Use linear backing store for hidden properties to save memory.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 8b22755c8230211278b9874ce97415cdb4fcf9f2..92b312456d80a4d6bae5a2e81efb395952639175 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -520,6 +520,11 @@ TYPE_CHECKER(FixedArray, FIXED_ARRAY_TYPE)
 TYPE_CHECKER(FixedDoubleArray, FIXED_DOUBLE_ARRAY_TYPE)
 
 
+bool Object::IsHiddenPropertiesArray() {
+  return IsFixedArray();
+}
+
+
 bool Object::IsDescriptorArray() {
   return IsFixedArray();
 }
@@ -1879,6 +1884,72 @@ Object** FixedArray::data_start() {
 }
 
 
+MaybeObject* HiddenPropertiesArray::Allocate() {
+  static const int initial_size = 2;
+  ASSERT(IS_POWER_OF_TWO(initial_size));
+  return Isolate::Current()->heap()->AllocateFixedArrayWithHoles(initial_size);
+}
+
+
+MaybeObject* HiddenPropertiesArray::Set(String* key, Object* value) {
+  for (int i = 0; i < length(); i += 2) {
+    if (is_the_hole(i) || String::cast(get(i))->Equals(key)) {
+      set(i, key);
+      set(i + 1, value);
+      return this;
+    }
+  }
+  // Need to extend the backing store.
+  int old_size = length();
+  int new_size = old_size << 1;
+  ASSERT(IS_POWER_OF_TWO(new_size));
+  FixedArray* new_array;
+  { MaybeObject* maybe_obj = GetHeap()->AllocateFixedArrayWithHoles(new_size);
+    if (!maybe_obj->To<FixedArray>(&new_array)) return maybe_obj;
+  }
+  CopyTo(0, new_array, 0, old_size);
+  new_array->set(old_size, key);
+  new_array->set(old_size + 1, value);
+  return new_array;
+}
+
+
+void HiddenPropertiesArray::Delete(String* key) {
+  // Linear search to find the key and delete the entry.
+  for (int i = 0; i < length(); i += 2) {
+    if (is_the_hole(i)) {
+      if (is_the_hole(i + 1)) {
+        // Both key and value are holes.  This is the end of the list.
+        return;
+      }
+    } else if (String::cast(get(i))->Equals(key)) {
+      // Set the value field to undefined (not the hole) to signal that the
+      // list may not end here.
+      set_the_hole(i);
+      set_undefined(i + 1);
+      return;
+    }
+  }
+}
+
+
+Object* HiddenPropertiesArray::Get(String* key) {
+  // Linear search to find the key and return the value.
+  for (int i = 0; i < length(); i += 2) {
+    if (is_the_hole(i)) {
+      if (is_the_hole(i + 1)) {
+        // Both key and value are holes.  This is the end of the list.
+        break;
+      }
+    } else if (String::cast(get(i))->Equals(key)) {
+      return get(i + 1);
+    }
+  }
+  // Not found, return undefined.
+  return GetHeap()->undefined_value();
+}
+
+
 bool DescriptorArray::IsEmpty() {
   ASSERT(length() >= kFirstIndex ||
          this == HEAP->empty_descriptor_array());
@@ -2219,6 +2290,7 @@ void SeededNumberDictionary::set_requires_slow_elements() {
 
 CAST_ACCESSOR(FixedArray)
 CAST_ACCESSOR(FixedDoubleArray)
+CAST_ACCESSOR(HiddenPropertiesArray)
 CAST_ACCESSOR(DescriptorArray)
 CAST_ACCESSOR(DeoptimizationInputData)
 CAST_ACCESSOR(DeoptimizationOutputData)