1 //===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file implements the LatencyPriorityQueue class, which is a 10 // SchedulingPriorityQueue that schedules using latency information to 11 // reduce the length of the critical path through the basic block. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "llvm/CodeGen/LatencyPriorityQueue.h" 16 #include "llvm/Config/llvm-config.h" 17 #include "llvm/Support/Debug.h" 18 #include "llvm/Support/raw_ostream.h" 19 using namespace llvm; 20 21 #define DEBUG_TYPE "scheduler" 22 23 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const { 24 // The isScheduleHigh flag allows nodes with wraparound dependencies that 25 // cannot easily be modeled as edges with latencies to be scheduled as 26 // soon as possible in a top-down schedule. 27 if (LHS->isScheduleHigh && !RHS->isScheduleHigh) 28 return false; 29 if (!LHS->isScheduleHigh && RHS->isScheduleHigh) 30 return true; 31 32 unsigned LHSNum = LHS->NodeNum; 33 unsigned RHSNum = RHS->NodeNum; 34 35 // The most important heuristic is scheduling the critical path. 36 unsigned LHSLatency = PQ->getLatency(LHSNum); 37 unsigned RHSLatency = PQ->getLatency(RHSNum); 38 if (LHSLatency < RHSLatency) return true; 39 if (LHSLatency > RHSLatency) return false; 40 41 // After that, if two nodes have identical latencies, look to see if one will 42 // unblock more other nodes than the other. 43 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum); 44 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum); 45 if (LHSBlocked < RHSBlocked) return true; 46 if (LHSBlocked > RHSBlocked) return false; 47 48 // Finally, just to provide a stable ordering, use the node number as a 49 // deciding factor. 50 return RHSNum < LHSNum; 51 } 52 53 54 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor 55 /// of SU, return it, otherwise return null. 56 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) { 57 SUnit *OnlyAvailablePred = nullptr; 58 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end(); 59 I != E; ++I) { 60 SUnit &Pred = *I->getSUnit(); 61 if (!Pred.isScheduled) { 62 // We found an available, but not scheduled, predecessor. If it's the 63 // only one we have found, keep track of it... otherwise give up. 64 if (OnlyAvailablePred && OnlyAvailablePred != &Pred) 65 return nullptr; 66 OnlyAvailablePred = &Pred; 67 } 68 } 69 70 return OnlyAvailablePred; 71 } 72 73 void LatencyPriorityQueue::push(SUnit *SU) { 74 // Look at all of the successors of this node. Count the number of nodes that 75 // this node is the sole unscheduled node for. 76 unsigned NumNodesBlocking = 0; 77 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 78 I != E; ++I) { 79 if (getSingleUnscheduledPred(I->getSUnit()) == SU) 80 ++NumNodesBlocking; 81 } 82 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking; 83 84 Queue.push_back(SU); 85 } 86 87 88 // scheduledNode - As nodes are scheduled, we look to see if there are any 89 // successor nodes that have a single unscheduled predecessor. If so, that 90 // single predecessor has a higher priority, since scheduling it will make 91 // the node available. 92 void LatencyPriorityQueue::scheduledNode(SUnit *SU) { 93 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 94 I != E; ++I) { 95 AdjustPriorityOfUnscheduledPreds(I->getSUnit()); 96 } 97 } 98 99 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just 100 /// scheduled. If SU is not itself available, then there is at least one 101 /// predecessor node that has not been scheduled yet. If SU has exactly ONE 102 /// unscheduled predecessor, we want to increase its priority: it getting 103 /// scheduled will make this node available, so it is better than some other 104 /// node of the same priority that will not make a node available. 105 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) { 106 if (SU->isAvailable) return; // All preds scheduled. 107 108 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU); 109 if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return; 110 111 // Okay, we found a single predecessor that is available, but not scheduled. 112 // Since it is available, it must be in the priority queue. First remove it. 113 remove(OnlyAvailablePred); 114 115 // Reinsert the node into the priority queue, which recomputes its 116 // NumNodesSolelyBlocking value. 117 push(OnlyAvailablePred); 118 } 119 120 SUnit *LatencyPriorityQueue::pop() { 121 if (empty()) return nullptr; 122 std::vector<SUnit *>::iterator Best = Queue.begin(); 123 for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()), 124 E = Queue.end(); I != E; ++I) 125 if (Picker(*Best, *I)) 126 Best = I; 127 SUnit *V = *Best; 128 if (Best != std::prev(Queue.end())) 129 std::swap(*Best, Queue.back()); 130 Queue.pop_back(); 131 return V; 132 } 133 134 void LatencyPriorityQueue::remove(SUnit *SU) { 135 assert(!Queue.empty() && "Queue is empty!"); 136 std::vector<SUnit *>::iterator I = find(Queue, SU); 137 assert(I != Queue.end() && "Queue doesn't contain the SU being removed!"); 138 if (I != std::prev(Queue.end())) 139 std::swap(*I, Queue.back()); 140 Queue.pop_back(); 141 } 142 143 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP) 144 LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const { 145 dbgs() << "Latency Priority Queue\n"; 146 dbgs() << " Number of Queue Entries: " << Queue.size() << "\n"; 147 for (const SUnit *SU : Queue) { 148 dbgs() << " "; 149 DAG->dumpNode(*SU); 150 } 151 } 152 #endif 153