GCC optimization/pt-br

Este guia fornece uma introdução à otimização de códigos usando CFLAGS e CXXFLAGS seguras e sensatas. Também descreve a teoria por trás da otimização em geral.

O que são CFLAGS e 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.

Elas podem ser usadas para diminuir a quantidade de mensagens de debug para um programa, aumentar os níveis de alertas de erro e, é claro, otimizar o código produzido. O Manual do GCC mantém uma lista completa das opções disponíveis e seus propósitos.

Como elas são usadas?
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. Most 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.

Equívocos
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.

Pronto?
Estando ciente dos riscos envolvidos, dê uma olhada em algumas otimizações seguras e sensatas. Estas serão mantidas em um bom lugar e serão agradáveis para os desenvolvedores da próxima vez que um problema for relatado no Bugzilla. (Os desenvolvedores irão, normalmente, solicitar aos usuários para recompilar um pacote com o mínimo de CFLAGS para ver se o problema persiste. Lembre-se: flags agressivas podem arruinar o código!)

O básico
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
A primeira e mais importante opção é. Isto informa ao compilador que código deve produzir para a arquitetura do processador (ou "arch"); ele informa ao GCC que deverá produzir código para um determinado tipo de CPU. Diferentes CPUs tem diferentes capacidades, suporta diferentes conjuntos de instruções e tem maneiras diferentes de execução de código. A flag  irá instruir o compilador para produzir um código específico para a CPU do sistema, com todas as suas capacidades, recursos, conjunto de instruções, particularidades e assim por diante.

Mesmo que a variável  no  especifique a arquitetura geral usada,   ainda deve ser usado de modo que os programas possam ser otimizados para o processador específico do sistema. CPUs x86 e x86-64 (entre outras) devem utilizar a flag.

Que tipo de CPU o sistema tem? Para encontrar, execute o seguinte comando:

Para obter maiores detalhes, incluindo os valores  e , dois comandos podem ser usados.


 * O primeiro comando informa ao compilador não fazer nenhuma ligação e, ao invés de interpretar a opção   para esclarecer as opções da linha de comando,  ele agora irá mostrar se certas opções estão habilitadas ou desabilitadas . Neste caso, as opções mostradas são aquelas habilitadas para o alvo selecionado:


 * O segundo comando irá mostrar as diretivas do compilador para construir o arquivo cabeçalho, mas sem realmente executar os passos e ao invés disto mostrá-los na tela . A saída final é o comando que contém todas as opções de otimização e seleção de arquitetura:

Agora vamos ver  em ação. Este exemplo é para um antigo chip Pentium III:

Aqui está outro para uma CPU AMD 64-bit:

Se o tipo da CPU é indeterminado, ou se o usuário não sabe qual configuração escolher, é possível usar a configuração. Quando esta flag é usada, o GCC tentará detectar o processador e automaticamente definir as flags apropriadas para ele. Entretanto, isto não deve ser usado quando se pretende compilar pacotes para diferentes CPUs!

Se compilar pacotes em um computador a fim de executá-los em um computador diferente (tal como quando se utiliza um computador rápido para construir para uma máquina antiga, lenta), então "não" use. "Native" significa que o código produzido executará "apenas" naquele tipo de CPU. As aplicações construídas com  em um AMD Athlon 64 CPU não serão capazes de executar em uma antiga CPU VIA C3.

Também estão disponíveis as flags  e. Estas flags são normalmente usadas apenas quando não há a opção  disponível; certas arquiteturas de processadores podem exigir   ou até mesmo. Infelizmente o comportamento do GCC não é muito consistente com a forma como cada flag se comporta de uma arquitetura para outra.

Em CPUs x86 e x86-64,  irá gerar um código especificamente para esta CPU usando seu conjunto de instruções disponíveis e a correta ABI; ele não terá compatibilidade com versões anteriores para antigas/diferentes CPUs. Considere usar  quando gerar códigos para antigas CPUs tais como i386 e i486. produz um código mais genérico do que ; mas isto vai ajustar o código para uma determinada CPU, ele não leva em conta os conjuntos de instruções disponíveis e ABI. Não use  em sistemas x86 e x86-64, de forma que ele é obsoleto para estas arquiteturas.

Apenas em CPUs não x86/x86-64 (como Sparc, Alpha e PowerPC) pode exigir  ou   ao invés de. Nestas arquiteturas,  /   às vezes se comportam como   (em x86/x86-64), com um nome de flag diferente. Mais uma vez, o comportamento do GCC e nomeação de flag não é consistente em todas as arquiteturas, por isso não deixe de verificar o manual do GCC para determinar qual deve ser usado.

-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.


 * : 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.


 * : 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 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 Extentions (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.

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".

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


 * The GCC online documentation


 * Gentoo Handbook - Configuring compile options


 * man make.conf


 * Wikipedia


 * The Gentoo Forums