LatencyPriorityQueue.cpp revision b4566a999970b514d7c6973d99e293a6625d3f70 
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#define DEBUG_TYPE "scheduler"
17#include "llvm/CodeGen/LatencyPriorityQueue.h"
18#include "llvm/Support/Debug.h"
19#include "llvm/Support/raw_ostream.h"
20using namespace llvm;
21
22bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
23  // The isScheduleHigh flag allows nodes with wraparound dependencies that
24  // cannot easily be modeled as edges with latencies to be scheduled as
25  // soon as possible in a top-down schedule.
26  if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
27    return false;
28  if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
29    return true;
30
31  unsigned LHSNum = LHS->NodeNum;
32  unsigned RHSNum = RHS->NodeNum;
33
34  // The most important heuristic is scheduling the critical path.
35  unsigned LHSLatency = PQ->getLatency(LHSNum);
36  unsigned RHSLatency = PQ->getLatency(RHSNum);
37  if (LHSLatency < RHSLatency) return true;
38  if (LHSLatency > RHSLatency) return false;
39
40  // After that, if two nodes have identical latencies, look to see if one will
41  // unblock more other nodes than the other.
42  unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
43  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
44  if (LHSBlocked < RHSBlocked) return true;
45  if (LHSBlocked > RHSBlocked) return false;
46
47  // Finally, just to provide a stable ordering, use the node number as a
48  // deciding factor.
49  return RHSNum < LHSNum;
50}
51
52
53/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
54/// of SU, return it, otherwise return null.
55SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
56  SUnit *OnlyAvailablePred = 0;
57  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
58       I != E; ++I) {
59    SUnit &Pred = *I->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 0;
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 (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
77       I != E; ++I) {
78    if (getSingleUnscheduledPred(I->getSUnit()) == SU)
79      ++NumNodesBlocking;
80  }
81  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
82
83  Queue.push_back(SU);
84}
85
86
87// ScheduledNode - As nodes are scheduled, we look to see if there are any
88// successor nodes that have a single unscheduled predecessor.  If so, that
89// single predecessor has a higher priority, since scheduling it will make
90// the node available.
91void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
92  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
93       I != E; ++I) {
94    AdjustPriorityOfUnscheduledPreds(I->getSUnit());
95  }
96}
97
98/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
99/// scheduled.  If SU is not itself available, then there is at least one
100/// predecessor node that has not been scheduled yet.  If SU has exactly ONE
101/// unscheduled predecessor, we want to increase its priority: it getting
102/// scheduled will make this node available, so it is better than some other
103/// node of the same priority that will not make a node available.
104void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
105  if (SU->isAvailable) return;  // All preds scheduled.
106
107  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
108  if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
109
110  // Okay, we found a single predecessor that is available, but not scheduled.
111  // Since it is available, it must be in the priority queue.  First remove it.
112  remove(OnlyAvailablePred);
113
114  // Reinsert the node into the priority queue, which recomputes its
115  // NumNodesSolelyBlocking value.
116  push(OnlyAvailablePred);
117}
118
119SUnit *LatencyPriorityQueue::pop() {
120  if (empty()) return NULL;
121  std::vector<SUnit *>::iterator Best = Queue.begin();
122  for (std::vector<SUnit *>::iterator I = llvm::next(Queue.begin()),
123       E = Queue.end(); I != E; ++I)
124    if (Picker(*Best, *I))
125      Best = I;
126  SUnit *V = *Best;
127  if (Best != prior(Queue.end()))
128    std::swap(*Best, Queue.back());
129  Queue.pop_back();
130  return V;
131}
132
133void LatencyPriorityQueue::remove(SUnit *SU) {
134  assert(!Queue.empty() && "Queue is empty!");
135  std::vector<SUnit *>::iterator I = std::find(Queue.begin(), Queue.end(), SU);
136  if (I != prior(Queue.end()))
137    std::swap(*I, Queue.back());
138  Queue.pop_back();
139}
140
141#ifdef NDEBUG
142void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {}
143#else
144void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
145  LatencyPriorityQueue q = *this;
146  while (!q.empty()) {
147    SUnit *su = q.pop();
148    dbgs() << "Height " << su->getHeight() << ": ";
149    su->dump(DAG);
150  }
151}
152#endif
153