GCC optimization

This guide provides Article description::an introduction to optimizing compiled code using safe, sane CFLAGS and CXXFLAGS . It also describes the theory behind optimizing in general.

What are CFLAGS and CXXFLAGS?
CFLAGS and CXXFLAGS are among the environment variables conventionally used to specify compiler options to a build system when compiling C and C++ code. While these variables are not standardized, their use is essentially ubiquitous and any correctly written build should understand these for passing extra or custom options when it invokes the compiler. See the GNU make info page for a list of some of the commonly used variables in this category.

Because such a large proportion of the packages that make up most Gentoo systems are written in C and C++, these are two variables administrators will definitely want to set correctly as they will greatly influence the way much of the system is built.

They can be used to decrease the amount of debug messages for a program, increase error warning levels and, of course, to optimize the code produced. The GCC manual maintains a complete list of available options and their purposes.

How are they used?
Normally, CFLAGS and CXXFLAGS would be set in the environment when invoking a configure script or with makefiles generated by the program. In Gentoo-based systems, set the CFLAGS and CXXFLAGS variables in. Variables set in this file will be exported to the environment of programs invoked by portage such that all packages will be compiled using these options as a base.

As seen in the example above, the CXXFLAGS variable is set to use all the options present in CFLAGS. Almost every system should be configured in this manner. Additional options for CXXFLAGS are less common and don't usually apply generally enough to deserve setting them globally.

Misconceptions
While compiler optimizations enabled by various CFLAGS can be an effective means of producing smaller and/or faster binaries, they can also impair the function of the code, bloat its size, slow down its execution time, or simply cause a build failure. The point of diminishing performance returns is reached rather quickly when dealing with CFLAGS. Don't set them arbitrarily.

Remember, the global CFLAGS configured in will be applied to every package on the system so administrators typically only set general, widely-applicable options. Individual packages further modify these options either in the ebuild or the build system itself to generate the final set of flags used when invoking the compiler.

Ready?
Being aware of the risks involved, take a look at some sane, safe optimizations. These will hold in good stead and will be endearing to developers the next time a problem is reported on Bugzilla. (Developers will usually request the user to recompile a package with minimal CFLAGS to see if the problem persists. Remember: aggressive flags can ruin code!)

The basics
The goal behind CFLAGS and CXXFLAGS is to create code tailor-made to the system; it should function perfectly while being lean and fast, if possible. Sometimes these conditions are mutually exclusive, so this guide will stick to combinations known to work well. Ideally, they are the best available for any CPU architecture. For informational purposes, aggressive flag use will be covered later. Not every option listed on the GCC manual (there are hundreds) will be discussed, but basic, most common flags will be reviewed.

-march
The first and most important option is. This tells the compiler what code it should produce for the system's processor architecture (or arch); it tells GCC that it should produce code for a certain kind of CPU. Different CPUs have different capabilities, support different instruction sets, and have different ways of executing code. The  flag will instruct the compiler to produce specific code for the system's CPU, with all its capabilities, features, instruction sets, quirks, and so on provided the source code is prepared to use them. For instance, to take benefit from AVX instructions, the source code needs to be adapted to support it.

is an ISA selection option; it tells the compiler that it may use the instructions from the ISA. On an Intel/AMD64 platform with  or lower optimization level, the code will likely end up with AVX instructions used but using shorter SSE XMM registers. To take full advantage of AVX YMM registers, the,   or   options should be used as well.

is an optimization option (default at  and  ), which attempts to vectorize loops using the selected ISA if possible. The reason it isn't enabled at  is that it doesn't always improve code, it can make code slower as well, and usually makes the code larger; it really depends on the loop etc.

Even though the CHOST variable in specifies the general architecture used,   should still be used so that programs can be optimized for the system specific processor. x86 and x86-64 CPUs (among others) should make use of the  flag.

What kind of CPU does the system have? To find out, run the following command:

or even install and add the available CPU-specific options to the  file, which the tool does through e.g. the CPU_FLAGS_X86 variable:

To get more details, including  and   values, two commands can be used.


