Red Hat ENTERPRISE LINUX 3 - DEVELOPER TOOLS GUIDE Guide de l'utilisateur télécharger pdf (Page 58)

Chapter 4. Compiling and Building

3. Recompile and optimize source.c with profiling information gathered in step one:

gcc source.c -fprofile-use -O2 -o executable

Multiple data collection runs, as seen in step two, will accumulate data into the profiling file instead

of replacing it. This allows the executable in step two to be run multiple times with additional

representative data in order to collect even more information.

The executable must run with representative levels of both the machine being used and a respective

data set large enough for the input needed. This ensures optimal results are achieved.

By default, GCC will generate the profile data into the directory where step one was performed. To

generate this information elsewhere, compile with -fprofile-dir=DIR where DIR is the preferred

output directory.

Warning

The format of the compiler feedback data file changes between compiler versions. It is imperative

that the program compilation is repeated with every new version of the compiler.

4.1.6.6. Using 32-bit compilers on a 64-bit host

On a 64-bit host, GCC will build executables that can only run on 64-bit hosts. However, GCC can be

used to build executables that will run both on 64-bit hosts and on 32-bit hosts.

To build 32-bit binaries on a 64-bit host, first install 32-bit versions of any supporting libraries the

executable may need. This must at least include supporting libraries for glibc and li/jointfilesconvert/280529/bgcc, and

possibly for libstdc++ if the program is a C++ program. On Intel 64 and AMD64, this can be done

with:

yum install glibc-devel.i686 li/jointfilesconvert/280529/bgcc.i686 libstdc++-devel.i686

There may be cases where it is useful to to install additional 32-bit libraries that a program may need.

For example, if a program uses the db4-devel libraries to build, the 32-bit version of these libraries

can be installed with:

yum install db4-devel.i686

Note

The .i686 suffix on the x86 platform (as opposed to x86-64) specifies a 32-bit version of the

given package. For PowerPC architectures, the suffix is ppc (as opposed to ppc64).

After the 32-bit libraries have been installed, the -m32 option can be passed to the compiler and

linker to produce 32-bit executables. Provided the supporting 32-bit libraries are installed on teh 64-bit

system, this executable will be able to run on both 32-bit systems and 64-bit systems.

Procedure 4.6. Compiling a 32-bit Program on a 64-bit Host

1. On a 64-bit system, compile hello.c into a 64-bit executable with:

gcc hello.c -o hello64

2. Ensure that the resulting executable is a 64-bit binary:

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Red Hat ENTERPRISE LINUX 3 - DEVELOPER TOOLS GUIDE Guide de l'utilisateur Page 58