aboutsummaryrefslogtreecommitdiffstats
path: root/lib/CodeGen/LatencyPriorityQueue.cpp
blob: cdf505e3e0c9cc030c346caf6833309516ce49a2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
//===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the LatencyPriorityQueue class, which is a
// SchedulingPriorityQueue that schedules using latency information to
// reduce the length of the critical path through the basic block.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/LatencyPriorityQueue.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

#define DEBUG_TYPE "scheduler"

bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
  // The isScheduleHigh flag allows nodes with wraparound dependencies that
  // cannot easily be modeled as edges with latencies to be scheduled as
  // soon as possible in a top-down schedule.
  if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
    return false;
  if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
    return true;

  unsigned LHSNum = LHS->NodeNum;
  unsigned RHSNum = RHS->NodeNum;

  // The most important heuristic is scheduling the critical path.
  unsigned LHSLatency = PQ->getLatency(LHSNum);
  unsigned RHSLatency = PQ->getLatency(RHSNum);
  if (LHSLatency < RHSLatency) return true;
  if (LHSLatency > RHSLatency) return false;

  // After that, if two nodes have identical latencies, look to see if one will
  // unblock more other nodes than the other.
  unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
  if (LHSBlocked < RHSBlocked) return true;
  if (LHSBlocked > RHSBlocked) return false;

  // Finally, just to provide a stable ordering, use the node number as a
  // deciding factor.
  return RHSNum < LHSNum;
}


/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
/// of SU, return it, otherwise return null.
SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
  SUnit *OnlyAvailablePred = nullptr;
  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
       I != E; ++I) {
    SUnit &Pred = *I->getSUnit();
    if (!Pred.isScheduled) {
      // We found an available, but not scheduled, predecessor.  If it's the
      // only one we have found, keep track of it... otherwise give up.
      if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
        return nullptr;
      OnlyAvailablePred = &Pred;
    }
  }

  return OnlyAvailablePred;
}

void LatencyPriorityQueue::push(SUnit *SU) {
  // Look at all of the successors of this node.  Count the number of nodes that
  // this node is the sole unscheduled node for.
  unsigned NumNodesBlocking = 0;
  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
       I != E; ++I) {
    if (getSingleUnscheduledPred(I->getSUnit()) == SU)
      ++NumNodesBlocking;
  }
  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;

  Queue.push_back(SU);
}


// scheduledNode - As nodes are scheduled, we look to see if there are any
// successor nodes that have a single unscheduled predecessor.  If so, that
// single predecessor has a higher priority, since scheduling it will make
// the node available.
void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
       I != E; ++I) {
    AdjustPriorityOfUnscheduledPreds(I->getSUnit());
  }
}

/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
/// scheduled.  If SU is not itself available, then there is at least one
/// predecessor node that has not been scheduled yet.  If SU has exactly ONE
/// unscheduled predecessor, we want to increase its priority: it getting
/// scheduled will make this node available, so it is better than some other
/// node of the same priority that will not make a node available.
void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
  if (SU->isAvailable) return;  // All preds scheduled.

  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
  if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;

  // Okay, we found a single predecessor that is available, but not scheduled.
  // Since it is available, it must be in the priority queue.  First remove it.
  remove(OnlyAvailablePred);

  // Reinsert the node into the priority queue, which recomputes its
  // NumNodesSolelyBlocking value.
  push(OnlyAvailablePred);
}

SUnit *LatencyPriorityQueue::pop() {
  if (empty()) return nullptr;
  std::vector<SUnit *>::iterator Best = Queue.begin();
  for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
       E = Queue.end(); I != E; ++I)
    if (Picker(*Best, *I))
      Best = I;
  SUnit *V = *Best;
  if (Best != std::prev(Queue.end()))
    std::swap(*Best, Queue.back());
  Queue.pop_back();
  return V;
}

void LatencyPriorityQueue::remove(SUnit *SU) {
  assert(!Queue.empty() && "Queue is empty!");
  std::vector<SUnit *>::iterator I = std::find(Queue.begin(), Queue.end(), SU);
  if (I != std::prev(Queue.end()))
    std::swap(*I, Queue.back());
  Queue.pop_back();
}

#ifdef NDEBUG
void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {}
#else
void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
  LatencyPriorityQueue q = *this;
  while (!q.empty()) {
    SUnit *su = q.pop();
    dbgs() << "Height " << su->getHeight() << ": ";
    su->dump(DAG);
  }
}
#endif