Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.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 tree_ir.optimization;
 
 /**
  * Performs the following transformations on the tree:
  * - Assignment propagation
  * - If-to-conditional conversion
@@ skipping 19 matching lines @@
  * seen, but are only processed once to keep this transformation linear in
  * the size of the tree.
  *
  * The transformation builds an environment containing [Assign] bindings that
  * are in scope.  These bindings have yet-untranslated definitions.  When a use
  * is encountered the transformation determines if it is safe and profitable
  * to propagate the definition to its use.  If so, it is removed from the
  * environment and the definition is recursively processed (in the
  * new environment at the use site) before being propagated.
  *
- * See [visitVariable] for the implementation of the heuristic for propagating
- * a definition.
+ * See [visitVariableUse] for the implementation of the heuristic for
+ * propagating a definition.
  *
  *
  * IF-TO-CONDITIONAL CONVERSION:
  * If-statement are converted to conditional expressions when possible.
  * For example:
  *
  *   if (v0) { v1 = foo(); break L } else { v1 = bar(); break L }
  *     ==>
  *   { v1 = v0 ? foo() : bar(); break L }
  *
@@ skipping 98 matching lines @@
       for (int i = node.arguments.length - 1; i >= 0; --i) {
         node.arguments[i] = visitStatement(node.arguments[i]);
         assert(environment.isEmpty);
       }
     });
     return node;
   }
 
   Expression visitExpression(Expression e) => e.processed ? e : e.accept(this);
 
-  Expression visitVariable(Variable node) {
+  @override
+  Expression visitVariableUse(VariableUse node) {
     // Propagate constant to use site.
-    Expression constant = constantEnvironment[node];
+    Expression constant = constantEnvironment[node.variable];
     if (constant != null) return constant;
 
     // Propagate a variable's definition to its use site if:
     // 1.  It has a single use, to avoid code growth and potential duplication
     //     of side effects, AND
     // 2.  It was the most recent expression evaluated so that we do not
     //     reorder expressions with side effects.
     if (!environment.isEmpty &&
-        environment.last.variable == node &&
+        environment.last.variable == node.variable &&
         environment.last.hasExactlyOneUse) {
       return visitExpression(environment.removeLast().definition);
     }
+
     // If the definition could not be propagated, leave the variable use.
     return node;
   }
 
   /// Returns true if [exp] has no side effects and has a constant value within
   /// any given activation of the enclosing method.
   bool isEffectivelyConstant(Expression exp) {
     // TODO(asgerf): Can be made more aggressive e.g. by checking conditional
     // expressions recursively. Determine if that is a valuable optimization
     // and/or if it is better handled at the CPS level.
     return exp is Constant ||
            exp is This ||
            exp is ReifyTypeVar ||
-           exp is Variable && constantEnvironment.containsKey(exp);
+           exp is VariableUse && constantEnvironment.containsKey(exp.variable);
   }
 
   Statement visitAssign(Assign node) {
     if (isEffectivelyConstant(node.definition) &&
         node.variable.writeCount == 1) {
       // Handle constant assignments specially.
       // They are always safe to propagate (though we should avoid duplication).
       // Moreover, they should not prevent other expressions from propagating.
       if (node.variable.readCount <= 1) {
         // A single-use constant should always be propagted to its use site.
@@ skipping 331 matching lines @@
         return new Return(e);
       }
     }
     return null;
   }
 
   /// Returns an expression equivalent to both [e1] and [e2].
   /// If non-null is returned, the caller must discard [e1] and [e2] and use
   /// the resulting expression in the tree.
   static Expression combineExpressions(Expression e1, Expression e2) {
-    if (e1 is Variable && e1 == e2) {
-      --e1.readCount; // Two references become one.
+    if (e1 is VariableUse && e2 is VariableUse && e1.variable == e2.variable) {
+      --e1.variable.readCount; // Two references become one.
       return e1;
     }
     if (e1 is Constant && e2 is Constant && e1.value == e2.value) {
       return e1;
     }
     return null;
   }
 
   /// Try to collapse nested ifs using && and || expressions.
   /// For example:
@@ skipping 73 matching lines @@
   }
 
   Expression makeCondition(Expression e, bool polarity) {
     return polarity ? e : new Not(e);
   }
 
   Statement getBranch(If node, bool polarity) {
     return polarity ? node.thenStatement : node.elseStatement;
   }
 }