> But thinking about this today, I have no idea what MPI_REPLACE is
> supposed to do in a collective reduction. Specifically -- what value
> should end up in the target buffer? It doesn't make sense.
> I think that this is a grey area in the MPI standard -- MPI_REPLACE is
> an MPI_Op, but it should *only* be used with MPI_ACCUMULATE, and not
> with the collective reduction functions.
> Any comments on this MPI interpretation? Agree / disagree?
Makes sense to limit its use.. so we should return err_arg if used in a
collective reduce op then.
(It has one interesting use, everybody does an allreduce on their rank,
and it then acts as an atomic election, except its repeatable and