Improve performance of Array.prototype.join and String.prototype.substring...

src/string.js

Index: src/string.js
===================================================================
--- src/string.js	(revision 3544)
+++ src/string.js	(working copy)
@@ -505,7 +505,7 @@
 // ECMA-262 section 15.5.4.14
 function StringSplit(separator, limit) {
   var subject = ToString(this);
-  limit = (limit === void 0) ? 0xffffffff : ToUint32(limit);
+  limit = (IS_UNDEFINED(limit)) ? 0xffffffff : TO_UINT32(limit);
   if (limit === 0) return [];
 
   // ECMA-262 says that if separator is undefined, the result should
@@ -604,22 +604,30 @@
 
 // ECMA-262 section 15.5.4.15
 function StringSubstring(start, end) {
-  var s = ToString(this);
+  var s = this;
+  if (!IS_STRING(s)) s = ToString(s);
   var s_len = s.length;
+
   var start_i = TO_INTEGER(start);
+  if (start_i < 0) {
+    start_i = 0;
+  } else if (start_i > s_len) {
+    start_i = s_len;
+  }
+
   var end_i = s_len;
-  if (!IS_UNDEFINED(end))
+  if (!IS_UNDEFINED(end)) {
     end_i = TO_INTEGER(end);
-
-  if (start_i < 0) start_i = 0;
-  if (start_i > s_len) start_i = s_len;
-  if (end_i < 0) end_i = 0;
-  if (end_i > s_len) end_i = s_len;
-
-  if (start_i > end_i) {
-    var tmp = end_i;
-    end_i = start_i;
-    start_i = tmp;
+    if (end_i > s_len) {
+      end_i = s_len;
+    } else {
+      if (end_i < 0) end_i = 0;
+      if (start_i > end_i) {
+        var tmp = end_i;
+        end_i = start_i;
+        start_i = tmp;
+      }
+    }
   }
 
   return SubString(s, start_i, end_i);
@@ -790,21 +798,14 @@
 }
 
 
-// StringBuilder support.
-
-function StringBuilder() {
-  this.elements = new $Array();
-}
-
-
+// ReplaceResultBuilder support.
 function ReplaceResultBuilder(str) {
   this.elements = new $Array();
   this.special_string = str;
 }
 
 
-ReplaceResultBuilder.prototype.add =
-StringBuilder.prototype.add = function(str) {
+ReplaceResultBuilder.prototype.add = function(str) {
   if (!IS_STRING(str)) str = ToString(str);
   if (str.length > 0) {
     var elements = this.elements;
@@ -828,13 +829,9 @@
 }
 
 
-StringBuilder.prototype.generate = function() {
-  return %StringBuilderConcat(this.elements, "");
-}
-
-
 ReplaceResultBuilder.prototype.generate = function() {
-  return %StringBuilderConcat(this.elements, this.special_string);
+  var elements = this.elements;
+  return %StringBuilderConcat(elements, elements.length, this.special_string);
 }