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

src/harmony-string.js

Index: src/harmony-string.js
diff --git a/src/harmony-string.js b/src/harmony-string.js
index ae13745cdbf55989ad78b288843396ee5fab36e9..69223447d70f065b5c344ddd9c0bcaa676f4ccdc 100644
--- a/src/harmony-string.js
+++ b/src/harmony-string.js
@@ -21,12 +21,21 @@ function StringRepeat(count) {
     throw MakeRangeError("invalid_count_value", []);
   }
 
-  var elements = new InternalArray(n);
-  for (var i = 0; i < n; i++) {
-    elements[i] = s;
+  // O(log n) algorithm
+
+  if (n < 1) return '';

[aandrey, 2014/07/30 09:04:01]

nit: use ""

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

[aandrey, 2014/07/30 09:04:01]

+1 for a simpler version, since string concatenations is far more heavy than a
comparison. e.g.

var res = "";
while (n) {
  if (n & 1) {
    res += s;
    --n;
  } else {
    s += s;
    n >>= 1;
  }
}
return res;

+      if (n & 1) res += s;
+      n >>= 1;
+      s += s;
   }
 
-  return %StringBuilderConcat(elements, n, "");
+  // unroll last iteration, no need to double the initial string again at the
+  // final iteration.
+  return res + s;
 }
 
 
@@ -50,8 +59,7 @@ function StringStartsWith(searchString /* position */) {  // length == 1
 
   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) {

[aandrey, 2014/07/30 09:04:01]

this should be a separate patch, to ease possible rollbacks

     return false;
   }
 
@@ -81,8 +89,7 @@ function StringEndsWith(searchString /* position */) {  // length == 1
   }
 
   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;
   }
@@ -111,8 +118,7 @@ function StringContains(searchString /* position */) {  // length == 1
 
   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;
   }