Improve fast to slow elements conversion:

src/objects.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1928 matching lines @@
 
   // Maximal number of fast properties for the JSObject. Used to
   // restrict the number of map transitions to avoid an explosion in
   // the number of maps for objects used as dictionaries.
   inline int MaxFastProperties();
 
   // Maximal number of elements (numbered 0 .. kMaxElementCount - 1).
   // Also maximal value of JSArray's length property.
   static const uint32_t kMaxElementCount = 0xffffffffu;
 
+  // Constants for heuristics controlling conversion of fast elements
+  // to slow elements.
+
+  // Maximal gap that can be introduced by adding an element beyond
+  // the current elements length.
   static const uint32_t kMaxGap = 1024;
-  static const int kMaxFastElementsLength = 5000;
+
+  // Maximal length of fast elements array that won't be checked for
+  // being dense enough on expansion.
+  static const int kMaxUncheckedFastElementsLength = 5000;
+
+  // Same as above but for old arrays. This limit is more strict. We
+  // don't want to be wasteful with long lived objects.
+  static const int kMaxUncheckedOldFastElementsLength = 500;
+
   static const int kInitialMaxFastElementArray = 100000;
   static const int kMaxFastProperties = 12;
   static const int kMaxInstanceSize = 255 * kPointerSize;
   // When extending the backing storage for property values, we increase
   // its size by more than the 1 entry necessary, so sequentially adding fields
   // to the same object requires fewer allocations and copies.
   static const int kFieldsAdded = 3;
 
   // Layout description.
   static const int kPropertiesOffset = HeapObject::kHeaderSize;
@@ skipping 45 matching lines @@
   bool ReferencesObjectFromElements(FixedArray* elements,
                                     ElementsKind kind,
                                     Object* object);
   bool HasElementInElements(FixedArray* elements,
                             ElementsKind kind,
                             uint32_t index);
 
   // Returns true if most of the elements backing storage is used.
   bool HasDenseElements();
 
+  // Gets the current elements capacity and the number of used elements.
+  void GetElementsCapacityAndUsage(int* capacity, int* used);
+
   bool CanSetCallback(String* name);
   MUST_USE_RESULT MaybeObject* SetElementCallback(
       uint32_t index,
       Object* structure,
       PropertyAttributes attributes);
   MUST_USE_RESULT MaybeObject* SetPropertyCallback(
       String* name,
       Object* structure,
       PropertyAttributes attributes);
   MUST_USE_RESULT MaybeObject* DefineGetterSetter(
@@ skipping 5148 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_