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 #define DEBUG_TYPE "scheduler" 17 #include "llvm/CodeGen/LatencyPriorityQueue.h" 18 #include "llvm/Support/Debug.h" 19 using namespace llvm; 20 21 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const { 22 unsigned LHSNum = LHS->NodeNum; 23 unsigned RHSNum = RHS->NodeNum; 24 25 // The most important heuristic is scheduling the critical path. 26 unsigned LHSLatency = PQ->getLatency(LHSNum); 27 unsigned RHSLatency = PQ->getLatency(RHSNum); 28 if (LHSLatency < RHSLatency) return true; 29 if (LHSLatency > RHSLatency) return false; 30 31 // After that, if two nodes have identical latencies, look to see if one will 32 // unblock more other nodes than the other. 33 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum); 34 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum); 35 if (LHSBlocked < RHSBlocked) return true; 36 if (LHSBlocked > RHSBlocked) return false; 37 38 // Finally, just to provide a stable ordering, use the node number as a 39 // deciding factor. 40 return LHSNum < RHSNum; 41 } 42 43 44 /// CalcNodePriority - Calculate the maximal path from the node to the exit. 45 /// 46 void LatencyPriorityQueue::CalcLatency(const SUnit &SU) { 47 int &Latency = Latencies[SU.NodeNum]; 48 if (Latency != -1) 49 return; 50 51 std::vector<const SUnit*> WorkList; 52 WorkList.push_back(&SU); 53 while (!WorkList.empty()) { 54 const SUnit *Cur = WorkList.back(); 55 bool AllDone = true; 56 unsigned MaxSuccLatency = 0; 57 for (SUnit::const_succ_iterator I = Cur->Succs.begin(),E = Cur->Succs.end(); 58 I != E; ++I) { 59 int SuccLatency = Latencies[I->getSUnit()->NodeNum]; 60 if (SuccLatency == -1) { 61 AllDone = false; 62 WorkList.push_back(I->getSUnit()); 63 } else { 64 unsigned NewLatency = SuccLatency + I->getLatency(); 65 MaxSuccLatency = std::max(MaxSuccLatency, NewLatency); 66 } 67 } 68 if (AllDone) { 69 Latencies[Cur->NodeNum] = MaxSuccLatency; 70 WorkList.pop_back(); 71 } 72 } 73 } 74 75 /// CalculatePriorities - Calculate priorities of all scheduling units. 76 void LatencyPriorityQueue::CalculatePriorities() { 77 Latencies.assign(SUnits->size(), -1); 78 NumNodesSolelyBlocking.assign(SUnits->size(), 0); 79 80 // For each node, calculate the maximal path from the node to the exit. 81 for (unsigned i = 0, e = SUnits->size(); i != e; ++i) 82 CalcLatency((*SUnits)[i]); 83 } 84 85 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor 86 /// of SU, return it, otherwise return null. 87 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) { 88 SUnit *OnlyAvailablePred = 0; 89 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end(); 90 I != E; ++I) { 91 SUnit &Pred = *I->getSUnit(); 92 if (!Pred.isScheduled) { 93 // We found an available, but not scheduled, predecessor. If it's the 94 // only one we have found, keep track of it... otherwise give up. 95 if (OnlyAvailablePred && OnlyAvailablePred != &Pred) 96 return 0; 97 OnlyAvailablePred = &Pred; 98 } 99 } 100 101 return OnlyAvailablePred; 102 } 103 104 void LatencyPriorityQueue::push_impl(SUnit *SU) { 105 // Look at all of the successors of this node. Count the number of nodes that 106 // this node is the sole unscheduled node for. 107 unsigned NumNodesBlocking = 0; 108 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 109 I != E; ++I) 110 if (getSingleUnscheduledPred(I->getSUnit()) == SU) 111 ++NumNodesBlocking; 112 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking; 113 114 Queue.push(SU); 115 } 116 117 118 // ScheduledNode - As nodes are scheduled, we look to see if there are any 119 // successor nodes that have a single unscheduled predecessor. If so, that 120 // single predecessor has a higher priority, since scheduling it will make 121 // the node available. 122 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) { 123 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 124 I != E; ++I) 125 AdjustPriorityOfUnscheduledPreds(I->getSUnit()); 126 } 127 128 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just 129 /// scheduled. If SU is not itself available, then there is at least one 130 /// predecessor node that has not been scheduled yet. If SU has exactly ONE 131 /// unscheduled predecessor, we want to increase its priority: it getting 132 /// scheduled will make this node available, so it is better than some other 133 /// node of the same priority that will not make a node available. 134 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) { 135 if (SU->isAvailable) return; // All preds scheduled. 136 137 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU); 138 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return; 139 140 // Okay, we found a single predecessor that is available, but not scheduled. 141 // Since it is available, it must be in the priority queue. First remove it. 142 remove(OnlyAvailablePred); 143 144 // Reinsert the node into the priority queue, which recomputes its 145 // NumNodesSolelyBlocking value. 146 push(OnlyAvailablePred); 147 } 148