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 #define DEBUG_TYPE "scheduler"
  17 #include "llvm/CodeGen/LatencyPriorityQueue.h"
  18 #include "llvm/Support/Debug.h"
  19 #include "llvm/Support/raw_ostream.h"
  20 using namespace llvm;
  21
  22 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
  23   // The isScheduleHigh flag allows nodes with wraparound dependencies that
  24   // cannot easily be modeled as edges with latencies to be scheduled as
  25   // soon as possible in a top-down schedule.
  26   if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
  27     return false;
  28   if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
  29     return true;
  30
  31   unsigned LHSNum = LHS->NodeNum;
  32   unsigned RHSNum = RHS->NodeNum;
  33
  34   // The most important heuristic is scheduling the critical path.
  35   unsigned LHSLatency = PQ->getLatency(LHSNum);
  36   unsigned RHSLatency = PQ->getLatency(RHSNum);
  37   if (LHSLatency < RHSLatency) return true;
  38   if (LHSLatency > RHSLatency) return false;
  39
  40   // After that, if two nodes have identical latencies, look to see if one will
  41   // unblock more other nodes than the other.
  42   unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
  43   unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
  44   if (LHSBlocked < RHSBlocked) return true;
  45   if (LHSBlocked > RHSBlocked) return false;
  46
  47   // Finally, just to provide a stable ordering, use the node number as a
  48   // deciding factor.
  49   return LHSNum < RHSNum;
  50 }
  51
  52
  53 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
  54 /// of SU, return it, otherwise return null.
  55 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
  56   SUnit *OnlyAvailablePred = 0;
  57   for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
  58        I != E; ++I) {
  59     SUnit &Pred = *I->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 0;
  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 (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
  93        I != E; ++I) {
  94     AdjustPriorityOfUnscheduledPreds(I->getSUnit());
  95   }
  96 }
  97
  98 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
  99 /// scheduled.  If SU is not itself available, then there is at least one
 100 /// predecessor node that has not been scheduled yet.  If SU has exactly ONE
 101 /// unscheduled predecessor, we want to increase its priority: it getting
 102 /// scheduled will make this node available, so it is better than some other
 103 /// node of the same priority that will not make a node available.
 104 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
 105   if (SU->isAvailable) return;  // All preds scheduled.
 106
 107   SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
 108   if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
 109
 110   // Okay, we found a single predecessor that is available, but not scheduled.
 111   // Since it is available, it must be in the priority queue.  First remove it.
 112   remove(OnlyAvailablePred);
 113
 114   // Reinsert the node into the priority queue, which recomputes its
 115   // NumNodesSolelyBlocking value.
 116   push(OnlyAvailablePred);
 117 }
 118
 119 SUnit *LatencyPriorityQueue::pop() {
 120   if (empty()) return NULL;
 121   std::vector<SUnit *>::iterator Best = Queue.begin();
 122   for (std::vector<SUnit *>::iterator I = llvm::next(Queue.begin()),
 123        E = Queue.end(); I != E; ++I)
 124     if (Picker(*Best, *I))
 125       Best = I;
 126   SUnit *V = *Best;
 127   if (Best != prior(Queue.end()))
 128     std::swap(*Best, Queue.back());
 129   Queue.pop_back();
 130   return V;
 131 }
 132
 133 void LatencyPriorityQueue::remove(SUnit *SU) {
 134   assert(!Queue.empty() && "Queue is empty!");
 135   std::vector<SUnit *>::iterator I = std::find(Queue.begin(), Queue.end(), SU);
 136   if (I != prior(Queue.end()))
 137     std::swap(*I, Queue.back());
 138   Queue.pop_back();
 139 }
 140
 141 #ifdef NDEBUG
 142 void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {}
 143 #else
 144 void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
 145   LatencyPriorityQueue q = *this;
 146   while (!q.empty()) {
 147     SUnit *su = q.pop();
 148     dbgs() << "Height " << su->getHeight() << ": ";
 149     su->dump(DAG);
 150   }
 151 }
 152 #endif