1 and higher. Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.

Perform loop vectorization on trees. This flag is enabled by default at -O2 and by -ftree-vectorize , -fprofile-use , and -fauto-profile. Perform basic block vectorization on trees. Initialize automatic variables with either a pattern or with zeroes to increase the security and predictability of a program by preventing uninitialized memory disclosure and use.

With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes.

However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the first case of a switch statement. Using -Wtrivial-auto-var-init to report all such cases. You can control this behavior for a specific variable by using the variable attribute uninitialized see Variable Attributes. Alter the cost model used for vectorization.

Alter the cost model used for vectorization of loops marked with the OpenMP simd directive. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.

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. Split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O3 and above.

Enables expression 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. A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block.

It also does not work at all on some architectures due to restrictions in the CSE pass. With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. Inline parts of functions. Perform predictive commoning optimization, i.

This option is enabled at level -O3. 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. Do not substitute constants for known return value of formatted output functions such as sprintf , snprintf , vsprintf , and vsnprintf but not printf of fprintf.

This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions 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 bit or smaller integer because the return value is guaranteed to be at most 8.

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. The -fprintf-return-value option is enabled by default. 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.

GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback -fprofile-arcs. These heuristics are based on the control flow graph. The default is -fguess-branch-probability at levels -O , -O2 , -O3 , -Os. Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

Use the specified algorithm for basic block reordering. 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. When -fsplit-stack is used this option is not enabled by default to avoid linker errors , but may be enabled explicitly if using a working linker.

Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections. hot for most frequently executed functions and. 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. Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled.

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. A character type may alias any other type.

Even with -fstrict-aliasing , type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. See Structures unions enumerations and bit-fields implementation. However, this code might not:. 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.

The -fstrict-aliasing option is enabled at levels -O2 , -O3 , -Os. Controls whether rules of -fstrict-aliasing are applied across function boundaries. Note that if multiple functions gets inlined into a single function the memory accesses are no longer considered to be crossing a function boundary. The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing. Align the start of functions to the next power-of-two greater than or equal to n , skipping up to m -1 bytes.

This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n -byte alignment boundary. If m2 is not specified, it defaults to n2. 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. The maximum allowed n option value is If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions , but not to skip more bytes than the size of the function.

Parameters of this option are analogous to the -falign-functions option. If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead. 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. 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. Allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads.

Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads. Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value.

Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization. This option is left for compatibility reasons. 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. Additionally -fno-toplevel-reorder implies -fno-section-anchors. This also affects any such calls implicitly generated by the compiler. 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.

Assume that the current compilation unit represents the whole program being compiled. This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information. This option runs the standard link-time optimizer.

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:. The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo. o and bar. The final invocation reads the GIMPLE bytecode from foo.

o , merges the two files into a single internal image, and compiles the result as usual. Since both foo. 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. 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: -fcommon , -fexceptions , -fnon-call-exceptions , -fgnu-tm and all the -m target flags. The following options -fPIC , -fpic , -fpie and -fPIE are combined based on the following scheme:. 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.

You can override them at link time. Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time.

When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units.

You can alternatively also specify assembler options at LTO link time. 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.

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. 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:. 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.

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:.

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. 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. This is useful when the Makefile calling GCC is already executing in parallel. This option likely only works if MAKE is GNU make. Specify the partitioning algorithm used by the link-time optimizer. This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode -flto.

GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 no compression to 19 maximum compression , while zlib supports values from 0 to 9.

Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used. 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. 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.

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. 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. After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic.

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

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

With -fprofile-use all portions of programs not executed during 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 train run is not representative but also leads to significantly bigger code.

Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:. Before you can use this option, you must first generate profiling information. See Instrumentation Options , for information about the -fprofile-generate option. By default, GCC emits an error message if the feedback profiles do not match the source code. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist see -Wmissing-profile.

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:. path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.

afdo in the current directory. You must also supply the unstripped binary for your program to this tool. The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. 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 where the floating registers of the 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. This option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

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

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. 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. Enables -fno-signed-zeros , -fno-trapping-math , -fassociative-math and -freciprocal-math. Allow re-association of operands in series of floating-point operations. 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.

For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. Note that this loses precision and increases the number of flops operating on the value.

Allow optimizations for floating-point arithmetic that ignore the signedness of zero. 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. 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. This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. 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 is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. The default is -ffp-int-builtin-inexact , allowing the exception to be raised, unless C2X or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect. Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.

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. Nevertheless, the option applies to all languages.

