Modify the Weak Reference Set creation API to make it easier to create

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 35211)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -365,16 +365,26 @@
     while (queue != NULL) {
       WeakReferenceSet* reference_set = WeakReferenceSet::Pop(&queue);
       ASSERT(reference_set != NULL);
+      intptr_t num_keys = reference_set->num_keys();
+      intptr_t num_values = reference_set->num_values();
+      if ((num_keys == 1) && (num_values == 1) &&
+          reference_set->SingletonKeyEqualsValue()) {
+        // We do not have to process sets that have just one key/value pair
+        // and the key and value are identical.
+        continue;
+      }
       bool is_unreachable = true;
       // Test each key object for reachability.  If a key object is
       // reachable, all value objects should be marked.
-      for (intptr_t k = 0; k < reference_set->num_keys(); ++k) {
+      for (intptr_t k = 0; k < num_keys; ++k) {
         if (!IsUnreachable(*reference_set->get_key(k))) {
-          for (intptr_t v = 0; v < reference_set->num_values(); ++v) {
+          for (intptr_t v = 0; v < num_values; ++v) {
             visitor->VisitPointer(reference_set->get_value(v));
           }
           is_unreachable = false;
-          delete reference_set;
+          // Since we have found a key object that is reachable and all
+          // value objects have been marked we can break out of iterating
+          // this set and move on to the next set.
           break;
         }
       }
@@ -389,17 +399,16 @@
       DrainMarkingStack(isolate, visitor);
     } else {
       // Break out of the loop if there has been no forward process.
+      // All key objects in the weak reference sets are unreachable
+      // so we reset the weak reference sets queue.
+      state->set_delayed_weak_reference_sets(NULL);
       break;
     }
   }
-  // Deallocate any unmarked references on the delay queue.
-  if (state->delayed_weak_reference_sets() != NULL) {
-    WeakReferenceSet* queue = state->delayed_weak_reference_sets();
-    state->set_delayed_weak_reference_sets(NULL);
-    while (queue != NULL) {
-      delete WeakReferenceSet::Pop(&queue);
-    }
-  }
+  ASSERT(state->delayed_weak_reference_sets() == NULL);
+  // All weak reference sets are zone allocated and unmarked references which
+  // were on the delay queue will be freed when the zone is released in the
+  // epilog callback.
 }