[MPI3 Fortran] MPI buffer arguments - void and call-by-reference

[Rolf Rabenseifner]

This is the second part of my two mails:

B) VOID buffer declaration
--------------------------

B.1 Current status

     INTEGER :: ibuf
     CALL MPI_SEND(ibuf,...)

     REAL, DIMENSION(100) :: rarr
     CALL MPI_SEND(rarr,...)
     CALL MPI_SEND(rarr(5:30:3),...)

     COMPLEX, DIMENSION(10,20,30) :: carr3dim
     CALL MPI_SEND(carr3dim,...)

     TYPE buff_type
      SEQUENCE
      INTEGER, DIMENSION(3):: i_val
      DOUBLE PRECISION, DIMENSION(5):: d_val
     END TYPE buff_type
     TYPE (buff_type) :: struct
     CALL MPI_SEND(struct,...)

     TYPE (buff_type), DIMENSION(10,20) :: struct2dim
     CALL MPI_SEND(struct2dim,...)

     ... ... ... ...

     All this works and the C-writte MPI_SEND receives the address
     of the first data element of each buffer.

     Currently, there isn't a F90 interface definition
     (i.e. only F77 style is used)

     MPI has its own interface to handle the data structure behind
     the starting pointer, the so called MPI datatypes.
     Such an MPI datatype  need not to describe the whole
     structure, i.e., there may be differences between the
     type information that was told the compiler and the MPI datatype
     information.


B.2 Goals

     B.2.1 To allow F90 interfaces that do not break eisting codes
     B.2.2 To minimize the burden for MPI libraries implementers
           and Fortran compiler implementers.

B.3 Maybe a simple but efficient solution

     SUBROUTINE MPI_SEND(buf,...
     VOID :: buf
     END

     This can be used only in a interface definition for routines
     that are implemented in C.
     There is no meaning if one wants to write the routine MPI_SEND
     in Fortran.

     Meaning of VOID buf:
     Exactly the same as if implicit (F77) interfaces are used.

     (Not needed for MPI, but a possible extension:

        Instead of VOID, one is using IMPLARG, meaning the
        same as VOID on the caller side and is also allowed
        on the soubroutine side.
        Meaning: Same argument handlich as with implicit interfaces,
                 i.e., handing over the pointer to the data or
                 in cases of noncontiguos subarrays only a pointer to
                 a contiguous copy of the data.
     )


C) Call by reference and not by in-copy-out-copy
------------------------------------------------

    It is clear, that F77 interfaces cannot do strided arrays
    directly with call-by-reference.

    For contiguous arrays and non-arrays, it isn't a problem.

    Fortran compilers are allowed to choose call-by-incopy-and-outcopy
    instead of call-by-reference.

    Maybe the following additional option may solve the problem:

    SUBROUTINE MPI_SEND(buf,...
    VOID, CALLBYREFERENCE :: buf

    Meaning: Same as what currently done, i.e., call-by-reference
    on the caller side, if call-by-reference is possible (i.e.,
    not for non-contiguous subarrays)


Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30)