Restrict floating control to minimal control-connected component.

src/compiler/control-equivalence.h

+// 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/v8.h"
+
+#include "src/compiler/graph.h"
+#include "src/compiler/node.h"
+#include "src/compiler/node-properties.h"
+#include "src/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 ControlEquivalence : public ZoneObject {
+ public:
+  ControlEquivalence(Zone* zone, Graph* graph)
+      : zone_(zone),
+        graph_(graph),
+        dfs_number_(0),
+        class_number_(1),
+        node_data_(graph->NodeCount(), EmptyData(), 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) {
+    if (GetClass(exit) != kInvalidClass) return;
+    DetermineParticipation(exit);
+    RunUndirectedDFS(exit);
+  }
+
+  // Retrieves a previously computed class number.
+  size_t ClassOf(Node* node) {
+    DCHECK(GetClass(node) != kInvalidClass);
+    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 {
+    size_t class_number;  // Equivalence class number assigned to node.
+    size_t dfs_number;    // Pre-order DFS number assigned to node.
+    bool on_stack;        // Indicates node is on DFS stack during walk.
+    bool participates;    // Indicates node participates in DFS walk.
+    BracketList blist;    // List of brackets per node.
+  };
+
+  // The per-node data computed during the DFS walk.
+  typedef ZoneVector<NodeData> Data;
+
+  // Called at pre-visit during DFS walk.
+  void VisitPre(Node* node) {
+    Trace("CEQ: Pre-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
+
+    // Dispense a new pre-order number.
+    SetNumber(node, NewDFSNumber());
+    Trace("  Assigned DFS number is %d\n", GetNumber(node));
+  }
+
+  // Called at mid-visit during DFS walk.
+  void VisitMid(Node* node, DFSDirection direction) {
+    Trace("CEQ: Mid-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
+    BracketList& blist = GetBracketList(node);
+
+    // Remove brackets pointing to this node [line:19].
+    BracketListDelete(blist, node, direction);
+
+    // Potentially introduce artificial dependency from start to end.
+    if (blist.empty()) {
+      DCHECK_EQ(graph_->start(), node);
+      DCHECK_EQ(kInputDirection, direction);
+      VisitBackedge(graph_->start(), graph_->end(), kInputDirection);
+    }
+
+    // Potentially start a new equivalence class [line:37].
+    BracketListTrace(blist);
+    Bracket* recent = &blist.back();
+    if (recent->recent_size != blist.size()) {
+      recent->recent_size = blist.size();
+      recent->recent_class = NewClassNumber();
+    }
+
+    // Assign equivalence class to node.
+    SetClass(node, recent->recent_class);
+    Trace("  Assigned class number is %d\n", GetClass(node));
+  }
+
+  // Called at post-visit during DFS walk.
+  void VisitPost(Node* node, Node* parent_node, DFSDirection direction) {
+    Trace("CEQ: Post-visit of #%d:%s\n", node->id(), node->op()->mnemonic());
+    BracketList& blist = GetBracketList(node);
+
+    // Remove brackets pointing to this node [line:19].
+    BracketListDelete(blist, node, direction);
+
+    // Propagate bracket list up the DFS tree [line:13].
+    if (parent_node != NULL) {
+      BracketList& parent_blist = GetBracketList(parent_node);
+      parent_blist.splice(parent_blist.end(), blist);
+    }
+  }
+
+  // Called when hitting a back edge in the DFS walk.
+  void VisitBackedge(Node* from, Node* to, DFSDirection direction) {
+    Trace("CEQ: Backedge from #%d:%s to #%d:%s\n", from->id(),
+          from->op()->mnemonic(), to->id(), to->op()->mnemonic());
+
+    // Push backedge onto the bracket list [line:25].
+    Bracket bracket = {direction, kInvalidClass, 0, from, to};
+    GetBracketList(from).push_back(bracket);
+  }
+
+  // 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) {
+    ZoneStack<DFSStackEntry> stack(zone_);
+    DFSPush(stack, exit, NULL, kInputDirection);
+    VisitPre(exit);
+
+    while (!stack.empty()) {  // Undirected depth-first backwards traversal.
+      DFSStackEntry& entry = stack.top();
+      Node* node = entry.node;
+
+      if (entry.direction == kInputDirection) {
+        if (entry.input != node->input_edges().end()) {
+          Edge edge = *entry.input;
+          Node* input = edge.to();
+          ++(entry.input);
+          if (NodeProperties::IsControlEdge(edge) &&
+              NodeProperties::IsControl(input)) {
+            // Visit next control input.
+            if (!GetData(input)->participates) continue;
+            if (GetData(input)->on_stack) {
+              // Found backedge if input is on stack.
+              if (input != entry.parent_node) {
+                VisitBackedge(node, input, kInputDirection);
+              }
+            } else {
+              // Push input onto stack.
+              DFSPush(stack, input, node, kInputDirection);
+              VisitPre(input);
+            }
+          }
+          continue;
+        }
+        if (entry.use != node->use_edges().end()) {
+          // Switch direction to uses.
