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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 #include "llvm/CodeGen/LatencyPriorityQueue.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 (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
     59        I != E; ++I) {
     60     SUnit &Pred = *I->getSUnit();
     61     if (!Pred.isScheduled) {
     62       // We found an available, but not scheduled, predecessor.  If it's the
     63       // only one we have found, keep track of it... otherwise give up.
     64       if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
     65         return nullptr;
     66       OnlyAvailablePred = &Pred;
     67     }
     68   }
     69 
     70   return OnlyAvailablePred;
     71 }
     72 
     73 void LatencyPriorityQueue::push(SUnit *SU) {
     74   // Look at all of the successors of this node.  Count the number of nodes that
     75   // this node is the sole unscheduled node for.
     76   unsigned NumNodesBlocking = 0;
     77   for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
     78        I != E; ++I) {
     79     if (getSingleUnscheduledPred(I->getSUnit()) == SU)
     80       ++NumNodesBlocking;
     81   }
     82   NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
     83 
     84   Queue.push_back(SU);
     85 }
     86 
     87 
     88 // scheduledNode - As nodes are scheduled, we look to see if there are any
     89 // successor nodes that have a single unscheduled predecessor.  If so, that
     90 // single predecessor has a higher priority, since scheduling it will make
     91 // the node available.
     92 void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
     93   for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
     94        I != E; ++I) {
     95     AdjustPriorityOfUnscheduledPreds(I->getSUnit());
     96   }
     97 }
     98 
     99 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
    100 /// scheduled.  If SU is not itself available, then there is at least one
    101 /// predecessor node that has not been scheduled yet.  If SU has exactly ONE
    102 /// unscheduled predecessor, we want to increase its priority: it getting
    103 /// scheduled will make this node available, so it is better than some other
    104 /// node of the same priority that will not make a node available.
    105 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
    106   if (SU->isAvailable) return;  // All preds scheduled.
    107 
    108   SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
    109   if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
    110 
    111   // Okay, we found a single predecessor that is available, but not scheduled.
    112   // Since it is available, it must be in the priority queue.  First remove it.
    113   remove(OnlyAvailablePred);
    114 
    115   // Reinsert the node into the priority queue, which recomputes its
    116   // NumNodesSolelyBlocking value.
    117   push(OnlyAvailablePred);
    118 }
    119 
    120 SUnit *LatencyPriorityQueue::pop() {
    121   if (empty()) return nullptr;
    122   std::vector<SUnit *>::iterator Best = Queue.begin();
    123   for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
    124        E = Queue.end(); I != E; ++I)
    125     if (Picker(*Best, *I))
    126       Best = I;
    127   SUnit *V = *Best;
    128   if (Best != std::prev(Queue.end()))
    129     std::swap(*Best, Queue.back());
    130   Queue.pop_back();
    131   return V;
    132 }
    133 
    134 void LatencyPriorityQueue::remove(SUnit *SU) {
    135   assert(!Queue.empty() && "Queue is empty!");
    136   std::vector<SUnit *>::iterator I = std::find(Queue.begin(), Queue.end(), SU);
    137   if (I != std::prev(Queue.end()))
    138     std::swap(*I, Queue.back());
    139   Queue.pop_back();
    140 }
    141