[MPI3 Fortran] Proposed solution for the volatile buffer problem

[Bill Long]

I've thought about the various proposed solutions to the problem of 
protecting a buffer used for a split send/receive - wait pair of calls.  
In particular, the need to prevent motion of code involving the buffer 
across the wait call. Most of them seem to have at least one defect.  
Here is a possible solution (not entirely original) that I think works, 
and involves minimal and localized changes in the Fortran spec:

Introduce a new subprogram prefix named VOLATILE.

A VOLATILE subroutine:

1) must also be BIND(C)

2) may modify any variable accessible in the program unit that calls the 
subroutine

The VOLATILE prefix is not required to match between an interface and 
the actual definition of the subprogram.

Restriction (1) gets around the objection that the concept really makes 
no sense in the Fortran-Fortran case.  Restriction (2) will require that 
variables be reloaded from memory after a call to a volatile subroutine, 
and that code motion across the call is effectively blocked. 

This scheme can be implemented entirely in a module with interfaces for 
the MPI routines, avoiding user code changes.  Vendors understand what 
"volatile" means for variables, and this usage is a very natural 
extension.  This should be easy to implement. And avoids adding more 
keywords to Fortran.

In practice, the VOLATILE prefix would be added to the interface for the 
"wait" subroutine. 

There is some concern about the performance hit from register spills 
before the CALL, but since you would possibly be waiting for the 
subroutine to complete anyway, it might not have an noticeable 
performance hit in actual practice.

Cheers,
Bill



-- 
Bill Long                                   longb at cray.com
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