Use linear backing store for hidden properties to save memory.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 7ed297340373326cc33c02e3d43173371bf1d04c..25759568a4e15fcf1bf1871e00db454f6f0b9bdf 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -74,6 +74,7 @@
 //       - FixedArrayBase
 //         - ByteArray
 //         - FixedArray
+//           - HiddenPropertiesArray
 //           - DescriptorArray
 //           - HashTable
 //             - Dictionary
@@ -799,6 +800,7 @@ class MaybeObject BASE_EMBEDDED {
   V(JSContextExtensionObject)                  \
   V(JSModule)                                  \
   V(Map)                                       \
+  V(HiddenPropertiesArray)                     \
   V(DescriptorArray)                           \
   V(TransitionArray)                           \
   V(DeoptimizationInputData)                   \
@@ -2235,16 +2237,15 @@ class JSObject: public JSReceiver {
       Object* setter,
       PropertyAttributes attributes);
 
-  // Returns the hidden properties backing store object, currently
-  // a StringDictionary, stored on this object.
+  // Returns the hidden properties backing store object stored on this object.
   // If no hidden properties object has been put on this object,
   // return undefined, unless create_if_absent is true, in which case
-  // a new dictionary is created, added to this object, and returned.
-  MUST_USE_RESULT MaybeObject* GetHiddenPropertiesDictionary(
+  // a new backing store is allocated, added to this object, and returned.
+  MUST_USE_RESULT MaybeObject* GetHiddenPropertiesArray(
       bool create_if_absent);
-  // Updates the existing hidden properties dictionary.
-  MUST_USE_RESULT MaybeObject* SetHiddenPropertiesDictionary(
-      StringDictionary* dictionary);
+  // Updates the existing hidden properties backing store.
+  MUST_USE_RESULT MaybeObject* SetHiddenPropertiesArray(
+      HiddenPropertiesArray* hidden_properties);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSObject);
 };
@@ -2455,6 +2456,29 @@ class FixedDoubleArray: public FixedArrayBase {
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedDoubleArray);
 };
 
+// HiddenPropertiesArray are fixed arrays used to store hidden properties.
+// Since we expect very few hidden properties, we use store the entries
+// linearly and use linear search for retrieval.
+class HiddenPropertiesArray: public FixedArray {
+ public:
+  // We allocate the backing store as FixedArray filled with holes.
+  MUST_USE_RESULT static inline MaybeObject* Allocate();
+
+  // Set a hidden property.  Return the current backing store afterwards
+  // (the backing store may be re-allocated when extended).
+  MUST_USE_RESULT inline MaybeObject* Set(String* key, Object* value);
+
+  // If the entry corresponding to the key is found, set the key of the entry
+  // to the hole and the value to undefined.  This way we can distinguish
+  // deleted entries from end of the list when we do linear search.
+  inline void Delete(String* key);
+  inline Object* Get(String* key);
+  static inline HiddenPropertiesArray* cast(Object* obj);
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(HiddenPropertiesArray);
+};
+
 
 // DescriptorArrays are fixed arrays used to hold instance descriptors.
 // The format of the these objects is: