Do not switch to two-byte string in String.fromCharCode if avoidable.

src/js/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
@@ skipping 764 matching lines @@
   var n = %_ArgumentsLength();
   if (n == 1) {
     if (!%_IsSmi(code)) code = TO_NUMBER(code);
     return %_StringCharFromCode(code & 0xffff);
   }
 
   var one_byte = %NewString(n, NEW_ONE_BYTE_STRING);
   var i;
   for (i = 0; i < n; i++) {
     var code = %_Arguments(i);
-    if (!%_IsSmi(code)) code = TO_NUMBER(code) & 0xffff;
-    if (code < 0) code = code & 0xffff;
+    if (!%_IsSmi(code)) code = TO_NUMBER(code);

[Benedikt Meurer, 2015/11/09 07:50:12]

As discussed offline: Please do not do this %_IsSmi and explicit TO_NUMBER
gymnastics here. The & does an implicit ToNumber, which is even optimized by the
IC.

+    code = code & 0xffff;
     if (code > 0xff) break;
     %_OneByteSeqStringSetChar(i, code, one_byte);
   }
   if (i == n) return one_byte;
   one_byte = %TruncateString(one_byte, i);
 
   var two_byte = %NewString(n - i, NEW_TWO_BYTE_STRING);
   for (var j = 0; i < n; i++, j++) {
     var code = %_Arguments(i);
     if (!%_IsSmi(code)) code = TO_NUMBER(code) & 0xffff;
@@ skipping 372 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })