LatencyPriorityQueue.cpp revision d34903a350e0bb37ab31d6cb8449b419535bfe8a 
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"
19using namespace llvm;
20
21bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
22  // The isScheduleHigh flag allows nodes with wraparound dependencies that
23  // cannot easily be modeled as edges with latencies to be scheduled as
24  // soon as possible in a top-down schedule.
25  if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
26    return false;
27  if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
28    return true;
29
30  unsigned LHSNum = LHS->NodeNum;
31  unsigned RHSNum = RHS->NodeNum;
32
33  // The most important heuristic is scheduling the critical path.
34  unsigned LHSLatency = PQ->getLatency(LHSNum);
35  unsigned RHSLatency = PQ->getLatency(RHSNum);
36  if (LHSLatency < RHSLatency) return true;
37  if (LHSLatency > RHSLatency) return false;
38
39  // After that, if two nodes have identical latencies, look to see if one will
40  // unblock more other nodes than the other.
41  unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
42  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
43  if (LHSBlocked < RHSBlocked) return true;
44  if (LHSBlocked > RHSBlocked) return false;
45
46  // Finally, just to provide a stable ordering, use the node number as a
47  // deciding factor.
48  return LHSNum < RHSNum;
49}
50
51
52/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
53/// of SU, return it, otherwise return null.
54SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
55  SUnit *OnlyAvailablePred = 0;
56  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
57       I != E; ++I) {
58    SUnit &Pred = *I->getSUnit();
59    if (!Pred.isScheduled) {
60      // We found an available, but not scheduled, predecessor.  If it's the
61      // only one we have found, keep track of it... otherwise give up.
62      if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
63        return 0;
64      OnlyAvailablePred = &Pred;
65    }
66  }
67
68  return OnlyAvailablePred;
69}
70
71void LatencyPriorityQueue::push_impl(SUnit *SU) {
72  // Look at all of the successors of this node.  Count the number of nodes that
73  // this node is the sole unscheduled node for.
74  unsigned NumNodesBlocking = 0;
75  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
76       I != E; ++I) {
77    if (getSingleUnscheduledPred(I->getSUnit()) == SU)
78      ++NumNodesBlocking;
79  }
80  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
81
82  Queue.push(SU);
83}
84
85
86// ScheduledNode - As nodes are scheduled, we look to see if there are any
87// successor nodes that have a single unscheduled predecessor.  If so, that
88// single predecessor has a higher priority, since scheduling it will make
89// the node available.
90void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
91  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
92       I != E; ++I) {
93    AdjustPriorityOfUnscheduledPreds(I->getSUnit());
94  }
95}
96
97/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
98/// scheduled.  If SU is not itself available, then there is at least one
99/// predecessor node that has not been scheduled yet.  If SU has exactly ONE
100/// unscheduled predecessor, we want to increase its priority: it getting
101/// scheduled will make this node available, so it is better than some other
102/// node of the same priority that will not make a node available.
103void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
104  if (SU->isAvailable) return;  // All preds scheduled.
105
106  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
107  if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
108
109  // Okay, we found a single predecessor that is available, but not scheduled.
110  // Since it is available, it must be in the priority queue.  First remove it.
111  remove(OnlyAvailablePred);
112
113  // Reinsert the node into the priority queue, which recomputes its
114  // NumNodesSolelyBlocking value.
115  push(OnlyAvailablePred);
116}
117