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 using namespace llvm;
  20
  21 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
  22   // The isScheduleHigh flag allows nodes with wraparound dependencies that
  23   // cannot easily be modeled as edges with latencies to be scheduled as
  24   // soon as possible in a top-down schedule.
  25   if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
  26     return false;
  27   if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
  28     return true;
  29
  30   unsigned LHSNum = LHS->NodeNum;
  31   unsigned RHSNum = RHS->NodeNum;
  32
  33   // The most important heuristic is scheduling the critical path.
  34   unsigned LHSLatency = PQ->getLatency(LHSNum);
  35   unsigned RHSLatency = PQ->getLatency(RHSNum);
  36   if (LHSLatency < RHSLatency) return true;
  37   if (LHSLatency > RHSLatency) return false;
  38
  39   // After that, if two nodes have identical latencies, look to see if one will
  40   // unblock more other nodes than the other.
  41   unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
  42   unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
  43   if (LHSBlocked < RHSBlocked) return true;
  44   if (LHSBlocked > RHSBlocked) return false;
  45
  46   // Finally, just to provide a stable ordering, use the node number as a
  47   // deciding factor.
  48   return LHSNum < RHSNum;
  49 }
  50
  51
  52 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
  53 /// of SU, return it, otherwise return null.
  54 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
  55   SUnit *OnlyAvailablePred = 0;
  56   for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
  57        I != E; ++I) {
  58     SUnit &Pred = *I->getSUnit();
  59     if (!Pred.isScheduled) {
  60       // We found an available, but not scheduled, predecessor.  If it's the
  61       // only one we have found, keep track of it... otherwise give up.
  62       if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
  63         return 0;
  64       OnlyAvailablePred = &Pred;
  65     }
  66   }
  67
  68   return OnlyAvailablePred;
  69 }
  70
  71 void LatencyPriorityQueue::push_impl(SUnit *SU) {
  72   // Look at all of the successors of this node.  Count the number of nodes that
  73   // this node is the sole unscheduled node for.
  74   unsigned NumNodesBlocking = 0;
  75   for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
  76        I != E; ++I)
  77     if (getSingleUnscheduledPred(I->getSUnit()) == SU)
  78       ++NumNodesBlocking;
  79   NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
  80
  81   Queue.push(SU);
  82 }
  83
  84
  85 // ScheduledNode - As nodes are scheduled, we look to see if there are any
  86 // successor nodes that have a single unscheduled predecessor.  If so, that
  87 // single predecessor has a higher priority, since scheduling it will make
  88 // the node available.
  89 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
  90   for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
  91        I != E; ++I)
  92     AdjustPriorityOfUnscheduledPreds(I->getSUnit());
  93 }
  94
  95 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
  96 /// scheduled.  If SU is not itself available, then there is at least one
  97 /// predecessor node that has not been scheduled yet.  If SU has exactly ONE
  98 /// unscheduled predecessor, we want to increase its priority: it getting
  99 /// scheduled will make this node available, so it is better than some other
 100 /// node of the same priority that will not make a node available.
 101 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
 102   if (SU->isAvailable) return;  // All preds scheduled.
 103
 104   SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
 105   if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
 106
 107   // Okay, we found a single predecessor that is available, but not scheduled.
 108   // Since it is available, it must be in the priority queue.  First remove it.
 109   remove(OnlyAvailablePred);
 110
 111   // Reinsert the node into the priority queue, which recomputes its
 112   // NumNodesSolelyBlocking value.
 113   push(OnlyAvailablePred);
 114 }