Add pointer cache field to external string for access in generated code.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 11819eaa09d13db19bf029f07c9b92a960c72c31..fda9f4d2d59f6951c21379aa80e498b4cec25d6f 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2297,6 +2297,11 @@ void ConsString::set_second(String* value, WriteBarrierMode mode) {
 }
 
 
+void ExternalString::clear_data_cache() {
+  WRITE_INTPTR_FIELD(this, kResourceDataOffset, NULL);
+}
+
+
 const ExternalAsciiString::Resource* ExternalAsciiString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }
@@ -2306,6 +2311,15 @@ void ExternalAsciiString::set_resource(
     const ExternalAsciiString::Resource* resource) {
   *reinterpret_cast<const Resource**>(
       FIELD_ADDR(this, kResourceOffset)) = resource;
+  clear_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();

[Lasse Reichstein, 2011/11/17 13:40:18]

Alternatively, we could ensure that the data_field is always set (by setting it
when creating the string object). That allows us to avoid checking it on every
access.

[Yang, 2011/11/17 17:06:23]

I thought lazy init is nice. I'll check in another CL whether this has
advantages.

+  return *data_field;
 }
 
 
@@ -2318,6 +2332,15 @@ void ExternalTwoByteString::set_resource(
     const ExternalTwoByteString::Resource* resource) {
   *reinterpret_cast<const Resource**>(
       FIELD_ADDR(this, kResourceOffset)) = resource;
+  clear_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;
 }