Introduce short external strings without pointer cache.

src/objects-inl.h

 // Copyright 2011 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 2279 matching lines @@
   return READ_FIELD(this, kSecondOffset);
 }
 
 
 void ConsString::set_second(String* value, WriteBarrierMode mode) {
   WRITE_FIELD(this, kSecondOffset, value);
   CONDITIONAL_WRITE_BARRIER(GetHeap(), this, kSecondOffset, value, mode);
 }
 
 
-void ExternalString::clear_data_cache() {
-  WRITE_INTPTR_FIELD(this, kResourceDataOffset, 0);
+bool ExternalString::is_short() {
+  InstanceType type = map()->instance_type();
+  return (type & kShortExternalStringMask) == kShortExternalStringTag;
 }
 
 
 const ExternalAsciiString::Resource* ExternalAsciiString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }
 
 
+void ExternalAsciiString::update_data_cache() {
+  if (is_short()) return;
+  const char** data_field =
+      reinterpret_cast<const char**>(FIELD_ADDR(this, kResourceDataOffset));
+  *data_field = resource()->data();
+}
+
+
 void ExternalAsciiString::set_resource(
     const ExternalAsciiString::Resource* resource) {
   *reinterpret_cast<const Resource**>(
       FIELD_ADDR(this, kResourceOffset)) = resource;
-  clear_data_cache();
+  if (resource != NULL) update_data_cache();
 }
 
 
 const char* ExternalAsciiString::GetChars() {
-  const char** data_field =
-      reinterpret_cast<const char**>(FIELD_ADDR(this, kResourceDataOffset));
-  if (*data_field == NULL) *data_field = resource()->data();
-  return *data_field;
+  return resource()->data();
 }
 
 
 uint16_t ExternalAsciiString::ExternalAsciiStringGet(int index) {
   ASSERT(index >= 0 && index < length());
   return GetChars()[index];
 }
 
 
 const ExternalTwoByteString::Resource* ExternalTwoByteString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }
 
 
+void ExternalTwoByteString::update_data_cache() {
+  if (is_short()) return;
+  const uint16_t** data_field =
+      reinterpret_cast<const uint16_t**>(FIELD_ADDR(this, kResourceDataOffset));
+  *data_field = resource()->data();
+}
+
+
 void ExternalTwoByteString::set_resource(
     const ExternalTwoByteString::Resource* resource) {
   *reinterpret_cast<const Resource**>(
       FIELD_ADDR(this, kResourceOffset)) = resource;
-  clear_data_cache();
+  if (resource != NULL) update_data_cache();
 }
 
 
 const uint16_t* ExternalTwoByteString::GetChars() {
-  const uint16_t** data_field =
-      reinterpret_cast<const uint16_t**>(FIELD_ADDR(this, kResourceDataOffset));
-  if (*data_field == NULL) *data_field = resource()->data();
-  return *data_field;
+  return resource()->data();
 }
 
 
 uint16_t ExternalTwoByteString::ExternalTwoByteStringGet(int index) {
   ASSERT(index >= 0 && index < length());
   return GetChars()[index];
 }
 
 
 const uint16_t* ExternalTwoByteString::ExternalTwoByteStringGetData(
@@ skipping 2309 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_