Optimize non-ASCII string splitting with single-character search pattern

src/objects.h

 // Copyright 2012 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 7171 matching lines @@
     Vector<const char> ToAsciiVector() {
       ASSERT_EQ(ASCII, state_);
       return Vector<const char>::cast(buffer_);
     }
     // Return the two-byte content of the string. Only use if IsTwoByte()
     // returns true.
     Vector<const uc16> ToUC16Vector() {
       ASSERT_EQ(TWO_BYTE, state_);
       return Vector<const uc16>::cast(buffer_);
     }
+    // Return the character length of the string.
+    unsigned Length() const {
+      if (state_ == ASCII) return buffer_.length();
+      if (state_ == TWO_BYTE) return buffer_.length() / 2;

[Yang, 2012/11/26 08:45:30]

you could do an
if (state_ == ASCII) {
  ...
} else {
  ASSERT(state == TWO_BYTE) {
  ...
}

to be cleaner.

Even cooler would be without any branching, like,

ASSERT(ASCII == 0 && TWO_BYTE == 1);
ASSERT(state_ == ASCII || state_ == TWO_BYTE);
return buffer_.length() >> state_;

(Make sure the enum elements have values explicitly assigned.)

+      UNREACHABLE();
+      return 0;  // Placate compiler.
+    }
 
    private:
     enum State { NON_FLAT, ASCII, TWO_BYTE };
 
     // Constructors only used by String::GetFlatContent().
     explicit FlatContent(Vector<const char> chars)
         : buffer_(Vector<const byte>::cast(chars)),
           state_(ASCII) { }
     explicit FlatContent(Vector<const uc16> chars)
         : buffer_(Vector<const byte>::cast(chars)),
@@ skipping 1801 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_