StringDictionary::FindEntry optimized for symbol strings.

Review URL: http://codereview.chromium.org/3020003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5086 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -7338,6 +7338,46 @@ int HashTable<Shape, Key>::FindEntry(Key key) {
 }
 
 
+// Find entry for key otherwise return kNotFound.
+int StringDictionary::FindEntry(String* key) {
+  if (!key->IsSymbol()) {
+    return HashTable<StringDictionaryShape, String*>::FindEntry(key);
+  }
+
+  // Optimized for symbol key. Knowledge of the key type allows:
+  // 1. Move the check if the key is a symbol out of the loop.
+  // 2. Avoid comparing hash codes in symbol to symbol comparision.
+  // 3. Detect a case when a dictionary key is not a symbol but the key is.
+  //    In case of positive result the dictionary key may be replaced by
+  //    the symbol with minimal performance penalty. It gives a chance to
+  //    perform further lookups in code stubs (and significant performance boost
+  //    a certain style of code).
+
+  // EnsureCapacity will guarantee the hash table is never full.
+  uint32_t capacity = Capacity();
+  uint32_t entry = FirstProbe(key->Hash(), capacity);
+  uint32_t count = 1;
+
+  while (true) {
+    int index = EntryToIndex(entry);
+    Object* element = get(index);
+    if (element->IsUndefined()) break;  // Empty entry.
+    if (key == element) return entry;
+    if (!element->IsSymbol() &&
+        !element->IsNull() &&
+        String::cast(element)->Equals(key)) {
+      // Replace a non-symbol key by the equivalent symbol for faster further
+      // lookups.
+      set(index, key);
+      return entry;
+    }
+    ASSERT(element->IsNull() || !String::cast(element)->Equals(key));
+    entry = NextProbe(entry, count++, capacity);
+  }
+  return kNotFound;
+}
+
+
 template<typename Shape, typename Key>
 Object* HashTable<Shape, Key>::EnsureCapacity(int n, Key key) {
   int capacity = Capacity();

src/objects.h

@@ -2012,7 +2012,7 @@ class HashTable: public FixedArray {
   static const int kMaxCapacity =
       (FixedArray::kMaxLength - kElementsStartOffset) / kEntrySize;
 
-  // Find entry for key otherwise return -1.
+  // Find entry for key otherwise return kNotFound.
   int FindEntry(Key key);
 
  protected:
@@ -2294,6 +2294,10 @@ class StringDictionary: public Dictionary<StringDictionaryShape, String*> {
   // For transforming properties of a JSObject.
   Object* TransformPropertiesToFastFor(JSObject* obj,
                                        int unused_property_fields);
+
+  // Find entry for key otherwise return kNotFound. Optimzed version of
+  // HashTable::FindEntry.
+  int FindEntry(String* key);
 };