To: J3                                                     J3/25-127
From: R. Bader & D. Bonachea & HPC
Subject: Edits for DIN1: Collectives over a specified team
Date: 2025-February-20
References:
  22-163: F202Y feature request - extend collective semantics
  25-125: F202Y feature request: collectives over a specified team
  25-007: Fortran 202Y Working Draft

Introduction
~~~~~~~~~~~~

This paper is a republication of 22-163, with the suggested edits in
the final section adjusted to apply against the current 25-007 Fortran
202Y Working Draft (as requested in J3 afternoon plenary session on
2025-Feb-19).  Aside from the addition of page and line numbers in the
suggested edits and this introduction section, the content of this
paper remains entirely unchanged from 22-163.

See also paper 25-125 for some expanded background and motivation.

Formal Requirement
~~~~~~~~~~~~~~~~~~

The collective subroutines from Fortran 2018 should support execution
in a team that is not the current team.

Rationale
~~~~~~~~~

If e.g. a reduction operation across the initial team's images is needed
within the context of a CHANGE TEAM construct, this is rather cumbersome
to do with the Fortran 2018/2023 semantics. Either it is necessary to
exit the CHANGE TEAM construct (this permits a single CO_REDUCE
invocation but can impose significant overhead on handling of team-
specific data), or the reduction must be manually performed in two
phases: first, a team-specific call to CO_REDUCE, followed by a manual
assembly across teams (this leads to code that is hard to understand
and maintain).

Also, the ability to perform a reduction on a subset of images
corresponding to a defined team variable without needing to execute
a CHANGE TEAM statement is considered useful.

Specification
~~~~~~~~~~~~~

Add an optional TEAM argument of type TEAM_TYPE to all collective
subroutines and say that execution of the subroutine applies for
the specified team.

Suggested edits (against 25-007)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

16.6 Collective subroutines:

[379:26] para 1:
 replace "the current team" by "of its specified team"

[379:28] para 1:
 replace "its current team" by "its specified team"

[379:30+] after para 1:
 Insert new paragraph reading:

"The team specified for an invocation of a collective subroutine is
 the current team if the TEAM argument is not present in that
 invocation. Otherwise, it is the team identified by the value of
 the TEAM argument; on all images that execute the collective
 subroutine, these values shall either be those of the initial
 team, or those of team variables defined by corresponding
 executions of the same FORM TEAM statement (11.7.9)."

[380:8,13,19,21] para 3, para 4 bullet 4, para 6 (twice):
 replace "the current team" by "the specified team".

16.9.54 CO_BROADCAST:

[407:29] in the heading:
 replace "[, STAT, ERRMSG]" by "[, STAT, ERRMSG, TEAM]"

[407:35-36] in the description of A:
 replace "the current team" by "the specified team"

[408:4] in the description of SOURCE_IMAGE:
 replace "the current team" by "the specified team"

[408:8+] after the description of ERRMSG in para 3:
 add "TEAM (optional) shall be a scalar of type TEAM_TYPE from the
      intrinsic module ISO_FORTRAN_ENV. It is an INTENT(IN) argument."

[408:9] in para 4:
 replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.55 CO_MAX:

[408:13] in the heading:
 replace "[, STAT, ERRMSG]" by "[, STAT, ERRMSG, TEAM]"

[408:27] in the description of RESULT_IMAGE:
 replace "the current team" by "the specified team"

[408:30+] after the description of ERRMSG in para 3:
 add "TEAM (optional) shall be a scalar of type TEAM_TYPE from the
      intrinsic module ISO_FORTRAN_ENV. It is an INTENT(IN) argument."

[408:31] in para 4:
 replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.56 CO_MIN:

[408:35] in the heading:
 replace "[, STAT, ERRMSG]" by "[, STAT, ERRMSG, TEAM]"

[409:11] in the description of RESULT_IMAGE:
 replace "the current team" by "the specified team"

[409:14+] after the description of ERRMSG in para 3:
 add "TEAM (optional) shall be a scalar of type TEAM_TYPE from the
      intrinsic module ISO_FORTRAN_ENV. It is an INTENT(IN) argument."

[409:15] in para 4:
 replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.57 CO_REDUCE:

[409:19] in the heading:
 replace "[, STAT, ERRMSG]" by "[, STAT, ERRMSG, TEAM]"

[410:3] in the description of RESULT_IMAGE:
 replace "the current team" by "the specified team"

[410:6+] after the description of ERRMSG in para 3:
 add "TEAM (optional) shall be a scalar of type TEAM_TYPE from the
      intrinsic module ISO_FORTRAN_ENV. It is an INTENT(IN) argument."

[410:7] in para 4:
 replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.58 CO_SUM:

[410:20] in the heading:
 replace "[, STAT, ERRMSG]" by "[, STAT, ERRMSG, TEAM]"

[410:35] in the description of RESULT_IMAGE:
 replace "the current team" by "the specified team"

[410:38+] after the description of ERRMSG in para 3:
 add "TEAM (optional) shall be a scalar of type TEAM_TYPE from the
      intrinsic module ISO_FORTRAN_ENV. It is an INTENT(IN) argument."

[410:39] in para 4:
 replace "and ERRMSG" by ", ERRMSG and TEAM"

===END===