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

Not really an answer, but we have the same problem and use something like this:

!> Number of digits for single precision numbers
integer, parameter, public :: single_prec = 6
!> Number of digits for double precision numbers
integer, parameter, public :: double_prec = 15
!> Number of digits for extended double precision numbers
integer, parameter, public :: xdble_prec = 18
!> Number of digits for quadruple precision numbers
integer, parameter, public :: quad_prec = 33

integer, parameter, public :: rk_prec = double_prec

!> The kind to select for default reals
integer, parameter, public :: rk = selected_real_kind(rk_prec)

And then have an initialization routine where we do:

!call mpi_type_create_f90_real(rk_prec, MPI_UNDEFINED, rk_mpi, iError)
!call mpi_type_create_f90_integer(long_prec, long_k_mpi, iError)
! Workaround shitty MPI-Implementations.
select case(rk_prec)
case(single_prec)
  rk_mpi = MPI_REAL
case(double_prec)
  rk_mpi = MPI_DOUBLE_PRECISION
case(quad_prec)
  rk_mpi = MPI_REAL16
case default
  write(*,*) 'unknown real type specified for mpi_type creation'
end select
long_k_mpi = MPI_INTEGER8

While this is not nice, it works reasonably well, and seems to be usable on Cray, IBM BlueGene and conventional Linux Clusters. Best thing to do is push sites and vendors to properly support this in MPI. As far as I know it has been fixed in OpenMPI and planned to be fixed in MPICH by 3.1.1. See OpenMPI Tickets 3432 and 3435 as well as MPICH Tickets 1769 and 1770.