[Mpi-forum] Big Fortran hole in MPI-4.0 Embiggening

[HOLMES Daniel]

Hi Rolf,

This is a (somewhat contrived, arguably) reason for taking another tiny step towards removing the “mpif.h” method of Fortran support and pushing users and implementations towards preferring the Fortran 2008 interface, which is a direction of travel that I fully support.

I think this might be seen as quite a big change for implementers, especially if it were to occur between the RCM and FRM.

Cheers,
Dan.
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On 10 Jan 2021, at 12:22, Rolf Rabenseifner via mpi-forum <mpi-forum at lists.mpi-forum.org<mailto:mpi-forum at lists.mpi-forum.org>> wrote:

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Dear MPI-Forum members,

MPI-3.1 and MPI-4.0 includes the following rule on top of the 2nd page
of the Language Binding chapter:

| MPI implementations providing a Fortran interface must provide
| one or both of the following:
| - The USE mpi_f08 Fortran support method.
| - The USE mpi and INCLUDE 'mpif.h' Fortran support methods.

The embiggening was included only in the C and mpi_f08 bindings.

Most implementors nowadays provide all three MPI Fortran support methods,
i.e., the mpi_f08 and the mpi module and mpif.h.

For all MPI-3.1 libraries that provide all three MPI Fortran support methods,
the most easiest and fastest way to provide MPI-4.0 for C and Fortran is,
- to implement the embiggening for C,
- and to remove the mpi_f08 module,
- and maybe months or years later, to provide mpi_f08 again, now embiggened.

This implementation path (by removing mpi_f08 from MPI-3.1 and to name
it MPI-4.0 without Fortran embiggening) was of course never intended,
when we decided to require the embiggened routines only for mpi_f08.
The goal was
- to not require additional work from the implementors for the old mpi module
- convince the users that it is a good idea to make a transition
  to mpi_f08.

The most simplest way to resolve this problem would be to require
mpi_f08 for MPI-4.0, i.e., to change the text to

| MPI implementations providing a Fortran interface
| - must provide the USE mpi_f08 Fortran support method,
| - and additionally may provide both,
|   the USE mpi and INCLUDE 'mpif.h' Fortran support methods.

What is your opinion?

I expect that we should discuss this next Wednesday at our MPI Forum telcon.

Best regards
Rolf

--
Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de<mailto:rabenseifner at hlrs.de> .
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 .
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 .
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner<http://www.hlrs.de/people/rabenseifner> .
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .
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