To:      J3                                               J3/14-147r1
From:    John Reid & Bill Long
Subject: Collectives
Date: 2014 June 23
References: N2007, N2013

Discussion
----------

In N2013, Malcolm Cohen says
"the collectives CO MAX, CO MIN, CO REDUCE, CO SUM, should be split
into two forms, one with RESULT, one without.  The one with RESULT
should have SOURCE as INTENT(IN), the one without should have SOURCE
as INTENT(INOUT).  RESULT must not be optional.  The SOURCE INTENT(IN)
form should have no coarray restrictions on SOURCE."
and Nick Maclaren says
"This TS introduces a conflict with Fortran 2008's concept
and specification of variable definition context.  This is definitely
fixable, but may need significant changes to the collectives."
and he suggests the remedy of splitting the collectives as in this
example
7.4.7 CO_MAX (SOURCE [, RESULT, RESULT_IMAGE, STAT, ERRMSG]) or
      CO_MAX_IN_PLACE (OBJECT [, RESULT_IMAGE, STAT, ERRMSG])
Malcolm suggests moving RESULT to the end of the argument list:
7.4.7 CO_MAX (SOURCE [, RESULT_IMAGE, STAT, ERRMSG, RESULT]) or
      CO_MAX (SOURCE [, RESULT_IMAGE, STAT, ERRMSG])
and has other suggestions that seem to me to be less attractive.

A simpler solution is to remove the argument RESULT and give the
argument SOURCE intent(inout). It does not involve a loss of
functionality since the call
         CALL CO_MAX (SOURCE,RESULT)
may be replaced by
         RESULT = SOURCE
         CALL CO_MAX (RESULT)
and the call
         CALL CO_MAX (SOURCE,RESULT,RESULT_IMAGE)
may be replaced by
         RESULT = SOURCE
         CALL CO_MAX (RESULT,RESULT_IMAGE)
The simple scheme might lead to a slight loss of performance because
of the copy, but the copy could be optimized away (if the compiler can
do non-inplace collectives efficiently), and in any case this should
be small since no communication is involved.

Furthermore, the names SOURCE and RESULT are potentially confusing
when the input argument is overwritten as the output. Instead, the
argument name A is used.

This paper contains edits for this change.


Edits to N2007:
---------------

[17:29] Replace "SOURCE or RESULT" by "A".

[17:38-39] Change "the RESULT argument becomes undefined ... otherwise"
to "the A argument becomes undefined".

[18:10+] In NOTE 7.1, replace the first "SOURCE" by "The argument A",
delete "with RESULT not present", and replace the second "SOURCE" by
"A".

[18:10+] Add new note:
All the collectives have an argument A with INTENT(INOUT) that
holds the original data on entry and the result on return. If it is
desired to retain the original data, this is readily obtained by
making a copy before entry. Here is an example:
    REDUCTION = ORIGINAL
    CALL CO_MIN(REDUCTION)

[20:15-21] Replace "SOURCE" by "A", but do not change "SOURCE_IMAGE".

[20:19, 20:35, 21:21, 22:7, and 23:1] After "current team." add
to "It is an INTENT(INOUT) argument.".

In 7.4.7 CO_MAX, 7.4.8 CO_MIN, 7.4.9 CO_REDUCE, and 7.4.10 CO_SUM
replace "SOURCE" by "A".

[20:30, 21:16, 22:1, and 22:40] Delete "RESULT, ".

[20:39-40, 21:25-26, 22:16-18, and 23:6-7] Delete
"RESULT ... current team.".

[21:5, 21:32, 22:24, and 23:13] Change
"If RESULT and RESULT_IMAGE are not present," to
"If RESULT_IMAGE is not present,".

[21:6, 21:33, 22:25, and 23:14] Change
"If RESULT is not present and" to "If".

[21:8-11, 21:35-38, 22:27-30, and 23:16-19] Delete
"If RESULT is present and ... not modified.".

[21:14-15, 21:41-42, 22:38, and 23:22-23] Change "RESULT" to "A"
and delete ", RESULT".

[34:14+] In the entries for CO_MAX, CO_MIN, CO_REDUCE and CO_SUM,
replace "SOURCE [, RESULT," by "[, ".