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

[Bill Long]

On Feb 9, 2015, at 12:26 PM, Daniel C Chen <cdchen at ca.ibm.com> wrote:

> Malcolm said:
> "HOWEVER, in the similar but different
> 
>    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."
> 
> As for the solution, can we


A ?solution? should not have the effect of making a currently standard-conforming program suddenly not conforming.   That would seem to disqualify option 2 below.   Van raised the possibility that BLOCK could accomplish option 1. 

Cheers,
Bill


> 
> 1. Add syntax to DO CONCURRENT to allow users specify variables that need to be localised, which is similar to OMP's PRIVATE/FIRSTPRIVATE clause. Or,
> 
> 2. Change the following restriction (similar changes should apply to Pointers, Allocatables)
> "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 more than one iteration becomes undefined when the loop terminates. "
> To
> "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.
> 
> 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
> 
> <graycol.gif>"Malcolm Cohen" ---02/05/2015 19:35:24---Van writes: >That was my first reaction too.  Then I realized that A is not changed
> 
> From:	"Malcolm Cohen" <malcolm at nag-j.co.jp>
> To:	"fortran standards email list for J3" <j3 at j3-fortran.org>
> Date:	02/05/2015 19:35
> Subject:	(j3.2006) Fw:  (j3-members.2015) Question about DO CONCURRENT
> Sent by:	j3-bounces at mailman.j3-fortran.org
> 
> 
> 
> Van writes:
> >That was my first reaction too.  Then I realized that A is not changed
> >during any iteration.  Therefore, either B is assigned in every
> >iteration, or not assigned in any.
> 
> Ah of course, thanks.  (Very easy to misread that code!)
> 
> Yes I realised some time ago that we didn't get the DO CONCURRENT specs "right"
> in the sense of being able to tell at compile time whether a variable was
> localised or global.  In
> 
>  DO CONCURRENT(...)
>      IF (condition) THEN
>         X = something
>      END IF
>      code using X
>   END DO
> 
> when the loop has more than one iteration, you can make X local to the loop,
> since condition has to be loop-invariant for this to be legal, and after
> execution, the "outer" X either has its original value (if the condition was
> false) or is undefined (if the condition was true).
> 
> HOWEVER, in the similar but different
> 
>    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.
> 
> Cheers,
> -- 
> ................................Malcolm Cohen, Nihon NAG, Tokyo. 
> 
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Bill Long                                                                       longb at cray.com
Fortran Technical Suport  &                                  voice:  651-605-9024
Bioinformatics Software Development                     fax:  651-605-9142
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