Implement control reducer, which reduces branches and phis together in a single fixpoint.

src/compiler/control-reducer.cc

 // 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.
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/control-reducer.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-matchers.h"
 #include "src/compiler/node-properties-inl.h"
 #include "src/zone-containers.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
-enum VisitState { kUnvisited, kOnStack, kRevisit, kVisited };
+enum VisitState { kUnvisited = 0, kOnStack = 1, kRevisit = 2, kVisited = 3 };
+enum Reachability { kFromStart = 8 };
 
 #define TRACE(x) \
   if (FLAG_trace_turbo) PrintF x
 
 class ControlReducerImpl {
  public:
   ControlReducerImpl(Zone* zone, JSGraph* jsgraph,
                      CommonOperatorBuilder* common)
       : zone_(zone),
         jsgraph_(jsgraph),
         common_(common),
         state_(jsgraph->graph()->NodeCount(), kUnvisited, zone_),
         stack_(zone_),
         revisit_(zone_),
         dead_(NULL) {}
 
   Zone* zone_;
   JSGraph* jsgraph_;
   CommonOperatorBuilder* common_;
   ZoneVector<VisitState> state_;
   ZoneDeque<Node*> stack_;
   ZoneDeque<Node*> revisit_;
   Node* dead_;
 
-  void Trim() {
-    //  Mark all nodes reachable from end.
+  void Reduce() {
+    Push(graph()->end());
+    do {
+      // Process the node on the top of the stack, potentially pushing more
+      // or popping the node off the stack.
+      ReduceTop();
+      // If the stack becomes empty, revisit any nodes in the revisit queue.
+      // If no nodes in the revisit queue, try removing dead loops.
+      // If no dead loops, then finish.
+    } while (!stack_.empty() || TryRevisit() || RepairAndRemoveLoops());
+  }
+
+  bool TryRevisit() {
+    while (!revisit_.empty()) {
+      Node* n = revisit_.back();
+      revisit_.pop_back();
+      if (state_[n->id()] == kRevisit) {  // state can change while in queue.
+        Push(n);
+        return true;
+      }
+    }
+    return false;
+  }
+
+  // Repair the graph after the possible creation of non-terminating or dead
+  // loops. Removing dead loops can produce more opportunities for reduction.
+  bool RepairAndRemoveLoops() {
+    // TODO(turbofan): we can skip this if the graph has no loops, but
+    // we have to be careful about proper loop detection during reduction.
+
+    // Gather all nodes backwards-reachable from end (through inputs).
+    state_.assign(graph()->NodeCount(), kUnvisited);
     NodeVector nodes(zone_);
-    state_.assign(jsgraph_->graph()->NodeCount(), kUnvisited);
-    Push(jsgraph_->graph()->end());
+    AddNodesReachableFromEnd(nodes);
+
+    // Walk forward through control nodes, looking for back edges to nodes
+    // that are not connected to end. Those are non-terminating loops (NTLs).
+    Node* start = graph()->start();
+    ZoneVector<byte> fw_reachability(graph()->NodeCount(), 0, zone_);
+    fw_reachability[start->id()] = kFromStart | kOnStack;
+    stack_.push_back(start);
+
     while (!stack_.empty()) {
-      Node* node = stack_[stack_.size() - 1];
-      stack_.pop_back();
-      state_[node->id()] = kVisited;
-      nodes.push_back(node);
+      Node* node = stack_.back();
+      TRACE(("ControlFw: #%d:%s\n", node->id(), node->op()->mnemonic()));
+      bool pop = true;
+      for (UseIter i = node->uses().begin(); i != node->uses().end(); ++i) {
+        Node* succ = *i;
+        byte reach = fw_reachability[succ->id()];
+        if ((reach & kOnStack) != 0 && state_[succ->id()] != kVisited) {
+          // {succ} is on stack and not reachable from end.
+          ConnectNTL(nodes, succ);
+          fw_reachability.resize(graph()->NodeCount(), 0);
+          pop = false;  // continue traversing inputs to this node.
+          break;
+        }
+        if ((reach & kFromStart) == 0 &&
+            IrOpcode::IsControlOpcode(succ->opcode())) {
+          // {succ} is a control node and not yet reached from start.
