[MPIWG Fortran] MPI Fortran ABI discussion scheduling and agenda

[Jeff Hammond]

Terse minutes from the meeting follow.  Please correct the record if I have
made an error.  Thanks to everyone who attended (I didn't transcript it,
but at least Jeff H, Quincy K, Bill W, Bill L, Ken R, Julien J, Joseph S,
Christian N, M Klemm, and Abraham N were there).

Decision 1:

The sizes of Fortran types is part of the MPI ABI and each
choice/combination of these requires a separate implementation.  The
stipulation that the user must use exactly one of these throughout the
application extends to the Fortran sizes.

Rationale: Every other option is extremely difficult to implement and
unpleasant to reason about.  If we figure out a more compact way to solve
this problem, we can standardize it in the future.  The O(n) solution is
practical because today no implementation implements n>2.

Corollary 1:

There is one mpi.h, one libmpi_abi.so and one of each for the Fortran
modules (and header, if supported) for every MPI C+Fortran ABI.  If an
implementation provides more than one MPI C+Fortran ABI, the implementation
should distinguish them using paths.

Decision 2:

The sizes prescribed by the external32 specification are required to be
supported in the ABI.  All other sets of sizes are optional.

Corollary 2:

User code must do one of the following:
1. Assume the external32 ABI.  If wrong SO is linked, that's a user error
and bad things will happen.
2. Query the Fortran size ABI to ensure that the MPI SO matches the Fortran
object code, and abort if they don't match.
2. Query the Fortran size ABI to ensure that the MPI SO matches the Fortran
object code, and handle any differences manually, e.g. using MPI_INTEGER8
instead of MPI_INTEGER.

Corollary 3:

MPI_Type_size must be callable before MPI_Init for all predefined
datatypes (and only predefined datatypes), because this is how a user can
detect potential ABI problems.

If people don't like this, then I'll add MPI_Abi_type_sizes that meets the
need, without supporting the generality of MPI_Type_size.

Jeff

On Fri, Aug 25, 2023 at 11:35 AM Jeff Hammond <jeff.science at gmail.com>
wrote:

> We need to meet to discuss some of the open problems with the MPI Fortran
> ABI.
>
> https://www.when2meet.com/?21032099-f9qjp
>
> The agenda is
> https://docs.google.com/document/d/1URUIbHhGQfUcoNg_KTX1lhWS-Z80LwJA9LH2QL7HCuo/edit?usp=sharing.
> You'll need to request access to comment/edit because this email list is
> public and I do not want to deal with vandalism.
>
> Jeff
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>


-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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