MPI communicator type in Fortran 2008

Question

I have been trying to build an application which seems to be mixing files that use mpi_f08 and files that use mpi. The problem is that it failed at compiling some MPI calls. It took me a while to figure out that it was a communicator data type the real problem:

error #6285: There is no matching specific subroutine for this generic subroutine call. [MPI_BCAST]

Here it is an extracted section:

module submod
  use mpi_f08
  implicit none

contains

  subroutine callmpi(pe, nproc, comm)
    integer :: pe,nproc
    integer :: comm
    integer :: ierr
    integer :: sbuf
    sbuf = 0
    call mpi_bcast( sbuf, 1, mpi_integer, 0, comm, ierr )
  end subroutine callmpi

end module submod

Changing the comm datatype from integer to type(mpi_comm) fixes the problem. Now, the application is huge and I was wondering if there was a compiler flag (Intel) that could do this without modifying the code.

Answer 1

Compiler flags are not magic.

In the Fortran 2008 MPI bindings the interface of MPI_Bcast is, as noted, not compatible with the "older" Fortran bindings. The (MPI standard) mpi_f08 module does not provide a generic MPI_Bcast with a specific procedure matching this other binding.

A Fortran compiler does not know how to map an integer to a different type and it's extreme to expect a Fortran compiler to special-case MPI.

Now, you modified your use mpi (or include 'mpif.h') code to move to use mpi_f08, so you can make further modifications. To incrementally update your codebase to the Fortran 2008 bindings you can use a renaming technique like:

  use mpi_f08, MPI_Bcast_unported => MPI_Bcast
  use mpi, only : MPI_Bcast

  call MPI_Init        ! F2008 binding, with optional ierror absent
  call MPI_Bcast(...)  ! Pre-F2008 binding call