[MPI3 Fortran] ASYNC attribute

[Craig Rasmussen]

I'm at the WG5 Fortran standards meeting and I think we've made  
significant advances regarding the MPI-3 Fortran API.  We are  
discussing adding a new variable attribute, ASYNC.  I'd like to get  
feedback from the MPI-3 Fortran working group on these possible changes.

ASYNC implies that a variable is potentially active in asynchronous  
operations outside of Fortran.  The use of this new attribute should  
give the compiler enough information to inhibit optimizations similar  
to inhibitions involved in Fortran asynchronous I/O, specifically  
copyin/copyout and code motion.  The compiler will likely have to  
inhibit code motion regarding any use of a variable with the ASYNC  
attribute and procedures other than intrinsics.  The affect is similar  
to the use of the volatile attribute (but not regarding loads and  
stores).

Usage is outlined below:

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

real, ASYNC_EXTERNAL :: buf(100,100)
type(MPI_Request) :: request

! initiate data transfer of boundary
CALL MPI_IRecv(buf(:,1),...,request,...)  ! no copyin/copyout will  
happen

! do work on interior of the buffer while transfer is in progress
CALL UPDATE_INTERIOR(buf)  ! dummy arg should have ASYNC attribute

! wait for the communication to finish
CALL MPI_Wait(request,...)   ! no code motion allowed with respect to  
buf

! finish work on buffer boundaries
CALL UPDATE_BOUNDARIES(buf)  ! should copyin/copyout be allowed here?

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

So how does this look?  Anything we've left out?  I'm also including  
the interface definition.

- craig

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

interface

SUBROUTINE MPI_Irecv(buf, count, datatype, source, tag, comm, request,  
err) BIND(C, name="MPI_Irecv")
    import MPI_Datatype, MPI_Comm, MPI_Request
    TYPE(*), DIMENSION(..), ASYNC   :: buf
    integer, value,     intent(in)  :: count
    type(MPI_Datatype), intent(in)  :: datatype
    integer, value,     intent(in)  :: source
    integer, value,     intent(in)  :: tag
    type(MPI_Comm),     intent(in)  :: comm
    type(MPI_Request),  intent(out) :: request
    integer,            intent(out) :: err
END SUBROUTINE MPI_Irecv

SUBROUTINE MPI_Isend(buf, count, datatype, dest, tag, comm, request,  
err) BIND(C, name="MPI_Isend")
    import MPI_Datatype, MPI_Comm, MPI_Request
    TYPE(*), DIMENSION(..), ASYNC   :: buf
    integer, value,     intent(in)  :: count
    type(MPI_Datatype), intent(in)  :: datatype
    integer, value,     intent(in)  :: dest
    integer, value,     intent(in)  :: tag
    type(MPI_Comm),     intent(in)  :: comm
    type(MPI_Request),  intent(out) :: request
    integer,            intent(out) :: err
END SUBROUTINE MPI_Isend

SUBROUTINE MPI_Wait(request, status, err) BIND(C, name="MPI_Wait")
    import MPI_Request, MPI_Status
    type(MPI_Request), intent(in)  :: request  !	inout?
    type(MPI_Status),  intent(out) :: status
    integer,           intent(out) :: err
END SUBROUTINE MPI_Wait

end interface