To: J3                                                     J3/25-166
Subject: US04: Syntax for Asynchronous Collective Subroutines
From: Brandon Cook & Damian Rouson & Dan Bonachea & Reuben D. Budiardja
Date: 2025-September-12
References: J3/25-162r1, J3/25-165, J3/25-127r1, WG5/N2249

1. Background
-------------
The current Fortran 202Y work list WG5/N2249 includes asynchronous
collective subroutines as accepted work item US04.

Paper J3/25-162r1 "US04: Requirements for Asynchronous Collective
Subroutines" presents an illustrative use case and a list of requirements.

Paper J3/25-165 "US04: Specifications for Asynchronous Collective
Subroutines" presents the proposed specifications and semantics for the new
asynchronous collective subroutines feature. Please consult that paper for
all semantic details, which will not be replicated here.

The current paper focuses exclusively on the proposed syntactic elements
for the asynchronous collective subroutines feature.

2. Completion Type Syntax
-------------------------

T1. New type COMPLETION_TYPE is added to the ISO_FORTRAN_ENV intrinsic
    module.

See J3/25-165 for detailed specifications and semantics.

3. Asynchronous Collective Initiation Syntax
--------------------------------------------

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

C1. CO_BROADCAST (A, SOURCE_IMAGE [, STAT, ERRMSG, COMPLETION])

C2. CO_MAX (A [, RESULT_IMAGE, STAT, ERRMSG, COMPLETION])

C3. CO_MIN (A [, RESULT_IMAGE, STAT, ERRMSG, COMPLETION])

C4. CO_REDUCE (A, OPERATION [, RESULT_IMAGE, STAT, ERRMSG, COMPLETION])

C5. CO_SUM (A [, RESULT_IMAGE, STAT, ERRMSG, COMPLETION])

See J3/25-165 for detailed specifications and semantics.

Note that J3/25-127r1 "DIN1: Collectives over a specified team,
req/spec/syntax/edits" also adds a different trailing optional argument
(TEAM) to each of the collective subroutines above, for work item DIN1.
These two features (US04 and DIN1) are entirely orthogonal, and their
corresponding new arguments could be specified in either order.

C6. Assuming that edits for both work items DIN1 and US04 are applied for
    Fortran 202Y, we propose that the new optional TEAM argument
    immediately precede the new optional COMPLETION argument in the syntax
    of each collective subroutine listed above.

4. Completion Wait Statement Syntax
-----------------------------------

A new <action-stmt> called COMPLETION WAIT is introduced, with the
following syntax:

W1. <completion-wait-stmt>  <<is>>
      COMPLETION WAIT ( <completion-item> )

W2. <completion-item>  <<is>>  <variable>

W3. The <completion-item> specifier to COMPLETION WAIT shall be a variable
    of type COMPLETION_TYPE from the intrinsic module ISO_FORTRAN_ENV.

W4. The COMPLETION WAIT statement contains no specifiers other than the
    <completion-item>.

See J3/25-165 for detailed specifications and semantics.

5. Completion Query Function Syntax
-----------------------------------

A new standard intrinsic elemental function called COMPLETION_QUERY is
introduced, with the following syntax:

Q1. COMPLETION_QUERY ( COMPLETION )

Q1. The COMPLETION argument to COMPLETION_QUERY() shall be a variable of
    type COMPLETION_TYPE from the intrinsic module ISO_FORTRAN_ENV.

Q3. The COMPLETION_QUERY function accepts no arguments other than the
    completion variable.

See J3/25-165 for detailed specifications and semantics.

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