1 //===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
14 //===----------------------------------------------------------------------===//
16 #define DEBUG_TYPE "scheduler"
17 #include "llvm/CodeGen/LatencyPriorityQueue.h"
18 #include "llvm/Support/Debug.h"
21 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
22 unsigned LHSNum = LHS->NodeNum;
23 unsigned RHSNum = RHS->NodeNum;
25 // The most important heuristic is scheduling the critical path.
26 unsigned LHSLatency = PQ->getLatency(LHSNum);
27 unsigned RHSLatency = PQ->getLatency(RHSNum);
28 if (LHSLatency < RHSLatency) return true;
29 if (LHSLatency > RHSLatency) return false;
31 // After that, if two nodes have identical latencies, look to see if one will
32 // unblock more other nodes than the other.
33 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
34 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
35 if (LHSBlocked < RHSBlocked) return true;
36 if (LHSBlocked > RHSBlocked) return false;
38 // Finally, just to provide a stable ordering, use the node number as a
40 return LHSNum < RHSNum;
44 /// CalcNodePriority - Calculate the maximal path from the node to the exit.
46 void LatencyPriorityQueue::CalcLatency(const SUnit &SU) {
47 int &Latency = Latencies[SU.NodeNum];
51 std::vector<const SUnit*> WorkList;
52 WorkList.push_back(&SU);
53 while (!WorkList.empty()) {
54 const SUnit *Cur = WorkList.back();
56 unsigned MaxSuccLatency = 0;
57 for (SUnit::const_succ_iterator I = Cur->Succs.begin(),E = Cur->Succs.end();
59 int SuccLatency = Latencies[I->getSUnit()->NodeNum];
60 if (SuccLatency == -1) {
62 WorkList.push_back(I->getSUnit());
64 unsigned NewLatency = SuccLatency + I->getLatency();
65 MaxSuccLatency = std::max(MaxSuccLatency, NewLatency);
69 Latencies[Cur->NodeNum] = MaxSuccLatency;
75 /// CalculatePriorities - Calculate priorities of all scheduling units.
76 void LatencyPriorityQueue::CalculatePriorities() {
77 Latencies.assign(SUnits->size(), -1);
78 NumNodesSolelyBlocking.assign(SUnits->size(), 0);
80 // For each node, calculate the maximal path from the node to the exit.
81 for (unsigned i = 0, e = SUnits->size(); i != e; ++i)
82 CalcLatency((*SUnits)[i]);
85 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
86 /// of SU, return it, otherwise return null.
87 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
88 SUnit *OnlyAvailablePred = 0;
89 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
91 SUnit &Pred = *I->getSUnit();
92 if (!Pred.isScheduled) {
93 // We found an available, but not scheduled, predecessor. If it's the
94 // only one we have found, keep track of it... otherwise give up.
95 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
97 OnlyAvailablePred = &Pred;
101 return OnlyAvailablePred;
104 void LatencyPriorityQueue::push_impl(SUnit *SU) {
105 // Look at all of the successors of this node. Count the number of nodes that
106 // this node is the sole unscheduled node for.
107 unsigned NumNodesBlocking = 0;
108 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
110 if (getSingleUnscheduledPred(I->getSUnit()) == SU)
112 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
118 // ScheduledNode - As nodes are scheduled, we look to see if there are any
119 // successor nodes that have a single unscheduled predecessor. If so, that
120 // single predecessor has a higher priority, since scheduling it will make
121 // the node available.
122 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
123 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
125 AdjustPriorityOfUnscheduledPreds(I->getSUnit());
128 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
129 /// scheduled. If SU is not itself available, then there is at least one
130 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
131 /// unscheduled predecessor, we want to increase its priority: it getting
132 /// scheduled will make this node available, so it is better than some other
133 /// node of the same priority that will not make a node available.
134 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
135 if (SU->isAvailable) return; // All preds scheduled.
137 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
138 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
140 // Okay, we found a single predecessor that is available, but not scheduled.
141 // Since it is available, it must be in the priority queue. First remove it.
142 remove(OnlyAvailablePred);
144 // Reinsert the node into the priority queue, which recomputes its
145 // NumNodesSolelyBlocking value.
146 push(OnlyAvailablePred);