Linking Homebrew-compiled openmpi (or mpich2) to Homebrew's gcc - c

Question

I'm new to Homebrew (I usually use Macports, but I'm trying out Homebrew on a 2nd computer), and I wish to install the openmpi (or mpich2) package. Steps are as follows (carried out on Mac OS X Yosemite with Xcode 6 installed):
brew install gcc
brew install openmpi
However, I suspect the linking may have been done incorrectly, due to the following reasons:
The symbolic link for /usr/local/bin/gcc is missing:
$ which gcc
/usr/bin/gcc
$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix
$ which gcc-4.9
/usr/local/bin/gcc-4.9
$ gcc-4.9 --version
gcc-4.9 (Homebrew gcc 4.9.2) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
mpicc may have been linked to the Apple gcc:
$ which mpicc
/usr/local/bin/mpicc
$ mpicc --version
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix
My questions are as follows:
How do I get the /usr/local/bin/gcc symbolic link? Or is Homebrew deliberately avoiding this for certain reasons? e.g. if Homebrew is compiling all its packages using Apple gcc, which is assumed to be in the path, would changing the path to gcc-4.9 mean that Homebrew now compiles its packages with gcc-4.9 instead?
Is the Homebrew-installed Open MPI linked to the Apple gcc (and not the Homebrew gcc)? If yes, is it possible (or advisable) to change the linking?
Alternatively, how necessary is it to fix the linkages? Could I run into certain problems if I choose not to fix it? For example, I'm considering using ln -s to forcibly create the /usr/local/bin/gcc symbolic link. But is this a good idea (*)?
(*) I understand there are likely to be issues when linking object files created by different compilers. So with (1), (2) and (3), I'm hoping to find a solution that avoids combinations whereby I'm creating object files with different compilers (some with gcc-4.9 and some with the Apple gcc).

Answer 1

You need to do the following:
1) Add environment variables for homebrew (you can also add these lines to your ~\.bashrc):
$ export HOMEBREW_CC=gcc-4.9
$ export HOMEBREW_CXX=g++-4.9
2) Rebuild and reinstall openmpi and its dependencies from source
$ brew reinstall openmpi --build-from-source
3) In the end you will get a message like:
==> Reinstalling open-mpi
==> Using Homebrew-provided fortran compiler.
This may be changed by setting the FC environment variable.
==> Downloading http://www.open-mpi.org/software/ompi/v1.8/downloads/openmpi-1.8.
Already downloaded: /Library/Caches/Homebrew/open-mpi-1.8.4.tar.bz2
==> ./configure --prefix=/usr/local/Cellar/open-mpi/1.8.4 --disable-silent-rules
==> make all
==> make check
==> make install
Warning: open-mpi dependency gcc was built with a different C++ standard
library (libstdc++ from clang). This may cause problems at runtime.
🍺 /usr/local/Cellar/open-mpi/1.8.4: 785 files, 23M, built in 41.2 minutes
$ mpicc --showme
gcc-4.9 -I/usr/local/Cellar/open-mpi/1.8.4/include -L/usr/local/opt/libevent/lib -L/usr/local/Cellar/open-mpi/1.8.4/lib -lmpi
On my MacBook I had some conflicts with XCode 6.2, which were solved following this instructions
If you want to install mpich
$ unlink openmpi
$ brew unlink gcc
$ brew install homebrew/versions/gcc5
$ brew install mpich --build-from-source
$ mpicc -v
mpicc for MPICH version 3.2
Using built-in specs.
COLLECT_GCC=gcc-5
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc5/5.4.0/libexec/gcc/x86_64-apple-darwin14.5.0/5.4.0/lto-wrapper
Target: x86_64-apple-darwin14.5.0
Configured with: ../configure --build=x86_64-apple-darwin14.5.0 --prefix=/usr/local/Cellar/gcc5/5.4.0 --libdir=/usr/local/Cellar/gcc5/5.4.0/lib/gcc/5 --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-5 --with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl014 --with-system-zlib --enable-libstdcxx-time=yes --enable-stage1-checking --enable-checking=release --enable-lto --disable-werror --with-pkgversion='Homebrew gcc5 5.4.0' --with-bugurl=https://github.com/Homebrew/homebrew-versions/issues --enable-plugin --disable-nls --enable-multilib
Thread model: posix
gcc version 5.4.0 (Homebrew gcc5 5.4.0)

Answer 2

It would probably be cleaner if you simply modify the configuration file for the Open MPI compiler wrapper and make it invoke gcc-4.9 instead of simply gcc. As I have no idea where exactly Homebrew puts Open MPI and therefore cannot give you the correct path directly, you should search for it:
$ find /usr/local -name mpicc-wrapper-data.txt
Once you have found mpicc-wrapper-data.txt, find the line that starts with compiler= and modify it to read:
compiler=gcc-4.9
You should also modify all other files that match the shell glob mpi*-wrapper-data.txt and modify the compiler=... line accordingly. Use g++-4.9 in mpic++, mpicxx, and mpiCC. Use gfortran-4.9 (if installed) in mpif77 and mpif90 (for Open MPI versions < 1.8) or for mpifort (for versions >= 1.8).
Mixing object code produced by different compilers on the same platform should be fine as long as all of them adhere to the same ABI, which is more or less the case with Clang and GCC. In the end, all programs are linked against the system libraries that come with OS X and those are compiled with Clang only.