Sharing of descriptor arrays.

src/handles.cc

 // Copyright 2012 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 681 matching lines @@
 
 Handle<JSArray> GetKeysFor(Handle<JSReceiver> object, bool* threw) {
   Isolate* isolate = object->GetIsolate();
   isolate->counters()->for_in()->Increment();
   Handle<FixedArray> elements =
       GetKeysInFixedArrayFor(object, INCLUDE_PROTOS, threw);
   return isolate->factory()->NewJSArrayWithElements(elements);
 }
 
 
+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();
+      // 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);
 
-      // 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 (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());
 
     if (map->instance_descriptors()->IsEmpty()) {
       isolate->counters()->enum_cache_hits()->Increment();
       if (cache_result) map->SetEnumLength(0);
       return isolate->factory()->empty_fixed_array();
     }
 
     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);
 
     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) {
             indices = Handle<FixedArray>();
           } else {
             int field_index = Descriptor::IndexFromValue(descs->GetValue(i));
             if (field_index >= map->inobject_properties()) {
               field_index = -(field_index - map->inobject_properties() + 1);
             }
             indices->set(index, Smi::FromInt(field_index));
           }
         }
         index++;
       }
     }
     ASSERT(index == storage->length());
 
     Handle<FixedArray> bridge_storage =
         isolate->factory()->NewFixedArray(
             DescriptorArray::kEnumCacheBridgeLength);
     DescriptorArray* desc = object->map()->instance_descriptors();
     desc->SetEnumCache(*bridge_storage,
                        *storage,
                        indices.is_null() ? Object::cast(Smi::FromInt(0))
                                          : Object::cast(*indices));
-    if (cache_result) {
-      object->map()->SetEnumLength(index);
-    }
-    return storage;
+    if (cache_result) object->map()->SetEnumLength(enum_size);

[Jakob Kummerow, 2012/09/11 14:24:25]

I don't feel strongly about it, but for the record: I don't think we should
convert if-statements to their one-line form.

[Toon Verwaest, 2012/09/11 14:49:23]

Done.

+
+    return ReduceFixedArrayTo(storage, enum_size);
   } else {
     Handle<StringDictionary> dictionary(object->property_dictionary());
 
     int length = dictionary->NumberOfElements();
     if (length == 0) {
       return Handle<FixedArray>(isolate->heap()->empty_fixed_array());
     }
 
     // The enumeration array is generated by allocating an array big enough to
     // hold all properties that have been seen, whether they are are deleted or
@@ skipping 243 matching lines @@
   data->next = prev_next_;
   data->limit = prev_limit_;
 #ifdef DEBUG
   handles_detached_ = true;
 #endif
   return deferred;
 }
 
 
 } }  // namespace v8::internal