Some string optimizations:

src/objects.h

 // Copyright 2006-2009 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 3908 matching lines @@
   inline int length();
   inline void set_length(int value);
 
   // Get and set the hash field of the string.
   inline uint32_t hash_field();
   inline void set_hash_field(uint32_t value);
 
   inline bool IsAsciiRepresentation();
   inline bool IsTwoByteRepresentation();
 
+  // Check whether this string is an external two-byte string that in
+  // fact contains only ascii characters.
+  //
+  // Such strings may appear when the embedder prefers two-byte
+  // representations even for ascii data.
+  inline bool IsExternalTwoByteStringWithAsciiChars();
+
   // Get and set individual two byte chars in the string.
   inline void Set(int index, uint16_t value);
   // Get individual two byte char in the string.  Repeated calls
   // to this method are not efficient unless the string is flat.
   inline uint16_t Get(int index);
 
   // Try to flatten the top level ConsString that is hiding behind this
   // string.  This is a no-op unless the string is a ConsString.  Flatten
   // mutates the ConsString and might return a failure.
   Object* SlowTryFlatten(PretenureFlag pretenure);
@@ skipping 1135 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_