(j3.2006) MPI usage problems in Fortran

[Craig Rasmussen]

On Mar 20, 2008, at 11:10 AM, Aleksandar Donev wrote:

>> As Bill says, this is very much like asynchronous I/O in
>> Fortran and should be treated similarly.
> Yes, but if only ASYNCHRONOUS were itself treated consistently, we  
> could have
> something to work from...
>
>> Perhaps it would be helpful
>> to have the ASYNCHRONOUS attribute apply to procedures, i.e.,
>>       ASYNCHRONOUS FUNCTION MPI_Isend(...)
> And what exactly does this mean?

A description of one problem (text partially borrowed from MPI  
documentation at Argonne):

-----------------------------

To prevent instruction reordering or the allocation of a buffer in a  
register there are two possibilities in portable Fortran code:

The compiler may be prevented from moving a reference to a buffer  
across a call to an MPI subroutine by surrounding the call by calls  
to an external subroutine with the buffer as an actual argument.

         call MPI_IRECV(buf,...,req)

         ....

         call MPI_WAIT(rec,...)

         call DD(buf)

with the separately compiled

         subroutine DD(buf)

           integer buf

         end

------------------------------

So with appropriate wording in the standard, could something like the  
following be made to work?

interface

     ASYNCHRONOUS function MPI_Irecv(...)     ! similar effect as  
asynchronous specifier (9.6.2.5) and

     ASYNCHRONOUS function MPI_Wait(...)      ! any statement to  
these functions involve asynchronous operations

end interface



integer, ASYNCHRONOUS :: buf(*)     ! ASYNCHRONOUS rather than TARGET  
to limit the effect to local scoping unit

call MPI_Irecv(C_LOC(buf),...,req)        ! C_LOC modified to include  
ASYNCHRONOUS variables

....

call MPI_Iwait(req,...)            ! last asynchronous function seen  
in this scoping unit so optimizations can be turned back on?



Regards,

Craig