Add optimized String.fromCharCodes path for Uint8List and Int8List.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index 9d5111bd9ea391f0a5d643bbfcdd81440fb9057b..ead4b8f23321a5ec944a465cf0e408f57c595326 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -34,6 +34,9 @@ class _StringBase {
    */
   static String createFromCharCodes(Iterable<int> charCodes) {
     if (charCodes != null) {
+      if (charCodes is Uint8List || charCodes is Int8List) {

[Anders Johnsen, 2013/11/18 20:26:31]

Should we use _classId here as well?

[srdjan, 2013/11/18 20:45:15]

Yes, using cid-s is much quicker.

[Anders Johnsen, 2013/11/19 07:38:43]

I encountered an issue here. These classes are in typed_data-patch, so:

- _Uint8List is not accessible from dart:core-patch.
- _cid is not accessible within dart:typed_data-patch.
- Uint8List._privateHelper is not accessible

What's your recommendation here?

[srdjan, 2013/11/19 16:47:38]

This would not slow down existing code:

if ((ccid != _List._classId) && (ccid != _GrowableList._classId) && (ccid !=
_ImmutableList._classId)) {
  if (charCodes is Uint8List || charCodes is Int8List) {
    return _OneByteString._allocateFromOneByteList(charCodes);
  }
  charCodes = new List<int>.from(charCodes, growable: false);
}

You may want to benefit from skipping the call to native for small 8-bit lists
and skip the isOneByteString test.

You could also check first the charCodes length:

if (charCodes.length >128) {
  return _OneByteString._allocateFromOneByteList(charCodes);
}

We plan to make all cid's available through an internal VM class. Until then use
the instanceOf test is OK.

+        return _OneByteString._allocateFromOneByteList(charCodes);
+      }
       // TODO(srdjan): Also skip copying of typed arrays.
       final ccid = charCodes._cid;
       if ((ccid != _List._classId) &&
@@ -53,11 +56,7 @@ class _StringBase {
         }
       }
       if (isOneByteString) {
-        var s = _OneByteString._allocate(charCodes.length);
-        for (int i = 0; i < charCodes.length; i++) {
-          s._setAt(i, charCodes[i]);
-        }
-        return s;
+        return _OneByteString._allocateFromOneByteList(charCodes);

[srdjan, 2013/11/18 20:45:15]

Calling to native is quite slow, i.e., for small lists this will be quite more
expensive. Could you benchmark? Any golem numbers on existing benchmarks?

[Anders Johnsen, 2013/11/18 21:10:51]

Before, we already did the runtime-call to allocate the list. So, doing this is
actually faster (~25% for a small local test). Will upload golem numbers
tomorrow at the office.

       }
     }
     return _createFromCodePoints(charCodes);
@@ -628,6 +627,10 @@ class _OneByteString extends _StringBase implements String {
   // set using _setAt.
   static _OneByteString _allocate(int length) native "OneByteString_allocate";
 
+
+  static _OneByteString _allocateFromOneByteList(List<int> list)
+      native "OneByteString_allocateFromOneByteList";
+
   // This is internal helper method. Code point value must be a valid
   // Latin1 value (0..0xFF), index must be valid.
   void _setAt(int index, int codePoint) native "OneByteString_setAt";