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 (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(Succ.getSUnit()) == SU)
78      ++NumNodesBlocking;
79  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
80
81  Queue.push_back(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(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(OnlyAvailablePred);
109
110  // Reinsert the node into the priority queue, which recomputes its
111  // NumNodesSolelyBlocking value.
112  push(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(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(Queue.end()))
124    std::swap(*Best, 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(Queue, SU);
132  assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
133  if (I != std::prev(Queue.end()))
134    std::swap(*I, 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