[MPI3 Fortran] Results of recent J3 Fortran meeting

Bill Long longb at cray.com
Thu Oct 21 17:59:42 CDT 2010

N.M. Maclaren wrote:
> On Oct 21 2010, Bill Long wrote:
>>> Except for the last sentence, that is true.  But one of MPI's major
>>> requirements is that it should be able to handle Fortran default types,
>>> and NOT force every program to use the C-interoperability KINDs to
>>> call MPI!  Sorry, but there IS an issue.
>> OK, I agree that there is a general issue for MPI with non-interoperable 
>> types.  We have a type code for that - CFI_type_unspecified.   I assume 
>> that, if the MPI routines are expecting data objects of Fortran types 
>> that have no corresponding C types (and hence there is little that can 
>> be done with them in C other than memory copies) then the actual intent 
>> is to only make memory copies.  In that case, knowing the size of a 
>> single character is unimportant. All you need is the base_addr, 
>> elem_len, rank, and dim[] information.
> That is true, but why do you assume that they have no corresponding C
> types?  C90 had a fixed number of integer types, but C99 has an arbitrary
> number.  It is also possible to translate between any number of bytes
> and an integer/character in C - all you need to know is how many there
> are.

I am assuming that the interoperable types are the ones we say are 
interoperable in Table 15.2 of the Fortran standard (and in the 
corresponding table in the TR).  If the table is missing some entries 
for types that correspond between Fortran and C, then we can expand the 
table.   The sort of games you can play with bits inside C is separate 
from interoperability of interfaces.

> What I said at the beginning, and which was my point, is that this approach
> is the least FUTURE-PROOF of the approaches, not that it was incompatible
> with the existing interoperability design.  And one of the things that I
> hope Fortran has learnt is that assuming that requirements aren't going to
> become important is a mistake.  It isn't possible to design for everything,
> but maximising the flexibility is definitely good.

I think we have good flexibility. The table of interoperable types is 
expandable, and we (with TYPE(*)) have provisions for non-interoperable 
types.  Flexibility is not the only goal. Another design goal is to 
avoid internal inconsistency and contradictions.  The rank expansion 
scheme fails that goal.


> Regards,
> Nick Maclaren.
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Bill Long                                           longb at cray.com
Fortran Technical Support    &                 voice: 651-605-9024
Bioinformatics Software Development            fax:   651-605-9142
Cray Inc./Cray Plaza, Suite 210/380 Jackson St./St. Paul, MN 55101

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