[turbofan] Reenable value numbering.

src/compiler/pipeline.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/pipeline.h"
 
 #include <sstream>
 
 #include "src/base/platform/elapsed-timer.h"
 #include "src/compiler/ast-graph-builder.h"
@@ skipping 272 matching lines @@
 
       VerifyAndPrintGraph(&graph, "Lowered simplified");
     }
     {
       // Lower changes that have been inserted before.
       PhaseStats lowering_stats(info(), PhaseStats::OPTIMIZATION,
                                 "change lowering");
       SourcePositionTable::Scope pos(&source_positions,
                                      SourcePosition::Unknown());
       Linkage linkage(info());
-      // TODO(turbofan): Value numbering disabled for now.
-      // ValueNumberingReducer vn_reducer(zone());
+      ValueNumberingReducer vn_reducer(zone());
       SimplifiedOperatorReducer simple_reducer(&jsgraph);
       ChangeLowering lowering(&jsgraph, &linkage);
       MachineOperatorReducer mach_reducer(&jsgraph);
       GraphReducer graph_reducer(&graph);
       // TODO(titzer): Figure out if we should run all reducers at once here.
-      // graph_reducer.AddReducer(&vn_reducer);
+      graph_reducer.AddReducer(&vn_reducer);
       graph_reducer.AddReducer(&simple_reducer);
       graph_reducer.AddReducer(&lowering);
       graph_reducer.AddReducer(&mach_reducer);
       graph_reducer.ReduceGraph();
 
       VerifyAndPrintGraph(&graph, "Lowered changes");
     }
   }
 
   {
@@ skipping 136 matching lines @@
 }
 
 
 void Pipeline::TearDown() {
   InstructionOperand::TearDownCaches();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8