1// Copyright 2015 the V8 project authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef V8_COMPILER_DEAD_CODE_ELIMINATION_H_
6#define V8_COMPILER_DEAD_CODE_ELIMINATION_H_
7
8#include "src/base/compiler-specific.h"
9#include "src/codegen/machine-type.h"
10#include "src/common/globals.h"
11#include "src/compiler/graph-reducer.h"
12
13namespace v8 {
14namespace internal {
15namespace compiler {
16
17// Forward declarations.
18class CommonOperatorBuilder;
19
20// Propagates {Dead} control and {DeadValue} values through the graph and
21// thereby removes dead code.
22// We detect dead values based on types, replacing uses of nodes with
23// {Type::None()} with {DeadValue}. A pure node (other than a phi) using
24// {DeadValue} is replaced by {DeadValue}. When {DeadValue} hits the effect
25// chain, a crashing {Unreachable} node is inserted and the rest of the effect
26// chain is collapsed. We wait for the {EffectControlLinearizer} to connect
27// {Unreachable} nodes to the graph end, since this is much easier if there is
28// no floating control.
29// {DeadValue} has an input, which has to have {Type::None()}. This input is
30// important to maintain the dependency on the cause of the unreachable code.
31// {Unreachable} has a value output and {Type::None()} so it can be used by
32// {DeadValue}.
33// {DeadValue} nodes track a {MachineRepresentation} so they can be lowered to a
34// value-producing node. {DeadValue} has the runtime semantics of crashing and
35// behaves like a constant of its representation so it can be used in gap moves.
36// Since phi nodes are the only remaining use of {DeadValue}, this
37// representation is only adjusted for uses by phi nodes.
38// In contrast to {DeadValue}, {Dead} can never remain in the graph.
39class V8_EXPORT_PRIVATE DeadCodeElimination final
40    : public NON_EXPORTED_BASE(AdvancedReducer) {
41 public:
42  DeadCodeElimination(Editor* editor, Graph* graph,
43                      CommonOperatorBuilder* common, Zone* temp_zone);
44  ~DeadCodeElimination() final = default;
45  DeadCodeElimination(const DeadCodeElimination&) = delete;
46  DeadCodeElimination& operator=(const DeadCodeElimination&) = delete;
47
48  const char* reducer_name() const override { return "DeadCodeElimination"; }
49
50  Reduction Reduce(Node* node) final;
51
52 private:
53  Reduction ReduceEnd(Node* node);
54  Reduction ReduceLoopOrMerge(Node* node);
55  Reduction ReduceLoopExit(Node* node);
56  Reduction ReduceNode(Node* node);
57  Reduction ReducePhi(Node* node);
58  Reduction ReduceEffectPhi(Node* node);
59  Reduction ReducePureNode(Node* node);
60  Reduction ReduceUnreachableOrIfException(Node* node);
61  Reduction ReduceEffectNode(Node* node);
62  Reduction ReduceDeoptimizeOrReturnOrTerminateOrTailCall(Node* node);
63  Reduction ReduceBranchOrSwitch(Node* node);
64
65  Reduction RemoveLoopExit(Node* node);
66  Reduction PropagateDeadControl(Node* node);
67
68  void TrimMergeOrPhi(Node* node, int size);
69
70  Node* DeadValue(Node* none_node,
71                  MachineRepresentation rep = MachineRepresentation::kNone);
72
73  Graph* graph() const { return graph_; }
74  CommonOperatorBuilder* common() const { return common_; }
75  Node* dead() const { return dead_; }
76
77  Graph* const graph_;
78  CommonOperatorBuilder* const common_;
79  Node* const dead_;
80  Zone* zone_;
81};
82
83}  // namespace compiler
84}  // namespace internal
85}  // namespace v8
86
87#endif  // V8_COMPILER_DEAD_CODE_ELIMINATION_H_
88