This section describes how to acquire and build llvm-gcc 4.2, which is based on the GCC 4.2.1 front-end. Supported languages are Ada, C, C++, Fortran, Objective-C and Objective-C++. Note that the instructions for building these front-ends are completely different (and much easier!) than those for building llvm-gcc3 in the past.

1. Retrieve the appropriate llvm-gcc-4.2-version.source.tar.gz archive from the LLVM web site.

   It is also possible to download the sources of the llvm-gcc front end from a read-only mirror using subversion. To check out the 4.2 code for first time use:

   svn co http://llvm.org/svn/llvm-project/llvm-gcc-4.2/trunk dst-directory
   

   After that, the code can be be updated in the destination directory using:

   svn update

   The mirror is brought up to date every evening.

2. Follow the directions in the top-level README.LLVM file for up-to-date instructions on how to build llvm-gcc. See below for building with support for Ada or Fortran.

Building with support for Ada amounts to following the directions in the top-level README.LLVM file, adding ",ada" to EXTRALANGS, for example: EXTRALANGS=,ada

There are some complications however:

1. The only platform for which the Ada front-end is known to build is 32 bit intel x86 running linux. It is unlikely to build for other systems without some work.

2. The build requires having a compiler that supports Ada, C and C++. The Ada front-end is written in Ada so an Ada compiler is needed to build it. Compilers known to work with the LLVM 2.5 release are gcc-4.2 and the 2005, 2006 and 2007 versions of the GNAT GPL Edition. GNAT GPL 2008, gcc-4.3 and later will not work. The LLVM parts of llvm-gcc are written in C++ so a C++ compiler is needed to build them. The rest of gcc is written in C. Some linux distributions provide a version of gcc that supports all three languages (the Ada part often comes as an add-on package to the rest of gcc). Otherwise it is possible to combine two versions of gcc, one that supports Ada and C (such as the 2007 GNAT GPL Edition) and another which supports C++, see below.

3. Because the Ada front-end is experimental, it is wise to build the compiler with checking enabled. This causes it to run much slower, but helps catch mistakes in the compiler (please report any problems using LLVM bugzilla).

4. The Ada front-end fails to bootstrap, due to lack of LLVM support for setjmp/longjmp style exception handling (used internally by the compiler), so you must specify --disable-bootstrap.

Supposing appropriate compilers are available, llvm-gcc with Ada support can be built on an x86-32 linux box using the following recipe:

1. Download the LLVM source and unpack it:

   wget http://llvm.org/releases/2.5/llvm-2.5.tar.gz
   tar xzf llvm-2.5.tar.gz
   mv llvm-2.5 llvm
   

   or check out the latest version from subversion:

   svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm

2. Download the llvm-gcc-4.2 source and unpack it:

   wget http://llvm.org/releases/2.5/llvm-gcc-4.2-2.5.source.tar.gz
   tar xzf llvm-gcc-4.2-2.5.source.tar.gz
   mv llvm-gcc4.2-2.5.source llvm-gcc-4.2
   

   or check out the latest version from subversion:

   svn co http://llvm.org/svn/llvm-project/llvm-gcc-4.2/trunk llvm-gcc-4.2
   

3. Make a build directory llvm-objects for llvm and make it the current directory:

   mkdir llvm-objects
   cd llvm-objects
   

4. Configure LLVM (here it is configured to install into /usr/local):

   ../llvm/configure --prefix=/usr/local --enable-optimized --enable-assertions
   

   If you have a multi-compiler setup and the C++ compiler is not the default, then you can configure like this:

   CXX=PATH_TO_C++_COMPILER ../llvm/configure --prefix=/usr/local --enable-optimized --enable-assertions
   

   To compile without checking (not recommended), replace --enable-assertions with --disable-assertions.

5. Build LLVM:

   make
   

6. Install LLVM (optional):

   make install
   

7. Make a build directory llvm-gcc-4.2-objects for llvm-gcc and make it the current directory:

   cd ..
   mkdir llvm-gcc-4.2-objects
   cd llvm-gcc-4.2-objects
   

8. Configure llvm-gcc (here it is configured to install into /usr/local). The --enable-checking flag turns on sanity checks inside the compiler. To turn off these checks (not recommended), replace --enable-checking with --disable-checking. Additional languages can be appended to the --enable-languages switch, for example --enable-languages=ada,c,c++.

   ../llvm-gcc-4.2/configure --prefix=/usr/local --enable-languages=ada,c \
                             --enable-checking --enable-llvm=$PWD/../llvm-objects \
   			  --disable-bootstrap --disable-multilib
   

   If you have a multi-compiler setup, then you can configure like this:

   export CC=PATH_TO_C_AND_ADA_COMPILER
   export CXX=PATH_TO_C++_COMPILER
   ../llvm-gcc-4.2/configure --prefix=/usr/local --enable-languages=ada,c \
                             --enable-checking --enable-llvm=$PWD/../llvm-objects \
   			  --disable-bootstrap --disable-multilib
   

9. Build and install the compiler:

   make
   make install
   

To build with support for Fortran, follow the directions in the top-level README.LLVM file, adding ",fortran" to EXTRALANGS, for example:

EXTRALANGS=,fortran

The LLVM GCC frontend is licensed to you under the GNU General Public License and the GNU Lesser General Public License. Please see the files COPYING and COPYING.LIB for more details.

More information is available in the FAQ.