Use direct copy and templates to speed up flattening of strings.

src/factory.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 61 matching lines @@
 
 
 Handle<String> Factory::NewRawTwoByteString(int length,
                                             PretenureFlag pretenure) {
   CALL_HEAP_FUNCTION(Heap::AllocateRawTwoByteString(length, pretenure), String);
 }
 
 
 Handle<String> Factory::NewConsString(Handle<String> first,
                                       Handle<String> second) {
+  if (first->length() == 0) return second;
+  if (second->length() == 0) return first;
   CALL_HEAP_FUNCTION(Heap::AllocateConsString(*first, *second), String);
 }
 
 
 Handle<String> Factory::NewStringSlice(Handle<String> str, int begin, int end) {
   CALL_HEAP_FUNCTION(str->Slice(begin, end), String);
 }
 
 
 Handle<String> Factory::NewExternalStringFromAscii(
@@ skipping 714 matching lines @@
     Execution::ConfigureInstance(instance,
                                  instance_template,
                                  pending_exception);
   } else {
     *pending_exception = false;
   }
 }
 
 
 } }  // namespace v8::internal