Short-circuit embedded cons strings.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 8b842a733928f7425f73870b8c61523bc03ac84c..10e70d909fdbf98be99fd436768170d9e621d3bb 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -259,6 +259,32 @@ Handle<String> Factory::NewConsString(Handle<String> first,
 }
 
 
+template<typename SinkChar, typename StringType>
+Handle<String> ConcatStringContent(Handle<StringType> result,
+                                   Handle<String> first,
+                                   Handle<String> second) {
+  DisallowHeapAllocation pointer_stays_valid;
+  SinkChar* sink = result->GetChars();
+  String::WriteToFlat(*first, sink, 0, first->length());
+  String::WriteToFlat(*second, sink + first->length(), 0, second->length());
+  return result;
+}
+
+
+Handle<String> Factory::NewFlatConcatString(Handle<String> first,
+                                            Handle<String> second) {
+  int total_length = first->length() + second->length();
+  if (first->IsOneByteRepresentationUnderneath() &&
+      second->IsOneByteRepresentationUnderneath()) {
+    return ConcatStringContent<uint8_t>(
+        NewRawOneByteString(total_length), first, second);
+  } else {
+    return ConcatStringContent<uc16>(
+        NewRawTwoByteString(total_length), first, second);
+  }
+}
+
+
 Handle<String> Factory::NewSubString(Handle<String> str,
                                      int begin,
                                      int end) {