1// Copyright 2014 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_CONTROL_EQUIVALENCE_H_
6#define V8_COMPILER_CONTROL_EQUIVALENCE_H_
7
8#include "src/base/compiler-specific.h"
9#include "src/common/globals.h"
10#include "src/compiler/graph.h"
11#include "src/compiler/node.h"
12#include "src/zone/zone-containers.h"
13
14namespace v8 {
15namespace internal {
16namespace compiler {
17
18// Determines control dependence equivalence classes for control nodes. Any two
19// nodes having the same set of control dependences land in one class. These
20// classes can in turn be used to:
21//  - Build a program structure tree (PST) for controls in the graph.
22//  - Determine single-entry single-exit (SESE) regions within the graph.
23//
24// Note that this implementation actually uses cycle equivalence to establish
25// class numbers. Any two nodes are cycle equivalent if they occur in the same
26// set of cycles. It can be shown that control dependence equivalence reduces
27// to undirected cycle equivalence for strongly connected control flow graphs.
28//
29// The algorithm is based on the paper, "The program structure tree: computing
30// control regions in linear time" by Johnson, Pearson & Pingali (PLDI94) which
31// also contains proofs for the aforementioned equivalence. References to line
32// numbers in the algorithm from figure 4 have been added [line:x].
33class V8_EXPORT_PRIVATE ControlEquivalence final
34    : public NON_EXPORTED_BASE(ZoneObject) {
35 public:
36  ControlEquivalence(Zone* zone, Graph* graph)
37      : zone_(zone),
38        graph_(graph),
39        dfs_number_(0),
40        class_number_(1),
41        node_data_(graph->NodeCount(), zone) {}
42
43  // Run the main algorithm starting from the {exit} control node. This causes
44  // the following iterations over control edges of the graph:
45  //  1) A breadth-first backwards traversal to determine the set of nodes that
46  //     participate in the next step. Takes O(E) time and O(N) space.
47  //  2) An undirected depth-first backwards traversal that determines class
48  //     numbers for all participating nodes. Takes O(E) time and O(N) space.
49  void Run(Node* exit);
50
51  // Retrieves a previously computed class number.
52  size_t ClassOf(Node* node) {
53    DCHECK_NE(kInvalidClass, GetClass(node));
54    return GetClass(node);
55  }
56
57 private:
58  static const size_t kInvalidClass = static_cast<size_t>(-1);
59  enum DFSDirection { kInputDirection, kUseDirection };
60
61  struct Bracket {
62    DFSDirection direction;  // Direction in which this bracket was added.
63    size_t recent_class;     // Cached class when bracket was topmost.
64    size_t recent_size;      // Cached set-size when bracket was topmost.
65    Node* from;              // Node that this bracket originates from.
66    Node* to;                // Node that this bracket points to.
67  };
68
69  // The set of brackets for each node during the DFS walk.
70  using BracketList = ZoneLinkedList<Bracket>;
71
72  struct DFSStackEntry {
73    DFSDirection direction;            // Direction currently used in DFS walk.
74    Node::InputEdges::iterator input;  // Iterator used for "input" direction.
75    Node::UseEdges::iterator use;      // Iterator used for "use" direction.
76    Node* parent_node;                 // Parent node of entry during DFS walk.
77    Node* node;                        // Node that this stack entry belongs to.
78  };
79
80  // The stack is used during the undirected DFS walk.
81  using DFSStack = ZoneStack<DFSStackEntry>;
82
83  struct NodeData : ZoneObject {
84    explicit NodeData(Zone* zone)
85        : class_number(kInvalidClass),
86          blist(BracketList(zone)),
87          visited(false),
88          on_stack(false) {}
89
90    size_t class_number;  // Equivalence class number assigned to node.
91    BracketList blist;    // List of brackets per node.
92    bool visited : 1;     // Indicates node has already been visited.
93    bool on_stack : 1;    // Indicates node is on DFS stack during walk.
94  };
95
96  // The per-node data computed during the DFS walk.
97  using Data = ZoneVector<NodeData*>;
98
99  // Called at pre-visit during DFS walk.
100  void VisitPre(Node* node);
101
102  // Called at mid-visit during DFS walk.
103  void VisitMid(Node* node, DFSDirection direction);
104
105  // Called at post-visit during DFS walk.
106  void VisitPost(Node* node, Node* parent_node, DFSDirection direction);
107
108  // Called when hitting a back edge in the DFS walk.
109  void VisitBackedge(Node* from, Node* to, DFSDirection direction);
110
111  // Performs and undirected DFS walk of the graph. Conceptually all nodes are
112  // expanded, splitting "input" and "use" out into separate nodes. During the
113  // traversal, edges towards the representative nodes are preferred.
114  //
115  //   \ /        - Pre-visit: When N1 is visited in direction D the preferred
116  //    x   N1      edge towards N is taken next, calling VisitPre(N).
117  //    |         - Mid-visit: After all edges out of N2 in direction D have
118  //    |   N       been visited, we switch the direction and start considering
119  //    |           edges out of N1 now, and we call VisitMid(N).
120  //    x   N2    - Post-visit: After all edges out of N1 in direction opposite
121  //   / \          to D have been visited, we pop N and call VisitPost(N).
122  //
123  // This will yield a true spanning tree (without cross or forward edges) and
124  // also discover proper back edges in both directions.
125  void RunUndirectedDFS(Node* exit);
126
127  void DetermineParticipationEnqueue(ZoneQueue<Node*>& queue, Node* node);
128  void DetermineParticipation(Node* exit);
129
130 private:
131  NodeData* GetData(Node* node) {
132    size_t const index = node->id();
133    if (index >= node_data_.size()) node_data_.resize(index + 1);
134    return node_data_[index];
135  }
136  void AllocateData(Node* node) {
137    size_t const index = node->id();
138    if (index >= node_data_.size()) node_data_.resize(index + 1);
139    node_data_[index] = zone_->New<NodeData>(zone_);
140  }
141
142  int NewClassNumber() { return class_number_++; }
143  int NewDFSNumber() { return dfs_number_++; }
144
145  bool Participates(Node* node) { return GetData(node) != nullptr; }
146
147  // Accessors for the equivalence class stored within the per-node data.
148  size_t GetClass(Node* node) { return GetData(node)->class_number; }
149  void SetClass(Node* node, size_t number) {
150    DCHECK(Participates(node));
151    GetData(node)->class_number = number;
152  }
153
154  // Accessors for the bracket list stored within the per-node data.
155  BracketList& GetBracketList(Node* node) {
156    DCHECK(Participates(node));
157    return GetData(node)->blist;
158  }
159  void SetBracketList(Node* node, BracketList& list) {
160    DCHECK(Participates(node));
161    GetData(node)->blist = list;
162  }
163
164  // Mutates the DFS stack by pushing an entry.
165  void DFSPush(DFSStack& stack, Node* node, Node* from, DFSDirection dir);
166
167  // Mutates the DFS stack by popping an entry.
168  void DFSPop(DFSStack& stack, Node* node);
169
170  void BracketListDelete(BracketList& blist, Node* to, DFSDirection direction);
171  void BracketListTRACE(BracketList& blist);
172
173  Zone* const zone_;
174  Graph* const graph_;
175  int dfs_number_;    // Generates new DFS pre-order numbers on demand.
176  int class_number_;  // Generates new equivalence class numbers on demand.
177  Data node_data_;    // Per-node data stored as a side-table.
178};
179
180}  // namespace compiler
181}  // namespace internal
182}  // namespace v8
183
184#endif  // V8_COMPILER_CONTROL_EQUIVALENCE_H_
185