The follwing discussion is (partially) independent from RMA. The example is about MPI_Irecv and MPI_Wait.
Comment (by gropp):
I fully agree with Dave. As I've said before, MPI should only talk about MPI. What the compiler might to with non-MPI is not in scope for us. Yes, this makes it difficult to provide examples that are guaranteed to work; that's part of the point of Boehm's article. But that doesn't change the fact that the MPI Forum must not specify the behavior of C (or Fortran) codes that happen to be accessing shared memory.
-- Ticket URL: <https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/456#comment:16>
For me it looks like that you give up with the important goal that the MPI standard should provide source code portability. I mean source code portability for MPI applications written in one of the supported language bindings. For me, the MPI standard has to specify all that is needed that a source code that is consistent with the MPI standard is portable from one MPI library with companion compiler to another MPI library with its companion compiler. With MPI-3.0 and the new Fortran interface, we tried to achieve this goal. Maybe that further investigation is needed. People always thought that there is no such problem with C. MPI-3.0 Section 17.1.20 "Comparison with C" tries to say this. As I learned in this discussion, such expectation about brave C is completely wrong, as seen in https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/456#comment:13 There is no source code portability for the MPI C Interface if C = C99 or older: Such a small code for(...) { MPI_Irecv(&buf,...,&rq) MPI_Wait(&rq,...) print(...) buf } may be compiled by C99 (not C11) into reg0=buf; for(...) { buf=reg0; MPI_Irecv(&buf,...,&rq) reg0=buf; buf=reg0; MPI_Wait(&rq,...) with internally: buf=received_value reg0=buf; print(...) reg0 } buf=reg0; And with an MPI library making progress between MPI_Irecv and MPI_Wait, the outcome may be reg0=buf; for(...) { buf=reg0; MPI_Irecv(&buf,...,&rq) reg0=buf; with internally by the MPI lib progress engine: buf=received_value buf=reg0; /* i.e. the received value is overwritten by the old value!!! */ MPI_Wait(&rq,...) reg0=buf; print(...) reg0 } buf=reg0; Or in other words, according to - MPI-3.0 Section 17.1.20, and - https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/456#comment:13 already MPI-1.1 for C required the C11/C11++ memory model, which does not allow such optimizations. This has direct influence on the wording we use in MPI-3.0 Section 2.6. Saying "ISO C is enough" is outdated, i.e., closing the eyes in front of the knowledge that exists. I would like that the MPI Forum gets this right, in C and Fortran. Rolf -- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)