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

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 535)
+++ src/objects.h	(working copy)
@@ -2965,7 +2965,7 @@
   inline bool StringIsSlicedString();
   inline bool StringIsConsString();
 
-  Vector<const char> ToAsciiVector();
+  Vector<const byte> ToAsciiVector();
   Vector<const uc16> ToUC16Vector();
 
   // Mark the string as an undetectable object. It only applies to
@@ -3043,6 +3043,8 @@
 
   // Get the representation tag.
   inline StringRepresentationTag representation_tag();
+  // Get the representation and ASCII tag.
+  inline int full_representation_tag();
   static inline StringRepresentationTag map_representation_tag(Map* map);
 
   // For use during stack traces.  Performs rudimentary sanity check.
@@ -3069,6 +3071,9 @@
   // Max ascii char code.
   static const int kMaxAsciiCharCode = 127;
 
+  // Minimum lenth for a cons or sliced string.
+  static const int kMinNonFlatLength = 13;
+
   // Mask constant for checking if a string has a computed hash code
   // and if it is an array index.  The least significant bit indicates
   // whether a hash code has been computed.  If the hash code has been
@@ -3106,11 +3111,11 @@
                                         unsigned* offset);
 
   // Helper function for flattening strings.
-  static void Flatten(String* source,
-                      String* sink,
-                      int from,
-                      int to,
-                      int sink_offset);
+  template <typename sinkchar>
+  static void WriteToFlat(String* source,
+                          sinkchar* sink,
+                          int from,
+                          int to);
 
  protected:
   class ReadBlockBuffer {
@@ -3301,8 +3306,6 @@
                                             unsigned chars);
 
 
-  // Minimum lenth for a cons string.
-  static const int kMinLength = 13;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ConsString);
@@ -3348,9 +3351,6 @@
                                               unsigned* offset_ptr,
                                               unsigned chars);
 
-  // Minimum lenth for a sliced string.
-  static const int kMinLength = 13;
-
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(SlicedString);
 };