Optimize algorithm for String.prototype.repeat(), fix value caching in others

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)) {
     throw MakeRangeError("invalid_count_value", []);
   }
 
-  var elements = new InternalArray(n);
-  for (var i = 0; i < n; i++) {
-    elements[i] = s;
+  if (n < 1) return '';

[Michael Starzinger, 2014/07/28 11:13:52]

This special case is not needed if comment below is addressed.

+
+  // Optimized O(log N) algorithm, without the added overhead of an extra array

[Michael Starzinger, 2014/07/28 11:13:51]

nit: Let's drop (at least) the second half of the comment. Not sure it's worth
referring to an array that no longer exists.

+  var res = '';
+
+  while (n > 1) {

[Michael Starzinger, 2014/07/28 11:13:51]

If we change the predicate to be "n > 0" most special cases resolve naturally.

+      if (n & 1) res += s;
+      n >>= 1;
+      s += s;
   }
-
-  return %StringBuilderConcat(elements, n, "");
+  return res + s;

[Michael Starzinger, 2014/07/28 11:13:51]

This addition is not needed if comment above is addressed.

 }
 
 
 // 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)) {
     throw MakeTypeError("first_argument_not_regexp",
                         ["String.prototype.startsWith"]);
   }
 
   var ss = TO_STRING_INLINE(searchString);
   var pos = 0;
   if (%_ArgumentsLength() > 1) {
     pos = %_Arguments(1);  // position
     pos = ToInteger(pos);
   }
 
   var s_len = s.length;
   var start = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+  if (ss.length + start > s_len) {
     return false;
   }
 
   return %StringIndexOf(s, ss, start) === start;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.7
 function StringEndsWith(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.endsWith");
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
     throw MakeTypeError("first_argument_not_regexp",
                         ["String.prototype.endsWith"]);
   }
 
   var ss = TO_STRING_INLINE(searchString);
   var s_len = s.length;
   var pos = s_len;
   if (%_ArgumentsLength() > 1) {
     var arg = %_Arguments(1);  // position
     if (!IS_UNDEFINED(arg)) {
       pos = ToInteger(arg);
     }
   }
 
   var end = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  var start = end - ss_len;
+  var start = end - ss.length;
   if (start < 0) {
     return false;
   }
 
   return %StringLastIndexOf(s, ss, start) === start;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.6
 function StringContains(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.contains");
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
     throw MakeTypeError("first_argument_not_regexp",
                         ["String.prototype.contains"]);
   }
 
   var ss = TO_STRING_INLINE(searchString);
   var pos = 0;
   if (%_ArgumentsLength() > 1) {
     pos = %_Arguments(1);  // position
     pos = ToInteger(pos);
   }
 
   var s_len = s.length;
   var start = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+  if (ss.length + start > s_len) {
     return false;
   }
 
   return %StringIndexOf(s, ss, start) !== -1;
 }
 
 
 // -------------------------------------------------------------------
 
 function ExtendStringPrototype() {
   %CheckIsBootstrapping();
 
   // Set up the non-enumerable functions on the String prototype object.
   InstallFunctions($String.prototype, DONT_ENUM, $Array(
     "repeat", StringRepeat,
     "startsWith", StringStartsWith,
     "endsWith", StringEndsWith,
     "contains", StringContains
   ));
 }
 
 ExtendStringPrototype();