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/compiler/graph.h" 10 #include "src/compiler/node.h" 11 #include "src/globals.h" 12 #include "src/zone/zone-containers.h" 13 14 namespace v8 { 15 namespace internal { 16 namespace 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]. 33 class 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(), EmptyData(), 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 typedef enum { kInputDirection, kUseDirection } DFSDirection; 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 typedef ZoneLinkedList<Bracket> BracketList; 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 typedef ZoneStack<DFSStackEntry> DFSStack; 82 83 struct NodeData { 84 size_t class_number; // Equivalence class number assigned to node. 85 size_t dfs_number; // Pre-order DFS number assigned to node. 86 bool visited; // Indicates node has already been visited. 87 bool on_stack; // Indicates node is on DFS stack during walk. 88 bool participates; // Indicates node participates in DFS walk. 89 BracketList blist; // List of brackets per node. 90 }; 91 92 // The per-node data computed during the DFS walk. 93 typedef ZoneVector<NodeData> Data; 94 95 // Called at pre-visit during DFS walk. 96 void VisitPre(Node* node); 97 98 // Called at mid-visit during DFS walk. 99 void VisitMid(Node* node, DFSDirection direction); 100 101 // Called at post-visit during DFS walk. 102 void VisitPost(Node* node, Node* parent_node, DFSDirection direction); 103 104 // Called when hitting a back edge in the DFS walk. 105 void VisitBackedge(Node* from, Node* to, DFSDirection direction); 106 107 // Performs and undirected DFS walk of the graph. Conceptually all nodes are 108 // expanded, splitting "input" and "use" out into separate nodes. During the 109 // traversal, edges towards the representative nodes are preferred. 110 // 111 // \ / - Pre-visit: When N1 is visited in direction D the preferred 112 // x N1 edge towards N is taken next, calling VisitPre(N). 113 // | - Mid-visit: After all edges out of N2 in direction D have 114 // | N been visited, we switch the direction and start considering 115 // | edges out of N1 now, and we call VisitMid(N). 116 // x N2 - Post-visit: After all edges out of N1 in direction opposite 117 // / \ to D have been visited, we pop N and call VisitPost(N). 118 // 119 // This will yield a true spanning tree (without cross or forward edges) and 120 // also discover proper back edges in both directions. 121 void RunUndirectedDFS(Node* exit); 122 123 void DetermineParticipationEnqueue(ZoneQueue<Node*>& queue, Node* node); 124 void DetermineParticipation(Node* exit); 125 126 private: 127 NodeData* GetData(Node* node) { 128 size_t const index = node->id(); 129 if (index >= node_data_.size()) node_data_.resize(index + 1, EmptyData()); 130 return &node_data_[index]; 131 } 132 int NewClassNumber() { return class_number_++; } 133 int NewDFSNumber() { return dfs_number_++; } 134 135 // Template used to initialize per-node data. 136 NodeData EmptyData() { 137 return {kInvalidClass, 0, false, false, false, BracketList(zone_)}; 138 } 139 140 // Accessors for the DFS number stored within the per-node data. 141 size_t GetNumber(Node* node) { return GetData(node)->dfs_number; } 142 void SetNumber(Node* node, size_t number) { 143 GetData(node)->dfs_number = number; 144 } 145 146 // Accessors for the equivalence class stored within the per-node data. 147 size_t GetClass(Node* node) { return GetData(node)->class_number; } 148 void SetClass(Node* node, size_t number) { 149 GetData(node)->class_number = number; 150 } 151 152 // Accessors for the bracket list stored within the per-node data. 153 BracketList& GetBracketList(Node* node) { return GetData(node)->blist; } 154 void SetBracketList(Node* node, BracketList& list) { 155 GetData(node)->blist = list; 156 } 157 158 // Mutates the DFS stack by pushing an entry. 159 void DFSPush(DFSStack& stack, Node* node, Node* from, DFSDirection dir); 160 161 // Mutates the DFS stack by popping an entry. 162 void DFSPop(DFSStack& stack, Node* node); 163 164 void BracketListDelete(BracketList& blist, Node* to, DFSDirection direction); 165 void BracketListTRACE(BracketList& blist); 166 167 Zone* const zone_; 168 Graph* const graph_; 169 int dfs_number_; // Generates new DFS pre-order numbers on demand. 170 int class_number_; // Generates new equivalence class numbers on demand. 171 Data node_data_; // Per-node data stored as a side-table. 172 }; 173 174 } // namespace compiler 175 } // namespace internal 176 } // namespace v8 177 178 #endif // V8_COMPILER_CONTROL_EQUIVALENCE_H_ 179
