Cache multiple ElementsKind map transition per map.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index ae0ec7a6fb3347ee5d9e3ecaaed4722324d7394a..6863c6511ebc9f67f060d5f2bce7dffe785272fc 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -180,7 +180,6 @@ class PropertyDetails BASE_EMBEDDED {
   PropertyDetails(PropertyAttributes attributes,
                   PropertyType type,
                   int index = 0) {
-    ASSERT(type != ELEMENTS_TRANSITION);
     ASSERT(TypeField::is_valid(type));
     ASSERT(AttributesField::is_valid(attributes));
     ASSERT(StorageField::is_valid(index));
@@ -194,23 +193,6 @@ class PropertyDetails BASE_EMBEDDED {
     ASSERT(index == this->index());
   }
 
-  PropertyDetails(PropertyAttributes attributes,
-                  PropertyType type,
-                  ElementsKind elements_kind) {
-    ASSERT(type == ELEMENTS_TRANSITION);
-    ASSERT(TypeField::is_valid(type));
-    ASSERT(AttributesField::is_valid(attributes));
-    ASSERT(StorageField::is_valid(static_cast<int>(elements_kind)));
-
-    value_ = TypeField::encode(type)
-        | AttributesField::encode(attributes)
-        | StorageField::encode(static_cast<int>(elements_kind));
-
-    ASSERT(type == this->type());
-    ASSERT(attributes == this->attributes());
-    ASSERT(elements_kind == this->elements_kind());
-  }
-
   // Conversion for storing details as Object*.
   explicit inline PropertyDetails(Smi* smi);
   inline Smi* AsSmi();
@@ -232,11 +214,6 @@ class PropertyDetails BASE_EMBEDDED {
 
   int index() { return StorageField::decode(value_); }
 
-  ElementsKind elements_kind() {
-    ASSERT(type() == ELEMENTS_TRANSITION);
-    return static_cast<ElementsKind>(StorageField::decode(value_));
-  }
-
   inline PropertyDetails AsDeleted();
 
   static bool IsValidIndex(int index) {
@@ -4176,6 +4153,8 @@ class Map: public HeapObject {
   // This is undone in MarkCompactCollector::ClearNonLiveTransitions().
   void CreateBackPointers();
 
+  void CreateOneBackPointer(Map* transition_target);
+
   // Set all map transitions from this map to dead maps to null.
   // Also, restore the original prototype on the targets of these
   // transitions, so that we do not process this map again while