+          fw_reachability[succ->id()] |= kFromStart | kOnStack;
+          stack_.push_back(succ);
+          pop = false;  // "recurse" into successor control node.
+          break;
+        }
+      }
+      if (pop) {
+        fw_reachability[node->id()] &= ~kOnStack;
+        stack_.pop_back();
+      }
+    }
+
+    // Trim references from dead nodes to live nodes first.
+    jsgraph_->GetCachedNodes(&nodes);
+    TrimNodes(nodes);
+
+    // Any control nodes not reachable from start are dead, even loops.
+    for (size_t i = 0; i < nodes.size(); i++) {
+      Node* node = nodes[i];
+      byte reach = fw_reachability[node->id()];
+      if ((reach & kFromStart) == 0 &&
+          IrOpcode::IsControlOpcode(node->opcode())) {
+        ReplaceNode(node, dead());  // uses will be added to revisit queue.
+      }
+    }
+    return TryRevisit();  // try to push a node onto the stack.
+  }
+
+  // Connect {loop}, the header of a non-terminating loop, to the end node.
+  void ConnectNTL(NodeVector& nodes, Node* loop) {
+    TRACE(("ConnectNTL: #%d:%s\n", loop->id(), loop->op()->mnemonic()));
+    CHECK_EQ(IrOpcode::kLoop, loop->opcode());
+    Node* to_add = loop;
+    Node* end = graph()->end();
+    CHECK_EQ(IrOpcode::kEnd, end->opcode());
+    Node* merge = end->InputAt(0);
+    if (merge->opcode() != IrOpcode::kMerge) {
+      if (merge->opcode() == IrOpcode::kDead) {
+        // end died; just connect end to the loop.
+        end->ReplaceInput(0, loop);
+        to_add = loop;
+      } else {
+        // Introduce a new merge for the end.
+        merge = graph()->NewNode(common_->Merge(2), merge, loop);
+        state_.resize(graph()->NodeCount(), kUnvisited);
+        end->ReplaceInput(0, merge);
+        to_add = merge;
+      }
+    } else {
+      // Append a new input to the final merge at the end.
+      merge->AppendInput(graph()->zone(), loop);
+      merge->set_op(common_->Merge(merge->InputCount()));
+    }
+    nodes.push_back(to_add);
+    state_[to_add->id()] = kVisited;
+    AddBackwardsReachableNodes(nodes, nodes.size() - 1);
+  }
+
+  void AddNodesReachableFromEnd(NodeVector& nodes) {
+    Node* end = graph()->end();
+    if (!end->IsDead()) {
+      state_[end->id()] = kVisited;
+      nodes.push_back(end);
+      AddBackwardsReachableNodes(nodes, nodes.size() - 1);
+    }
+  }
+
+  void AddBackwardsReachableNodes(NodeVector& nodes, size_t cursor) {
+    while (cursor < nodes.size()) {
+      Node* node = nodes[cursor++];
       for (InputIter i = node->inputs().begin(); i != node->inputs().end();
            ++i) {
-        Recurse(*i);  // pushes node onto the stack if necessary.
+        Node* input = *i;
+        if (state_[input->id()] != kVisited) {
+          state_[input->id()] = kVisited;
+          nodes.push_back(input);
+        }
       }
     }
+  }
+
+  void Trim() {
+    // Gather all nodes backwards-reachable from end through inputs.
+    state_.assign(graph()->NodeCount(), kUnvisited);
+    NodeVector nodes(zone_);
+    AddNodesReachableFromEnd(nodes);
+
     // Process cached nodes in the JSGraph too.
     jsgraph_->GetCachedNodes(&nodes);
+    TrimNodes(nodes);
+  }
+
+  void TrimNodes(NodeVector& nodes) {
     // Remove dead->live edges.
     for (size_t j = 0; j < nodes.size(); j++) {
       Node* node = nodes[j];
       for (UseIter i = node->uses().begin(); i != node->uses().end();) {
         size_t id = static_cast<size_t>((*i)->id());
         if (state_[id] != kVisited) {
           TRACE(("DeadLink: #%d:%s(%d) -> #%d:%s\n", (*i)->id(),
                  (*i)->op()->mnemonic(), i.index(), node->id(),
                  node->op()->mnemonic()));
           i.UpdateToAndIncrement(NULL);
@@ skipping 11 matching lines @@
         CHECK_NE(NULL, *i);
       }
       for (UseIter i = node->uses().begin(); i != node->uses().end(); ++i) {
         size_t id = static_cast<size_t>((*i)->id());
         CHECK_EQ(kVisited, state_[id]);
       }
     }
 #endif
   }
 
