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

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 0d5ed0fc4297c49b0f79917de5a9e6b6a2758c4d..04688bda49401d766834132162edfddbe4104d9c 100644
--- a/src/string.js
+++ b/src/string.js
@@ -711,12 +711,13 @@ function StringSubstr(start, n) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.substr");
 
   var s = TO_STRING_INLINE(this);
+  var s_len = s.length;
   var len;
 
   // Correct n: If not given, set to string length; if explicitly
   // set to undefined, zero, or negative, returns empty string.
   if (IS_UNDEFINED(n)) {
-    len = s.length;
+    len = s_len;
   } else {
     len = TO_INTEGER(n);
     if (len <= 0) return '';
@@ -730,17 +731,17 @@ function StringSubstr(start, n) {
     start = TO_INTEGER(start);
     // If positive, and greater than or equal to the string length,
     // return empty string.
-    if (start >= s.length) return '';
+    if (start >= s_len) return '';
     // If negative and absolute value is larger than the string length,
     // use zero.
     if (start < 0) {
-      start += s.length;
+      start += s_len;
       if (start < 0) start = 0;
     }
   }
 
   var end = start + len;
-  if (end > s.length) end = s.length;
+  if (end > s_len) end = s_len;
 
   return %_SubString(s, start, end);
 }