dart2js cps: Add CharCodeAt builtin.

pkg/compiler/lib/src/cps_ir/builtin_operator.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library builtin_operator;
 // This is shared by the CPS and Tree IRs.
 // Both cps_ir_nodes and tree_ir_nodes import and re-export this file.
 
 /// An operator supported natively in the CPS and Tree IRs using the
 /// `ApplyBuiltinOperator` instructions.
 ///
@@ skipping 52 matching lines @@
   /// Compiles to `(~x) >>> 0`.
   NumBitNot,
 
   /// Concatenates any number of strings.
   ///
   /// Takes any number of arguments, and each argument must be a string.
   ///
   /// Returns the empty string if no arguments are given.
   StringConcatenate,
 
+  /// Corresponds to `a.charCodeAt(b)`. `a' must be a String. The index `b` must
+  /// be in range `0 <= b < a.length`.
+  CharCodeAt,

[sra1, 2015/11/23 20:39:32]

Question: will GVN hoist this above the bounds check?
There is no edge from the bounds check to the bounds use to prevent that.

[asgerf, 2015/11/24 11:28:31]

Yes, nice catch. We should disable lifting for these. We could just make it a
BuiltinMethod until we land a better infrastructure to keep track of these
things.

We might want to add "witness" nodes like in the SSA (essentially non-unary
refinements), but unless BCE is going to hoist witness nodes and follow up with
another LICM pass, I think the effect will be same as just disabling lifting for
the nodes that need such witnesses.

+
   /// Returns true if the two arguments are the same value, and that value is
   /// not NaN, or if one argument is +0 and the other is -0.
   ///
   /// Compiled as a static method call.
   Identical,
 
   /// Like [Identical], except at most one argument may be null.
   ///
   /// Compiles to `===`.
   StrictEq,
@@ skipping 85 matching lines @@
   /// Compiles to `object.push(x1, ..., xN)`.
   Push,
 
   /// Remove and return the last item from a native list.
   /// 
   /// Takes no arguments.
   /// 
   /// Compiles to `object.pop()`.
   Pop,
 }