Introduce short external strings without pointer cache.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 9da480ea743a6df283a4228a51669acccafa37ad..2883a8d775642c1bca1d9fb64aaa35254e909679 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2297,8 +2297,9 @@ void ConsString::set_second(String* value, WriteBarrierMode 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;
 }
 
 
@@ -2307,19 +2308,24 @@ const ExternalAsciiString::Resource* ExternalAsciiString::resource() {
 }
 
 
+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();
 }
 
 
@@ -2334,19 +2340,24 @@ const ExternalTwoByteString::Resource* ExternalTwoByteString::resource() {
 }
 
 
+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();
 }