 * The first command tells the compiler not to do any linking, and instead of interpreting the  option for clarifying command line options, it now shows if certain options are enabled or disabled . In this case, the options shown are those enabled for the selected target: 


 * The second command will show the compiler directives for building the header file, but without actually performing the steps and instead showing them on the screen . The final output line is the command that holds all the optimization options and architecture selection:


 * The glibc-hwcaps feature (>=sys-libs/glibc-2.33) can be used to define  for a more general processor architecture (for >=sys-devel/gcc-11):

In this example cpu supports x86-64-v3 psABI x86_64 and you can use it for

Now lets see  in action. This example is for an older AMD Athlon 64 chip:

Here's another one for a common Intel processor:

If the type of CPU is undetermined, or if the user does not know what setting to choose, it is possible use the  setting. When this flag is used, GCC will attempt to detect the processor and automatically set appropriate flags for it. However, this should not be used when intending to compile packages for different CPUs!

If compiling packages on one computer in order to run them on a different computer (such as when using a fast computer to build for an older, slower machine), then do not use. "Native" means that the code produced will run only on that type of CPU. The applications built with  on an Intel Core CPU will not be able to run on an old Intel Atom CPU.

Also available are the  and   flags. These flags are normally only used when there is no available  option; certain processor architectures may require   or even. Unfortunately, GCC's behavior isn't very consistent with how each flag behaves from one architecture to the next.

On x86 and x86-64 CPUs,  will generate code specifically for that CPU using its available instruction sets and the correct ABI; it will have no backwards compatibility for older/different CPUs. Consider using  when generating code for older CPUs such as i386 and i486. produces more generic code than ; though it will tune code for a certain CPU, it does not take into account available instruction sets and ABI. Do not use  on x86 or x86-64 systems, as it is deprecated for those architectures.

Only non-x86/x86-64 CPUs (such as ARM, SPARC, Alpha, and PowerPC) may require  or   instead of. On these architectures,  /   will sometimes behave just like   (on x86/x86-64) but with a different flag name. Again, GCC's behavior and flag naming is not consistent across architectures, so be sure to check the GCC manual to determine which one should be used.

-O
Next up is the  variable. This variable controls the overall level of optimization. Changing this value will make the code compilation take more time and will use much more memory, especially as the level of optimization is increased.

There are seven  settings: ,  ,  ,  ,  ,  , and. Only use one of them in.

With the exception of, the   settings each activate several additional flags, so be sure to read the GCC manual's chapter on optimization options to learn which flags are activated at each   level, as well as some explanations as to what they do.

Let us examine each optimization level:


 * : This level (that is the letter "O" followed by a zero) turns off optimization entirely and is the default if no  level is specified in CFLAGS or CXXFLAGS . This reduces compilation time and can improve debugging info, but some applications will not work properly without optimization enabled. This option is not recommended except for debugging purposes.


 * : the most basic optimization level. The compiler will try to produce faster, smaller code without taking much compilation time. It is basic, but it should get the job done all the time.


 * : A step up from . The recommended level of optimization unless the system has special needs.   will activate a few more flags in addition to the ones activated by  . With , the compiler will attempt to increase code performance without compromising on size, and without taking too much compilation time. SSE or AVX may be utilized at this level but no YMM registers will be used unless   is also enabled.


 * : the highest level of optimization possible. It enables optimizations that are expensive in terms of compile time and memory usage. Compiling with   is not a guaranteed way to improve performance, and in fact, in many cases, can slow down a system due to larger binaries and increased memory usage.   is also known to break several packages. Using   is not recommended. However, it also enables   so that loops in the code get vectorized and will use AVX YMM registers.


 * : optimizes code for size. It activates all  options that do not increase the size of the generated code. It can be useful for machines that have extremely limited disk storage space and/or CPUs with small cache sizes.


