Let Uint16List.setRange detect a String.codeUnits object, and copy directly from the string.

runtime/lib/typed_data.dart

Index: runtime/lib/typed_data.dart
diff --git a/runtime/lib/typed_data.dart b/runtime/lib/typed_data.dart
index 84905ef08c751aa4dba7f396f2d8e182614fb240..f4ae012d46913b4c9b81cbb5cd5af1d599dc58c7 100644
--- a/runtime/lib/typed_data.dart
+++ b/runtime/lib/typed_data.dart
@@ -829,6 +829,29 @@ abstract class _TypedList extends _TypedListBase {
   Float64x2 _getFloat64x2(int offsetInBytes) native "TypedData_GetFloat64x2";
   void _setFloat64x2(int offsetInBytes, Float64x2 value)
       native "TypedData_SetFloat64x2";
+
+  /** Writes the code units of `string.substring(start, end)` as Uint16. */
+  void _setStringAt(int byteOffset, String string, int start, int end) {
+    // This can be optimized to a mem-move if string is two-byte.
+    int length = end - start;
+    assert(byteOffset + length * Uint16List.BYTES_PER_ELEMENT <=
+           this.lengthInBytes);
+    for (int i = 0; i < length; i++) {
+      _setUint16(byteOffset + i * Uint16List.BYTES_PER_ELEMENT,
+                 string.codeUnitAt(start + i));
+    }
+  }
+
+  void _setCodeUnits(CodeUnits units, int start, int end, int skipCount) {
+    String string = CodeUnits.stringOf(units);
+    end = RangeError.checkValidRange(start, end, length);
+    int sliceLength = end - start;
+    int sliceEnd = skipCount + sliceLength;
+    RangeError.checkValidRange(skipCount, sliceEnd,
+                               string.length,
+                               "skipCount", "skipCount + length");
+    this._setStringAt(start * elementSizeInBytes, string, skipCount, sliceEnd);
+  }
 }
 
 
@@ -1029,6 +1052,13 @@ class _Uint16Array extends _TypedList with _IntListMixin implements Uint16List {
     _setIndexedUint16(index, _toUint16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) != CodeUnits.cid) {
+      super.setRange(start, end, iterable, skipCount);
+    } else {
+      _setCodeUnits(iterable, start, end, skipCount);

[Vyacheslav Egorov (Google), 2015/01/29 13:04:54]

Maybe we should also have an optimization for Int8Array/Uint8Array and one byte
string in this CL?

It's also unclear to me why Int16Array is not covered by this optimization?

[Lasse Reichstein Nielsen, 2015/01/29 13:13:27]

I was planning to do that in a later CL.
Good point. Writing to an Int16Array and an Uint16Array is completely equivalent
(it's the same bits getting written in either case, it's only reading them back
that differs).

I'll add the setRange to Int16Array/View too.

+    }
+  }
 
   // Method(s) implementing the TypedData interface.
 
@@ -2453,7 +2483,7 @@ class _TypedListView extends _TypedListBase implements TypedData {
     return _typedData.buffer;
   }
 
-  final TypedData _typedData;
+  final _TypedList _typedData;
   final int offsetInBytes;
   final int length;
 }
@@ -2681,6 +2711,13 @@ class _Uint16ArrayView extends _TypedListView with _IntListMixin implements Uint
                           _toUint16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) != CodeUnits.cid) {
+      super.setRange(start, end, iterable, skipCount);
+    } else {
+      _typed_Data._setCodeUnits(iterable, start, end, skipCount);
+    }
+  }
 
   // Method(s) implementing TypedData interface.
 
@@ -2688,7 +2725,6 @@ class _Uint16ArrayView extends _TypedListView with _IntListMixin implements Uint
     return Uint16List.BYTES_PER_ELEMENT;
   }
 
-
   // Internal utility methods.
 
   Uint16List _createList(int length) {