Make String.prototype.replace a tiny bit faster by avoiding...

src/string.js

 // 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 178 matching lines @@
 // This has the same size as the lastMatchInfo array, and can be used for
 // functions that expect that structure to be returned.  It is used when the
 // needle is a string rather than a regexp.  In this case we can't update
 // lastMatchArray without erroneously affecting the properties on the global
 // RegExp object.
 var reusableMatchInfo = [2, "", "", -1, -1];
 
 
 // ECMA-262, section 15.5.4.11
 function StringReplace(search, replace) {
-  var subject = ToString(this);
+  var subject = IS_STRING(this) ? this : ToString(this);
 
   // Delegate to one of the regular expression variants if necessary.
   if (IS_REGEXP(search)) {
     %_Log('regexp', 'regexp-replace,%0r,%1S', [search, subject]);
     if (IS_FUNCTION(replace)) {
       return StringReplaceRegExpWithFunction(subject, search, replace);
     } else {
       return StringReplaceRegExp(subject, search, replace);
     }
   }
 
   // Convert the search argument to a string and search for it.
-  search = ToString(search);
+  search = IS_STRING(search) ? search : ToString(search);
   var start = %StringIndexOf(subject, search, 0);
   if (start < 0) return subject;
   var end = start + search.length;
 
   var builder = new ReplaceResultBuilder(subject);
   // prefix
   builder.addSpecialSlice(0, start);
 
   // Compute the string to replace with.
   if (IS_FUNCTION(replace)) {
     builder.add(replace.call(null, search, start, subject));
   } else {
     reusableMatchInfo[CAPTURE0] = start;
     reusableMatchInfo[CAPTURE1] = end;
-    ExpandReplacement(ToString(replace), subject, reusableMatchInfo, builder);
+    if (!IS_STRING(replace)) replace = ToString(replace);
+    ExpandReplacement(replace, subject, reusableMatchInfo, builder);
   }
 
   // suffix
   builder.addSpecialSlice(end, subject.length);
 
   return builder.generate();
 }
 
 
 // Helper function for regular expressions in String.prototype.replace.
@@ skipping 653 matching lines @@
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup,
     "toJSON", StringToJSON
   ));
 }
 
 
 SetupString();