Introduce read buffer for external strings when using charAt (ia32).

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 11819eaa09d13db19bf029f07c9b92a960c72c31..9946c88c13a99f5c0741ab44106a6ba46e15c687 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -256,11 +256,13 @@ bool Object::IsExternalTwoByteString() {
          String::cast(this)->IsTwoByteRepresentation();
 }
 
+
 bool Object::HasValidElements() {
   // Dictionary is covered under FixedArray.
   return IsFixedArray() || IsFixedDoubleArray() || IsExternalArray();
 }
 
+
 StringShape::StringShape(String* str)
   : type_(str->map()->instance_type()) {
   set_valid();
@@ -2143,7 +2145,7 @@ String* String::TryFlattenGetString(PretenureFlag pretenure) {
 }
 
 
-uint16_t String::Get(int index) {
+uint16_t String::Get(int index, ExternalStringBufferFlag flag) {
   ASSERT(index >= 0 && index < length());
   switch (StringShape(this).full_representation_tag()) {
     case kSeqStringTag | kAsciiStringTag:
@@ -2152,14 +2154,16 @@ uint16_t String::Get(int index) {
       return SeqTwoByteString::cast(this)->SeqTwoByteStringGet(index);
     case kConsStringTag | kAsciiStringTag:
     case kConsStringTag | kTwoByteStringTag:
-      return ConsString::cast(this)->ConsStringGet(index);
+      return ConsString::cast(this)->ConsStringGet(index, flag);
     case kExternalStringTag | kAsciiStringTag:
-      return ExternalAsciiString::cast(this)->ExternalAsciiStringGet(index);
+      return ExternalAsciiString::cast(this)->ExternalAsciiStringGet(index,
+                                                                     flag);
     case kExternalStringTag | kTwoByteStringTag:
-      return ExternalTwoByteString::cast(this)->ExternalTwoByteStringGet(index);
+      return ExternalTwoByteString::cast(this)->ExternalTwoByteStringGet(index,
+                                                                         flag);
     case kSlicedStringTag | kAsciiStringTag:
     case kSlicedStringTag | kTwoByteStringTag:
-      return SlicedString::cast(this)->SlicedStringGet(index);
+      return SlicedString::cast(this)->SlicedStringGet(index, flag);
     default:
       break;
   }
@@ -2297,6 +2301,32 @@ void ConsString::set_second(String* value, WriteBarrierMode mode) {
 }
 
 
+bool ExternalString::IsBuffered() {
+  uint32_t type = map()->instance_type();
+  ASSERT(StringShape(this).IsExternal());
+  return (type & kBufferedStringMask) == kBufferedStringTag;
+}
+
+
+int ExternalString::buffer_index() {
+  ASSERT(IsBuffered());
+  Object* value = READ_FIELD(this, kBufferIndexOffset);
+  return Smi::cast(value)->value();
+}
+
+
+void ExternalString::set_buffer_index(int value) {
+  ASSERT(IsBuffered());
+  WRITE_FIELD(this, kBufferIndexOffset, Smi::FromInt(value));
+}
+
+
+Address ExternalString::buffer() {
+  return reinterpret_cast<Address>(
+      HeapObject::RawField(this, kBufferContentOffset));
+}
+
+
 const ExternalAsciiString::Resource* ExternalAsciiString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }