[MPI3 Fortran] MPI buffer arguments - void and call-by-reference

[Aleksandar Donev]

Hi,

> Um.  While that is true, to a first approximation, it isn't really.  The
> reason is that all three of those have subtly different constraints on
> when a compiler must not use copy-in/copy-out and other optimisations. 
While this is true, I am really not sure what your message was meant to 
illustrate or clarify...it was confusing even to me (and I am at least 
familiar with the subtleties).

> The only one of those that ABSOLUTELY blocks copy-in/copy-out and the
> more-or-less equivalent optimisations is VOLATILE.  ASYNCHRONOUS does
> the least blocking, and is therefore the most efficient.
As far as copy in/out is concerned, VOLATILE and ASYNCHRONOUS are 
identical as far as the standard is concerned. Specifically, a situation 
where copy in/out may be required to be performed is prohibited by 
compile-time constraints. This is not quite identical to saying "though 
shall not perform copy in/out", but the reason the compile-time 
constraints are there is so as to ensure that the argument passing can 
be done by pass-by-reference.

I never objected to using ASYNCHRONOUS to handle non-blocking data 
transfers, as you know well. However, I want to (partially) agree with 
Bill that to truly properly integrate non-blocking operations with the 
Fortran standard *both* the TARGET and the ASYNCHRONOUS attribute are 
required for the buffer, both the dummy *and* the actual. The 
ASYNCHRONOUS attribute does not have guarantees about pointer 
association persisting across argument passing (this is equivalent to 
guarantees against copy in/out)...only TARGET does and it is the 
intended way to allow routines to save pointers to the actual inside a 
routine (which MPI does). At the same time, TARGET does not allow you to 
asynchronously change the value of the target. So both are needed.

That said, the ASYNCHRONOUS attribute, or a new attribute with similar 
semantics, can be extended to include the effects of pointer association 
argument passing semantics of TARGET.

Best,
Aleks