Internalize strings in-place

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 65f12cadfb318271696cc9c192ecfee5a0a0e7db..6a787fa42fe3eb61c6379c7bac9e780359d1121d 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -275,6 +275,11 @@ bool HeapObject::IsConsString() const {
   return StringShape(String::cast(this)).IsCons();
 }
 
+bool HeapObject::IsThinString() const {
+  if (!IsString()) return false;
+  return StringShape(String::cast(this)).IsThin();
+}
+
 bool HeapObject::IsSlicedString() const {
   if (!IsString()) return false;
   return StringShape(String::cast(this)).IsSliced();
@@ -698,6 +703,7 @@ CAST_ACCESSOR(StringTable)
 CAST_ACCESSOR(Struct)
 CAST_ACCESSOR(Symbol)
 CAST_ACCESSOR(TemplateInfo)
+CAST_ACCESSOR(ThinString)
 CAST_ACCESSOR(Uint16x8)
 CAST_ACCESSOR(Uint32x4)
 CAST_ACCESSOR(Uint8x16)
@@ -841,6 +847,10 @@ bool StringShape::IsCons() {
   return (type_ & kStringRepresentationMask) == kConsStringTag;
 }
 
+bool StringShape::IsThin() {
+  return (type_ & kStringRepresentationMask) == kThinStringTag;
+}
+
 bool StringShape::IsSliced() {
   return (type_ & kStringRepresentationMask) == kSlicedStringTag;
 }
@@ -3662,10 +3672,19 @@ bool String::Equals(Handle<String> one, Handle<String> two) {
 
 
 Handle<String> String::Flatten(Handle<String> string, PretenureFlag pretenure) {
-  if (!string->IsConsString()) return string;
-  Handle<ConsString> cons = Handle<ConsString>::cast(string);
-  if (cons->IsFlat()) return handle(cons->first());
-  return SlowFlatten(cons, pretenure);
+  if (string->IsConsString()) {
+    Handle<ConsString> cons = Handle<ConsString>::cast(string);
+    if (cons->IsFlat()) {
+      string = handle(cons->first());
+    } else {
+      return SlowFlatten(cons, pretenure);
+    }
+  }
+  if (string->IsThinString()) {
+    string = handle(Handle<ThinString>::cast(string)->actual());
+    DCHECK(!string->IsConsString());
+  }
+  return string;
 }
 
 
@@ -3686,6 +3705,9 @@ uint16_t String::Get(int index) {
     case kSlicedStringTag | kOneByteStringTag:
     case kSlicedStringTag | kTwoByteStringTag:
       return SlicedString::cast(this)->SlicedStringGet(index);
+    case kThinStringTag | kOneByteStringTag:
+    case kThinStringTag | kTwoByteStringTag:
+      return ThinString::cast(this)->ThinStringGet(index);
     default:
       break;
   }
@@ -3717,6 +3739,7 @@ String* String::GetUnderlying() {
   DCHECK(this->IsFlat());
   DCHECK(StringShape(this).IsIndirect());
   STATIC_ASSERT(ConsString::kFirstOffset == SlicedString::kParentOffset);
+  STATIC_ASSERT(ConsString::kFirstOffset == ThinString::kActualOffset);
   const int kUnderlyingOffset = SlicedString::kParentOffset;
   return String::cast(READ_FIELD(this, kUnderlyingOffset));
 }
@@ -3768,6 +3791,11 @@ ConsString* String::VisitFlat(Visitor* visitor,
       case kConsStringTag | kTwoByteStringTag:
         return ConsString::cast(string);
 
+      case kThinStringTag | kOneByteStringTag:
+      case kThinStringTag | kTwoByteStringTag:
+        string = ThinString::cast(string)->actual();
+        continue;
+
       default:
         UNREACHABLE();
         return NULL;
@@ -3899,6 +3927,7 @@ void ConsString::set_second(String* value, WriteBarrierMode mode) {
   CONDITIONAL_WRITE_BARRIER(GetHeap(), this, kSecondOffset, value, mode);
 }
 
+ACCESSORS(ThinString, actual, String, kActualOffset);
 
 bool ExternalString::is_short() {
   InstanceType type = map()->instance_type();