lib/CodeGen/LatencyPriorityQueue.cpp

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