+  // Reduce the node on the top of the stack.
+  // If an input {i} is not yet visited or needs to be revisited, push {i} onto
+  // the stack and return. Otherwise, all inputs are visited, so apply
+  // reductions for {node} and pop it off the stack.
+  void ReduceTop() {
+    int height = stack_.size();
+    Node* node = stack_.back();
+
+    if (node->IsDead()) return Pop();  // Node was killed while on stack.
+
+    TRACE(("ControlReduce: #%d:%s\n", node->id(), node->op()->mnemonic()));
+
+    // Recurse on an input if necessary.
+    for (InputIter i = node->inputs().begin(); i != node->inputs().end(); ++i) {

[Benedikt Meurer, 2014/10/23 10:45:25]

for (Node* const input : node->inputs()) if (Recurse(input)) return;

[titzer, 2014/10/23 11:23:44]

Done.

+      if (Recurse(*i)) return;
+    }
+
+    // All inputs should be visited or on stack. Apply reductions to node.
+    Node* replacement = ReduceNode(node);
+    if (replacement != node) ReplaceNode(node, replacement);
+
+    // After reducing the node, pop it off the stack.
+    CHECK_EQ(height, stack_.size());

[Benedikt Meurer, 2014/10/23 10:45:25]

Windows builder will probably complain about this comparison. How about some try
jobs?

[titzer, 2014/10/23 11:23:44]

Fixed with static_cast

+    Pop();
+
+    // If there was a replacement, reduce it after popping {node}.
+    if (replacement != node) Recurse(replacement);
+  }
+
   // Push a node onto the stack if its state is {kUnvisited} or {kRevisit}.
   bool Recurse(Node* node) {
     size_t id = static_cast<size_t>(node->id());
     if (id < state_.size()) {
       if (state_[id] != kRevisit && state_[id] != kUnvisited) return false;
     } else {
       state_.resize((3 * id) / 2, kUnvisited);
     }
     Push(node);
     return true;
   }
 
   void Push(Node* node) {
     state_[node->id()] = kOnStack;
     stack_.push_back(node);
   }
+
+  void Pop() {
+    int pos = stack_.size() - 1;
+    DCHECK_GE(pos, 0);
+    DCHECK_EQ(kOnStack, state_[stack_[pos]->id()]);
+    state_[stack_[pos]->id()] = kVisited;
+    stack_.pop_back();
+  }
+
+  // Queue a node to be revisited if it has been visited once already.
+  void Revisit(Node* node) {
+    size_t id = static_cast<size_t>(node->id());
+    if (id < state_.size() && state_[id] == kVisited) {
+      TRACE(("  Revisit #%d:%s\n", node->id(), node->op()->mnemonic()));
+      state_[id] = kRevisit;
+      revisit_.push_back(node);
+    }
+  }
+
+  Node* dead() {
+    if (dead_ == NULL) dead_ = graph()->NewNode(common_->Dead());
+    return dead_;
+  }
+
+  //===========================================================================
+  // Reducer implementation: perform reductions on a node.
+  //===========================================================================
+  Node* ReduceNode(Node* node) {
+    if (OperatorProperties::GetControlInputCount(node->op()) == 1) {
+      // If a node has only one control input and it is dead, replace with dead.
+      Node* control = NodeProperties::GetControlInput(node);
+      if (control->opcode() == IrOpcode::kDead) {
+        TRACE(("ControlDead: #%d:%s\n", node->id(), node->op()->mnemonic()));
+        return control;
+      }
+    }
+
+    // Reduce branches, phis, and merges.
+    switch (node->opcode()) {
+      case IrOpcode::kBranch:
+        return ReduceBranch(node);
+      case IrOpcode::kLoop:
+      case IrOpcode::kMerge:
+        return ReduceMerge(node);
+      case IrOpcode::kPhi:
+      case IrOpcode::kEffectPhi:
+        return ReducePhi(node);
+      default:
+        return node;
+    }
+  }
+
+  // Reduce redundant phis.
+  Node* ReducePhi(Node* node) {
+    int n = node->InputCount();
+    DCHECK_EQ(1, OperatorProperties::GetControlInputCount(node->op()));
+    if (node->opcode() == IrOpcode::kPhi) {
+      DCHECK_EQ(n, 1 + OperatorProperties::GetValueInputCount(node->op()));
+    }
+    if (node->opcode() == IrOpcode::kEffectPhi) {
+      DCHECK_EQ(n, 1 + OperatorProperties::GetEffectInputCount(node->op()));
+    }
+    if (n <= 1) return dead();            // No non-control inputs.
+    if (n == 2) return node->InputAt(0);  // Only one non-control input.
+
+    Node* replacement = NULL;
+    Node::Inputs inputs = node->inputs();
+    for (InputIter it = inputs.begin(); n > 1; --n, ++it) {
+      Node* input = *it;
+      if (input->opcode() == IrOpcode::kDead) continue;  // ignore dead inputs.
+      if (input != node && input != replacement) {       // non-redundant input.
+        if (replacement != NULL) return node;
+        replacement = input;
+      }
+    }
+    return replacement == NULL ? dead() : replacement;
+  }
+
+  // Reduce merges by trimming away dead inputs from the merge and phis.
+  Node* ReduceMerge(Node* node) {
+    // Count the number of live inputs.
+    int live = 0;
+    int live_index = 0;
+    for (InputIter i = node->inputs().begin(); i != node->inputs().end(); ++i) {
+      if ((*i)->opcode() != IrOpcode::kDead) {
+        live++;
+        live_index = i.index();
+      }
+    }
+
+    if (live > 1 && live == node->InputCount()) return node;  // nothing to do.
+
+    TRACE(("ReduceMerge: #%d:%s (%d live)\n", node->id(),
+           node->op()->mnemonic(), live));
+
+    if (live == 0) return dead();  // no remaining inputs.
+
+    // Gather phis and effect phis to be edited.
+    ZoneDeque<Node*> phis(zone_);

