On Dec 30 2008, Bill Long wrote:

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.

Sorry, but that dog won't hunt. Firstly, MPI non-blocking buffers have ASYNCHRONOUS semantics, and so can be updated at any time between the isend/irecv and the wait. In particular, many programs put wrappers around the MPI calls (often to enable RDMA transports as an option), and so the buffer variable may not be visible in the procedure that calls the wait. Secondly, there are big problems with all visible variables. Some variables are syntactically read-only, but that is the easy bit. What about DO-loop indices surrounding the wait, or arguments associated with expressions? That specification makes such usages undefined. Thirdly, it is horribly unstructured and leads to catastrophic inefficiency if extended to allow asynchronous update (as would be needed), because all visible variables would be effectively VOLATILE. Regards, Nick Maclaren, University of Cambridge Computing Service, New Museums Site, Pembroke Street, Cambridge CB2 3QH, England. Email: [email protected] Tel.: +44 1223 334761 Fax: +44 1223 334679