"Jeff Squyres \(jsquyres\) via mpi-forum" <[email protected]> writes:

Let me ask a simple question: how will users to write portable MPI programs in C with large count values?

Answer: they will explicitly call MPI_Send_x(), and not rely on C11 _Generic.

Few packages will accept a hard dependency on MPI-4 for at least 10 years. MS-MPI still doesn't fully support MPI-2.1, for example, and PETSc only recently began requiring MPI-2.0. Instead, each package that wishes to upgrade to MPI_Count args will write configure tests (autoconf, etc.) to detect availability of MPI_Send_x (as individual functions, not as MPI_VERSION == 4) and define macros/wrappers OurMPI_Send() that forwards to MPI_Send_x (when available) or MPI_Send (otherwise). When the implementation doesn't provide MPI_Send_x, they'll either have a smart wrapper that errors at run-time on truncation or (likely most apps) will silently fail with an FAQ that suggests using a compliant MPI-4 implementation. I'm not taking a position on C11 _Generic in the standard, but it would significantly reduce the configure complexity for apps to upgrade to MPI_Count without dropping support for previous standards.

Which then raises the question: what's the point of using C11 _Generic? Its main functionality can lead to [potentially] silent run-time errors, and/or require additional error checking in every single code path by the implementation. That just seems like bad design, IMNSHO. That's why the WG decided to bring this to the Forum list (especially given the compressed timeframe for MPI-4).