GCC optimization/es

Esta guía ofrece una introducción al código compilado de forma óptima usando CFLAGS y CXXFLAGS seguras y sanas. También describe la teoría detrás de la optimización en general.

¿Qué son CFLAGS y CXXFLAGS?
CFLAGS y CXXFLAGS son variables de entorno usadas para indicar a la Colección de Compiladores de GNU,, qué tipo de parámetros usar cuando compila código fuente. Las CFLAGS se aplican al código escrito en C, mientras que CXXFLAGS son para código escrito en C++.

Pueden usarse para disminuir la cantidad de mensajes de depuración de un programa, aumentar los niveles de aviso de errores, y por supuesto, optimizar el código producido. El manualde GCC ofrece una lista completa opciones disponibles y sus aplicaciones.

¿Cómo se utilizan?
CFLAGS y CXXFLAGS se pueden usar de dos formas. La primera, por programa con los ficheros Makefile generados por automake.

Sin embargo, esto no debería hacerse cuando instalamos paquetes que se encuentran en el árbol Portage. En su lugar, establezca sus CFLAGS y CXXFLAGS en. De esta manera todos los paquetes se compilarán con las opciones que especifique.

CFLAGS en /etc/portage/make.conf

Como puede ver, CXXFLAGS se establece para usar todas las opciones presentes en CFLAGS. Casi seguro que es lo que se desea. Normalmente no necesitará especificar opciones adicionales en CXXFLAGS.

Confusiones
Aunque CFLAGS y CXXFLAGS pueden ser muy efectivos tomando el código fuente para producir binarios pequeños o rápidos, también pueden deteriorar la función de su código, inflar su tamaño, ralentizar su ejecución, ¡O incluso causar errores de compilación!

CFLAGS no es una solución mágica; no hará que su sistema corra más rápido o sus binarios sean más pequeños automáticamente. Añadir más y más parámetros en un intento de optimización (o "apretar") su sistema es una receta segura para fracasar. Hay un punto en el cual alcanzará resultados de peor calidad.

A pesar de las recomendaciones que se pueden encontrar en Internet, unas variables CFLAGS y CXXFLAGS agresivas están más cerca de dañar sus programas que de hacerles algún bien. Recuerde que la razón para la cual existen los parámetros en primer lugar es porque están diseñadas para usarse en sitios específicos para propósitos específicos. ¡Solo porque una CFLAG particular sea buena para un fragmento de código no significa que esté diseñada para compilar todo lo que quiera instalar en su máquina!

¿Preparado?
Ahora que le hemos advertido de algunos de los riesgos involucrados, echemos un vistazo a algunas optimizaciones sanas y seguras para su computadora. Esto le será útil y los desarrolladores lo agradecerán la próxima vez que informe de un problema en Bugzilla. (Los desarrolladores suelen pedir que recompile un paquete con los CFLAGS mínimos para ver si el problema persiste. Recuerde que los parámetros agresivos pueden arruinar el código.)

Conceptos básicos
El objetivo de usar CFLAGS y CXXFLAGS es crear código específico para su sistema; debería funcionar perfectamente y ser ligero y rápido, si es posible. Algunas veces estás condiciones son mutuamente excluyentes, pero nosotros trabajaremos con combinaciones que sabemos que funcionan bien. Idealmente, las mejores están disponibles para cada arquitectura de CPU. Mencionaremos más adelante ajustes más agresivos para que se sepa con cuales debe tener cuidado. No discutiremos cada opción listada en el manual de  (hay cientos), pero hablaremos de las básicas, las más comunes.

-march
La primera y más importante opción es. Esta le dice al compilador que código debería producir para su arquitectura de procesador (o arch); dice que debería producir código para un cierto tipo de CPU. Diferentes CPUs tienen diferentes características, soportan diferentes conjunto de instrucciones y tienen diferentes formas de ejecutar código. La opción  indicará al compilador que produzca código específico para su CPU, tomando en cuenta todas sus capacidades, características, conjuntos de instrucciones, caprichos y demás.

A pesar que la variable CHOST en especifica la arquitectura general utilizada,   también se usa para que sus programas sean optimizados para su procesador específico. Las arquitecturas x86 y x86-64 (entre otras) también deberían utilizar la opción.

