When constant folding x == y take reachability of blocks into account.

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"
 
@@ skipping 327 matching lines @@
 
   // Used to initialize the abstract value of definitions.
   static RawObject* Unknown() { return Object::unknown_constant().raw(); }
 
  private:
   void Analyze();
   void Transform();
   void EliminateRedundantBranches();
 
   void SetReachable(BlockEntryInstr* block);
-  void SetValue(Definition* definition, const Object& value);
+  bool SetValue(Definition* definition, const Object& value);
+
+  Definition* UnwrapPhi(Definition* defn);
+  void MarkPhi(Definition* defn);
 
   // Assign the join (least upper bound) of a pair of abstract values to the
   // first one.
   void Join(Object* left, const Object& right);
 
   bool IsUnknown(const Object& value) {
     return value.raw() == unknown_.raw();
   }
   bool IsNonConstant(const Object& value) {
     return value.raw() == non_constant_.raw();
@@ skipping 15 matching lines @@
   FlowGraph* graph_;
 
   // Sentinels for unknown constant and non-constant values.
   const Object& unknown_;
   const Object& non_constant_;
 
   // Analysis results. For each block, a reachability bit.  Indexed by
   // preorder number.
   BitVector* reachable_;
 
-  // Definitions can move up the lattice twice, so we use a mark bit to
-  // indicate that they are already on the worklist in order to avoid adding
-  // them again.  Indexed by SSA temp index.
-  BitVector* definition_marks_;
+  BitVector* marked_phis_;
 
   // Worklists of blocks and definitions.
   GrowableArray<BlockEntryInstr*> block_worklist_;
-  GrowableArray<Definition*> definition_worklist_;
+  DefinitionWorklist definition_worklist_;
 };
 
 
 // Rewrite branches to eliminate materialization of boolean values after
 // inlining, and to expose other optimizations (e.g., constant folding of
 // branches, unreachable code elimination).
 class BranchSimplifier : public AllStatic {
  public:
   static void Simplify(FlowGraph* flow_graph);
 
@@ skipping 101 matching lines @@
 // Optimize spill stores inside try-blocks by identifying values that always
 // contain a single known constant at catch block entry.
 class TryCatchAnalyzer : public AllStatic {
  public:
   static void Optimize(FlowGraph* flow_graph);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_