How do I retain precision for a Fortran MPI program in a portable way?

Question

I have a Fortran program where I specify the kind of the numeric data types in an attempt to retain a minimum level of precision, regardless of what compiler is

Answer 1

Use the Fortran 2008 intrinsic STORAGE_SIZE to determine the number bytes that each number requires and send as bytes. Note that STORAGE_SIZE returns the size in bits, so you will need to divide by 8 to get the size in bytes.

This solution works for moving data but does not help you use reductions. For that you will have to implement a user-defined reduction operation. If that's important to you, I will update my answer with the details.

For example:

program main

   use mpi

   implicit none

   integer, parameter :: rsp = selected_real_kind(16)
   integer :: err
   integer :: rank

   real(rsp) :: real_var

   call MPI_Init(err)
   call MPI_Comm_rank(MPI_COMM_WORLD,rank,err)

   if (rank.eq.0) then
      real_var = 1.123456789012345
      call MPI_Send(real_var,storage_size(real_var)/8,MPI_BYTE,1,5,MPI_COMM_WORLD,err)
   else
      call MPI_Recv(real_var,storage_size(real_var)/8,MPI_BYTE,0,5,MPI_COMM_WORLD,&
         MPI_STATUS_IGNORE,err)
   end if

   print *, rank, real_var

   call MPI_Finalize(err)

end program main

I confirmed that this change corrects the problem and the output I see is:

   0   1.12345683574676513672      
   1   1.12345683574676513672