[MPI3 Fortran] Proposing changes to Fortran 2008
donev1 at llnl.gov
Thu Mar 27 12:01:11 CDT 2008
On Thursday 27 March 2008 07:53, Hubert Ritzdorf wrote:
> This is what some vendors already implemented (using different names).
> When I remember (and understood)
> it correctly, you mentioned that they didn't it right.
They have not integrated it with the rest of the standard. You can do many
things that apparently work fine until you try them with polymorphic
assumed-shape arrays of a parameterized derived type (yes, Fortran has
those). A standard has to be internally fully self-consistent...it cannot
just say "oh, no one will do that, just make it work".
> Since the
> behaviour should be well-defined if an
> interface description is not available and the vendors have to implement
> it, and it should be possible
> to specify it for a single argument.
The existing standard says the following about implicit interfaces. A
routine's interface can be omitted, however, the interface exists and there
is a *unique* interface (modulo some attributes that can be omitted). That
is, an MPI_Send routine, which exists somewhere, according to the Fortran
standard, has to either accept an integer buffer or a real buffer. It cannot
accept both. There is a *unique* interface. If the interface is implicit, it
is up to the user to ensure the actual arguments actually match the type/rank
etc. of the dummy. If it is explicit, the compiler helps check it. That is
the ONLY difference between implicit and explicit interfaces.
Now, compilers often allow users to violate the standard and call MPI_Send
with both a real and an integer buffer array, and they either do not do any
analysis to detect it, or if they do, they offer a compiler switch to
say "close your eyes please". It is however, not conforming to the Fortran
> There are 2 general problems for MPI with these copy-in/copy-out behaviour:
I understand the problems it causes with MPI. But, it also make a lot of
wonderful things work in Fortran. You cannot remove it from Fortran just to
accommodate MPI. Saying "don't do copy in/out" will only work if you specify,
in details, what to do about situations where copy in/out is required. Should
we disallow those situations (i.e., put rules like "the actual argument shall
not be..."). Do some existing Fortran codes rely on copy in/out, for example:
CALL MPI_Send(my_array(1,:),...) ! Will copy column 1 into a contig buffer
Should we now make those programs non-conforming? I am not offering answers
here, I am pointing out that explicit answers are required.
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