[mpich-devel] MPIR_ERRTEST_ARGNULL and count=0 (or type_size=0)

[Jeff Hammond]

It seems that I didn't understand MPI_BOTTOM completely, so the error
checks needed to be different than described below, but the high-level
issues are the same.

Interested parties may consult
http://trac.mpich.org/projects/mpich/ticket/1863 for details and
further discussion.

Jeff

On Wed, May 29, 2013 at 7:41 AM, Jeff Hammond <jhammond at alcf.anl.gov> wrote:
> I see Jim used MPIR_ERRTEST_ARGNULL in (r)get_accumulate, which makes
> sense (I am working on verifying that they should be in the others as
> well).  However, these calls are not guarded by "if (count)", which
> means that one presumably (I have not written a test for it but it
> seems obvious) cannot use addr=NULL with count=0.  It seems to me that
> 0-byte transfers are find with NULL buffers (this was recently
> verified for MPI_Ssend because it's a common way to implement p2p
> synchronization).
>
> It seems prudent to change something like this:
>
>             MPIR_ERRTEST_COUNT(origin_count, mpi_errno);
>             MPIR_ERRTEST_DATATYPE(origin_datatype, "origin_datatype",
> mpi_errno);
>             MPIR_ERRTEST_ARGNULL(origin_addr, "origin_addr", mpi_errno);
>
> To this:
>
>             MPIR_ERRTEST_COUNT(origin_count, mpi_errno);
>             MPIR_ERRTEST_DATATYPE(origin_datatype, "origin_datatype",
> mpi_errno);
>             if (origin_count) MPIR_ERRTEST_ARGNULL(origin_addr,
> "origin_addr", mpi_errno);
>
> Then I got to thinking, what if someone - almost certainly not
> intentionally but because of a general implementation - uses
> MPI_Type_vector to create a zero-length vector type.  I don't see why
> one can't do a count>0 transfer with with a NULL buffer if the
> datatype is empty.
>
> I wrote the following test of a 0-length vector and see that it works
> just fine with bcast.
>
> #include <stdio.h>
> #include <stdlib.h>
>
> #include <mpi.h>
>
> int main(int argc, char ** argv) {
>   int      rank, nproc;
>   MPI_Datatype emptyvec;
>   MPI_Aint lb, ext;
>
>   MPI_Init(&argc, &argv);
>
>   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
>   MPI_Comm_size(MPI_COMM_WORLD, &nproc);
>
>   if (rank == 0) printf("Starting MPI datatype test with %d
> processes\n", nproc);
>
>   MPI_Type_vector(0,0,0,MPI_DOUBLE,&emptyvec);
>   MPI_Type_commit(&emptyvec);
>   MPI_Type_get_extent(emptyvec, &lb, &ext);
>   MPI_Type_size(emptyvec, &size);
>   if (rank == 0) printf("lb = %zu ext = %zu size = %d \n", lb, ext, size);
>
>   MPI_Bcast(NULL, 0, emptyvec, 0, MPI_COMM_WORLD);
>   MPI_Bcast(NULL, 1, emptyvec, 0, MPI_COMM_WORLD);
>
>   MPI_Type_free(&emptyvec);
>
>   if (rank == 0) printf("Test complete: PASS.\n");
>
>   MPI_Finalize();
>
>   return 0;
> }
>
>  This suggests that it should be valid to use addr=NULL and count>0 as
> long as the extent of the type is 0, hence the aforementioned code
> should really be:
>
>             int origin_datatype_size;
>             ...
>             MPIR_ERRTEST_COUNT(origin_count, mpi_errno);
>             MPIR_ERRTEST_DATATYPE(origin_datatype, "origin_datatype",
> mpi_errno);
>             MPID_Datatype_get_size_macro(origin_datatype, origin_datatype_size);
>             if (origin_count && origin_datatype_size)
> MPIR_ERRTEST_ARGNULL(origin_addr, "origin_addr", mpi_errno);
>
> Do people agree with this reasoning?
>
> Thanks,
>
> Jeff
>
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> University of Chicago Computation Institute
> jhammond at alcf.anl.gov / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
> ALCF docs: http://www.alcf.anl.gov/user-guides



-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
ALCF docs: http://www.alcf.anl.gov/user-guides