1//===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements the LatencyPriorityQueue class, which is a
10// SchedulingPriorityQueue that schedules using latency information to
11// reduce the length of the critical path through the basic block.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/CodeGen/LatencyPriorityQueue.h"
16#include "llvm/Config/llvm-config.h"
17#include "llvm/Support/Debug.h"
18#include "llvm/Support/raw_ostream.h"
19using namespace llvm;
20
21#define DEBUG_TYPE "scheduler"
22
23bool 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(NodeNum: LHSNum);
37 unsigned RHSLatency = PQ->getLatency(NodeNum: 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(NodeNum: LHSNum);
44 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(NodeNum: 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.
56SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
57 SUnit *OnlyAvailablePred = nullptr;
58 for (const SDep &P : SU->Preds) {
59 SUnit &Pred = *P.getSUnit();
60 if (!Pred.isScheduled) {
61 // We found an available, but not scheduled, predecessor. If it's the
62 // only one we have found, keep track of it... otherwise give up.
63 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
64 return nullptr;
65 OnlyAvailablePred = &Pred;
66 }
67 }
68
69 return OnlyAvailablePred;
70}
71
72void LatencyPriorityQueue::push(SUnit *SU) {
73 // Look at all of the successors of this node. Count the number of nodes that
74 // this node is the sole unscheduled node for.
75 unsigned NumNodesBlocking = 0;
76 for (const SDep &Succ : SU->Succs)
77 if (getSingleUnscheduledPred(SU: Succ.getSUnit()) == SU)
78 ++NumNodesBlocking;
79 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
80
81 Queue.push_back(x: 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.
89void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
90 for (const SDep &Succ : SU->Succs)
91 AdjustPriorityOfUnscheduledPreds(SU: Succ.getSUnit());
92}
93
94/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
95/// scheduled. If SU is not itself available, then there is at least one
96/// predecessor node that has not been scheduled yet. If SU has exactly ONE
97/// unscheduled predecessor, we want to increase its priority: it getting
98/// scheduled will make this node available, so it is better than some other
99/// node of the same priority that will not make a node available.
100void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
101 if (SU->isAvailable) return; // All preds scheduled.
102
103 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
104 if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
105
106 // Okay, we found a single predecessor that is available, but not scheduled.
107 // Since it is available, it must be in the priority queue. First remove it.
108 remove(SU: OnlyAvailablePred);
109
110 // Reinsert the node into the priority queue, which recomputes its
111 // NumNodesSolelyBlocking value.
112 push(SU: OnlyAvailablePred);
113}
114
115SUnit *LatencyPriorityQueue::pop() {
116 if (empty()) return nullptr;
117 std::vector<SUnit *>::iterator Best = Queue.begin();
118 for (std::vector<SUnit *>::iterator I = std::next(x: Queue.begin()),
119 E = Queue.end(); I != E; ++I)
120 if (Picker(*Best, *I))
121 Best = I;
122 SUnit *V = *Best;
123 if (Best != std::prev(x: Queue.end()))
124 std::swap(a&: *Best, b&: Queue.back());
125 Queue.pop_back();
126 return V;
127}
128
129void LatencyPriorityQueue::remove(SUnit *SU) {
130 assert(!Queue.empty() && "Queue is empty!");
131 std::vector<SUnit *>::iterator I = find(Range&: Queue, Val: SU);
132 assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
133 if (I != std::prev(x: Queue.end()))
134 std::swap(a&: *I, b&: Queue.back());
135 Queue.pop_back();
136}
137
138#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
139LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
140 dbgs() << "Latency Priority Queue\n";
141 dbgs() << " Number of Queue Entries: " << Queue.size() << "\n";
142 for (const SUnit *SU : Queue) {
143 dbgs() << " ";
144 DAG->dumpNode(*SU);
145 }
146}
147#endif
148