To: J3                                                     J3/24-145r1
From: John Reid & Reinhold Bader & Malcolm Cohen
Subject: Interp. on allocation segments
Date: 2024-June-26
References: 24-007

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NUMBER: F23/016
TITLE: Segments associated with allocation
KEYWORDS: segment, allocate, coarray
DEFECT TYPE: Clarification
STATUS: J3 consideration in progress

QUESTION:

Is it possible for a coarray to be allocated on an image in the
current team without there being a corresponding allocated coarray on
another active image in the current team?

For example, consider the program

PROGRAM ALLOCATION
   IMPLICIT NONE
   INTEGER :: I
   REAL, ALLOCATABLE :: A[:]
   ALLOCATE (A[*])
   A = THIS_IMAGE()
   SYNC ALL
   DO I = 2,THIS_IMAGE()
      IF (A[I]/=I) WRITE(*,*) "Value incorrect on image", I
   END DO
   DEALLOCATE (A)
END

Is it possible that an execution of the IF statement fails because the
coarray A has already been deallocated on image I?

ANSWER:

No, it is not possible.

For the ALLOCATE statement, the standard says
    "execution on the active images of the segment (11.7.2) following
     the statement is delayed until all other active images in the
     current team have executed the same statement the same number of
     times in this team."

That means that after execution of the ALLOCATE statement on an image,
the coarray is allocated on all the images (in the team).

For the DEALLOCATE statement, the standard says
    "When a statement that deallocates a coarray or an object with a
     coarray potential subobject component is executed, there is an
     implicit synchronization of all active images in the current
     team."
It then goes on to say
    "A coarray shall not become deallocated on an image unless it is
     successfully deallocated on all active images in this team."

That means that the DEALLOCATE statement cannot actually deallocate a
coarray on the executing image until all other active images have
reached that DEALLOCATE and confirmed that they can deallocate their
corresponding coarray.

Furthermore,
    "execution on the active images of the segment (11.7.2) following
     the statement is delayed until all other active images in the
     current team have executed the same statement the same number of
     times in this team"
means that after the DEALLOCATE statement execution is complete on one
image, the coarray is unallocated on all active images.

EDIT to 24-007.

None.

SUBMITTED BY: Reinhold Bader

HISTORY: 24-145   m233  Submitted
         24-145r1 m233  Revised

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