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(LHSNum);
37  unsigned RHSLatency = PQ->getLatency(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(LHSNum);
44  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(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 (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
59       I != E; ++I) {
60    SUnit &Pred = *I->getSUnit();
61    if (!Pred.isScheduled) {
62      // We found an available, but not scheduled, predecessor.  If it's the
63      // only one we have found, keep track of it... otherwise give up.
64      if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
65        return nullptr;
66      OnlyAvailablePred = &Pred;
67    }
68  }
69
70  return OnlyAvailablePred;
71}
72
73void LatencyPriorityQueue::push(SUnit *SU) {
74  // Look at all of the successors of this node.  Count the number of nodes that
75  // this node is the sole unscheduled node for.
76  unsigned NumNodesBlocking = 0;
77  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
78       I != E; ++I) {
79    if (getSingleUnscheduledPred(I->getSUnit()) == SU)
80      ++NumNodesBlocking;
81  }
82  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
83
84  Queue.push_back(SU);
85}
86
87
88// scheduledNode - As nodes are scheduled, we look to see if there are any
89// successor nodes that have a single unscheduled predecessor.  If so, that
90// single predecessor has a higher priority, since scheduling it will make
91// the node available.
92void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
93  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
94       I != E; ++I) {
95    AdjustPriorityOfUnscheduledPreds(I->getSUnit());
96  }
97}
98
99/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
100/// scheduled.  If SU is not itself available, then there is at least one
101/// predecessor node that has not been scheduled yet.  If SU has exactly ONE
102/// unscheduled predecessor, we want to increase its priority: it getting
103/// scheduled will make this node available, so it is better than some other
104/// node of the same priority that will not make a node available.
105void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
106  if (SU->isAvailable) return;  // All preds scheduled.
107
108  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
109  if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
110
111  // Okay, we found a single predecessor that is available, but not scheduled.
112  // Since it is available, it must be in the priority queue.  First remove it.
113  remove(OnlyAvailablePred);
114
115  // Reinsert the node into the priority queue, which recomputes its
116  // NumNodesSolelyBlocking value.
117  push(OnlyAvailablePred);
118}
119
120SUnit *LatencyPriorityQueue::pop() {
121  if (empty()) return nullptr;
122  std::vector<SUnit *>::iterator Best = Queue.begin();
123  for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
124       E = Queue.end(); I != E; ++I)
125    if (Picker(*Best, *I))
126      Best = I;
127  SUnit *V = *Best;
128  if (Best != std::prev(Queue.end()))
129    std::swap(*Best, Queue.back());
130  Queue.pop_back();
131  return V;
132}
133
134void LatencyPriorityQueue::remove(SUnit *SU) {
135  assert(!Queue.empty() && "Queue is empty!");
136  std::vector<SUnit *>::iterator I = find(Queue, SU);
137  assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
138  if (I != std::prev(Queue.end()))
139    std::swap(*I, Queue.back());
140  Queue.pop_back();
141}
142
143#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
144LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
145  dbgs() << "Latency Priority Queue\n";
146  dbgs() << "  Number of Queue Entries: " << Queue.size() << "\n";
147  for (const SUnit *SU : Queue) {
148    dbgs() << "    ";
149    DAG->dumpNode(*SU);
150  }
151}
152#endif
153