Small optimization or zero length messages.

This commit was SVN r12414.

ompi/mca/mtl/base/mtl_base_datatype.h

@@ -35,9 +35,15 @@ ompi_mtl_datatype_pack(struct ompi_convertor_t *convertor,
     size_t max_data;
 
     ompi_convertor_get_packed_size(convertor, &max_data);
+    if( 0 == max_data ) {
+        *freeAfter  = false;
+        *buffer     = NULL;
+        *buffer_len = 0;
+        return OMPI_SUCCESS;
+    }
     iov.iov_len = max_data;
 
-    if (max_data > 0 && ompi_convertor_need_buffers(convertor)) {
+    if (ompi_convertor_need_buffers(convertor)) {
         iov.iov_base = malloc(max_data);
         if (NULL == iov.iov_base) return OMPI_ERR_OUT_OF_RESOURCE;
         *freeAfter = true;
@@ -67,7 +73,13 @@ ompi_mtl_datatype_recv_buf(struct ompi_convertor_t *convertor,
 
     ompi_convertor_get_packed_size(convertor, &max_data);
 
-    if (max_data > 0 && ompi_convertor_need_buffers(convertor)) {
+    if( 0 == max_data ) {
+        *buffer = NULL;
+        *free_on_error = false;
+        *buffer_len = 0;
+        return OMPI_SUCCESS;
+    }
+    if (ompi_convertor_need_buffers(convertor)) {
         *buffer = malloc(max_data);
         *free_on_error = true;
     } else {
@@ -95,7 +107,6 @@ ompi_mtl_datatype_unpack(struct ompi_convertor_t *convertor,
     iov.iov_base = buffer;
     max_data = iov.iov_len;
 
-    
     if (max_data > 0 && ompi_convertor_need_buffers(convertor)) {
         ompi_convertor_unpack(convertor, &iov, &iov_count, &max_data );