[turbofan] Allow ReplaceWithValue to kill control.

src/compiler/graph-reducer.h

 // 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.
 
 #ifndef V8_COMPILER_GRAPH_REDUCER_H_
 #define V8_COMPILER_GRAPH_REDUCER_H_
 
 #include "src/compiler/node-marker.h"
 #include "src/zone-containers.h"
 
@@ skipping 59 matching lines @@
    public:
     virtual ~Editor() {}
 
     // Replace {node} with {replacement}.
     virtual void Replace(Node* node, Node* replacement) = 0;
     // Revisit the {node} again later.
     virtual void Revisit(Node* node) = 0;
     // Replace value uses of {node} with {value} and effect uses of {node} with
     // {effect}. If {effect == NULL}, then use the effect input to {node}. All
     // control uses will be relaxed assuming {node} cannot throw.
-    virtual void ReplaceWithValue(Node* node, Node* value,
-                                  Node* effect = nullptr,
-                                  Node* control = nullptr) = 0;
+    virtual void ReplaceWithValue(Node* node, Node* value, Node* effect,
+                                  Node* control) = 0;
   };
 
   explicit AdvancedReducer(Editor* editor) : editor_(editor) {}
 
  protected:
   // Helper functions for subclasses to produce reductions for a node.
   static Reduction Replace(Node* node) { return Reducer::Replace(node); }
 
   // Helper functions for subclasses to edit the graph.
   void Replace(Node* node, Node* replacement) {
     DCHECK_NOT_NULL(editor_);
     editor_->Replace(node, replacement);
   }
   void Revisit(Node* node) {
     DCHECK_NOT_NULL(editor_);
     editor_->Revisit(node);
   }
   void ReplaceWithValue(Node* node, Node* value, Node* effect = nullptr,
                         Node* control = nullptr) {
     DCHECK_NOT_NULL(editor_);
     editor_->ReplaceWithValue(node, value, effect, control);
   }
 
   // Relax the effects of {node} by immediately replacing effect and control
   // uses of {node} with the effect and control input to {node}.
   // TODO(turbofan): replace the effect input to {node} with {graph->start()}.
   void RelaxEffectsAndControls(Node* node) {
-    DCHECK_NOT_NULL(editor_);
-    editor_->ReplaceWithValue(node, node, nullptr, nullptr);
+    ReplaceWithValue(node, node, nullptr, nullptr);
   }
 
   // Relax the control uses of {node} by immediately replacing them with the
   // control input to {node}.
   void RelaxControls(Node* node) {
-    DCHECK_NOT_NULL(editor_);
-    editor_->ReplaceWithValue(node, node, node, nullptr);
+    ReplaceWithValue(node, node, node, nullptr);
   }
 
  private:
   Editor* const editor_;
 };
 
 
 // Performs an iterative reduction of a node graph.
-class GraphReducer final : public AdvancedReducer::Editor {
+class GraphReducer : public AdvancedReducer::Editor {
  public:
-  GraphReducer(Graph* graph, Zone* zone);
-  ~GraphReducer() final;
+  GraphReducer(Zone* zone, Graph* graph, Node* dead_value = nullptr,
+               Node* dead_control = nullptr);
+  ~GraphReducer();
 
   Graph* graph() const { return graph_; }
 
   void AddReducer(Reducer* reducer);
 
   // Reduce a single node.
   void ReduceNode(Node* const);
   // Reduce the whole graph.
   void ReduceGraph();
 
  private:
   enum class State : uint8_t;
   struct NodeState {
     Node* node;
     int input_index;
   };
 
   // Reduce a single node.
   Reduction Reduce(Node* const);
   // Reduce the node on top of the stack.
   void ReduceTop();
 
   // Replace {node} with {replacement}.
   void Replace(Node* node, Node* replacement) final;
 
   // Replace value uses of {node} with {value} and effect uses of {node} with
   // {effect}. If {effect == NULL}, then use the effect input to {node}. All
   // control uses will be relaxed assuming {node} cannot throw.
-  void ReplaceWithValue(Node* node, Node* value, Node* effect = nullptr,
-                        Node* control = nullptr) final;
+  void ReplaceWithValue(Node* node, Node* value, Node* effect,
+                        Node* control) final;
 
   // Replace all uses of {node} with {replacement} if the id of {replacement} is
   // less than or equal to {max_id}. Otherwise, replace all uses of {node} whose
   // id is less than or equal to {max_id} with the {replacement}.
   void Replace(Node* node, Node* replacement, NodeId max_id);
 
   // Node stack operations.
   void Pop();
   void Push(Node* node);
 
   // Revisit queue operations.
   bool Recurse(Node* node);
   void Revisit(Node* node) final;
 
   Graph* const graph_;
+  Node* dead_value_;
+  Node* dead_control_;
   NodeMarker<State> state_;
   ZoneVector<Reducer*> reducers_;
   ZoneStack<Node*> revisit_;
   ZoneStack<NodeState> stack_;
 
   DISALLOW_COPY_AND_ASSIGN(GraphReducer);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_GRAPH_REDUCER_H_