To: J3                                                     J3/25-127r1
From: Reinhold Bader & Dan Bonachea & HPC
Subject: DIN1: Collectives over a specified team, req/spec/syntax/edits
Date: 2025-June-20
References:
  J3/22-163: F202Y feature request - extend collective semantics
  J3/25-125r1: F202Y feature request: collectives over a specified team
  WG5/N-2339: Results of WG Consultation on Conditionally Approved Work
              Items for F202Y
  J3/25-007: Fortran 202Y Working Draft

1. Background
~~~~~~~~~~~~~

This paper is an updated version of J3/22-163, with the suggested edits in
the final section adjusted to apply against the current J3/25-007 Fortran
202Y Working Draft, and some other minor updates. Revision 1 of this paper
(J3/25-127r1) makes minor adjustments to the proposed edits (based on
suggestions received from Malcolm Cohen) and fills in some additional
detail in earlier sections.

Paper J3/25-125r1 provides expanded background and motivation, and was
discussed at the February 2025 INCITS/Fortran meeting #235.  At the time,
work item DIN1 (Collectives over a specified team) was a WG5 Conditionally
Accepted work item. Paper J3/25-125r1 recommends to WG5 that work item DIN1
be promoted to the Approved work list for F202Y, and was passed at
INCITS/Fortran meeting #235 with a vote of 10 in favor, 0 opposed, 4
absent. WG5 subsequently voted via letter ballot in Apr/May 2025, and
in WG5/N-2339 formally accepted work item DIN1 as an Approved work item,
with a vote of 23 Yes, 3 No, 0 Undecided.

This paper contains formal requirements, specifications, syntax and edits
for Fortran 202Y proposal DIN1, collectives over a specified team.

The Requirements, Specifications, Syntax and Edits will be moved
separately, in order.

2. Requirements
~~~~~~~~~~~~~~~

R1. The collective subroutines of Fortran should support execution over a
    team that is not the current team.

2.1 Rationale

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.


3. Specifications
~~~~~~~~~~~~~~~~~

S1 Add an optional TEAM argument of type TEAM_TYPE to all collective
   subroutines and specify that execution of the subroutine applies over
   the specified team.

S2. The executing image shall be a member of the specified team.

S3. As required by 16.6 "Collective subroutines", all active images in the
    specified team must invoke the same collective subroutine by the same
    statement in segments that are not ordered with respect to each other.


4. Syntax and Semantics
~~~~~~~~~~~~~~~~~~~~~~~

4.1 Syntax

An optional TEAM argument of type TEAM_TYPE is added as the final argument
to each of the five collective subroutines, yielding the following:

X1. CO_BROADCAST (A, SOURCE_IMAGE [, STAT, ERRMSG, TEAM])

X2. CO_MAX (A [, RESULT_IMAGE, STAT, ERRMSG, TEAM])

X3. CO_MIN (A [, RESULT_IMAGE, STAT, ERRMSG, TEAM])

X4. CO_REDUCE (A, OPERATION [, RESULT_IMAGE, STAT, ERRMSG, TEAM])

X5. CO_SUM (A [, RESULT_IMAGE, STAT, ERRMSG, TEAM])

4.2 Semantics

X10. If the TEAM argument is absent in a given invocation of a collective
     subroutine, the specified team is the current team.

X11. Otherwise, the specified team is the team identified by the value of
     the TEAM argument.

X12. All active images in the specified team shall invoke the collective
     subroutine in the same statement and specify the same team in
     corresponding references.


5. Suggested Edits (against J3/25-007)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

16.6 Collective subroutines:

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

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

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

"The team specified by 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 participating in the same collective
 computation, the specified team shall be the same team; either the
 initial team, or the same team created by 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===