Don't invoke back-end coll functions if the count is zero (ddt

functions don't behave well when called with 0 counts)

This commit was SVN r2889.

src/mpi/c/allgather.c

@@ -46,6 +46,14 @@ int MPI_Allgather(void *sendbuf, int sendcount, MPI_Datatype sendtype,
         OMPI_ERRHANDLER_CHECK(err, comm, err, FUNC_NAME);
     }
 
+    /* Can we optimize?  Everyone had to give the same send signature,
+       which means that everyone must have given a sendcount > 0 if
+       there's anything to send. */
+
+    if (sendcount == 0) {
+      return MPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 
     err = comm->c_coll.coll_allgather(sendbuf, sendcount, sendtype, 

src/mpi/c/gatherv.c

@@ -114,6 +114,12 @@ int MPI_Gatherv(void *sendbuf, int sendcount, MPI_Datatype sendtype,
         }
     }
 
+    /* If we have nothing to do, just return */
+
+    if (0 == sendcount && ompi_comm_rank(comm) != root) {
+        return MPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 	
     err = comm->c_coll.coll_gatherv(sendbuf, sendcount, sendtype, recvbuf,

src/mpi/c/scatter.c

@@ -90,6 +90,13 @@ int MPI_Scatter(void *sendbuf, int sendcount, MPI_Datatype sendtype,
         }
     }
 
+    /* If we have nothing to receive, return success (everyone must
+       have given the same recvcount) */
+
+    if (0 == recvcount) {
+        return MPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 	
     err = comm->c_coll.coll_scatter(sendbuf, sendcount, sendtype, recvbuf,

src/mpi/c/scatterv.c

@@ -118,6 +118,12 @@ int MPI_Scatterv(void *sendbuf, int *sendcounts, int *displs,
         }
     }
 
+    /* If we have nothing to do, just return */
+
+    if (0 == recvcount && ompi_comm_rank(comm) != root) {
+        return MPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 	
     err = comm->c_coll.coll_scatterv(sendbuf, sendcounts, displs, sendtype,