 * : In GCC 4.8, a new general optimization level,, has been introduced. It addresses the need for fast compilation and a superior debugging experience while providing a reasonable level of runtime performance. Overall experience for development should be better than the default optimization level  .  Note that   does not imply  , it simply disables optimizations that may interfere with debugging.


 * : New in GCC 4.7, consists of  plus ,  , and  . This option breaks strict standards compliance, and is not recommended for use.

As previously mentioned,  is the recommended optimization level. If package compilation fails and while not using, try rebuilding with that option. As a fallback option, try setting the CFLAGS and CXXFLAGS to a lower optimization level, such as  or even   (for error reporting and checking for possible problems).

-pipe
A common flag is. This flag has no effect on the generated code, but it makes the compilation process faster. It tells the compiler to use pipes instead of temporary files during the different stages of compilation, which uses more memory. On systems with low memory, GCC might get killed. In those cases do not use this flag.

-fomit-frame-pointer
This is a very common flag designed to reduce generated code size. It is turned on at all levels of  (except  ) on architectures where doing so does not interfere with debugging (such as x86-64), but it may need to be activated. In that case add it to the flags. Though the GCC manual does not specify all architectures, it is turned on by using the  option. It's still necessary to explicitly enable the  option, to activate it on x86-32 with GCC up to version 4.6, or when using   on x86-32 with any version of GCC. However, using  will make debugging hard or impossible.

In particular, it makes troubleshooting applications written in Java and compiled by gcj much harder, though Java is not the only code affected by using this flag. So while the flag can help, it also makes debugging harder; backtraces in particular will be useless. When not doing software debugging and no other debugging-related CFLAGS such as  have been used, then try using.

-msse, -msse2, -msse3, -mmmx, -m3dnow
These flags enable the Streaming SIMD Extensions (SSE), SSE2, SSE3, MMX, and 3DNow! instruction sets for x86 and x86-64 architectures. These are useful primarily in multimedia, gaming, and other floating point-intensive computing tasks, though they also contain several other mathematical enhancements. These instruction sets are found in more modern CPUs.

Normally none of these flags need to be added to, as long as the system is using the correct  (for example,   implies  ). Some notable exceptions are newer VIA and AMD64 CPUs that support instructions not implied by  (such as SSE3). For CPUs like these additional flags will need to be enabled where appropriate after checking.

Hardening optimizations
Hardening an otherwise unhardened system, like when using a desktop profile, can be considered a GCC optimization as well, especially in the light of security vulnerabilities such as Meltdown and Spectre.

Some packages feature an individual  USE flag, enabling tested security enhancements (like CFLAGs/CXXFLAGs). It may be a good idea to set this system-wide in.

Overflow protection
Optimizing CFLAGS/CXXFLAGS for overflow protection can be a good idea if security concerns outweigh speed optimization. This may be the case on a daily-use desktop system, while e.g. on an optimized gaming PC it will be considered counterproductive.

ASLR
Address Space Layout Randomization (ASLR) is a state-of-the-art measure to increase security by randomly placing each function and buffer in memory. This makes it harder for attack vectors to succeed.

For full ASLR it is recommended to switch to a hardened profile, as ASLR is only fully applied in combination with PaX and PIE/PIC. See Hardened/Introduction to Hardened Gentoo for further details.

But I get better performance with -funroll-loops -fomg-optimize!
No, people only think they do because someone has convinced them that more flags are better. Aggressive flags will only hurt applications when used system-wide. Even the GCC manual says that using  and   will make code larger and run more slowly. Yet for some reason, these two flags, along with,  ,  , and similar flags, continue to be very popular among ricers who want the biggest bragging rights.

The truth of the matter is that they are dangerously aggressive flags. Take a good look around the Gentoo Forums and Bugzilla to see what those flags do: nothing good!

These flags are not needed globally in CFLAGS or CXXFLAGS. They will only hurt performance. They might bring on the idea of having a high-performance system running on the bleeding edge, but they don't do anything but bloat the code and get bugs marked INVALID or WONTFIX.

