Some string optimizations:

src/objects-inl.h

 // 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 237 matching lines @@
 bool String::IsTwoByteRepresentation() {
   uint32_t type = map()->instance_type();
   if ((type & kStringRepresentationMask) == kConsStringTag &&
              ConsString::cast(this)->second()->length() == 0) {
     return ConsString::cast(this)->first()->IsTwoByteRepresentation();
   }
   return (type & kStringEncodingMask) == kTwoByteStringTag;
 }
 
 
+bool String::IsExternalTwoByteStringWithAsciiChars() {
+  if (!IsExternalTwoByteString()) return false;
+  const uc16* data = ExternalTwoByteString::cast(this)->resource()->data();
+  for (int i = 0, len = length(); i < len; i++) {
+    if (data[i] > kMaxAsciiCharCode) return false;
+  }
+  return true;
+}
+
+
 bool StringShape::IsCons() {
   return (type_ & kStringRepresentationMask) == kConsStringTag;
 }
 
 
 bool StringShape::IsExternal() {
   return (type_ & kStringRepresentationMask) == kExternalStringTag;
 }
 
 
@@ skipping 2817 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_