Improve String.repeat.

src/harmony-string.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 // var $Array = global.Array;
 
 // -------------------------------------------------------------------
 
 // ES6 draft 01-20-14, section 21.1.3.13
 function StringRepeat(count) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.repeat");
 
   var s = TO_STRING_INLINE(this);
   var n = ToInteger(count);
-  if (n < 0 || !NUMBER_IS_FINITE(n)) {
+  // The maximum string length is stored in a smi, so a longer repeat
+  // must result in a range error.
+  if (n < 0 || n > %_MaxSmi()) {
     throw MakeRangeError("invalid_count_value", []);
   }
 
-  var elements = new InternalArray(n);
-  for (var i = 0; i < n; i++) {
-    elements[i] = s;
+  var r = "";
+  while (true) {
+    if (n & 1) r += s;
+    n >>= 1;
+    if (n === 0) return r;
+    s += s;
   }
-
-  return %StringBuilderConcat(elements, n, "");
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.18
 function StringStartsWith(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.startsWith");
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
@@ skipping 142 matching lines @@
   InstallFunctions($String.prototype, DONT_ENUM, $Array(
     "codePointAt", StringCodePointAt,
     "contains", StringContains,
     "endsWith", StringEndsWith,
     "repeat", StringRepeat,
     "startsWith", StringStartsWith
   ));
 }
 
 ExtendStringPrototype();