Speed up join on arrays.

src/array.js

 // Copyright 2006-2008 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 59 matching lines @@
 // Optimized for sparse arrays if separator is ''.
 function SparseJoin(array, len, convert) {
   var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
   var builder = new StringBuilder();
   var last_key = -1;
   var keys_length = keys.length;
   for (var i = 0; i < keys_length; i++) {
     var key = keys[i];
     if (key != last_key) {
       var e = array[key];
-      builder.add(convert(e));
+      if (typeof(e) !== 'string') e = convert(e);
+      builder.add(e);
       last_key = key;
     }
   }
   return builder.generate();
 }
 
 
 function UseSparseVariant(object, length, is_array) {
    return is_array &&
        length > 1000 &&
@@ skipping 16 matching lines @@
   // Attempt to convert the elements.
   try {
     if (UseSparseVariant(array, length, is_array) && separator === '') {
       return SparseJoin(array, length, convert);
     }
 
     // Fast case for one-element arrays.
     if (length == 1) {
       var e = array[0];
       if (!IS_UNDEFINED(e) || (0 in array)) {
+        if (typeof(e) === 'string') return e;
         return convert(e);
       }
     }
 
     var builder = new StringBuilder();
 
-    for (var i = 0; i < length; i++) {
-      var e = array[i];
-      if (i != 0) builder.add(separator);
-      if (!IS_UNDEFINED(e) || (i in array)) {
-        builder.add(convert(e));
+    // We pull the empty separator check outside the loop for speed!
+    if (separator.length == 0) {
+      for (var i = 0; i < length; i++) {
+        var e = array[i];
+        if (!IS_UNDEFINED(e) || (i in array)) {
+          if (typeof(e) !== 'string') e = convert(e);
+          if (e.length > 0) {
+            var elements = builder.elements;
+            elements[elements.length] = e;
+          }
+        }
+      }
+    } else {
+      for (var i = 0; i < length; i++) {
+        var e = array[i];
+        if (i != 0) builder.add(separator);
+        if (!IS_UNDEFINED(e) || (i in array)) {
+          if (typeof(e) !== 'string') e = convert(e);
+          if (e.length > 0) {
+            var elements = builder.elements;
+            elements[elements.length] = e;
+          }
+        }
       }
     }
     return builder.generate();
   } finally {
     // Make sure to pop the visited array no matter what happens.
     if (is_array) visited_arrays.pop();
   }
 }
 
 
 function ConvertToString(e) {
+  if (typeof(e) === 'string') return e;
   if (e == null) return '';
   else return ToString(e);
 }
 
 
 function ConvertToLocaleString(e) {
+  if (typeof(e) === 'string') return e;
   if (e == null) return '';
   else {
     // e_obj's toLocaleString might be overwritten, check if it is a function.
     // Call ToString if toLocaleString is not a function.
     // See issue 877615.
     var e_obj = ToObject(e);
     if (IS_FUNCTION(e_obj.toLocaleString))
       return e_obj.toLocaleString();
     else
       return ToString(e);
@@ skipping 966 matching lines @@
     ArrayIndexOf: 1,
     ArrayLastIndexOf: 1,
     ArrayPush: 1,
     ArrayReduce: 1,
     ArrayReduceRight: 1
   });
 }
 
 
 SetupArray();