[MPIWG Fortran] Type of MPI_Status

Junchao Zhang jczhang at mcs.anl.gov
Mon Mar 17 14:10:41 CDT 2014

On Mon, Mar 17, 2014 at 1:52 PM, Bill Long <longb at cray.com> wrote:

> On Mar 17, 2014, at 1:39 PM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:
> >
> > On Mon, Mar 17, 2014 at 11:52 AM, Craig Rasmussen <
> rasmus at cas.uoregon.edu> wrote:
> > Jeff and I had a similar conversation last week.  It is my viewpoint
> that the MPI-3 interfaces are formal interfaces that can be adapted by an
> implementation in a way that formally meets the specification.
> >
> > +1 for this view.
> >
> > For Open MPI I believe that Jeff will add configuration magic to check
> is INTEGER(C_INT) and default INTEGER have the same KIND value.  This
> should be true for 100% of the compilers.  If not, then that compiler will
> not be able to build Open MPI.
> >
> > Supporting different INTEGER and c_int 1) is useless;  2) adds code
> complexity; 3) adds extra runtime overhead (in MPICH implementation) even
> when INTEGER and c_int are the same.  I prefer stopping the support and
> doing configuration time check.
> >
> There is a problem with this approach.  If the normal INTEGER and c_int
> are the same, then INTEGER compiled with a switch like -i8 is almost
> assured to be different from c_int.   It is not that hard to write the
> module such that a single source can be used to compile both -i4 and -i8
> versions.  There should not be measurable overhead with this, since the
> checks are all on compile-time constants, so the checks and the "unused"
> branch of the test will be eliminated by the compiler.

The compiler eliminates the unused branch in an MPI implementation at the
library build time. But later, you use a different sized INTEGER at user
code compilation time (through mpif90 options?). That is erroneous.
Why there is runtime overhead when INTEGER = c_int, see my previous reply
to Rolf on comparison of designs with INTEGER MPI_Status or with
INTEGER(c_int) MPI_Status.

> Note that the -i4/-i8 problem is one of the arguments why using default
> INTEGER in the Fortran interfaces should have been prohibited  by the MPI
> spec.
> Cheers,
> Bill
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