Normally from the MPI point of view the count == 0 is LEGAL only for MPI_UB and MPI_LB. AND the MPI level

is responsible for checking this condition. At the datatype engine, I accept it just as a nice way to set
the soft UB for a data (without using a real UB marker). This approach can be used to create
the subarray and darray datatype. However from the MPI level this function should never be
called directly with a count set to 0.

This commit was SVN r4512.

src/datatype/dt_add.c

@@ -71,20 +71,25 @@ int ompi_ddt_add( dt_desc_t* pdtBase, const dt_desc_t* pdtAdd,
         }
     } else {
         place_needed = pdtAdd->desc.used;
-        if( count != 1 ) place_needed += 2;
+        if( count != 1 ) place_needed += 2;  /* for the loop markers */
     }
 
-    /* the count == 0 is LEGAL for MPI_UB and MPI_LB */
+    /* 
+     * the count == 0 is LEGAL only for MPI_UB and MPI_LB. I accept it just as a nice way to set
+     * the soft UB for a data (without using a real UB marker). This approach can be used to create
+     * the subarray and darray datatype. However from the MPI level this function should never be
+     * called directly with a count set to 0.
+     */
     if( count == 0 ) {
         if( pdtBase->desc.used == 0 ) {  /* empty datatype */
             pdtBase->lb = 0;
             pdtBase->ub = 0;
             pdtBase->true_lb = 0;
             pdtBase->true_ub = 0;
-        }
-        return 0;
+        } else
+            pdtBase->ub += disp;
+        return OMPI_SUCCESS;
     }
-
     /* compute the new memory alignement */
     pdtBase->align = IMAX( pdtBase->align, pdtAdd->align );
     
@@ -165,6 +170,7 @@ int ompi_ddt_add( dt_desc_t* pdtBase, const dt_desc_t* pdtAdd,
         /* should I add some space until the extent of this datatype ? */
     }
 
+    /* Recompute the new true_lb and true_ub */
     if( 0 == pdtBase->nbElems ) old_true_ub = disp;
     else                        old_true_ub = pdtBase->true_ub;
     pdtBase->size += count * pdtAdd->size;