[MPIWG Fortran] Provide Fortran datatypes if Fortran bindings not provided?

[Jeff Squyres (jsquyres)]

On Feb 18, 2016, at 3:55 PM, Rolf Rabenseifner <rabenseifner at hlrs.de> wrote:
> 
> I would prefer a solution where an application or middlware can check at
> runtime whether the Fortran MPI datatypes exist, i.e., I like mpich's
> solution. It is more simple than all methods that People must check prior
> to compile and must remove or Change parts of there software.

The reason that JeffH and Jed hate this solution is because it makes it quite difficult for an application to know whether the MPI has Fortran support or not -- they basically have to compile an MPI application *and then run it* to see if the datatypes are actually MPI_DATATYPE_NULL.

Doing this inside a GNU Autoconf/configure script is very, very difficult because of the huge differences in how you launch MPI applications across different HPC environments.

Their (strong) preference is that you can have a simple compilation test in the GNU Autoconf/configure script that just tries to compile something like the following:

-----
#include <mpi.h>
MPI_Datatype foo = MPI_INTEGER;
-----

If that compiles, then your MPI has Fortran support.
If it doesn't compile, your MPI doesn't have Fortran support.

> In the moment,in the case of a missing accompanying Fortran Compiler,
> an application or middleware that wants to handle
> MPI Fortran datatypes must handle both cases
> (1) mpi.h does not provide a specific MPI Fortran datatype
> (2) mpi.h provides this MPI Fortran datatype as MPI_DATATYPE_NULL
> 
> The additional case, that it is mapped to a real datatype is not
> an exception, because it defines some sort of accompanying Fortran Compiler.
> 
> I would prefer that the MPI standard defines, which of the two
> options should be implemented.

I think JeffH and Jed want (1).

I think I want (1), too.

> As said at the beginning, I would prefer (1), 
> - as mpich is doing it in general, and
> - OMPI is doing it partially,  
> whereas (2) was not done by mpich nor by OMPI.

I think you have (1) and (2) backwards in this paragraph.

-- 
Jeff Squyres
jsquyres at cisco.com
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