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 // The isScheduleHigh flag allows nodes with wraparound dependencies that 23 // cannot easily be modeled as edges with latencies to be scheduled as 24 // soon as possible in a top-down schedule. 25 if (LHS->isScheduleHigh && !RHS->isScheduleHigh) 26 return false; 27 if (!LHS->isScheduleHigh && RHS->isScheduleHigh) 28 return true; 29 30 unsigned LHSNum = LHS->NodeNum; 31 unsigned RHSNum = RHS->NodeNum; 32 33 // The most important heuristic is scheduling the critical path. 34 unsigned LHSLatency = PQ->getLatency(LHSNum); 35 unsigned RHSLatency = PQ->getLatency(RHSNum); 36 if (LHSLatency < RHSLatency) return true; 37 if (LHSLatency > RHSLatency) return false; 38 39 // After that, if two nodes have identical latencies, look to see if one will 40 // unblock more other nodes than the other. 41 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum); 42 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum); 43 if (LHSBlocked < RHSBlocked) return true; 44 if (LHSBlocked > RHSBlocked) return false; 45 46 // Finally, just to provide a stable ordering, use the node number as a 47 // deciding factor. 48 return LHSNum < RHSNum; 49 } 50 51 52 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor 53 /// of SU, return it, otherwise return null. 54 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) { 55 SUnit *OnlyAvailablePred = 0; 56 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end(); 57 I != E; ++I) { 58 SUnit &Pred = *I->getSUnit(); 59 if (!Pred.isScheduled) { 60 // We found an available, but not scheduled, predecessor. If it's the 61 // only one we have found, keep track of it... otherwise give up. 62 if (OnlyAvailablePred && OnlyAvailablePred != &Pred) 63 return 0; 64 OnlyAvailablePred = &Pred; 65 } 66 } 67 68 return OnlyAvailablePred; 69 } 70 71 void LatencyPriorityQueue::push_impl(SUnit *SU) { 72 // Look at all of the successors of this node. Count the number of nodes that 73 // this node is the sole unscheduled node for. 74 unsigned NumNodesBlocking = 0; 75 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 76 I != E; ++I) 77 if (getSingleUnscheduledPred(I->getSUnit()) == SU) 78 ++NumNodesBlocking; 79 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking; 80 81 Queue.push(SU); 82 } 83 84 85 // ScheduledNode - As nodes are scheduled, we look to see if there are any 86 // successor nodes that have a single unscheduled predecessor. If so, that 87 // single predecessor has a higher priority, since scheduling it will make 88 // the node available. 89 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) { 90 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end(); 91 I != E; ++I) 92 AdjustPriorityOfUnscheduledPreds(I->getSUnit()); 93 } 94 95 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just 96 /// scheduled. If SU is not itself available, then there is at least one 97 /// predecessor node that has not been scheduled yet. If SU has exactly ONE 98 /// unscheduled predecessor, we want to increase its priority: it getting 99 /// scheduled will make this node available, so it is better than some other 100 /// node of the same priority that will not make a node available. 101 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) { 102 if (SU->isAvailable) return; // All preds scheduled. 103 104 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU); 105 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return; 106 107 // Okay, we found a single predecessor that is available, but not scheduled. 108 // Since it is available, it must be in the priority queue. First remove it. 109 remove(OnlyAvailablePred); 110 111 // Reinsert the node into the priority queue, which recomputes its 112 // NumNodesSolelyBlocking value. 113 push(OnlyAvailablePred); 114 } 115