Deprecate MPI_PROD

[Wee-Free-Scot]

Problem

There are no known usages of MPI_PROD in application codes. Implementing MPI_PROD in RMA efficiently requires hardware that does not exist; being forced to implement a slow-path MPI_PROD for RMA compromises the fast-path implementation of commonly-used operators in RMA. Deprecating for RMA but not for reductions and scans would suggest there is a use-case for MPI_PROD in collectives, but there is no such use-case.

Proposal

Deprecate MPI_PROD throughout the MPI Standard, with a view to removing it in future.

Changes to the Text

Remove MPI_PROD from the table of predefined reduction operators (page 215&216, section 6.9.2).
Remove MPI_PROD from the table of collective operations [sic] (page 824, Appendix A).
Add a new section into the Deprecated Interfaces chapter recording the MPI_PROD constant (page 750, section 16.4).

Impact on Implementations

Joy and celebration.

Impact on Users

Meh.

References and Pull Requests

https://github.com/mpi-forum/mpi-standard/pull/703

[Wee-Free-Scot]

Here is a PDF with the proposed changes:
mpi40-report_Issue567_Deprecate_MPI_PROD.pdf

Removals on pages 215, 216, and 824, addition on page 750, adjustment to constant index on page 1054.

[wesbland]

[devreal]

@RolfRabenseifner dug up this code snippet which seems legit (i.e., not just a wrapper or test case): https://github.com/manolillo/palm4u/blob/1751eca6b334bb29c23f85fdb1d78f22da34eb7a/current_version/trunk/SOURCE/spectra_mod.f90#L690