Avoid unnecessary cloning of regexp answer

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 152 matching lines @@
 
 
 // ECMA-262 section 15.5.4.10
 function StringMatch(regexp) {
   if (!IS_REGEXP(regexp)) regexp = new $RegExp(regexp);
   var subject = TO_STRING_INLINE(this);
 
   if (!regexp.global) return regexp.exec(subject);
 
   var cache = regExpCache;
+  var saveAnswer = false;
 
   if (%_ObjectEquals(cache.type, 'match') &&
       %_ObjectEquals(cache.regExp, regexp) &&
       %_ObjectEquals(cache.subject, subject)) {
-    var last = cache.answer;
-    if (last == null) {
-      return last;
+    if (cache.answerSaved) {
+      return CloneRegexpAnswer(cache.answer);
     } else {
-      return CloneRegexpAnswer(last);
+      saveAnswer = true;
     }
   }
 
   %_Log('regexp', 'regexp-match,%0S,%1r', [subject, regexp]);
   // lastMatchInfo is defined in regexp.js.
   var result = %StringMatch(subject, regexp, lastMatchInfo);
   cache.type = 'match';
   cache.regExp = regexp;
   cache.subject = subject;
-  cache.answer = result;
-  if (result == null) {
-    return result;
-  } else {
-    return CloneRegexpAnswer(result);
-  }
+  if (saveAnswer) cache.answer = CloneRegexpAnswer(result);
+  cache.answerSaved = saveAnswer;
+  return result;
 }
 
 
 // SubString is an internal function that returns the sub string of 'string'.
 // If resulting string is of length 1, we use the one character cache
 // otherwise we call the runtime system.
 function SubString(string, start, end) {
   // Use the one character string cache.
   if (start + 1 == end) {
     var char_code = %_FastCharCodeAt(string, start);
@@ skipping 382 matching lines @@
 
     // If the separator string is empty then return the elements in the subject.
     if (separator_length === 0) return %StringToArray(subject);
 
     var result = %StringSplit(subject, separator, limit);
 
     return result;
   }
 
   var cache = regExpCache;
+  var saveAnswer = false;
 
   if (%_ObjectEquals(cache.type, 'split') &&
       %_ObjectEquals(cache.regExp, separator) &&
       %_ObjectEquals(cache.subject, subject)) {
-    return CloneRegexpAnswer(cache.answer);
+    if (cache.answerSaved) {
+      return CloneRegexpAnswer(cache.answer);
+    } else {
+      saveAnswer = true;
+    }
   }
 
   cache.type = 'split';
   cache.regExp = separator;
   cache.subject = subject;
 
   %_Log('regexp', 'regexp-split,%0S,%1r', [subject, separator]);
 
   if (length === 0) {
+    cache.answerSaved = true;
     if (splitMatch(separator, subject, 0, 0) != null) {
       cache.answer = [];
       return [];
     }
     cache.answer = [subject];
     return [subject];
   }
 
   var currentIndex = 0;
   var startIndex = 0;
   var result = [];
 
+  outer_loop:
   while (true) {
 
     if (startIndex === length) {
       result[result.length] = subject.slice(currentIndex, length);
-      cache.answer = result;
-      return CloneRegexpAnswer(result);
+      break;
     }
 
     var matchInfo = splitMatch(separator, subject, currentIndex, startIndex);
 
     if (IS_NULL(matchInfo)) {
       result[result.length] = subject.slice(currentIndex, length);
-      cache.answer = result;
-      return CloneRegexpAnswer(result);
+      break;
     }
 
     var endIndex = matchInfo[CAPTURE1];
 
     // We ignore a zero-length match at the currentIndex.
     if (startIndex === endIndex && endIndex === currentIndex) {
       startIndex++;
       continue;
     }
 
     result[result.length] = SubString(subject, currentIndex, matchInfo[CAPTURE0]);
-    if (result.length === limit) {
-      cache.answer = result;
-      return CloneRegexpAnswer(result);
-    }
+    if (result.length === limit) break;
 
     var num_captures = NUMBER_OF_CAPTURES(matchInfo);
     for (var i = 2; i < num_captures; i += 2) {
       var start = matchInfo[CAPTURE(i)];
       var end = matchInfo[CAPTURE(i + 1)];
       if (start != -1 && end != -1) {
         result[result.length] = SubString(subject, start, end);
       } else {
         result[result.length] = void 0;
       }
-      if (result.length === limit) {
-        cache.answer = result;
-        return CloneRegexpAnswer(result);
-      }
+      if (result.length === limit) break outer_loop;
     }
 
     startIndex = currentIndex = endIndex;
   }
+  if (saveAnswer) cache.answer = CloneRegexpAnswer(result);
+  cache.answerSaved = saveAnswer;
+  return result;
+  
 }
 
 
 // ECMA-262 section 15.5.4.14
 // Helper function used by split.  This version returns the matchInfo
 // instead of allocating a new array with basically the same information.
 function splitMatch(separator, subject, current_index, start_index) {
   var matchInfo = DoRegExpExec(separator, subject, start_index);
   if (matchInfo == null) return null;
   // Section 15.5.4.14 paragraph two says that we do not allow zero length
@@ skipping 303 matching lines @@
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup,
     "toJSON", StringToJSON
   ));
 }
 
 
 SetupString();