Building the LLVM GCC Front-End

This section describes how to acquire and build llvm-gcc 4.0 and 4.2, which are based on the GCC 4.0.1/4.2.1 front-ends respectively. Both front-ends support C, C++, Objective-C and Objective-C++. The 4.2 front-end also supports Ada and Fortran to some extent. Note that the instructions for building these front-ends are completely different (and much easier!) than those for building llvm-gcc3 in the past.

Retrieve the appropriate llvm-gcc4.x-y.z.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 4.0 code for first time use:
```
svn co http://llvm.org/svn/llvm-project/llvm-gcc-4.0/trunk dst-directory
```
To check out the 4.2 code 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.
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:

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.
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. What is more, the Ada compiler must not be more recent than what it is trying to compile, otherwise the build will fail. This rules out gcc-4.3 (but not gcc-4.2) and also the 2007 GNAT GPL Edition. 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 2006 GNAT GPL Edition) and another which supports C++, see below.
Because the Ada front-end is experimental, it is wise to build the compiler with checking enabled. This causes it to run slower, but helps catch mistakes in the compiler (please report any problems using LLVM bugzilla).

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

Download the LLVM source and unpack it:

wget http://llvm.org/releases/2.2/llvm-2.2.tar.gz
tar xzf llvm-2.2.tar.gz
mv llvm-2.2 llvm

or check out the latest version from subversion:

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

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

wget http://llvm.org/releases/2.2/llvm-gcc4.2-2.2.source.tar.gz
tar xzf llvm-gcc4.2-2.2.source.tar.gz
mv llvm-gcc4.2-2.2.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

Make a build directory llvm-objects for llvm and make it the current directory:
```
mkdir llvm-objects
cd llvm-objects
```
Configure LLVM (here it is configured to install into /usr/local):
```
../llvm/configure --prefix=/usr/local
```
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
```
Build LLVM with checking enabled (use ENABLE_OPTIMIZED=1 to build without checking):
```
make ENABLE_OPTIMIZED=0
```
Install LLVM (optional):
```
make install
```
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
```

Configure llvm-gcc (here it is configured to install into /usr/local). The --enable-checking flag turns on sanity checks inside the compiler. If you omit it then LLVM must be built with make ENABLE_OPTIMIZED=1. 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-shared --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-shared --disable-bootstrap --disable-multilib

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.