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