(j3.2006) Fw: (j3-members.2015) Question about DO CONCURRENT

[Daniel C Chen]

Shall we allocate some time to talk about it on #206? Do we need a paper to
initiate the discussion?

Thanks,

Daniel

XL Fortran Development - IBM Toronto Software Lab
Phone: 905-413-3056
Tie: 969-3056
Email: cdchen at ca.ibm.com
http://www.ibm.com/software/awdtools/fortran/xlfortran



From:	"Malcolm Cohen" <malcolm at nag-j.co.jp>
To:	"fortran standards email list for J3"
            <j3 at mailman.j3-fortran.org>
Date:	02/09/2015 19:34
Subject:	Re: (j3.2006) Fw:  (j3-members.2015) Question about DO
            CONCURRENT
Sent by:	j3-bounces at mailman.j3-fortran.org



I wrote:
>    DO CONCURRENT(...)
>      IF (condition) THEN
>           X = something
>           code using X
>      END IF
>      code that doesn't use X
>   END DO
>
> you cannot localise X, because if "condition" is true exactly once, the
> assignment affects the "outer" X.  OTOH, if condition is true more than
once,
> you cannot execute in parallel without localising X.  Rather UGH."

Bill Long writes:
>A ?solution? should not have the effect of making a currently
>standard-conforming program suddenly not conforming.

This was only introduced in the current standard.  Therefore it is fixable
via
interp *IF* we agree that such cases should not be conforming.

Obviously there are various ways of rewriting the code.  "Option 2" works
if the
compiler supports both DO CONCURRENT and BLOCK.  Changing the name of X to
something that appears nowhere else in the program unit also works.
Neither of
these are particularly likely to be favoured by many users.

Bill continues:
>That would seem to disqualify option 2 below.

Not in itself, however, option 2 below is sadly unworkable anyway...

Daniel Chen suggested:
> "A variable that is referenced in an iteration shall either be previously

> defined during that iteration, or shall not be defined or become
undefined
> during any other iteration. A variable that is defined or becomes
undefined by
> one or more iterations becomes undefined when the loop terminates. "  --
This
> will allow compiler to localised x in the above example.

Unfortunately that would mean the DO CONCURRENT loop cannot achieve
anything
other than i/o.  Bathwater meet baby.

I don't see any obvious solution, at least without additional syntax, that
is
better than telling the user "don't do that", unsatisfactory though it
seems.

Cheers,
--
................................Malcolm Cohen, Nihon NAG, Tokyo.

_______________________________________________
J3 mailing list
J3 at mailman.j3-fortran.org
http://mailman.j3-fortran.org/mailman/listinfo/j3
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.j3-fortran.org/pipermail/j3/attachments/20150210/3d5fba9d/attachment.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: graycol.gif
Type: image/gif
Size: 105 bytes
Desc: not available
Url : http://mailman.j3-fortran.org/pipermail/j3/attachments/20150210/3d5fba9d/attachment.gif