Improve array to string conversion.

src/string.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 783 matching lines @@
 function StringTrimRight() {
   if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
     throw MakeTypeError("called_on_null_or_undefined",
                         ["String.prototype.trimRight"]);
   }
   return %StringTrim(TO_STRING_INLINE(this), false, true);
 }
 
 var static_charcode_array = new InternalArray(4);
 
+
+function TruncateString(string, new_length) {
+  if (new_length == 0) return "";
+  return %TruncateString(string, new_length);
+}
+
+
 // ECMA-262, section 15.5.3.2
 function StringFromCharCode(code) {
   var n = %_ArgumentsLength();
   if (n == 1) {
     if (!%_IsSmi(code)) code = ToNumber(code);
     return %_StringCharFromCode(code & 0xffff);
   }
 
-  // NOTE: This is not super-efficient, but it is necessary because we
-  // want to avoid converting to numbers from within the virtual
-  // machine. Maybe we can find another way of doing this?
-  var codes = static_charcode_array;
-  for (var i = 0; i < n; i++) {
+  var one_byte = %NewString(n, true);

[Toon Verwaest, 2012/12/05 14:29:12]

Can we have a javascript macro that indicates that this is "ONE_BYTE" vs
"TWO_BYTE"? That would be much more readable.

+  var i;
+  for (i = 0; i < n; i++) {
     var code = %_Arguments(i);
-    if (!%_IsSmi(code)) code = ToNumber(code);
-    codes[i] = code;
+    if (!%_IsSmi(code)) code = ToNumber(code) & 0xffff;
+    if (code > 0x7f) break;
+    %_OneByteSeqStringSetChar(one_byte, i, code);
   }
-  codes.length = n;
-  return %StringFromCharCodeArray(codes);
+  if (i == n) return one_byte;
+  one_byte = TruncateString(one_byte, i);
+
+  var two_byte = %NewString(n - i, false);
+  for (var j = 0; i < n; i++, j++) {
+    var code = %_Arguments(i);
+    if (!%_IsSmi(code)) code = ToNumber(code) & 0xffff;
+    %_TwoByteSeqStringSetChar(two_byte, j, code);
+  }
+  return one_byte + two_byte;
 }
 
 
 // Helper function for very basic XSS protection.
 function HtmlEscape(str) {
   return TO_STRING_INLINE(str).replace(/</g, "&lt;")
                               .replace(/>/g, "&gt;")
                               .replace(/"/g, "&quot;")
                               .replace(/'/g, "&#039;");
 }
@@ skipping 150 matching lines @@
     "fixed", StringFixed,
     "italics", StringItalics,
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup
   ));
 }
 
 SetUpString();