To: J3                                                     J3/22-163
From: R. Bader
Subject: F202Y feature request - extend collective semantics
Date: 2022-July-14
References: 22-007r1

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 22-007r1)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

16.6 Collective subroutines, para 1:

replace "the current team" by "of its specified team";
replace "its current team" by "its specified team";

16.6 Collective subroutines, after para 1: add a new para 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)."

16.6 Collective subroutines
para 3, para 4 bullet 4, para 6 (twice):
replace "the current team" by "the specified team".


16.9.54 CO_BROADCAST
 in the heading, replace "[, STAT, ERRMSG]" by
 "[, STAT, ERRMSG, TEAM]"
 in the description of A, replace "the current team" by
 "the specified team"
 in the description of SOURCE_IMAGE, replace "the current team" by
 "the specified team"
 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."
 in para 4, replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.55 CO_MAX
 in the heading, replace "[, RESULT_IMAGE, STAT, ERRMSG]" by
 "[, RESULT_IMAGE, STAT, ERRMSG, TEAM]"
 in the description of RESULT_IMAGE, replace "the current team" by
 "the specified team"
 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."
 in para 4, replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.56 CO_MIN
 in the heading, replace "[, RESULT_IMAGE, STAT, ERRMSG]" by
 "[, RESULT_IMAGE, STAT, ERRMSG, TEAM]"
 in the description of RESULT_IMAGE, replace "the current team" by
 "the specified team"
 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."
 in para 4, replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.57 CO_REDUCE
 in the heading, replace "[, RESULT_IMAGE, STAT, ERRMSG]" by
 "[, RESULT_IMAGE, STAT, ERRMSG, TEAM]"
 in the description of RESULT_IMAGE, replace "the current team" by
 "the specified team"
 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."
 in para 4, replace "and ERRMSG" by ", ERRMSG and TEAM"

16.9.58 CO_SUM
 in the heading, replace "[, RESULT_IMAGE, STAT, ERRMSG]" by
 "[, RESULT_IMAGE, STAT, ERRMSG, TEAM]"
 in the description of RESULT_IMAGE, replace "the current team" by
 "the specified team"
 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."
 in para 4, replace "and ERRMSG" by ", ERRMSG and TEAM"