early exit when every proc allocates 0

fixes issue nwchemgit#13

Author: jeffhammond

src/gmr.c

@@ -30,7 +30,6 @@ static pthread_mutex_t gmr_list_mutex = PTHREAD_MUTEX_INITIALIZER;
   */
 gmr_t *gmr_create(gmr_size_t local_size, void **base_ptrs, ARMCI_Group *group) {
   int           i;
-  gmr_size_t    aggregate_size;
   int           alloc_me, alloc_nproc;
   int           world_me, world_nproc;
   MPI_Group     world_group, alloc_group;
@@ -40,6 +39,21 @@ gmr_t *gmr_create(gmr_size_t local_size, void **base_ptrs, ARMCI_Group *group) {
   ARMCII_Assert(local_size >= 0);
   ARMCII_Assert(group != NULL);
 
+  /* determine if the GMR construction is pointless and exit early */
+  {
+    gmr_size_t max_local_size = 1<<30;
+
+    /* if gmr_size_t changes from long, this needs to change... */
+    MPI_Allreduce(&local_size, &max_local_size, 1, MPI_LONG, MPI_MAX, group->comm);
+
+    if (max_local_size==0) {
+      for (i = 0; i < alloc_nproc; i++) {
+        base_ptrs[i] = NULL;
+      }
+      return NULL;
+    }
+  }
+
   MPI_Comm_rank(group->comm, &alloc_me);
   MPI_Comm_size(group->comm, &alloc_nproc);
   MPI_Comm_rank(ARMCI_GROUP_WORLD.comm, &world_me);
@@ -114,24 +128,6 @@ gmr_t *gmr_create(gmr_size_t local_size, void **base_ptrs, ARMCI_Group *group) {
   MPI_Allgather(  &gmr_slice, sizeof(gmr_slice_t), MPI_BYTE,
                  alloc_slices, sizeof(gmr_slice_t), MPI_BYTE, group->comm);
 
-  /* Check for a global size 0 allocation */
-  for (i = aggregate_size = 0; i < alloc_nproc; i++) {
-    aggregate_size += alloc_slices[i].size;
-  }
-
-  /* Everyone asked for 0 bytes, return a NULL vector */
-  if (aggregate_size == 0) {
-    free(alloc_slices);
-    free(mreg->slices);
-    MPI_Win_free(&mreg->window);
-    free(mreg);
-
-    for (i = 0; i < alloc_nproc; i++)
-      base_ptrs[i] = NULL;
-
-    return NULL;
-  }
-
   /* Populate the base pointers array */
   for (i = 0; i < alloc_nproc; i++)
     base_ptrs[i] = alloc_slices[i].base;