Completed requests are set to MPI_REQUEST_NULL. Since MPI_Ibarrier (and MPI_Barrier) is a no-op for a communicator of size 1, it is trivially complete and it would seem that MPICH is optimizing away the unnecessary request object here.

Why don't you use MPI_Test instead of direct comparison?

Jeff

On Sun, Dec 6, 2015 at 10:04 PM, Jed Brown <jed@jedbrown.org> wrote:

In MPICH 3.2, MPI_Ibarrier(MPI_COMM_SELF,&r) returns r ==
MPI_REQUEST_NULL. This is also the case for other communicators of size
1. Previous versions of MPICH and also all versions of Open MPI return
a valid request not equal to MPI_REQUEST_NULL. This bit me because it
was natural to compare to MPI_REQUEST_NULL in a conditional for an
Ibarrier/Issend/Iprobe algorithm. It can be worked around with a
slightly more complicate conditional, but is this change intentional?

The call returns a request handle, which must be passed to a
completion call.
-- MPI-3 §5.12

This does not say "unless the communicator has size 1".

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Jeff Hammond
jeff.science@gmail.com
http://jeffhammond.github.io/