llvm/lib/CodeGen/LatencyPriorityQueue.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-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the LatencyPriorityQueue class, which is a
  10 // SchedulingPriorityQueue that schedules using latency information to
  11 // 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"
  19 using namespace llvm;
  20
  21 #define DEBUG_TYPE "scheduler"
  22
  23 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
  24   // The isScheduleHigh flag allows nodes with wraparound dependencies that
  25   // cannot easily be modeled as edges with latencies to be scheduled as
  26   // 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 will
  42   // 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 a
  49   // deciding factor.
  50   return RHSNum < LHSNum;
  51 }
  52
  53
  54 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
  55 /// of SU, return it, otherwise return null.
  56 SUnit *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 the
  62       // 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
  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 (const SDep &Succ : SU->Succs)
  93     AdjustPriorityOfUnscheduledPreds(Succ.getSUnit());
  94 }
  95
  96 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
  97 /// scheduled.  If SU is not itself available, then there is at least one
  98 /// predecessor node that has not been scheduled yet.  If SU has exactly ONE
  99 /// unscheduled predecessor, we want to increase its priority: it getting
 100 /// scheduled will make this node available, so it is better than some other
 101 /// node of the same priority that will not make a node available.
 102 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
 103   if (SU->isAvailable) return;  // All preds scheduled.
 104
 105   SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
 106   if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
 107
 108   // Okay, we found a single predecessor that is available, but not scheduled.
 109   // Since it is available, it must be in the priority queue.  First remove it.
 110   remove(OnlyAvailablePred);
 111
 112   // Reinsert the node into the priority queue, which recomputes its
 113   // NumNodesSolelyBlocking value.
 114   push(OnlyAvailablePred);
 115 }
 116
 117 SUnit *LatencyPriorityQueue::pop() {
 118   if (empty()) return nullptr;
 119   std::vector<SUnit *>::iterator Best = Queue.begin();
 120   for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
 121        E = Queue.end(); I != E; ++I)
 122     if (Picker(*Best, *I))
 123       Best = I;
 124   SUnit *V = *Best;
 125   if (Best != std::prev(Queue.end()))
 126     std::swap(*Best, Queue.back());
 127   Queue.pop_back();
 128   return V;
 129 }
 130
 131 void LatencyPriorityQueue::remove(SUnit *SU) {
 132   assert(!Queue.empty() && "Queue is empty!");
 133   std::vector<SUnit *>::iterator I = find(Queue, SU);
 134   assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
 135   if (I != std::prev(Queue.end()))
 136     std::swap(*I, Queue.back());
 137   Queue.pop_back();
 138 }
 139
 140 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 141 LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
 142   dbgs() << "Latency Priority Queue\n";
 143   dbgs() << "  Number of Queue Entries: " << Queue.size() << "\n";
 144   for (const SUnit *SU : Queue) {
 145     dbgs() << "    ";
 146     DAG->dumpNode(*SU);
 147   }
 148 }
 149 #endif