Complex multiplication and division follow 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. After running a program compiled with -fprofile-arcs see Instrumentation Options , 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 information in this 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.

See details about the file naming in -fprofile-arcs. These can be used to improve optimization. Currently, they are only used in one place: in reorg. 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. Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order. 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.

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. Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job. Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.

It also turns on complete loop peeling i. 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. Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. Peels loops for which there is enough information that they do not roll much from profile feedback or static analysis. complete removal of loops with small constant number of iterations.

Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og. Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration.

Note for this option to have an effect -ftree-loop-im has to be enabled as well. 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. If a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. This is particularly useful for assumed-shape arrays in Fortran where for example it allows better vectorization assuming contiguous accesses.

Place each function or data item into its own section in the output file if the target supports arbitrary sections. 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. 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. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. 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.

Zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming ROP attacks or preventing information leakage through registers.

In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions.

You can control some of these constants on the command line using the --param option. The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. In each case, the value is an integer. The following choices of name are recognized for all targets:.

When branch is predicted to be taken with probability lower than this threshold in percent , then it is considered well predictable. RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion.

The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable. RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not.

The maximum number of incoming edges to consider for cross-jumping. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.

The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The maximum number of instructions to duplicate to a block that jumps to a computed goto.

Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching.

Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information.

Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization.

If more memory than specified is required, the optimization is not done. 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. The maximum number of pending dependencies scheduling allows before flushing the current state and starting over.

Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.

Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once.

Several parameters control the tree inliner used in GCC. When you use -finline-functions included in -O3 , a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different more restrictive limit compared to functions declared inline can be applied --param max-inline-insns-auto.

This is bound applied to calls which are considered relevant with -finline-small-functions. This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. Number of instructions accounted by inliner for function overhead such as function prologue and epilogue. Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. The scale in percents applied to inline-insns-single , inline-insns-single-O2 , inline-insns-auto when inline heuristics hints that inlining is very profitable will enable later optimizations.

Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks. The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. Specifies maximal growth of large function caused by inlining in percents.

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Next: Instrumentation Options , Previous: Debugging Options , Up: Invoking GCC [ Contents ][ Index ]. 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. The compiler performs optimization based on the knowledge it has of the program.

Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. See Overall Options , for examples. 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.

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O , this option increases both compilation time and the performance of the generated code.

It also turns on the following optimization flags:. Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. Optimize yet more. Optimize for size. It also enables -finline-functions , causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.

Disregard strict standards compliance. It also enables optimizations that are not valid for all standard-compliant programs.

It turns on -ffast-math , -fallow-store-data-races and the Fortran-specific -fstack-arrays , unless -fmax-stack-var-size is specified, and -fno-protect-parens. It turns off -fsemantic-interposition. Optimize debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. Like -O0 , -Og completely disables a number of optimization passes so that individual options controlling them have no 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 increase the number of instructions executed if those instructions require fewer bytes to encode. If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Options of the form -f flag specify machine-independent flags. Most flags 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 you typically use. The following options control specific optimizations. They are either activated by -O options or are related to ones that are.

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 several function calls and pop them all at once. 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 -O1 , -O2 , -O3 , -Os. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted. Several targets always omit the frame pointer in leaf functions. Optimize various standard C string functions e. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute. 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. This inlining applies to all functions, even those not declared inline. Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining.

This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are 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 levels -O2 , -O3 , -Os. Also enabled by -fprofile-use and -fauto-profile. 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 at levels -O1 , -O2 , -O3 and -Os , but not -Og. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.

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. 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. 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.

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

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. This option implies -fmerge-constants. In addition to -fmerge-constants this considers e. even constant initialized arrays or initialized constant variables with integral or floating-point types. Perform swing modulo scheduling immediately before the first scheduling pass.

This pass looks at innermost loops and reorders their instructions by overlapping different iterations. Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences 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. The default is -fbranch-count-reg at -O1 and higher, except for -Og. 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.

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.

Perform optimizations that 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. When using a type that occupies multiple registers, such as long long on a bit system, split the registers apart and allocate them independently.

This normally generates better code for those types, but may make debugging more difficult. Fully split wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on. 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 an else clause, CSE follows the jump when the condition tested is false.

Controls whether rules of -fstrict-aliasing are applied across function boundaries. Buy Tickets Now! This option is intended to support existing code that relies on a particular ordering. This option has no effect unless -fsplit-wide-types is turned on. The maximum number of instructions to consider when looking for an instruction to fill a delay slot. Larger values result in larger compilation times and may result in more removed dead stores. Bound on size of expressions used in the scalar evolutions analyzer.

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