Add a simple dominator based redundancy elimination.

runtime/vm/flow_graph_optimizer.h

 // Copyright (c) 2012, 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.
 
 #ifndef VM_FLOW_GRAPH_OPTIMIZER_H_
 #define VM_FLOW_GRAPH_OPTIMIZER_H_
 
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph.h"
 
 namespace dart {
 
 template <typename T> class GrowableArray;
+template <typename T> class DirectChainedHashMap;
 
 class FlowGraphOptimizer : public FlowGraphVisitor {
  public:
   explicit FlowGraphOptimizer(const FlowGraph& flow_graph)
       : FlowGraphVisitor(flow_graph.reverse_postorder()) {}
   virtual ~FlowGraphOptimizer() {}
 
   void ApplyICData();
 
   void OptimizeComputations();
@@ skipping 74 matching lines @@
   virtual void VisitParameter(ParameterInstr* param);
   virtual void VisitPushArgument(PushArgumentInstr* bind);
 
  private:
   const ParsedFunction& parsed_function_;
   bool still_changing_;
   DISALLOW_COPY_AND_ASSIGN(FlowGraphTypePropagator);
 };
 
 
-class LocalCSE : public ValueObject {
+// A simple common subexpression elimination based
+// on the dominator tree.
+class DominatorBasedCSE : public AllStatic {
  public:
-  explicit LocalCSE(const FlowGraph& flow_graph)
-      : blocks_(flow_graph.reverse_postorder()) { }
-
-  void Optimize();
+  static void Optimize(BlockEntryInstr* graph_entry);
 
  private:
-  const GrowableArray<BlockEntryInstr*>& blocks_;
-
-  DISALLOW_COPY_AND_ASSIGN(LocalCSE);
+  static void OptimizeRecursive(
+      BlockEntryInstr* entry,
+      DirectChainedHashMap<BindInstr*>* map);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_