[MPI3 Fortran] Summary of items on the table

[Craig Rasmussen]

There has been quite a lot of traffic in the past couple of months  
regarding possible directions for a new MPI binding for Fortran.  I  
thought now would be a good time to summarize and try to get a  
consensus on what direction we should pursue.  Some choices will  
require changes to the Fortran standard and this input we need to get  
to the Fortran standards committee within the next two weeks.

The major choice we need to make is how to declare an MPI "choice  
buffer" (void *) in a Fortran interface.  I'll list the options below  
with advantages and disadvantages as I see them.  Please rank these  
options (they all will use F2003 BIND(C)).

1. Map void* to C_PTR.  Upside: this will work now in most compilers  
and likely all soon.  Downside: users must change the call site to  
use C_LOC to obtain the C address; C_LOC requires the POINTER or  
TARGET attribute and may reduce performance and require additional  
code changes.

2. Map void* to a new Fortran type C_VOID.  Upside: no changes to  
current code (I think). Downside: would require a change in the  
Fortran standard and couldn't be done until Fortran 20xx (a long time  
before we see it in compilers).

3. Map void* to type(C_VOID) defined in the ISO_C_BINDING intrinsic  
module and add LANG=C to the Fortran standard.  Upside: no changes to  
current code other than to USE the MPI3 module.  Downside: it will  
require changes to the Fortran standard but can get to users  
relatively soon (before F2008 appears in compilers).

4. Add an IGNORE attribute with parameters T (type) K (kind) R (rank)  
for BIND(C) interfaces.  This pretty much (as I understand it) does  
the same thing as #3, just with different syntax and with slightly  
more flexibility (partial type checking).  Upside: see #3.  Downside:  
see #3.

Aleks has submitted a proposal to the Fortran J3 committee to improve  
on the ASYNCHRONOUS attribute.  Hopefully this can make it into the  
F2008 standard and fix problems associated with copyin/copyout  
semantics and with code motion associated with asynchronous MPI  
calls.  A question I have is there a problem with copyin/copyout for  
synchronous MPI?

I'm currently at the April MPI Forum where I'll collect votes.   
Others please add your feedback (rank the options) in email (actually  
it would be helpful for the record to get email from everyone within  
shouting distance).

A question just came up about passing CHARACTER parameters.  Comments?

Regards,
Craig