coll/base: optimize handling of zero-byte datatypes in mca_coll_base_alltoallv_intra_basic_inplace()

Signed-off-by: Gilles Gouaillardet <[email protected]>

(back-ported from commit d1c5955)

Author: ggouaillardet

ompi/mca/coll/base/coll_base_alltoallv.c

@@ -45,17 +45,16 @@ mca_coll_base_alltoallv_intra_basic_inplace(const void *rbuf, const int *rcounts
     int i, j, size, rank, err=MPI_SUCCESS;
     ompi_request_t *req;
     char *allocated_buffer, *tmp_buffer;
-    size_t max_size, rdtype_size;
-    OPAL_PTRDIFF_TYPE ext, gap;
+    size_t max_size;
+    OPAL_PTRDIFF_TYPE ext, gap = 0;
 
     /* Initialize. */
 
     size = ompi_comm_size(comm);
     rank = ompi_comm_rank(comm);
-    ompi_datatype_type_size(rdtype, &rdtype_size);
 
     /* If only one process, we're done. */
-    if (1 == size || 0 == rdtype_size) {
+    if (1 == size) {
         return MPI_SUCCESS;
     }
 
@@ -67,6 +66,10 @@ mca_coll_base_alltoallv_intra_basic_inplace(const void *rbuf, const int *rcounts
     }
     /* The gap will always be the same as we are working on the same datatype */
 
+    if (OPAL_UNLIKELY(0 == max_size)) {
+        return MPI_SUCCESS;
+    }
+
     /* Allocate a temporary buffer */
     allocated_buffer = calloc (max_size, 1);
     if (NULL == allocated_buffer) {
@@ -78,7 +81,7 @@ mca_coll_base_alltoallv_intra_basic_inplace(const void *rbuf, const int *rcounts
     /* in-place alltoallv slow algorithm (but works) */
     for (i = 0 ; i < size ; ++i) {
         for (j = i+1 ; j < size ; ++j) {
-            if (i == rank && 0 != rcounts[j] && 0 != rdtype_size) {
+            if (i == rank && 0 != rcounts[j]) {
                 /* Copy the data into the temporary buffer */
                 err = ompi_datatype_copy_content_same_ddt (rdtype, rcounts[j],
                                                            tmp_buffer, (char *) rbuf + rdisps[j] * ext);
@@ -93,7 +96,7 @@ mca_coll_base_alltoallv_intra_basic_inplace(const void *rbuf, const int *rcounts
                                           j, MCA_COLL_BASE_TAG_ALLTOALLV, MCA_PML_BASE_SEND_STANDARD,
                                           comm));
                 if (MPI_SUCCESS != err) { goto error_hndl; }
-            } else if (j == rank && 0 != rcounts[i] && 0 != rdtype_size) {
+            } else if (j == rank && 0 != rcounts[i]) {
                 /* Copy the data into the temporary buffer */
                 err = ompi_datatype_copy_content_same_ddt (rdtype, rcounts[i],
                                                            tmp_buffer, (char *) rbuf + rdisps[i] * ext);