[MPI3 Fortran] WG mission statement / bullets

[Bill Long]

Iain Bason wrote:
> 
> On Sep 3, 2009, at 5:14 AM, N.M. Maclaren wrote:
> 
>> I haven't seen the mission statement for the same reason as Malcolm,
>> but would favour MPI using Fortran default integers whereever they
>> make sense.  Fortran's semantic specification of them is still useful
>> in a way that C's isn't.
> 
> I believe that the idea behind using C int (or, more generally, 
> something like a MPI_DEFAULT_INT kind) was to allow people to build a 
> library that would work regardless of whether the application is 
> compiled with -i8.  As it stands, the MPI Fortran library has to be 
> compiled with -i8 if the application is.  If people use an explicit kind 
> for their integers, then one library can serve applications with and 
> without -i8.

With the current MPI setup, we (as a vendor) supply two sets of 
module/library, compiled with 32-bit and 64-bit defaults respectively. 
The compiler figures out which to use based on what the meaning of 
"default" is based on the compile options. This is extra work to 
maintain, but it works for the user (mostly). Until some clown wants -r8 
-i4, in which case a third version is needed.

The advantage of explicitly specifying a KIND for the MPI dummy 
arguments is that it encourages (forces) users to supply actual 
arguments with the correct kinds.  This avoids hidden problems of 
supplying an actual argument with a value that is too large to fit into 
the 'int' that is used in the actual implementation routine.

The downside is that some existing codes will start getting error 
messages about mismatched interfaces.


> 
> There are, of course, other possible solutions to the problem.  Is there 
> one in particular that you favor?  Or do you think the problem shouldn't 
> be solved?


I think the better alternative is to make the user-called names generic, 
and have the interface specify two specific interfaces, one for 32-bit 
and one for 64-bit.  Both specifics would correspond to wrapper 
routines; in the wrappers you would cast the integer arguments to 
integer(c_int) before passing them on to the underlying C MPI functions. 
Perhaps you would need 3 specifics if you want to cover the dreaded 
-r8/-i4 case as well.

The advantage of the generic version is that the vendor need only supply 
one MPI module and one library.  This simplifies the implementation and 
also avoids errors such as the user forgetting to include the -i8 on the 
link command to trigger getting the 'right' library.

Cheers,
Bill



> 
> Iain
> 
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-- 
Bill Long                                   longb at cray.com
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