Sharing of descriptor arrays.

src/handles.cc

Index: src/handles.cc
diff --git a/src/handles.cc b/src/handles.cc
index 19db5eb021e109f06187bfc2d9c1d8c56d1fa978..aca17552de68f2d2995e844bbea8ac3c948ce98d 100644
--- a/src/handles.cc
+++ b/src/handles.cc
@@ -699,24 +699,46 @@ Handle<JSArray> GetKeysFor(Handle<JSReceiver> object, bool* threw) {
 }
 
 
+Handle<FixedArray> ReduceFixedArrayTo(Handle<FixedArray> array, int length) {
+  ASSERT(array->length() >= length);
+  if (array->length() == length) return array;
+
+  Handle<FixedArray> new_array =
+      array->GetIsolate()->factory()->NewFixedArray(length);
+  for (int i = 0; i < length; ++i) new_array->set(i, array->get(i));
+  return new_array;
+}
+
+
 Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
                                        bool cache_result) {
   Isolate* isolate = object->GetIsolate();
   if (object->HasFastProperties()) {
     if (object->map()->instance_descriptors()->HasEnumCache()) {
       int own_property_count = object->map()->EnumLength();
-
-      // Mark that we have an enum cache if we are allowed to cache it.
-      if (cache_result && own_property_count == Map::kInvalidEnumCache) {
-        int num_enum = object->map()->NumberOfDescribedProperties(DONT_ENUM);
-        object->map()->SetEnumLength(num_enum);
+      // If we have an enum cache, but the enum length of the given map is set
+      // to kInvalidEnumCache, this means that the map itself has never used the
+      // present enum cache. The first step to using the cache is to set the
+      // enum length of the map by counting the number of own descriptors that
+      // are not DONT_ENUM.
+      if (own_property_count == Map::kInvalidEnumCache) {
+        own_property_count = object->map()->NumberOfDescribedProperties(
+            OWN_DESCRIPTORS, DONT_ENUM);
+
+        if (cache_result) object->map()->SetEnumLength(own_property_count);
       }
 
       DescriptorArray* desc = object->map()->instance_descriptors();
       Handle<FixedArray> keys(FixedArray::cast(desc->GetEnumCache()), isolate);
 
-      isolate->counters()->enum_cache_hits()->Increment();
-      return keys;
+      // In case the number of properties required in the enum are actually
+      // present, we can reuse the enum cache. Otherwise, this means that the
+      // enum cache was generated for a previous (smaller) version of the
+      // Descriptor Array. In that case we regenerate the enum cache.
+      if (own_property_count <= keys->length()) {
+        isolate->counters()->enum_cache_hits()->Increment();
+        return ReduceFixedArrayTo(keys, own_property_count);
+      }
     }
 
     Handle<Map> map(object->map());
@@ -728,8 +750,7 @@ Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
     }
 
     isolate->counters()->enum_cache_misses()->Increment();
-
-    int num_enum = map->NumberOfDescribedProperties(DONT_ENUM);
+    int num_enum = map->NumberOfDescribedProperties(ALL_DESCRIPTORS, DONT_ENUM);
 
     Handle<FixedArray> storage = isolate->factory()->NewFixedArray(num_enum);
     Handle<FixedArray> indices = isolate->factory()->NewFixedArray(num_enum);
@@ -737,10 +758,14 @@ Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
     Handle<DescriptorArray> descs =
         Handle<DescriptorArray>(object->map()->instance_descriptors(), isolate);
 
+    int real_size = map->NumberOfOwnDescriptors();
+    int enum_size = 0;
     int index = 0;
+
     for (int i = 0; i < descs->number_of_descriptors(); i++) {
       PropertyDetails details = descs->GetDetails(i);
       if (!details.IsDontEnum()) {
+        if (i < real_size) ++enum_size;
         storage->set(index, descs->GetKey(i));
         if (!indices.is_null()) {
           if (details.type() != FIELD) {
@@ -767,9 +792,10 @@ Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
                        indices.is_null() ? Object::cast(Smi::FromInt(0))
                                          : Object::cast(*indices));
     if (cache_result) {
-      object->map()->SetEnumLength(index);
+      object->map()->SetEnumLength(enum_size);
     }
-    return storage;
+
+    return ReduceFixedArrayTo(storage, enum_size);
   } else {
     Handle<StringDictionary> dictionary(object->property_dictionary());