1 //===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements the LatencyPriorityQueue class, which is a 11 // SchedulingPriorityQueue that schedules using latency information to 12 // reduce the length of the critical path through the basic block. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "llvm/CodeGen/LatencyPriorityQueue.h" 17 #include "llvm/Config/llvm-config.h" 18 #include "llvm/Support/Debug.h" 19 #include "llvm/Support/raw_ostream.h" 20 using namespace llvm; 21 22 #define DEBUG_TYPE "scheduler" 23 24 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const { 25 // The isScheduleHigh flag allows nodes with wraparound dependencies that 26 // cannot easily be modeled as edges with latencies to be scheduled as 27 // soon as possible in a top-down schedule. 28 if (LHS->isScheduleHigh && !RHS->isScheduleHigh) 29 return false; 30 if (!LHS->isScheduleHigh && RHS->isScheduleHigh) 31 return true; 32 33 unsigned LHSNum = LHS->NodeNum; 34 unsigned RHSNum = RHS->NodeNum; 35 36 // The most important heuristic is scheduling the critical path. 37 unsigned LHSLatency = PQ->getLatency(LHSNum); 38 unsigned RHSLatency = PQ->getLatency(RHSNum); 39 if (LHSLatency < RHSLatency) return true; 40 if (LHSLatency > RHSLatency) return false; 41 42 // After that, if two nodes have identical latencies, look to see if one will 43 // unblock more other nodes than the other. 44 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum); 45 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum); 46 if (LHSBlocked < RHSBlocked) return true; 47 if (LHSBlocked > RHSBlocked) return false; 48 49 // Finally, just to provide a stable ordering, use the node number as a 50 // deciding factor. 51 return RHSNum < LHSNum; 52 } 53 54 55 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor 56 /// of SU, return it, otherwise return null. 57 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) { 58 SUnit *OnlyAvailablePred = nullptr; 59 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end(); 60 I != E; ++I) { 61 SUnit &Pred = *I->getSUnit(); 62 if (!Pred.isScheduled) { 63 // We found an available, but not scheduled, predecessor. If it's the 64 // only one we have found, keep track of it... otherwise give up. 65 if (OnlyAvailablePred && OnlyAvailablePred != &Pred) 66 return nullptr; 67 OnlyAvailablePred = &Pred; 68 } 69 } 70 71 return OnlyAvailablePred; 72 } 73 74 void LatencyPriorityQueue::push(SUnit *SU) { 75 // Look at all of the successors of this node. Count the number of nodes that 76 // this node is the sole unscheduled node for. 77 unsigned NumNodesBlocking = 0; 78 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 79 I != E; ++I) { 80 if (getSingleUnscheduledPred(I->getSUnit()) == SU) 81 ++NumNodesBlocking; 82 } 83 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking; 84 85 Queue.push_back(SU); 86 } 87 88 89 // scheduledNode - As nodes are scheduled, we look to see if there are any 90 // successor nodes that have a single unscheduled predecessor. If so, that 91 // single predecessor has a higher priority, since scheduling it will make 92 // the node available. 93 void LatencyPriorityQueue::scheduledNode(SUnit *SU) { 94 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 95 I != E; ++I) { 96 AdjustPriorityOfUnscheduledPreds(I->getSUnit()); 97 } 98 } 99 100 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just 101 /// scheduled. If SU is not itself available, then there is at least one 102 /// predecessor node that has not been scheduled yet. If SU has exactly ONE 103 /// unscheduled predecessor, we want to increase its priority: it getting 104 /// scheduled will make this node available, so it is better than some other 105 /// node of the same priority that will not make a node available. 106 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) { 107 if (SU->isAvailable) return; // All preds scheduled. 108 109 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU); 110 if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return; 111 112 // Okay, we found a single predecessor that is available, but not scheduled. 113 // Since it is available, it must be in the priority queue. First remove it. 114 remove(OnlyAvailablePred); 115 116 // Reinsert the node into the priority queue, which recomputes its 117 // NumNodesSolelyBlocking value. 118 push(OnlyAvailablePred); 119 } 120 121 SUnit *LatencyPriorityQueue::pop() { 122 if (empty()) return nullptr; 123 std::vector<SUnit *>::iterator Best = Queue.begin(); 124 for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()), 125 E = Queue.end(); I != E; ++I) 126 if (Picker(*Best, *I)) 127 Best = I; 128 SUnit *V = *Best; 129 if (Best != std::prev(Queue.end())) 130 std::swap(*Best, Queue.back()); 131 Queue.pop_back(); 132 return V; 133 } 134 135 void LatencyPriorityQueue::remove(SUnit *SU) { 136 assert(!Queue.empty() && "Queue is empty!"); 137 std::vector<SUnit *>::iterator I = find(Queue, SU); 138 assert(I != Queue.end() && "Queue doesn't contain the SU being removed!"); 139 if (I != std::prev(Queue.end())) 140 std::swap(*I, Queue.back()); 141 Queue.pop_back(); 142 } 143 144 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP) 145 LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const { 146 dbgs() << "Latency Priority Queue\n"; 147 dbgs() << " Number of Queue Entries: " << Queue.size() << "\n"; 148 for (auto const &SU : Queue) { 149 dbgs() << " "; 150 SU->dump(DAG); 151 } 152 } 153 #endif 154
