[mpiwg-rma] same_op_no_op

[Jeff Hammond]

If we thought it was a mistake in MPI-2 to disallow it, wouldn't we
have tried to relax it in MPI-3?  Brian had commented before that we
might have been confused when defining same_op_no_op and really meant
same_op_no_op_replace, which is a perfectly reasonable default
semantic.

I would argue that we should admit the latter mistake and fix this in
MPI-3.1.  I know of no implementation that need to change because of
this.

same_op_no_op_replace might not be completely SHMEM-compliant but it
almost certainly meets the needs of reasonable SHMEM programs.  Right
now, the incompatibility of REPLACE and <any reduce op> puts me in a
very bad place for essentially all PGAS models.

Jeff

On Thu, Mar 13, 2014 at 2:40 PM, Balaji, Pavan <balaji at anl.gov> wrote:
>
> On Mar 13, 2014, at 2:00 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>> Do you think a user can determine from MPI-3 that it is an erroneous
>> program to have accumulate to same loc with diff ops using
>> _reasonable_ effort?
>
> I’m not arguing as to whether it is convenient for the user.  I’m just stating that the standard disallows it.  IIRC, it was the explicit intention of the WG to disallow it, because we thought MPI-2 disallowed it as well (which was a mistake).
>
>   — Pavan
>
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-- 
Jeff Hammond
jeff.science at gmail.com