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/Config/llvm-config.h"
18 #include "llvm/Support/Debug.h"
19 #include "llvm/Support/raw_ostream.h"
20 using namespace llvm;
21
22 #define DEBUG_TYPE "scheduler"
23
operator ()(const SUnit * LHS,const SUnit * RHS) const24 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
25 // The isScheduleHigh flag allows nodes with wraparound dependencies that
26 // cannot easily be modeled as edges with latencies to be scheduled as
27 // soon as possible in a top-down schedule.
28 if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
29 return false;
30 if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
31 return true;
32
33 unsigned LHSNum = LHS->NodeNum;
34 unsigned RHSNum = RHS->NodeNum;
35
36 // The most important heuristic is scheduling the critical path.
37 unsigned LHSLatency = PQ->getLatency(LHSNum);
38 unsigned RHSLatency = PQ->getLatency(RHSNum);
39 if (LHSLatency < RHSLatency) return true;
40 if (LHSLatency > RHSLatency) return false;
41
42 // After that, if two nodes have identical latencies, look to see if one will
43 // unblock more other nodes than the other.
44 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
45 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
46 if (LHSBlocked < RHSBlocked) return true;
47 if (LHSBlocked > RHSBlocked) return false;
48
49 // Finally, just to provide a stable ordering, use the node number as a
50 // deciding factor.
51 return RHSNum < LHSNum;
52 }
53
54
55 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
56 /// of SU, return it, otherwise return null.
getSingleUnscheduledPred(SUnit * SU)57 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
58 SUnit *OnlyAvailablePred = nullptr;
59 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
60 I != E; ++I) {
61 SUnit &Pred = *I->getSUnit();
62 if (!Pred.isScheduled) {
63 // We found an available, but not scheduled, predecessor. If it's the
64 // only one we have found, keep track of it... otherwise give up.
65 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
66 return nullptr;
67 OnlyAvailablePred = &Pred;
68 }
69 }
70
71 return OnlyAvailablePred;
72 }
73
push(SUnit * SU)74 void LatencyPriorityQueue::push(SUnit *SU) {
75 // Look at all of the successors of this node. Count the number of nodes that
76 // this node is the sole unscheduled node for.
77 unsigned NumNodesBlocking = 0;
78 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
79 I != E; ++I) {
80 if (getSingleUnscheduledPred(I->getSUnit()) == SU)
81 ++NumNodesBlocking;
82 }
83 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
84
85 Queue.push_back(SU);
86 }
87
88
89 // scheduledNode - As nodes are scheduled, we look to see if there are any
90 // successor nodes that have a single unscheduled predecessor. If so, that
91 // single predecessor has a higher priority, since scheduling it will make
92 // the node available.
scheduledNode(SUnit * SU)93 void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
94 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
95 I != E; ++I) {
96 AdjustPriorityOfUnscheduledPreds(I->getSUnit());
97 }
98 }
99
100 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
101 /// scheduled. If SU is not itself available, then there is at least one
102 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
103 /// unscheduled predecessor, we want to increase its priority: it getting
104 /// scheduled will make this node available, so it is better than some other
105 /// node of the same priority that will not make a node available.
AdjustPriorityOfUnscheduledPreds(SUnit * SU)106 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
107 if (SU->isAvailable) return; // All preds scheduled.
108
109 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
110 if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
111
112 // Okay, we found a single predecessor that is available, but not scheduled.
113 // Since it is available, it must be in the priority queue. First remove it.
114 remove(OnlyAvailablePred);
115
116 // Reinsert the node into the priority queue, which recomputes its
117 // NumNodesSolelyBlocking value.
118 push(OnlyAvailablePred);
119 }
120
pop()121 SUnit *LatencyPriorityQueue::pop() {
122 if (empty()) return nullptr;
123 std::vector<SUnit *>::iterator Best = Queue.begin();
124 for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
125 E = Queue.end(); I != E; ++I)
126 if (Picker(*Best, *I))
127 Best = I;
128 SUnit *V = *Best;
129 if (Best != std::prev(Queue.end()))
130 std::swap(*Best, Queue.back());
131 Queue.pop_back();
132 return V;
133 }
134
remove(SUnit * SU)135 void LatencyPriorityQueue::remove(SUnit *SU) {
136 assert(!Queue.empty() && "Queue is empty!");
137 std::vector<SUnit *>::iterator I = find(Queue, SU);
138 assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
139 if (I != std::prev(Queue.end()))
140 std::swap(*I, Queue.back());
141 Queue.pop_back();
142 }
143
144 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
dump(ScheduleDAG * DAG) const145 LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
146 dbgs() << "Latency Priority Queue\n";
147 dbgs() << " Number of Queue Entries: " << Queue.size() << "\n";
148 for (auto const &SU : Queue) {
149 dbgs() << " ";
150 SU->dump(DAG);
151 }
152 }
153 #endif
154