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 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor 45 /// of SU, return it, otherwise return null. 46 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) { 47 SUnit *OnlyAvailablePred = 0; 48 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end(); 49 I != E; ++I) { 50 SUnit &Pred = *I->getSUnit(); 51 if (!Pred.isScheduled) { 52 // We found an available, but not scheduled, predecessor. If it's the 53 // only one we have found, keep track of it... otherwise give up. 54 if (OnlyAvailablePred && OnlyAvailablePred != &Pred) 55 return 0; 56 OnlyAvailablePred = &Pred; 57 } 58 } 59 60 return OnlyAvailablePred; 61 } 62 63 void LatencyPriorityQueue::push_impl(SUnit *SU) { 64 // Look at all of the successors of this node. Count the number of nodes that 65 // this node is the sole unscheduled node for. 66 unsigned NumNodesBlocking = 0; 67 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 68 I != E; ++I) 69 if (getSingleUnscheduledPred(I->getSUnit()) == SU) 70 ++NumNodesBlocking; 71 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking; 72 73 Queue.push(SU); 74 } 75 76 77 // ScheduledNode - As nodes are scheduled, we look to see if there are any 78 // successor nodes that have a single unscheduled predecessor. If so, that 79 // single predecessor has a higher priority, since scheduling it will make 80 // the node available. 81 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) { 82 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 83 I != E; ++I) 84 AdjustPriorityOfUnscheduledPreds(I->getSUnit()); 85 } 86 87 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just 88 /// scheduled. If SU is not itself available, then there is at least one 89 /// predecessor node that has not been scheduled yet. If SU has exactly ONE 90 /// unscheduled predecessor, we want to increase its priority: it getting 91 /// scheduled will make this node available, so it is better than some other 92 /// node of the same priority that will not make a node available. 93 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) { 94 if (SU->isAvailable) return; // All preds scheduled. 95 96 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU); 97 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return; 98 99 // Okay, we found a single predecessor that is available, but not scheduled. 100 // Since it is available, it must be in the priority queue. First remove it. 101 remove(OnlyAvailablePred); 102 103 // Reinsert the node into the priority queue, which recomputes its 104 // NumNodesSolelyBlocking value. 105 push(OnlyAvailablePred); 106 } 107