To: J3                                                     J3/23-173
From: Jeff Hammond
Subject: MPI Liason Report
Date: 2023-June-11

Two tickets related to Fortran that have passed their
second and final vote, and therefore will be part of
the next release of MPI (4.1) are:

 1. MPI will refer to Fortran 2018 instead of Fortran
    2008 + TS 29113 where appropriate, which reflects
    the current state of Fortran.

    https://github.com/mpi-forum/mpi-issues/issues/565

 2. The mpif.h interface will be deprecated.  This
    interace has many flaws (listed in the MPI document).
    The API will still be available from mpi.mod,
    which we cannot deprecate even though mpi_f08.mod
    is unconditionally better.

    https://github.com/mpi-forum/mpi-issues/issues/561

The ABI standardization effort is going well.  The
current goal is an ABI-only release as MPI 4.2 in
2024.  The latest draft of the text addresses C and
Fortran, and will have zero overhead in Fortran for
all common use cases, which is currently true in
MPICH but not Open MPI.

A paper on ABI standardization efforts has been
submitted to EuroMPI 2023, and is available to
WG5/J3 members upon request.

Implementing the full feature set required by MPI 3.0
for Fortran subarrays has proved more difficult than
expected and may be the subject of clarifications in
future releases of MPI.  In particular, the combination
of non-contiguous subarray buffers and user-defined
datatypes - an unlikely use case - appears to require
an inefficient implementation, or at least extensive
metadata processing.