To:      J3                                           J3/16-280r2
From:    R. Bader & Bill Long
Subject: coarray dummy arguments and aliasing rules
Date: 2016 October 13
References: 16-007r2, TS18508, 16-174

Introduction:
~~~~~~~~~~~~~

This paper is a rewrite of 16-174 that attempts to resolve a problem
with coindexed write accesses to dummy arguments. Paper 16-174 was
submitted to meeting 210 and had "no action".

Discussion:
~~~~~~~~~~~

Consider the following example program, to be executed with at least
2 images:

PROGRAM example_1
  INTEGER :: a[*], il

  il = this_image()
  CALL sub(a, il)
CONTAINS
  SUBROUTINE sub(b, il)
    INTEGER :: b[*], il

    SELECT CASE (this_image())
    CASE (1)
      b[2] = il     ! (X)
      SYNC ALL
    CASE (2)
      SYNC ALL
      il = b        ! (Y)
      WRITE(*,*) il
    CASE DEFAULT
      SYNC ALL
    END SELECT
  END SUBROUTINE
END PROGRAM

Question 1A: Was it intended that the program example_1 is conforming?

Answer 1A: Yes, it is intended that this example is conforming. It
compiles with no errors and executes:

> ftn test.f90
> srun -n2 ./a.out
 1


{It appears to me that this example currently is not conforming based
 on the F2008 as well as 16-007r2 specifications, because the statement
 marked (X) constitutes a violation of the anti-aliasing rule
 16-007r2/15.5.2.13 para 1 item (3) on image 2: The object a on image
 2 is modified through something else than image's 2 dummy argument.
 If this reasoning is correct and the intent is for the program to
 be conforming, edits are needed to fix the problem.}

Question 1B: Was it intended that the program example_1 is conforming
 if statement (Y) is replaced by "il = a"?

Answer 1B: No, with this change the code would violate the
anti-aliasing rules.

{If the answer to 1A is "no", this question is of course trivial.}



Next, consider a second example program, again for at least 2 images:

PROGRAM example_2
  USE, INTRINSIC :: iso_fortran_env
  INTEGER(atomic_int_kind) :: a[*]=0, il

  il = this_image()

  SELECT CASE (this_image())
  CASE (1)
    CALL sub_1(a, il)
  CASE (2)
    CALL sub_2(a, il)
    WRITE(*,*) il
  END SELECT
CONTAINS
  SUBROUTINE sub_1(b, il)
    INTEGER(atomic_int_kind) :: b[*], il
    CALL ATOMIC_DEFINE(b[2],il)          ! (Z)
  END SUBROUTINE
  SUBROUTINE sub_2(b, il)
    INTEGER(atomic_int_kind) :: b[*], il
    il = 0
    DO WHILE (il == 0)
      CALL ATOMIC_REF(il, b)
    END DO
  END SUBROUTINE
END PROGRAM

Question 2: Was it intended that the program example_2 is conforming?

Answer 2: Yes, it is intended that this example is conforming. It
compiles with no errors and executes:

> ftn test2.f90
> srun -n2 ./a.out
 1

{To my knowledge, the intent was that it should be possible to
 coordinate actions between two procedures in the manner illustrated
 by the code above. See also MR&C, Modern Fortran Explained, Section
 19.6, last two paras. However, again the rule 16-007r2/15.5.2.13
 para 1 item (3) appears to be violated, namely by statement (Z).}


Edits to 16-007r2:
------------------

[xix] Introduction, para 2, at the end of the bullet section Program
units and Procedures, append "A coarray dummy argument can be
referenced or defined by another image."

[324:14] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, delete "or".

[324:16] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, change the final "." to ", or".

[324:16+] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, Add another item to (3):

  "(d) the dummy argument is a coarray and the action is a coindexed
  definition of the corresponding ultimate argument coarray from a
  different image."

[324:24] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, delete "or".

[324:26] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, change the final "." to ", or".

[324:26+] In 15.5.2.13 Restrictions on entities associated with dummy
arguments, Add another item to (4):

  "(d) the dummy argument is a coarray and the reference is a
  coindexed reference of its corresponding ultimate argument coarray
  from a different image."

[326:1-] At the end of 15.5.2.13 Restrictions on entities associated
with dummy arguments, After NOTE 15.40 add

  "NOTE 15.40+1 The exception to the aliasing restrictions for dummy
  coarrays enables cross-image access while the procedure is
  executing.  Because nonatomic accesses from different images
  typically must be separated by an image control statement, code
  optimization within segments is not unduly inhibited."