(j3.2006) (SC22WG5.4074) Some minor points on the Draft Final CD

[N.M. Maclaren]

I have started to look at this and, because I had stepped back from the 
trees, have noticed a few minor loopholes. I should appreciate comments and 
corrections before formally passing them on to the UK.



1) I thought that we had discussed this, but now can't find any wording
that forbids it, nor what it means if it is allowed:

    ! running on either 10 (too few) or 27 (not a multiple) images
    INTEGER, SAVE :: coarray(10)[5,5,*]
    ! and similarly for ALLOCATE

Unless I have missed something, this needs a new paragraph in 5.3.6.3
(Explicit-shape coarray), such as:

    The product of (upper-cobound - lower-cobound + 1) for all
    codimensions except the last shall be a factor of the number of
    images.

and in 6.7.1.1 (ALLOCATE statement: Syntax):

    The product of (upper-cobound-expr - lower-cobound-expr + 1) for
    all codimensions except the last shall be a factor of the number of
    images.

If not, there needs to be something (even just a NOTE) clarifying what
happens in these cases.



2) We seem to have let this one slip through; should we allow it or not?
It has essentially the effect of a SYNC ALL, but no image can tell that
without synchronising with the others.

    SYNC_IMAGES( (/ MOD(THIS_IMAGE(),NUM_IMAGES())+1,    &
        MOD(THIS_IMAGE()+NUM_IMAGES()-2,NUM_IMAGES())+1 /) )

I suggest not, on the grounds that it will inconvenience implementors,
isn't a particularly sane usage, and could be added later.  In 8.5.4
(SYNC IMAGES statement), add a new paragraph:

    If two SYNC IMAGES statements correspond, the two image sets
    (extended by adding the executing image if that is not present)
    shall be the same.



3) I think that the current wording overspecifies error termination in
paragraph 4 of 2.3.5 (Execution sequence).  Specifically, requiring one
image to be able to force with others into termination without them
executing any special statement is a heavy burden on implementors, and
may not always be feasible.  On the other hand, when it isn't possible,
the standard can't say anything useful.  To put it in other words, is
"error termination" intended to mean that the program has failed, or
does it really mean "unilateral termination"?

If people agree with me, I suggest changing:

    The synchronization step is executed by all images.

to:

    For normal termination, the synchronization step is executed by all
    images; for error termination, the synchronization of images is
    processor dependent.



Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679