Merge pull request #11103 from drwootton/mpi_bcast_zerodivide

Fix divide by zero error in calculate_num_nodes_up_to_level using MPI_Bcast with 2 tasks

Author: jjhursey

ompi/mca/coll/adapt/coll_adapt_topocache.c

@@ -59,7 +59,7 @@ static ompi_coll_tree_t *create_topology(
         {
             int fanout = ompi_comm_size(comm) - 1;
             ompi_coll_tree_t *tree;
-            if (fanout < 1) {
+            if (fanout <= 1) {
                 tree = ompi_coll_base_topo_build_chain(1, comm, root);
             } else if (fanout <= MAXTREEFANOUT) {
                 tree = ompi_coll_base_topo_build_tree(ompi_comm_size(comm) - 1, comm, root);

ompi/mca/coll/base/coll_base_topo.c

@@ -59,7 +59,12 @@ static int calculate_num_nodes_up_to_level( int fanout, int level )
 {
     /* just use geometric progression formula for sum:
        a^0+a^1+...a^(n-1) = (a^n-1)/(a-1) */
-    return ((pown(fanout,level) - 1)/(fanout - 1));
+    if (1 == fanout) {
+        return level;
+    }
+    else {
+        return ((pown(fanout,level) - 1)/(fanout - 1));
+    }
 }
 
 /*