[Mpi-forum] MPI Ain't and MPI 3.1

N.M. Maclaren nmm1 at cam.ac.uk
Wed Jul 17 12:03:23 CDT 2013

On Jul 17 2013, Jim Dinan wrote:
>I've just gotten around to updating the ticket with feedback from the June
>meeting.  My apologies, the ticket was way out of sync with the progress on
>this issue.  I'm hopeful that the new proposal text addresses your concerns
>with respect to segmented and tagged architectures:


>The solution you described is similar to "proposed solution #2" on the
>ticket, which won't work for Fortran, because Fortran lacks the unsigned
>integers we would need for the MPI_Disp type.

Not really.  I wasn't addressing that point - not least because I regard
C unsigned types as an abomination.  The key point was a handle to identify
the window, for the reasons that David Goodell describes (though I gave
some of the underlying ones rather than the standardese).

The signed/unsigned distinction is a red herring because (for those reasons)
integer arithmetic on even uintptr_t is not guaranteed to work in the way
that I think you expect.  All that the standard requires is that it can be
converted to and from a pointer without losing the address.

The point is that an address is inherently a combination of a handle and
an integer.  The handle can be ignored for flat memory architectures, but
not all are like that, and it is quite likely that we will see structured
ones return in not too long from now.  How likely?  My guess is perhaps a
30% chance within a decade.

Nick Maclaren.

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