1. install llvm by
$ yum install llvm
2. download llvm-gcc source code from here. I downloaded llvm-gcc4.2-2.5.source.tar.gz. Let's say you downloaded it to /path/to/llvm-gcc
3. now extract the tarball by
$ cd /path/to/llvm-gcc
$ tar -zxvf llvm-gcc-version.tar.gz
4. make temp directories for compilation
$ cd /path/to/llvm-gcc/llvm-gcc4.2-2.5.source
$ mkdir install;mkdir obj
5. now comes the fun part of customizing everything to suit your needs. I did
$ cd obj
$ ../configure --prefix=`pwd`/../install --program-prefix=llvm- --enable-llvm=/usr --enable-languages=c,c++,fortran --disable-multilib
The extras here are fortran, disabled multilib for native x86-64 builds and the prefix directory for installation.
6. Now compile and install
$ make -j2
On your machine, it might be a little different depending on how many cpus you can spare for installation.
7. Finally, install it with
$ make install
Now all your stuff will be installed int the $prefix/install/bin directory. Here, $prefix is the path you used to run ./configure, and not some arbitrary environment variable. You may place symlinks at your preferred places.