[Benedikt Meurer, 2014/10/23 10:45:25]

Hm, I'd like to have a ZoneStack here, i.e. a class for std::stack<ZoneDeque>

[titzer, 2014/10/23 11:23:44]

Order doesn't really matter here, so I could as well use NodeVector.

+    for (UseIter i = node->uses().begin(); i != node->uses().end(); ++i) {

[Benedikt Meurer, 2014/10/23 10:45:25]

for (Node* const to : node->uses()) { ... }

[titzer, 2014/10/23 11:23:44]

Done.

+      Node* to = *i;
+      if (to->opcode() == IrOpcode::kPhi ||
+          to->opcode() == IrOpcode::kEffectPhi) {
+        phis.push_back(to);
+      }
+    }
+
+    // Remove dead inputs from merge and corresponding phis.
+    while (!phis.empty()) {
+      Node* phi = phis.back();
+      phis.pop_back();
+      TRACE(("  PhiInMerge: #%d:%s (%d live)\n", phi->id(),
+             phi->op()->mnemonic(), live));
+      if (live == 1) {
+        Node* result = phi->InputAt(live_index);
+        ReplaceNode(phi, result);  // phi is redundant.
+      } else {
+        RemoveDeadInputs(node, phi);
+        Revisit(phi);  // re-reduce every phi.
+      }
+    }
+
+    // If only one input is remaining, remove the merge altogether.
+    if (live == 1) return node->InputAt(live_index);
+    RemoveDeadInputs(node, node);
+    return node;
+  }
+
+  // Reduce branches with constant inputs.
+  Node* ReduceBranch(Node* node) {
+    Node* cond = node->InputAt(0);
+    bool is_true;
+    switch (cond->opcode()) {
+      case IrOpcode::kInt32Constant:
+        is_true = !Int32Matcher(cond).Is(0);
+        break;
+      case IrOpcode::kNumberConstant:
+        is_true = !NumberMatcher(cond).Is(0);
+        break;
+      case IrOpcode::kHeapConstant: {
+        Handle<Object> object =
+            HeapObjectMatcher<Object>(cond).Value().handle();
+        if (object->IsTrue())
+          is_true = true;
+        else if (object->IsFalse())
+          is_true = false;
+        else
+          return node;  // TODO(turbofan): fold branches on strings, objects.
+        break;
+      }
+      default:
+        return node;
+    }
+
+    TRACE(("BranchReduce: #%d:%s = %s\n", node->id(), node->op()->mnemonic(),
+           is_true ? "true" : "false"));
+
+    // Replace IfTrue and IfFalse projections from this branch.
+    Node* control = NodeProperties::GetControlInput(node);
+    for (UseIter i = node->uses().begin(); i != node->uses().end();) {
+      Node* to = *i;
+      if (to->opcode() == IrOpcode::kIfTrue) {
+        TRACE(("  IfTrue: #%d:%s\n", to->id(), to->op()->mnemonic()));
+        i.UpdateToAndIncrement(NULL);
+        ReplaceNode(to, is_true ? control : dead());
+      } else if (to->opcode() == IrOpcode::kIfFalse) {
+        TRACE(("  IfFalse: #%d:%s\n", to->id(), to->op()->mnemonic()));
+        i.UpdateToAndIncrement(NULL);
+        ReplaceNode(to, is_true ? dead() : control);
+      } else {
+        ++i;
+      }
+    }
+    return control;
+  }
+
+  // Remove inputs to {node} corresponding to the dead inputs to {merge}
+  // and compact the remaining inputs, updating the operator.
