To: J3                                                     J3/25-125
From: D. Bonachea & HPC
Subject: F202Y feature request: collectives over a specified team
Date: 2025-February-18
References:
  22-163: F202Y feature request - extend collective semantics
  WG5-N2234:  WORK ITEMS FROM THE HYBRID WG5 MEETING

Formal Requirement (from 22-163, lightly edited)
~~~~~~~~~~~~~~~~~~

The collective subroutines of Fortran should support execution in a
specified team that is not the current team.

Background
----------

As of Fortran 2023, the collective subroutine intrinsics
(CO_BROADCAST, CO_MAX, CO_MIN, CO_REDUCE, and CO_SUM) may only be
executed over the current team, as defined by the CHANGE TEAM
construct. This becomes very awkward when one needs to execute such a
collective over an ancestor team; because there is no way to directly
express that without closing the CHANGE TEAM construct, and invoking
END TEAM may have undesired side-effects such as deallocating
team-specific coarrays. It would also be convenient to allow
collectives directly over a child team without forcing the
synchronization side effects associated with a CHANGE TEAM to that
child team.

Collective subroutines are not image control statements, and they are
entirely decoupled from the coarray feature that motivates the design
of CHANGE TEAM and the current team stack. There is no fundamental
reason to disallow execution of a collective subroutine over any named
team, provided of course that all the members of that specified team
execute the same collective subroutine statement together. The
SYNC TEAM statement allows synchronization over any specified ancestor
team or child team passed as a TEAM_TYPE argument, which is analogous
to the feature proposed here except for data exchange instead of
synchronization.

Team collective operations have been a standard part of MPI for
decades. Every MPI collective operation takes a communicator argument
that names the team of processes participating in that collective.
HPC programmers familiar with subset teams expect this feature as a
natural capability, and are disappointed to find it currently missing
from Fortran's multi-image feature set.

Status
------

WG5-N2234 added this to the "CONDITIONALLY ACCEPTED" work list as Item
DIN1:

- DIN1  Add execution of collective procedures on a specified team
        Ref: N2230 "DIN Suggestions for F202Y"

The full formal requirement, specification and set of suggested edits
are detailed in 22-163 as originally proposed by R. Bader.  In
summary, these add an optional TEAM argument of type TEAM_TYPE to the
five collective subroutines and specifies that execution of the
subroutine applies for the specified team if the argument is present
(or the current team if absent, preserving backwards compatibility).

The contents of 22-163 are believed to remain complete and correct,
and should be advanced as written there.