Correctly recognize external strings.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index a019c9d561f92a00ece1efd263e5d0a0c1deb987..26f09d5a3899ce62d48ea8fe8f0b979f24667447 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1897,6 +1897,7 @@ class V8_EXPORT String : public Primitive {
   void VerifyExternalStringResourceBase(ExternalStringResourceBase* v,
                                         Encoding encoding) const;
   void VerifyExternalStringResource(ExternalStringResource* val) const;
+  static internal::Object* GetRedirectedString(internal::Object* cons);
   static void CheckCast(v8::Value* obj);
 };
 
@@ -5388,9 +5389,12 @@ class Internals {
   static const int kContextHeaderSize = 2 * kApiPointerSize;
   static const int kContextEmbedderDataIndex = 65;
   static const int kFullStringRepresentationMask = 0x07;
+  static const int kStringRepresentationMask = 0x03;
   static const int kStringEncodingMask = 0x4;
+  static const int kConsTwoByteRepresentationTag = 0x01;
   static const int kExternalTwoByteRepresentationTag = 0x02;
-  static const int kExternalAsciiRepresentationTag = 0x06;
+  static const int kConsRepresentationTag = 0x01;
+  static const int kExternalRepresentationTag = 0x02;
 
   static const int kIsolateEmbedderDataOffset = 0 * kApiPointerSize;
   static const int kIsolateRootsOffset = 5 * kApiPointerSize;
@@ -5454,11 +5458,6 @@ class Internals {
     return SmiValue(ReadField<O*>(obj, kOddballKindOffset));
   }
 
-  V8_INLINE static bool IsExternalTwoByteString(int instance_type) {
-    int representation = (instance_type & kFullStringRepresentationMask);
-    return representation == kExternalTwoByteRepresentationTag;
-  }
-
   V8_INLINE static uint8_t GetNodeFlag(internal::Object** obj, int shift) {
       uint8_t* addr = reinterpret_cast<uint8_t*>(obj) + kNodeFlagsOffset;
       return *addr & static_cast<uint8_t>(1U << shift);
@@ -5991,12 +5990,16 @@ String::ExternalStringResource* String::GetExternalStringResource() const {
   typedef internal::Object O;
   typedef internal::Internals I;
   O* obj = *reinterpret_cast<O**>(const_cast<String*>(this));
-  String::ExternalStringResource* result;
-  if (I::IsExternalTwoByteString(I::GetInstanceType(obj))) {
+  String::ExternalStringResource* result = NULL;
+  int type = I::GetInstanceType(obj) & I::kFullStringRepresentationMask;
+  // Resolve redirected string via cons string.
+  if (type == I::kConsTwoByteRepresentationTag) {
+    obj = GetRedirectedString(obj);
+    type = I::GetInstanceType(obj) & I::kFullStringRepresentationMask;
+  }
+  if (type == I::kExternalTwoByteRepresentationTag) {
     void* value = I::ReadField<void*>(obj, I::kStringResourceOffset);
     result = reinterpret_cast<String::ExternalStringResource*>(value);
-  } else {
-    result = NULL;
   }
 #ifdef V8_ENABLE_CHECKS
   VerifyExternalStringResource(result);
@@ -6010,11 +6013,16 @@ String::ExternalStringResourceBase* String::GetExternalStringResourceBase(
   typedef internal::Object O;
   typedef internal::Internals I;
   O* obj = *reinterpret_cast<O**>(const_cast<String*>(this));
-  int type = I::GetInstanceType(obj) & I::kFullStringRepresentationMask;
-  *encoding_out = static_cast<Encoding>(type & I::kStringEncodingMask);
+  int instance_type = I::GetInstanceType(obj);
+  int string_type = instance_type & I::kStringRepresentationMask;
+  *encoding_out = static_cast<Encoding>(instance_type & I::kStringEncodingMask);
   ExternalStringResourceBase* resource = NULL;
-  if (type == I::kExternalAsciiRepresentationTag ||
-      type == I::kExternalTwoByteRepresentationTag) {
+  // Resolve redirected string via cons string.
+  if (string_type == I::kConsRepresentationTag) {
+    obj = GetRedirectedString(obj);
+    string_type = I::GetInstanceType(obj) & I::kStringRepresentationMask;
+  }
+  if (string_type == I::kExternalRepresentationTag) {
     void* value = I::ReadField<void*>(obj, I::kStringResourceOffset);
     resource = static_cast<ExternalStringResourceBase*>(value);
   }