¿Qué tipo de CPU tiene? Para averiguarlo, ejecute la siguiente orden:

Ahora veamos a  en acción. Este ejemplo es para un antiguo Pentium III:

/etc/portage/make.conf: Pentium III

Aquí hay otro para una CPU AMD de 64 bits:

/etc/portage/make.conf: AMD64

Si todavía no está seguro qué tipo de CPU tiene, tal vez quiera usar la opción. Al usarla, GCC detectará el procesador y automáticamente usará las opciones apropiadas. ¡Sin embargo, no use esta opción si la intención es ¡compilar paquetes para un CPU diferente!

So if you're compiling packages on one computer, but intend 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 AMD Athlon 64 CPU will not be able to run on an old VIA C3 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, '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 all its available instruction sets and the correct ABI; it will have no backwards compatibility for older/different CPUs. If you don't need to execute code on anything other than the system you're running Gentoo on, continue to use. You should only consider using  when you need to generate code for older CPUs such as i386 and i486. produces more generic code than ; though it will tune code for a certain CPU, it doesn't take into account available instruction sets and ABI. Don't use  on x86 or x86-64 systems, as it is deprecated for those arches.

Only non-x86/x86-64 CPUs (such as 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, 's behavior and flag naming just isn't consistent across architectures, so be sure to check the   manual to determine which one you should use for your system.

-O
Next up is the  variable. This controls the overall level of optimization. This makes the code compilation take somewhat more time, and can take up much more memory, especially as you increase the level of optimization.

There are seven  settings: ,  ,  ,  ,  ,  , and. You should use only 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's examine each optimization level:


 * : This level (that's 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.


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


 * : A step up from . This is the recommended level of optimization unless you have 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.


 * : This is 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.  Therefore, using   is not recommended.


 * : This option will optimize your code for size. It activates all  options that don't increase the size of the generated code. It can be useful for machines that have extremely limited disk storage space and/or have 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 you aren't using, try rebuilding with that option. As a fallback option, try setting your 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 actually 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 that case, 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 you may need to activate it yourself by adding it to your flags. Though the  manual does not specify all architectures it is turned on by using , you will need to explicitly activate it on x86. However, using this flag will make debugging hard to 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. However, if you don't plan to do much software debugging and haven't added any other debugging-related CFLAGS such as, then you can try using.

-msse, -msse2, -msse3, -mmmx, -m3dnow
These flags enable the 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.

You normally don't need to add any of these flags to as long as you are 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 you'll need to enable additional flags where appropriate after checking the output of.

But I get better performance with -funroll-loops -fomg-optimize!
No, you only think you do because someone has convinced you that more flags are better. Aggressive flags will only hurt your applications when used system-wide. Even the  manual says that using   and   makes 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!

You don't need to use those flags globally in CFLAGS or CXXFLAGS. They will only hurt performance. They may make you sound like you have a high-performance system running on the bleeding edge, but they don't do anything but bloat your code and get your bugs marked INVALID or WONTFIX.

You don't need dangerous flags like these. 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:

-O source code

As you can see, any value higher than 3 is treated as just.

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:

Specifying redundant CFLAGS

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 your whole system with  if some of the flags turned on by that level will cause problems with certain packages. Therefore, you shouldn't try to "outsmart" the developers who maintain those packages. Trust the developers. Flag filtering and replacing is done for your benefit! If an ebuild specifies alternative flags, then don't try to get around it.

You will most likely continue to run into problems when you build a package with unacceptable flags. When you report your troubles on Bugzilla, the flags you use in will be readily visible and you will be told to recompile without those flags. Save yourself the trouble of recompiling by not using redundant flags in the first place! Don't just automatically assume that you know better than the developers.

What about LDFLAGS?
The Gentoo developers have already set basic, safe LDFLAGS in the base profiles, so you don't need to change them.

Can I use per-package flags?
Puede encontrarse información acerca de como utilizar las variables de entorno por paquete (incluyendo CFLAGS) en el Manual de Gentoo, "Variables de entorno por paquete".

Recursos
Los siguientes recursos pueden ser de ayuda para comprender la optimización:


 * La documentación en línea de GCC


 * El capítulo 5 de los manuales de instalación de Gentoo




 * Wikipedia


 * Los Foros de Gentoo

Agradecimientos
Nos gustaría dar las gracias a los siguientes autores y editores por sus contribuciones a esta guía:


 * nightmorph