[MPI3 Fortran] MPI-2.1: Fortran 90 bindings

[Jeff Squyres]

The current MPI-2 F90 bindings are un-implementable because they  
require almost 7 *million* interface functions: with 15 intrinsic  
Fortran types, each with 7 possible dimensions, the current F90  
interface *requires*:

- 50 MPI functions with one choice buffer: 15 * 7 * 50 = 5,250 functions
- 25 MPI functions with two choice buffers: (15 * 7 * 25)^2 = 6.8M  
functions
- ...and a few hundred more MPI functions with no choice buffers

This is clearly broken.

An MPI-2.1-worthy solution could be to add a global statement saying  
that MPI functions with two choice buffers are explicitly not included  
in the F90 bindings.  F90 MPI applications can transparently fall back  
to the F77 bindings (although they won't get the strong type checking,  
etc.).  This idea therefore fits the requirement of not breaking any  
existing MPI codes.

Specifically: if we remove the requirement to provide all MPI  
functions with two choice buffers, 5,000+ F90 interface functions is  
[a pain but] implementable.  I doubt that any current MPI  
implementation provides more than the no-choice-buffers plus one- 
choice-buffer functions anyway...

If this seems like a good idea, I can write a proper proposal for it.   
Comments?

-- 
Jeff Squyres
Cisco Systems