Longer types for use with MPI_MINLOC and MPI_MAXLOC

[mpiforumbot]

Originally by jedbrown on 2012-01-28 12:39:35 -0600

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Migrated to mpi-forum/mpi-issues#18

[mpiforumbot]

Originally by jhammond on 2015-11-09 10:56:40 -0600

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I withdrew my related ticket but I want to add the text here because I think there is value to adding the word "AND" to these types to make them more clear.

Replying to [#342 jhammond]:

MPI_FLOAT_INT, MPI_LONG_INT, MPI_DOUBLE_INT, MPI_SHORT_INT, MPI_2INT and MPI_LONG_DOUBLE_INT are an inadequate set of built-in datatypes for use with MPI_{MAX,MIN}LOC, especially since MPI_Accumulate does not permit user-defined reductions. There are very good use cases for MPI_2LONG, for example, in sparse graph work by the PETSc team (Jed Brown and Matt Knepley).

The set of types that are valid for use with MPI_{MAX,MIN}LOC should be expanded to include all pairs of built-in datatypes with one another. The reason is that the location (the second element in the pair) need not be a location in a computer science sense, i.e. an offset or address, but rather can be an arbitrary tag.

I see from Ticket #145 that this might cause issues with name uniqueness, hence it may be sensible to add "AND" between the two types when they are different. Thus MPI_LONG_AND_LONG_INT is distinguished from MPI_LONG_LONG_AND_INT as well as the scalar type MPI_LONG_LONG_INT.

If people don't like LOC being a non-integral type, at least let LOC be LONG in addition to INT for all existing pair types.