148 lines · cpp
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-exception6//7//===----------------------------------------------------------------------===//8//9// This file implements the LatencyPriorityQueue class, which is a10// SchedulingPriorityQueue that schedules using latency information to11// 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"19using namespace llvm;20 21#define DEBUG_TYPE "scheduler"22 23bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {24 // The isScheduleHigh flag allows nodes with wraparound dependencies that25 // cannot easily be modeled as edges with latencies to be scheduled as26 // 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 will42 // 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 a49 // deciding factor.50 return RHSNum < LHSNum;51}52 53 54/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor55/// of SU, return it, otherwise return null.56SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {57 SUnit *OnlyAvailablePred = nullptr;58 for (const SDep &P : SU->Preds) {59 SUnit &Pred = *P.getSUnit();60 if (!Pred.isScheduled) {61 // We found an available, but not scheduled, predecessor. If it's the62 // only one we have found, keep track of it... otherwise give up.63 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)64 return nullptr;65 OnlyAvailablePred = &Pred;66 }67 }68 69 return OnlyAvailablePred;70}71 72void LatencyPriorityQueue::push(SUnit *SU) {73 // Look at all of the successors of this node. Count the number of nodes that74 // this node is the sole unscheduled node for.75 unsigned NumNodesBlocking = 0;76 for (const SDep &Succ : SU->Succs)77 if (getSingleUnscheduledPred(Succ.getSUnit()) == SU)78 ++NumNodesBlocking;79 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;80 81 Queue.push_back(SU);82}83 84 85// scheduledNode - As nodes are scheduled, we look to see if there are any86// successor nodes that have a single unscheduled predecessor. If so, that87// single predecessor has a higher priority, since scheduling it will make88// the node available.89void LatencyPriorityQueue::scheduledNode(SUnit *SU) {90 for (const SDep &Succ : SU->Succs)91 AdjustPriorityOfUnscheduledPreds(Succ.getSUnit());92}93 94/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just95/// scheduled. If SU is not itself available, then there is at least one96/// predecessor node that has not been scheduled yet. If SU has exactly ONE97/// unscheduled predecessor, we want to increase its priority: it getting98/// scheduled will make this node available, so it is better than some other99/// node of the same priority that will not make a node available.100void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {101 if (SU->isAvailable) return; // All preds scheduled.102 103 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);104 if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;105 106 // Okay, we found a single predecessor that is available, but not scheduled.107 // Since it is available, it must be in the priority queue. First remove it.108 remove(OnlyAvailablePred);109 110 // Reinsert the node into the priority queue, which recomputes its111 // NumNodesSolelyBlocking value.112 push(OnlyAvailablePred);113}114 115SUnit *LatencyPriorityQueue::pop() {116 if (empty()) return nullptr;117 std::vector<SUnit *>::iterator Best = Queue.begin();118 for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),119 E = Queue.end(); I != E; ++I)120 if (Picker(*Best, *I))121 Best = I;122 SUnit *V = *Best;123 if (Best != std::prev(Queue.end()))124 std::swap(*Best, Queue.back());125 Queue.pop_back();126 return V;127}128 129void LatencyPriorityQueue::remove(SUnit *SU) {130 assert(!Queue.empty() && "Queue is empty!");131 std::vector<SUnit *>::iterator I = find(Queue, SU);132 assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");133 if (I != std::prev(Queue.end()))134 std::swap(*I, Queue.back());135 Queue.pop_back();136}137 138#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)139LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {140 dbgs() << "Latency Priority Queue\n";141 dbgs() << " Number of Queue Entries: " << Queue.size() << "\n";142 for (const SUnit *SU : Queue) {143 dbgs() << " ";144 DAG->dumpNode(*SU);145 }146}147#endif148