[es6] Optimize String{Starts, Ends}With

src/string.js

 // Copyright 2012 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.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var ArrayIndexOf;
 var ArrayJoin;
 var GlobalRegExp = global.RegExp;
 var GlobalString = global.String;
 var InternalArray = utils.InternalArray;
 var InternalPackedArray = utils.InternalPackedArray;
-var MathMax;
-var MathMin;
 var RegExpExec;
 var RegExpExecNoTests;
 var RegExpLastMatchInfo;
 var SymbolToString;
 var ToNumber;
 var ToString;
 
 utils.Import(function(from) {
   ArrayIndexOf = from.ArrayIndexOf;
   ArrayJoin = from.ArrayJoin;
-  MathMax = from.MathMax;
-  MathMin = from.MathMin;
   RegExpExec = from.RegExpExec;
   RegExpExecNoTests = from.RegExpExecNoTests;
   RegExpLastMatchInfo = from.RegExpLastMatchInfo;
   SymbolToString = from.SymbolToString;
   ToNumber = from.ToNumber;
   ToString = from.ToString;
 });
 
 //-------------------------------------------------------------------
 
@@ skipping 942 matching lines @@
   }
 
   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);
+  if (pos < 0) pos = 0;
+  if (pos > s_len) pos = s_len;
   var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+
+  if (ss_len + pos > s_len) {
     return false;
   }
 
-  return %_SubString(s, start, start + ss_len) === ss;
+  for (var i = 0; i < ss_len; i++) {
+    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i))
+      return false;
+  }
+
+  return true;
 }
 
 
 // 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(kFirstArgumentNotRegExp, "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);
+  if (pos < 0) pos = 0;
+  if (pos > s_len) pos = s_len;
   var ss_len = ss.length;
-  var start = end - ss_len;
-  if (start < 0) {
+  pos = pos - ss_len;
+
+  if (pos < 0) {
     return false;
   }
 
-  return %_SubString(s, start, start + ss_len) === ss;
+  for (var i = 0; i < ss_len; i++) {
+    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i))
+      return false;
+  }
+
+  return true;

[Dan Ehrenberg, 2015/09/08 07:27:58]

There's some code duplication between these two functions. Can you factor any of
it out? For example, lines 1035 through 1041 are identical to 998-1003. If it's
marked inline, then I would expect that it wouldn't change performance.

[Jakob Kummerow, 2015/09/08 08:13:03]

How do you mark a JavaScript function inline? :-)

I'd leave this as it is.

 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.6
 function StringIncludes(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.includes");
 
   var string = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
@@ skipping 164 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })