Consistent handling of zero counts in the MPI API.

Signed-off-by: George Bosilca <bosilca@icl.utk.edu>

ompi/mpi/c/ibcast.c

@@ -96,6 +96,13 @@ int MPI_Ibcast(void *buffer, int count, MPI_Datatype datatype,
       }
     }
 
+    /* If there's only one node, or if the count is 0, we're done */
+
+    if ((OMPI_COMM_IS_INTRA(comm) && ompi_comm_size(comm) <= 1) ||
+        0 == count) {
+        return MPI_SUCCESS;
+    }
+
     OPAL_CR_ENTER_LIBRARY();
 
     /* Invoke the coll component to perform the back-end operation */

ompi/mpi/c/ireduce_scatter_block.c

@@ -97,6 +97,11 @@ int MPI_Ireduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
         OMPI_ERRHANDLER_CHECK(err, comm, err, FUNC_NAME);
     }
 
+    if (0 == recvcount) {
+        *request = &ompi_request_empty;
+        return MPI_SUCCESS;
+    }
+
     OPAL_CR_ENTER_LIBRARY();
 
     /* Invoke the coll component to perform the back-end operation */

ompi/mpi/c/reduce_scatter_block.c

@@ -94,6 +94,9 @@ int MPI_Reduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
         OMPI_CHECK_DATATYPE_FOR_SEND(err, datatype, recvcount);
         OMPI_ERRHANDLER_CHECK(err, comm, err, FUNC_NAME);
     }
+    if (0 == recvcount) {
+        return MPI_SUCCESS;
+    }
 
     OPAL_CR_ENTER_LIBRARY();