Red Hat ENTERPRISE LINUX 3 - DEVELOPER TOOLS GUIDE Guide de l'utilisateur télécharger pdf (Page 56)

Chapter 4. Compiling and Building

4.1.6.4. Recommended Optimization Options

Different projects require different optimization options. There is no one-size-fits-all approach when it

comes to optimization, but here are a few guidelines to keep in mind.

Instruction selection and tuning

It is very important to chose the correct architecture for instruction scheduling. By default GCC

produces code is optimized for the most common processors, but if the CPU on which your code will

run is known, the corresponding -mtune= option to optimize the instruction scheduling, and -march=

option to optimize the instruction selection should be used.

The option -mtune= optimizes instruction scheduling to fit your architecture by tuning everything

except the ABI and the available instruction set. This option will not chose particular instructions,

but instead will tune your program in such a way that executing on a particular architecture will be

optimized. For example, if an Intel Core2 CPU will predominantly be used, choose -march=core2.

If the wrong choice is made, the program will still run, but not optimally on the given architecture. The

architecture on which the program will most likely run should always be chosen.

The option -march= optimizes instruction selection. As such, it is important to choose correctly as

choosing incorrectly will cause your program to fail. This option selects the instruction set used when

generating code. For example, if the program will be run on an AMD K8 core based CPU, choose -

march=k8. Specifying the architecture with this option will imply -mtune=.

The -mtune= and -march= commands should only be used for tuning and selecting instructions

within a given architecture, not to generate code for a different architecture (also known as cross-

compiling). For example, this is not to be used to generate PowerPC code from an Intel 64 and

AMD64 platform.

For a complete list of the available options for both -march= and -mtune=, refer to the GCC

documention available here: GCC 4.4.4 Manual: Hardware Models and Configurations

General purpose optimization flags

The compiler flag -O2 is a good middle of the road option to generate fast code. It produces the best

optimized code when the resulting code size is not large. Use this when unsure what would best suit.

When code size is not an issue, -O3 is preferrable. This option produces code that is slightly larger

but runs faster because of a more frequent inline of functions. This is ideal for floating point intensive

code.

The other general purpose optimization flag is -Os. This flag also optimizes for size, and produces

faster code in situations where a smaller footprint will increase code locality, thereby reducing cache

misses.

Use -frecord-gcc-switches when compiling objects. This records the options used to build

objects into objects themselves. After an object is built, it determines which set of options were used

to build it. The set of options are then recorded in a section called .GCC.command.line within the

object and can be examined with the following:

$ gcc -frecord-gcc-switches -O3 -Wall hello.c -o hello

$ readelf --string-dump=.GCC.command.line hello

http://gcc.gnu.org/onlinedocs/gcc-4.4.4/gcc/Submodel-Options.html#Submodel-Options

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Red Hat ENTERPRISE LINUX 3 - DEVELOPER TOOLS GUIDE Guide de l'utilisateur Page 56