Introduce HTransitionElementsKind instruction.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 2d471cc294962012df6315462c24cc93101cda96..d68774ec3ba0c690896edf5fde66a617c77ba65e 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -4101,12 +4101,40 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
     type_todo[i] = false;
   }
 
+  // Elements_kind transition support.
+  MapList transition_target(maps->length());
+  if (is_store) {
+    // Collect possible transition targets.
+    MapList possible_transitioned_maps(maps->length());
+    for (int i = 0; i < maps->length(); ++i) {
+      Handle<Map> map = maps->at(i);
+      ElementsKind elements_kind = map->elements_kind();
+      if (elements_kind == FAST_DOUBLE_ELEMENTS ||
+          elements_kind == FAST_ELEMENTS) {
+        possible_transitioned_maps.Add(*map);
+      }
+    }
+    // Get transition target for each map (NULL == no transition).
+    for (int i = 0; i < maps->length(); ++i) {
+      Handle<Map> map = maps->at(i);
+      Map* transitioned_map =
+          map->FindTransitionedMap(&possible_transitioned_maps);
+      transition_target.Add(transitioned_map);
+    }
+  }
+
   for (int i = 0; i < maps->length(); ++i) {
-    ASSERT(maps->at(i)->IsMap());
-    type_todo[maps->at(i)->elements_kind()] = true;
-    if (maps->at(i)->elements_kind()
-        >= FIRST_EXTERNAL_ARRAY_ELEMENTS_KIND) {
-      todo_external_array = true;
+    Handle<Map> map = maps->at(i);
+    ASSERT(map->IsMap());
+    ASSERT(!is_store || (transition_target.length() == maps->length()));
+    if (is_store && transition_target.at(i) != NULL) {
+      object = AddInstruction(new(zone()) HTransitionElementsKind(
+          object, map, Handle<Map>(transition_target.at(i))));
+    } else {
+      type_todo[map->elements_kind()] = true;
+      if (map->elements_kind() >= FIRST_EXTERNAL_ARRAY_ELEMENTS_KIND) {
+        todo_external_array = true;
+      }
     }
   }