Dangerous flags like these are not needed. Don't use them. Stick to the basics:,  , and.

What about -O levels higher than 3?
Some users boast about even better performance obtained by using,  , and so on, but the reality is that   levels higher than 3 have no effect. The compiler may accept CFLAGS like, but it actually doesn't do anything with them. It only performs the optimizations for, nothing more.

Need more proof? Examine the source code:

As can be seen, any value higher than 3 is treated as just.

What about compiling outside the target machine?
Some readers might wonder if compiling outside the target machine with a strictly inferior CPU or GCC sub-architecture will result in inferior optimization results (compared to a native compilation). The answer is simple: No. Regardless of the actual hardware on which the compilation takes place and the CHOST for which GCC was built, as long as the same arguments are used (except for ) and the same version of GCC is used (although minor version might be different), the resulting optimizations are strictly the same.

To exemplify, if Gentoo is installed on a machine whose GCC's CHOST is i686-pc-linux-gnu, and a Distcc server is setup on another computer whose GCC's CHOST is i486-linux-gnu, then there is no need to be afraid that the results would be less optimal because of the strictly inferior sub-architecture of the remote compiler and/or hardware. The result would be as optimized as a native build, as long as the same options are passed to both compilers (and the  parameter doesn't get a   argument). In this particular case the target architecture needs to be specified explicitly as explained in Distcc and -march=native.

The only difference in behavior between two GCC versions built targeting different sub-architectures is the implicit default argument for the  parameter, which is derived from the GCC's CHOST when not explicitly provided in the command line.

What about redundant flags?
Oftentimes CFLAGS and CXXFLAGS that are turned on at various  levels are specified redundantly in. Sometimes this is done out of ignorance, but it is also done to avoid flag filtering or flag replacing.

Flag filtering/replacing is done in many of the ebuilds in the Portage tree. It is usually done because packages fail to compile at certain  levels, or when the source code is too sensitive for any additional flags to be used. The ebuild will either filter out some or all CFLAGS and CXXFLAGS, or it may replace  with a different level.

The Gentoo Developer Manual outlines where and how flag filtering/replacing works.

It's possible to circumvent  filtering by redundantly listing the flags for a certain level, such as , by doing things like:

However, this is not a smart thing to do. CFLAGS are filtered for a reason! When flags are filtered, it means that it is unsafe to build a package with those flags. Clearly, it is not safe to compile the whole system with  if some of the flags turned on by that level will cause problems with certain packages. Therefore, don't try to "outsmart" the developers who maintain those packages. Trust the developers. Flag filtering and replacing is done to ensure stability of the system and application! If an ebuild specifies alternative flags, then don't try to get around it.

Building packages with unacceptable flags will most likely lead to problems. When reporting problems on Bugzilla, the flags that are used in will be readily visible and developers will ask to recompile without those flags. Save the trouble of recompiling by not using redundant flags in the first place! Don't just automatically assume to be more knowledgeable than the developers.

What about LDFLAGS?
The Gentoo developers have already set basic, safe LDFLAGS in the base profiles, so they do not need to be changed.

Can I use per-package flags?
Information on how to use per-package environment variables (including CFLAGS ) is described in the Gentoo Handbook, "Per-Package Environment Variables".

Profile Guided Optimization (PGO)
Profile guided optimization (PGO) consists of compiling and profiling a program to assess hot paths in the code. Optimizations are then applied based on this analysis.

performance may improve via PGO, although it may as much as double the compile times. To enable for :

Firefox also supports PGO although sometimes it may break the build.

Link Time Optimization (LTO)
LTO is still experimental. LTO may need to be disabled before reporting bugs because it is a common source of problems.

To enable for building itself with LTO:

There is a guide available on the Clang page but there is also a popular overlay (gentooLTO) which lists workarounds needed to make LTO build for the whole system and not break binaries during compilation or execution.

External resources
The following resources are of some help in further understanding optimization:


 * GCC online documentation