[MPI3 Fortran] Proposing changes to Fortran 2008

[Jeff Squyres]

On Mar 18, 2008, at 10:08 AM, Craig Rasmussen wrote:

>> I admit that I know so little about Fortran that much of your mail
>> went over my head.  :-(  But a question about the above: is it
>> possible to allow non-contiguous datatypes to be passed through
>> without having the compiler copy them into contiguous areas?  It may
>> be possible, for example, for the MPI datatype to accurately describe
>> the non-contiguousness of the data, and MPI is well-equipped to  
>> handle
>> such situations.
>
> I'm not sure I understand MPI datatypes to respond coherently, but
> that's never stopped me before :-)
>
> I think you really want contiguous"ness", otherwise you wouldn't know
> how to stride through the data with constant offsets.  Right?


No.

MPI datatypes are a rich abstraction that allow the description of a  
variety of different layouts in memory (such as constant strides,  
etc.).  Specifically: in C, there is no requirement for a contiguous  
buffer.  As long as the accompanying MPI datatype accurately describes  
the buffer, you're good to go.

This is one of the reasons that MPI requires a datatype argument --  
because even though modern languages such as C++ and Fortran can  
derive a type from the buffer argument, what if the actual memory  
layout differs from that of the buffer argument type?  For example:

struct foo {
   char c;
   double d;
}

struct foo instance;
MPI_Send(&instance->c, ...);

What is passed is a pointer to a character.  But what you really want  
to send is the whole struct (which likely has a big hole in its memory  
map).  Likewise, the type in the following call doesn't tell anything  
to MPI, and an explicit datatype is required:

MPI_Send(&instance, ...)

And note that &instance == &instance->c.

-- 
Jeff Squyres
Cisco Systems