LatencyPriorityQueue.cpp revision 3f23744df4809eba94284e601e81489212c974d4 
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  unsigned LHSNum = LHS->NodeNum;
23  unsigned RHSNum = RHS->NodeNum;
24
25  // The most important heuristic is scheduling the critical path.
26  unsigned LHSLatency = PQ->getLatency(LHSNum);
27  unsigned RHSLatency = PQ->getLatency(RHSNum);
28  if (LHSLatency < RHSLatency) return true;
29  if (LHSLatency > RHSLatency) return false;
30
31  // After that, if two nodes have identical latencies, look to see if one will
32  // unblock more other nodes than the other.
33  unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
34  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
35  if (LHSBlocked < RHSBlocked) return true;
36  if (LHSBlocked > RHSBlocked) return false;
37
38  // Finally, just to provide a stable ordering, use the node number as a
39  // deciding factor.
40  return LHSNum < RHSNum;
41}
42
43
44/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
45/// of SU, return it, otherwise return null.
46SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
47  SUnit *OnlyAvailablePred = 0;
48  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
49       I != E; ++I) {
50    SUnit &Pred = *I->getSUnit();
51    if (!Pred.isScheduled) {
52      // We found an available, but not scheduled, predecessor.  If it's the
53      // only one we have found, keep track of it... otherwise give up.
54      if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
55        return 0;
56      OnlyAvailablePred = &Pred;
57    }
58  }
59
60  return OnlyAvailablePred;
61}
62
63void LatencyPriorityQueue::push_impl(SUnit *SU) {
64  // Look at all of the successors of this node.  Count the number of nodes that
65  // this node is the sole unscheduled node for.
66  unsigned NumNodesBlocking = 0;
67  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
68       I != E; ++I)
69    if (getSingleUnscheduledPred(I->getSUnit()) == SU)
70      ++NumNodesBlocking;
71  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
72
73  Queue.push(SU);
74}
75
76
77// ScheduledNode - As nodes are scheduled, we look to see if there are any
78// successor nodes that have a single unscheduled predecessor.  If so, that
79// single predecessor has a higher priority, since scheduling it will make
80// the node available.
81void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
82  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
83       I != E; ++I)
84    AdjustPriorityOfUnscheduledPreds(I->getSUnit());
85}
86
87/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
88/// scheduled.  If SU is not itself available, then there is at least one
89/// predecessor node that has not been scheduled yet.  If SU has exactly ONE
90/// unscheduled predecessor, we want to increase its priority: it getting
91/// scheduled will make this node available, so it is better than some other
92/// node of the same priority that will not make a node available.
93void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
94  if (SU->isAvailable) return;  // All preds scheduled.
95
96  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
97  if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
98
99  // Okay, we found a single predecessor that is available, but not scheduled.
100  // Since it is available, it must be in the priority queue.  First remove it.
101  remove(OnlyAvailablePred);
102
103  // Reinsert the node into the priority queue, which recomputes its
104  // NumNodesSolelyBlocking value.
105  push(OnlyAvailablePred);
106}
107