Incorrect intent on Fortran 08 interfaces to MPI_All* routines

[PhilippOtte]

The intent of the receive buffer is declared incorrectly as intent(in) in all MPI_All* routines in the mpi-f08 module on the master branch of version 3.1. According to version 3.1 of the MPI standard (appendix A.3.3), the intent of the receive buffers should be intent(inout) (default). Note that the intent of the receive buffer in MPI_Recv is set according to the standard.

If the receive buffer is of intent(in), the compiler is free to optimize the routine in illegal ways.
For example, the compiler is allowed to interchange the following two calls:

call MPI_Alltoall(sndbuf,100,MPI_INT,recvbuf,100,MPI_INT,MPI_COMM_WORLD)
call MPI_Allreduce(sndbuf,recvbuf,1,MPI_INT,MPI_MAX,MPI_COMM_WORLD)

into:

call MPI_Allreduce(sndbuf,recvbuf,1,MPI_INT,MPI_MAX,MPI_COMM_WORLD)
call MPI_Alltoall(sndbuf,100,MPI_INT,recvbuf,100,MPI_INT,MPI_COMM_WORLD)

The problem could easily be fixed by setting the intent of recvbuf in all MPI_All* routines to intent(inout).

[ggouaillardet]

@PhilippOtte thanks for the bug report !

Fortunately, things are not so bad as they seem.

The mpi_f08.mod module is correctly generated since it is based on ompi/mpi/fortran/use-mpi-f08/mod/mpi-f08-interfaces.F90 (which is correct w.r.t. recvbuf intent).

The issue is in the function definition (e.g. ompi/mpi/fortran/use-mpi-f08/alltgather_f08.F90 and friends), but this is only used to build the libmpi_usempif08.so library.

Do you want to issue a PR by yourself ?

[PhilippOtte]

Thank you Gilles for the explanation. I will pose a PR with corrected function definitions for the MPI_All* and MPI_Iall* interfaces and their ompi interfaces.

[PhilippOtte]

I filed a PR. Please note that I was very careful while editing but was not able to compile the files as I did not manage to compile required parts of the code on my machine.

[jsquyres]

@ggouaillardet I made a v3.1.x (#5781) and v4.0.x (#5780) PRs corresponding the master PR you made (#5453). But I ran into conflicts trying to make a v3.0.x PR.

Could you make a backport PR for v3.0.x?