[MPIWG Fortran] Type of MPI_Status

[Bill Long]

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.   

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


> 
> The use of INTEGER(C_INT) to replace the default INTEGER in the MPI types is needed to stopped compiler warning from GNU.  We did this work (adding BIND(C) to gfortran) at Los Alamos and I think we perhaps got carried away with the warning messages.
> 
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Bill Long                                                                       longb at cray.com
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