Improve array to string conversion.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 6115930b6c8ff69568f425b8b996a821bc93cb87..b698a2af5ba351e584b3dc7143be044ffe82ba23 100644
--- a/src/string.js
+++ b/src/string.js
@@ -801,6 +801,13 @@ function StringTrimRight() {
 
 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();
@@ -809,17 +816,24 @@ function StringFromCharCode(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);
+  }
+  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);
   }
-  codes.length = n;
-  return %StringFromCharCodeArray(codes);
+  return one_byte + two_byte;
 }