J3/16-216r1
To: J3
From: Bill Long & Malcolm Cohen
Subject: Noncontiguous array with SHARED locality
Date: 2016 June 07

Discussion:
-----------

Para 3 of 8.1.7.5 Additional semantics for DO CONCURRENT constructs
specifies limitations on how a program can use a variable with SHARED
locality. Basically, the actions on the variable in one iteration
should not affect the value seen in a different iteration.  However,
there is a case that is not covered by explicit definition and
reference of the shared variable.

Suppose X is a discontiguous array that is specified to have SHARED( )
locality.  If X is passed to a subroutine that has a dummy argument
that is contiguous, there will be a copy-in/copy-out of the actual
(discontiguous) actual argument. In particular, the copy-out has the
effect of redefining all the values of the elements of X, some of
which might have been also defined by a different iteration.  This
results in unacceptable behavior, and needs to be avoided.  For example:

subroutine outer (X)
  real :: x(:)  ! Potentially discontiguous array
  ...
  do concurrent (i=1:n) shared(X)
    ...
    call sub (X,i,b)  ! Copy-in/copy-out of all of X here
    c = X(i)
    ...
  end do
  ...
end subroutine outer

pure subroutine sub (x,i,b)
  real,intent(inout) :: x(*) ! Contiguous dummy array
  integer,intent(in) :: i
  real,intent(in) :: b
  x(i) = b
end subroutine sub

Here the subroutine only disturbs X(i) in the do concurrent construct,
so satisfies the current limitations.  However, the copy-out of ALL of
the elements of X causes a problem.

An edit is suggested to disallow this case.

Additional Discussion
---------------------

The same thing happens with
   REAL X(100)
   ...
   ...
   CALL SUB(X(::2),I/2,1.5)
The edit below also addresses this case.

Similar bad things happen with unspecified locality,
but this is even more hideous and difficult to fix.
No edit is supplied.


Edit to J3/16-007r1:
--------------------

[185:24] In 8.1.7.5 Additional semantics for DO CONCURRENT constructs,
at the end of para 3, add a new sentence: "A noncontiguous array with
SHARED locality shall not be supplied as an actual argument
corresponding to a contiguous INTENT (INOUT) dummy argument.".

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