Add CPS IR transformation to make the JavaScript backend specific semantics explicit in the tree.

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

 // Copyright (c) 2014, 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.
 
 part of dart2js.optimizers;
 
 /**
  * Propagates constants throughout the IR, and replaces branches with fixed
  * jumps as well as side-effect free expressions with known constant results.
  * Should be followed by the [ShrinkingReducer] pass.
@@ skipping 10 matching lines @@
   // arguments.
   final dart2js.ConstantSystem _constantSystem;
 
   ConstantPropagator(this._compiler, this._constantSystem);
 
   void rewrite(FunctionDefinition root) {
     if (root.isAbstract) return;
 
     // Set all parent pointers.
 
-    new _ParentVisitor().visit(root);
+    new ParentVisitor().visit(root);
 
     // Analyze. In this phase, the entire term is analyzed for reachability
     // and the constant status of each expression.
 
     _ConstPropagationVisitor analyzer =
         new _ConstPropagationVisitor(_compiler, _constantSystem);
     analyzer.analyze(root);
 
     // Transform. Uses the data acquired in the previous analysis phase to
     // replace branches with fixed targets and side-effect-free expressions
@@ skipping 554 matching lines @@
       setReachable(node.body);
     }
   }
 
   // Conditions.
 
   void visitIsTrue(IsTrue node) {
     Branch branch = node.parent;
     visitBranch(branch);
   }
+
+  // JavaScript specific nodes.
+
+  void visitIdentical(Identical node) {
+    setReachable(node.left.definition);

[Kevin Millikin (Google), 2014/11/25 14:27:23]

I need to understand the code in this file a bit better.  But I think the safe
thing here is to have:

void visitIdentical(Identical node) {
  setValue(node, _ConstnessLattice.NonConst);
}

[karlklose, 2014/11/25 14:38:46]

I am not sure if it is okay to not set the parameters as reachable.

I added the call to mark the node itself as non-constant.

+    setReachable(node.right.definition);
+  }
 }
 
 /// Represents the constant-state of a variable at some point in the program.
 /// UNKNOWN: may be some as yet undetermined constant.
 /// CONSTANT: is a constant as stored in the local field.
 /// NONCONST: not a constant.
 class _ConstnessLattice {
   static const int UNKNOWN  = 0;
   static const int CONSTANT = 1;
   static const int NONCONST = 2;
@@ skipping 41 matching lines @@
       return that;
     }
 
     if (this.constant == that.constant) {
       return this;
     }
 
     return NonConst;
   }
 }