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 int 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();
55 unsigned CurLatency = Cur->Latency;
57 unsigned MaxSuccLatency = 0;
58 for (SUnit::const_succ_iterator I = Cur->Succs.begin(),E = Cur->Succs.end();
60 int SuccLatency = Latencies[I->getSUnit()->NodeNum];
61 if (SuccLatency == -1) {
63 WorkList.push_back(I->getSUnit());
65 // This assumes that there's no delay for reusing registers.
66 unsigned NewLatency = SuccLatency + CurLatency;
67 MaxSuccLatency = std::max(MaxSuccLatency, NewLatency);
71 Latencies[Cur->NodeNum] = MaxSuccLatency;
79 /// CalculatePriorities - Calculate priorities of all scheduling units.
80 void LatencyPriorityQueue::CalculatePriorities() {
81 Latencies.assign(SUnits->size(), -1);
82 NumNodesSolelyBlocking.assign(SUnits->size(), 0);
84 // For each node, calculate the maximal path from the node to the exit.
85 std::vector<std::pair<const SUnit*, unsigned> > WorkList;
86 for (unsigned i = 0, e = SUnits->size(); i != e; ++i) {
87 const SUnit *SU = &(*SUnits)[i];
88 if (SU->Succs.empty())
89 WorkList.push_back(std::make_pair(SU, 0U));
92 while (!WorkList.empty()) {
93 const SUnit *SU = WorkList.back().first;
94 unsigned SuccLat = WorkList.back().second;
96 int &Latency = Latencies[SU->NodeNum];
97 if (Latency == -1 || (SU->Latency + SuccLat) > (unsigned)Latency) {
98 Latency = SU->Latency + SuccLat;
99 for (SUnit::const_pred_iterator I = SU->Preds.begin(),E = SU->Preds.end();
101 WorkList.push_back(std::make_pair(I->getSUnit(), Latency));
106 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
107 /// of SU, return it, otherwise return null.
108 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
109 SUnit *OnlyAvailablePred = 0;
110 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
112 SUnit &Pred = *I->getSUnit();
113 if (!Pred.isScheduled) {
114 // We found an available, but not scheduled, predecessor. If it's the
115 // only one we have found, keep track of it... otherwise give up.
116 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
118 OnlyAvailablePred = &Pred;
122 return OnlyAvailablePred;
125 void LatencyPriorityQueue::push_impl(SUnit *SU) {
126 // Look at all of the successors of this node. Count the number of nodes that
127 // this node is the sole unscheduled node for.
128 unsigned NumNodesBlocking = 0;
129 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
131 if (getSingleUnscheduledPred(I->getSUnit()) == SU)
133 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
139 // ScheduledNode - As nodes are scheduled, we look to see if there are any
140 // successor nodes that have a single unscheduled predecessor. If so, that
141 // single predecessor has a higher priority, since scheduling it will make
142 // the node available.
143 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
144 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
146 AdjustPriorityOfUnscheduledPreds(I->getSUnit());
149 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
150 /// scheduled. If SU is not itself available, then there is at least one
151 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
152 /// unscheduled predecessor, we want to increase its priority: it getting
153 /// scheduled will make this node available, so it is better than some other
154 /// node of the same priority that will not make a node available.
155 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
156 if (SU->isAvailable) return; // All preds scheduled.
158 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
159 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
161 // Okay, we found a single predecessor that is available, but not scheduled.
162 // Since it is available, it must be in the priority queue. First remove it.
163 remove(OnlyAvailablePred);
165 // Reinsert the node into the priority queue, which recomputes its
166 // NumNodesSolelyBlocking value.
167 push(OnlyAvailablePred);