| // 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_CONTROL_EQUIVALENCE_H_ |
| #define V8_COMPILER_CONTROL_EQUIVALENCE_H_ |
| |
| #include "src/base/compiler-specific.h" |
| #include "src/compiler/graph.h" |
| #include "src/compiler/node.h" |
| #include "src/globals.h" |
| #include "src/zone/zone-containers.h" |
| |
| namespace v8 { |
| namespace internal { |
| namespace compiler { |
| |
| // Determines control dependence equivalence classes for control nodes. Any two |
| // nodes having the same set of control dependences land in one class. These |
| // classes can in turn be used to: |
| // - Build a program structure tree (PST) for controls in the graph. |
| // - Determine single-entry single-exit (SESE) regions within the graph. |
| // |
| // Note that this implementation actually uses cycle equivalence to establish |
| // class numbers. Any two nodes are cycle equivalent if they occur in the same |
| // set of cycles. It can be shown that control dependence equivalence reduces |
| // to undirected cycle equivalence for strongly connected control flow graphs. |
| // |
| // The algorithm is based on the paper, "The program structure tree: computing |
| // control regions in linear time" by Johnson, Pearson & Pingali (PLDI94) which |
| // also contains proofs for the aforementioned equivalence. References to line |
| // numbers in the algorithm from figure 4 have been added [line:x]. |
| class V8_EXPORT_PRIVATE ControlEquivalence final |
| : public NON_EXPORTED_BASE(ZoneObject) { |
| public: |
| ControlEquivalence(Zone* zone, Graph* graph) |
| : zone_(zone), |
| graph_(graph), |
| dfs_number_(0), |
| class_number_(1), |
| node_data_(graph->NodeCount(), zone) {} |
| |
| // Run the main algorithm starting from the {exit} control node. This causes |
| // the following iterations over control edges of the graph: |
| // 1) A breadth-first backwards traversal to determine the set of nodes that |
| // participate in the next step. Takes O(E) time and O(N) space. |
| // 2) An undirected depth-first backwards traversal that determines class |
| // numbers for all participating nodes. Takes O(E) time and O(N) space. |
| void Run(Node* exit); |
| |
| // Retrieves a previously computed class number. |
| size_t ClassOf(Node* node) { |
| DCHECK_NE(kInvalidClass, GetClass(node)); |
| return GetClass(node); |
| } |
| |
| private: |
| static const size_t kInvalidClass = static_cast<size_t>(-1); |
| typedef enum { kInputDirection, kUseDirection } DFSDirection; |
| |
| struct Bracket { |
| DFSDirection direction; // Direction in which this bracket was added. |
| size_t recent_class; // Cached class when bracket was topmost. |
| size_t recent_size; // Cached set-size when bracket was topmost. |
| Node* from; // Node that this bracket originates from. |
| Node* to; // Node that this bracket points to. |
| }; |
| |
| // The set of brackets for each node during the DFS walk. |
| typedef ZoneLinkedList<Bracket> BracketList; |
| |
| struct DFSStackEntry { |
| DFSDirection direction; // Direction currently used in DFS walk. |
| Node::InputEdges::iterator input; // Iterator used for "input" direction. |
| Node::UseEdges::iterator use; // Iterator used for "use" direction. |
| Node* parent_node; // Parent node of entry during DFS walk. |
| Node* node; // Node that this stack entry belongs to. |
| }; |
| |
| // The stack is used during the undirected DFS walk. |
| typedef ZoneStack<DFSStackEntry> DFSStack; |
| |
| struct NodeData : ZoneObject { |
| explicit NodeData(Zone* zone) |
| : class_number(kInvalidClass), |
| blist(BracketList(zone)), |
| visited(false), |
| on_stack(false) {} |
| |
| size_t class_number; // Equivalence class number assigned to node. |