+  void RemoveDeadInputs(Node* merge, Node* node) {
+    int pos = 0;
+    for (int i = 0; i < node->InputCount(); i++) {
+      // skip dead inputs.
+      if (i < merge->InputCount() &&
+          merge->InputAt(i)->opcode() == IrOpcode::kDead)
+        continue;
+      // compact live inputs.
+      if (pos != i) node->ReplaceInput(pos, node->InputAt(i));
+      pos++;
+    }
+    node->TrimInputCount(pos);
+    if (node->opcode() == IrOpcode::kPhi) {
+      node->set_op(common_->Phi(OpParameter<MachineType>(node->op()), pos - 1));
+    } else if (node->opcode() == IrOpcode::kEffectPhi) {
+      node->set_op(common_->EffectPhi(pos - 1));
+    } else if (node->opcode() == IrOpcode::kMerge) {
+      node->set_op(common_->Merge(pos));
+    } else if (node->opcode() == IrOpcode::kLoop) {
+      node->set_op(common_->Loop(pos));
+    } else {
+      UNREACHABLE();
+    }
+  }
+
+  void ReplaceNode(Node* node, Node* replacement) {
+    if (node == replacement) return;
+    TRACE(("  Replace: #%d:%s with #%d:%s\n", node->id(),
+           node->op()->mnemonic(), replacement->id(),
+           replacement->op()->mnemonic()));
+    for (UseIter i = node->uses().begin(); i != node->uses().end(); ++i) {
+      // Don't revisit this node if it refers to itself.
+      if (*i != node) Revisit(*i);
+    }
+    node->ReplaceUses(replacement);
+    node->Kill();
+  }
+
+  Graph* graph() { return jsgraph_->graph(); }
 };
 
+
 void ControlReducer::ReduceGraph(Zone* zone, JSGraph* jsgraph,
                                  CommonOperatorBuilder* common) {
-  ControlReducerImpl impl(zone, jsgraph, NULL);
+  ControlReducerImpl impl(zone, jsgraph, common);
   // Only trim the graph for now. Control reduction can reduce non-terminating
   // loops to graphs that are unschedulable at the moment.
+  impl.Reduce();
   impl.Trim();
 }
 
 
 void ControlReducer::TrimGraph(Zone* zone, JSGraph* jsgraph) {
   ControlReducerImpl impl(zone, jsgraph, NULL);
   impl.Trim();
 }
+
+
+Node* ControlReducer::ReducePhiForTesting(JSGraph* jsgraph,
+                                          CommonOperatorBuilder* common,
+                                          Node* node) {
+  Zone zone(jsgraph->graph()->zone()->isolate());
+  ControlReducerImpl impl(&zone, jsgraph, common);
+  return impl.ReducePhi(node);
+}
+
+
+Node* ControlReducer::ReduceMergeForTesting(JSGraph* jsgraph,
+                                            CommonOperatorBuilder* common,
+                                            Node* node) {
+  Zone zone(jsgraph->graph()->zone()->isolate());
+  ControlReducerImpl impl(&zone, jsgraph, common);
+  return impl.ReduceMerge(node);
+}
+
+
+Node* ControlReducer::ReduceBranchForTesting(JSGraph* jsgraph,
+                                             CommonOperatorBuilder* common,
+                                             Node* node) {
+  Zone zone(jsgraph->graph()->zone()->isolate());
+  ControlReducerImpl impl(&zone, jsgraph, common);
+  return impl.ReduceBranch(node);
+}
 }
 }
 }  // namespace v8::internal::compiler