===== Gnu Compiler Collection =====

Here are some hints about the [[http://gcc.gnu.org|Gcc]] compiler. The main purpose of this page is to help my bad memory.


==== Installation ====
Gcc manages sources throught a SVN repository, located at [[svn://gcc.gnu.org/svn/gcc]]. There exists several development branches, the main, as usual, is located in the ''trunk'' branch.

To get your copy of the latest Gcc sources, performs a checkout:

  svn checkout svn://gcc.gnu.org/svn/gcc/trunk gcc-trunk

The compiler is installed using standard Gnu tools:

  ./configure
  make
  make install

Here are a list of usefull configure options:

  * ''--prefix'' installation prefix
  * ''--enable-languages=lang[,lang]*'' compilers/runtimes to generates
  * ''--disable-nls'' disable Native Language Support

This example configures Gcc to be installed in the ''$MY_GCC_ROOT'' directory. The compiler will be built with C and Fortran support, while Native Language Support is disabled.

  ./configure --prefix=$MY_GCC_ROOT \
              --enable-languages=c,fortran \
              --disable-nls

After configuration, the compiler building process is started with the ''make'' command. If you have enought computational power, a parallel build can be performed:

  make -j 17

On many cases the number of parallel workers can be set to twice the number of cpus/cores plus one.

On a two-cpu, quad-core Intel Xeon 2 Ghz the compilation process took approximatly 20 minutes.

At last, the compiler is installed throught the ''make install'' command.




==== Source Tree Layout ====
The source tree contains both compiler and runtime sources.

By compiler we intend the various front-ends, optization passes and back-ends. They can be found in the ''gcc'' directory.

In the other hand, directories starting with the ''lib'' prefix seem contain libraries helping gcc providing translation and runtime services.

The following table summarize some of them:

^  Directory  ^  Contains  ^
|libcpp       |the C pre-processor                                 |
|libgcc       |functionalities that cannot be covered with assembly|
|libgomp      |the Gnu implementation of the Omp runtime           |

=== libgcc ===
Some functionalities, for example a call to ''memcmp'', may or may be not handled through direct assembly generation. For example, in an architecute there exists a ''MEMCMP'' instructions, allowing to map the call to ''memcp'' to this instruction. On other architectures, in order to provide an implementation of ''memcmp'', the compiler emit a call to a custom version of ''memcmp'', provided by itself.

The implementation of these routines are provided through the libgcc library.

=== libgomp ===
Contains the implementation of the Omp runtime. The main interface, knonw as Gnu Omp, is exposed through the ''libgomp.h'' file.

In the root directory, it is possible to find all architecture-indipendent code. The sources of a specific architecture, like locks or barriers low-level implementations, are stored in the ''config'' subdirectory.

For example the ''barrier.c'' file implements the ''GOMP_barrier'' functions, who uses ''gmp_team_barrier_wait'' to wait other threads at a barrier. This function is implemented many times, one for each architecture.

There can be multiple level of indirections. Indeed the ''linux'' architecture contains processor-specific code, that is isolated in a private directory (e.g ''linux/x86'').