[MPI3 Fortran] Serious problem with alignment of MPI_DOUBLE_PRECISION

[N.M. Maclaren]

On Sep 28 2011, William Gropp wrote:
>
>This entire problem comes from the natural but mistaken idea that the  
>MPI library should assign some default padding when creating a  
>datatype.  This is a bad idea because the alignment rules can depend  
>on compiler switches or other compiler-specific choices; for example,  
>some compilers allow the user to select between always 8 byte  
>alignment, leading to structures that might have gaps but permit the  
>uniform use of faster load operations or 4 byte alignment, but at a  
>potential performance cost.  In other words, even for a single  
>compiler, there need not be a unique choice of extent, and hence this  
>problem is insoluble.

Yes.  In fact, for Fortran derived types other than SEQUENCE and BIND(C)
and for all C structs, there is no reason for the rules to be the same
for every type even in a single file and compilation.  The only cases
where the rules must be the same for C are for compatible structs and
ones with a common initial sequence.

> Further, it is not necessary - the user can  
>either create the datatype with the appropriate extent (by using  
>MPI_UB) or resize it appropriately (with MPI_TYPE_CREATE_RESIZED since  
>MPI 2.0).  Rather than continue to get this wrong, I recommend simply  
>telling the user not to assume that the MPI library can always divine  
>how the compiler is going to chose to pad a derived type or struct.   
>We can add an advice to users here that explains the problem and  
>recommends using the existing MPI features to handle this problem.   
>This is better coding and will affect few users.

It assuredly avoids MPI getting tangled up in a particularly foul area
of the languages more than is necessary, and would help to future-proof
the specification!

As far as users are concerned, my limited survey of open-source scientific
codes found no examples that would be affected, and I have never seen a
user code that would, either.  However, that's merely my experience, and
I don't claim that it is representative of all MPI communities.

The issue of Fortran DOUBLE PRECISION versus C double in simple arrays
remains, but Fab Tillier has described what the Fortran standard itself
says about the matter.


Regards,
Nick Maclaren.