[Beowulf] compilers vs mpi?

Prentice Bisbal prentice at ias.edu
Tue Jul 20 14:05:51 EDT 2010

Mark Hahn wrote:
> Hi all,
> I'm interested in hearing about experiences with mixing compilers
> between the application and MPI.  that is, I would like to be able
> to compile MPI (say, OpenMPI) with gcc, and expect it to work correctly
> with apps compiled with other compilers.  I guess I'm reasoning by
> analogy to normal distro libs.
> the OpenMPI FAQ has this comment:
> NOTE: The Open MPI team recommends using a single compiler suite whenever
> possible. Unexpeced or undefined behavior can occur when you mix compiler
> suites in unsupported ways (e.g., mixing Fortran 77 and Fortran 90
> compilers
> between different compiler suites is almost guaranteed not to work).
> and there are complaints elsewhere in the FAQ about f90 bindings.  I'd
> appreciate it if someone could help a humble C/C++/perl hacker
> understand the issues here...
> thanks, mark hahn.
> PS: we have a large and diverse user base, so tend to have to support
> gcc, intel, pathscale and pgi.  we even have people who want to use
> intel's damned synthetic 128b FP over MPI :(


I'm not a developer, but I do spend a lot of my time compiling codes
(mostly C and Fortran) for users, and I've dealt with the problem plent
of times.

Here's the library compatibility problem as I understand it:

The C programming language standard defines the syntax for symbol names
in libraries, so if you have a function named printf, when compiled into
a library, the symbol for it will always be printf.

>From what I've heard, For C++ the standard isn't as strict as C, but
usually doesn't cause any problems. I rarely compile C++ code, so I have
no real experience with this.

For Fortran, the standard doesn't define a standard naming convention
for library symbols, so the compilers have more freedom with symbol
naming. Usually, the symbol names will have 0, 1, or two underscores
before/after the symbol name, and it can be in all caps or all lower case.

This is why gfortran has these options:


In theory, these options should work, but they only really work if your
using one compiler to link against libraries compiled by only one other
compilers (linking with gfortran to ifort-compiled libraries, for
example). Once you have libraries from two different compilers, the
libraries from one compiler might need --fno_underscoring, while the
other needs -fcase-lower and fsecond-underscore.

I highly recommend you read the man pages for g77 and/or gfortran where
these switches are explained. There is a much better explanation of why
they're needed there.

>From my experience, it's easier to just compile the libraries again
using a different compiler suite, and put them in a separate location to
make it clear.  For example, I have compiled Open MPI with GNU, Intel,
and PGI compilers,

$ pwd

$ ls -ld *
lrwxrwxrwx 1 root root    9 Jul 17  2009 gcc -> gcc-4.1.2
drwxr-xr-x 3 root root 4096 Feb 10  2009 gcc-4.1.2
lrwxrwxrwx 1 root root    8 Jul 17  2009 intel -> intel-11
drwxr-xr-x 3 root root 4096 Feb  5  2009 intel-11
lrwxrwxrwx 1 root root    7 Jul 17  2009 pgi -> pgi-8.0
drwxr-xr-x 3 root root 4096 Jan 28  2009 pgi-8.0

As long as users specify the correct paths to include and library files
in their compiler commands, they can compile using whatever compiler
they want. To save work, I only do this for libraries that I absolutely
know that users will be using with Fortran.

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