+          entry.direction = kUseDirection;
+          VisitMid(node, kInputDirection);
+          continue;
+        }
+      }
+
+      if (entry.direction == kUseDirection) {
+        if (entry.use != node->use_edges().end()) {
+          Edge edge = *entry.use;
+          Node* use = edge.from();
+          ++(entry.use);
+          if (NodeProperties::IsControlEdge(edge) &&
+              NodeProperties::IsControl(use)) {
+            // Visit next control use.
+            if (!GetData(use)->participates) continue;
+            if (GetData(use)->on_stack) {
+              // Found backedge if use is on stack.
+              if (use != entry.parent_node) {
+                VisitBackedge(node, use, kUseDirection);
+              }
+            } else {
+              // Push use onto stack.
+              DFSPush(stack, use, node, kUseDirection);
+              VisitPre(use);
+            }
+          }
+          continue;
+        }
+        if (entry.input != node->input_edges().end()) {
+          // Switch direction to inputs.
+          entry.direction = kInputDirection;
+          VisitMid(node, kUseDirection);
+          continue;
+        }
+      }
+
+      // Pop node from stack when done with all inputs and uses.
+      DCHECK(entry.input == node->input_edges().end());
+      DCHECK(entry.use == node->use_edges().end());
+      DFSPop(stack, node);
+      VisitPost(node, entry.parent_node, entry.direction);
+    }
+  }
+
+  void DetermineParticipationEnqueue(ZoneQueue<Node*>& queue, Node* node) {
+    if (!GetData(node)->participates) {
+      GetData(node)->participates = true;
+      queue.push(node);
+    }
+  }
+
+  void DetermineParticipation(Node* exit) {
+    ZoneQueue<Node*> queue(zone_);
+    DetermineParticipationEnqueue(queue, exit);
+    while (!queue.empty()) {  // Breadth-first backwards traversal.
+      Node* node = queue.front();
+      queue.pop();
+      int max = NodeProperties::PastControlIndex(node);
+      for (int i = NodeProperties::FirstControlIndex(node); i < max; i++) {
+        DetermineParticipationEnqueue(queue, node->InputAt(i));
+      }
+    }
+  }
+
+ private:
+  NodeData* GetData(Node* node) { return &node_data_[node->id()]; }
+  int NewClassNumber() { return class_number_++; }
+  int NewDFSNumber() { return dfs_number_++; }
+
+  // Template used to initialize per-node data.
+  NodeData EmptyData() {
+    return {kInvalidClass, 0, false, false, BracketList(zone_)};
+  }
+
+  // Accessors for the DFS number stored within the per-node data.
+  size_t GetNumber(Node* node) { return GetData(node)->dfs_number; }
+  void SetNumber(Node* node, size_t number) {
+    GetData(node)->dfs_number = number;
+  }
+
+  // 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) {
+    GetData(node)->class_number = number;
+  }
+
+  // Accessors for the bracket list stored within the per-node data.
+  BracketList& GetBracketList(Node* node) { return GetData(node)->blist; }
+  void SetBracketList(Node* node, BracketList& list) {
+    GetData(node)->blist = list;
+  }
+
+  // Mutates the DFS stack by pushing an entry.
+  void DFSPush(DFSStack& stack, Node* node, Node* from, DFSDirection dir) {
+    DCHECK(GetData(node)->participates);
+    GetData(node)->on_stack = true;
+    Node::InputEdges::iterator input = node->input_edges().begin();
+    Node::UseEdges::iterator use = node->use_edges().begin();
+    stack.push({dir, input, use, from, node});
+  }
+
+  // Mutates the DFS stack by popping an entry.
+  void DFSPop(DFSStack& stack, Node* node) {
+    DCHECK_EQ(stack.top().node, node);
+    GetData(node)->on_stack = false;
+    GetData(node)->participates = false;
+    stack.pop();
+  }
+
+  // TODO(mstarzinger): Optimize this to avoid linear search.
+  void BracketListDelete(BracketList& blist, Node* to, DFSDirection direction) {
+    for (BracketList::iterator i = blist.begin(); i != blist.end(); /*nop*/) {
+      if (i->to == to && i->direction != direction) {
+        Trace("  BList erased: {%d->%d}\n", i->from->id(), i->to->id());
+        i = blist.erase(i);
+      } else {
+        ++i;
+      }
+    }
+  }
+
+  void BracketListTrace(BracketList& blist) {
+    if (FLAG_trace_turbo_scheduler) {
+      Trace("  BList: ");
+      for (Bracket bracket : blist) {
+        Trace("{%d->%d} ", bracket.from->id(), bracket.to->id());
+      }
+      Trace("\n");
+    }
+  }
+
+  void Trace(const char* msg, ...) {
+    if (FLAG_trace_turbo_scheduler) {
+      va_list arguments;
+      va_start(arguments, msg);
+      base::OS::VPrint(msg, arguments);
+      va_end(arguments);
+    }
+  }
+
+  Zone* zone_;
+  Graph* 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_