[turbofan] Proper dead code elimination as regular reducer.

src/compiler/dead-code-elimination.h

+// Copyright 2015 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_DEAD_CODE_ELIMINATION_H_
+#define V8_COMPILER_DEAD_CODE_ELIMINATION_H_
+
+#include "src/compiler/graph-reducer.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Forward declarations.
+class CommonOperatorBuilder;
+
+
+// Propagates {Dead} control through the graph and thereby removes dead code.
+// Note that this does not include trimming dead uses from the graph, and it
+// also does not include detecting dead code by any other means than seeing a
+// {Dead} control input; that is left to other reducers.
+class DeadCodeElimination final : public AdvancedReducer {
+ public:
+  DeadCodeElimination(Editor* editor, Graph* graph,
+                      CommonOperatorBuilder* common);
+  ~DeadCodeElimination() final {}
+
+  Reduction Reduce(Node* node) final;
+
+ private:
+  Reduction ReduceEnd(Node* node);
+  Reduction ReduceLoopOrMerge(Node* node);
+  Reduction ReduceNode(Node* node);
+
+  void TrimMergeOrPhi(Node* node, int size);
+
+  Graph* graph() const { return graph_; }
+  CommonOperatorBuilder* common() const { return common_; }
+  Node* dead() const { return dead_; }
+
+  Graph* const graph_;
+  CommonOperatorBuilder* const common_;
+  Node* const dead_;
+
+  DISALLOW_COPY_AND_ASSIGN(DeadCodeElimination);
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_DEAD_CODE_ELIMINATION_H_