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      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
    153