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

[Jeff Hammond]

On Thu, Feb 18, 2016 at 1:31 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com
> wrote:

> 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.
>
>
Well, technically, I can look up how every implementation implements this
when it isn't supported and do a compile-time check for it, but this is a
truly terrible solution (which I have demonstrated below).  I do not want
an MPI application to compile if it is certain to not work.

#if defined(MPICH)
#if defined(MPI_INTEGER) && MPI_INTEGER=="((MPI_Datatype)MPI_DATATYPE_NULL)"
#error If you want Fortran, get a Fortran!
#endif
#endif

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.
>
>
Indeed.  It makes cross-compilation impossible on systems like Blue Gene
and Cray, which do not allow one to launch an MPI application on the login
node, and likely not the service node.


> 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.
>
>
Exactly.


> > 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
>

This is what I want.


> > (2) mpi.h provides this MPI Fortran datatype as MPI_DATATYPE_NULL
> >
>

I think this is what is implemented today and it is the only reasonable
implementation of what the text says (and doesn't say) right now.


> > 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.
>
>
You are as intelligent as your name suggests :-)


> > 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.
>
>
As I showed earlier, MPICH does 2.  Do I misunderstand you?


> I think you have (1) and (2) backwards in this paragraph.
>
>
I think I am confused now.

Jeff


> --
> Jeff Squyres
> jsquyres at cisco.com
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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