[mpiwg-tools] Mini example of single tool working with multiple MPI implementations

[Matthew LeGendre]

At the end of the call, Jeff asked about a simple example that illustrates 
building a tool with QMPI that works with multiple MPI implementations. 
I've attached a tarball that illustrates what I was picturing.

This example builds a pair of libraries, libfakeopenmpi.so and 
libfakempich.so, which each export "mpisend" functions with different 
signatures as well as a pair of fake apps, app_openmpi and app_mpich, that 
use each library.  It also builds a single libtool.so, which contains 
wrapper functions for each mpisend and dynamically chooses which one is 
used.

Note that the tool library still has files built against each MPI 
implementation, but produces a single library that works with either 
implementation.

On the phone I speculated that you'd need the callback registration 
functions to be ABI compatible across all MPI implementations, but this 
didn't turn up being necessary.  You can get away with only the MPI 
identification function having a standard ABI.  In this example I called 
the function get_mpi_implementation().

It also turned up being very convenient if the MPI header exported the 
return value of get_mpi_implementation() though a #define (which I called 
MPI_IMPL).  This allowed for an easy runtime test of whether the a tool 
implementation was compiled for the running MPI.

Also, On the phone I speculated that getting per-compiler tools would be 
difficult.  You actually could solve part of the problem by including a 
compiler name and version in the MPI_IMPL #define--most compilers have 
preprocessor macros that exports that info.  Though this still wouldn't 
solve any tool incompatibilities with runtime libraries like libstdc++.so.

-Matt


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