| BracketList blist; // List of brackets per node. |
| bool visited : 1; // Indicates node has already been visited. |
| bool on_stack : 1; // Indicates node is on DFS stack during walk. |
| }; |
| |
| // The per-node data computed during the DFS walk. |
| typedef ZoneVector<NodeData*> Data; |
| |
| // Called at pre-visit during DFS walk. |
| void VisitPre(Node* node); |
| |
| // Called at mid-visit during DFS walk. |
| void VisitMid(Node* node, DFSDirection direction); |
| |
| // Called at post-visit during DFS walk. |
| void VisitPost(Node* node, Node* parent_node, DFSDirection direction); |
| |
| // Called when hitting a back edge in the DFS walk. |
| void VisitBackedge(Node* from, Node* to, DFSDirection direction); |
| |
| // Performs and undirected DFS walk of the graph. Conceptually all nodes are |
| // expanded, splitting "input" and "use" out into separate nodes. During the |
| // traversal, edges towards the representative nodes are preferred. |
| // |
| // \ / - Pre-visit: When N1 is visited in direction D the preferred |
| // x N1 edge towards N is taken next, calling VisitPre(N). |
| // | - Mid-visit: After all edges out of N2 in direction D have |
| // | N been visited, we switch the direction and start considering |
| // | edges out of N1 now, and we call VisitMid(N). |
| // x N2 - Post-visit: After all edges out of N1 in direction opposite |
| // / \ to D have been visited, we pop N and call VisitPost(N). |
| // |
| // This will yield a true spanning tree (without cross or forward edges) and |
| // also discover proper back edges in both directions. |
| void RunUndirectedDFS(Node* exit); |
| |
| void DetermineParticipationEnqueue(ZoneQueue<Node*>& queue, Node* node); |
| void DetermineParticipation(Node* exit); |
| |
| private: |
| NodeData* GetData(Node* node) { |
| size_t const index = node->id(); |
| if (index >= node_data_.size()) node_data_.resize(index + 1); |
| return node_data_[index]; |
| } |
| void AllocateData(Node* node) { |
| size_t const index = node->id(); |
| if (index >= node_data_.size()) node_data_.resize(index + 1); |
| node_data_[index] = new (zone_) NodeData(zone_); |
| } |
| |
| int NewClassNumber() { return class_number_++; } |
| int NewDFSNumber() { return dfs_number_++; } |
| |
| bool Participates(Node* node) { return GetData(node) != nullptr; } |
| |
| // Accessors for the equivalence class stored within the per-node data. |
| size_t GetClass(Node* node) { return GetData(node)->class_number; } |
| void SetClass(Node* node, size_t number) { |
| DCHECK(Participates(node)); |
| GetData(node)->class_number = number; |
| } |
| |
| // Accessors for the bracket list stored within the per-node data. |
| BracketList& GetBracketList(Node* node) { |
| DCHECK(Participates(node)); |
| return GetData(node)->blist; |
| } |
| void SetBracketList(Node* node, BracketList& list) { |
| DCHECK(Participates(node)); |
| GetData(node)->blist = list; |
| } |
| |
| // Mutates the DFS stack by pushing an entry. |
| void DFSPush(DFSStack& stack, Node* node, Node* from, DFSDirection dir); |
| |
| // Mutates the DFS stack by popping an entry. |
| void DFSPop(DFSStack& stack, Node* node); |
| |
| void BracketListDelete(BracketList& blist, Node* to, DFSDirection direction); |
| void BracketListTRACE(BracketList& blist); |
| |
| Zone* const zone_; |
| Graph* const graph_; |
| int dfs_number_; // Generates new DFS pre-order numbers on demand. |
| int class_number_; // Generates new equivalence class numbers on demand. |
| Data node_data_; // Per-node data stored as a side-table. |
| }; |
| |
| } // namespace compiler |
| } // namespace internal |
| } // namespace v8 |
| |
| #endif // V8_COMPILER_CONTROL_EQUIVALENCE_H_ |