LLVM  6.0.0svn
LatencyPriorityQueue.cpp
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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 
17 #include "llvm/Support/Debug.h"
19 using namespace llvm;
20 
21 #define DEBUG_TYPE "scheduler"
22 
23 bool 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.
56 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
57  SUnit *OnlyAvailablePred = nullptr;
58  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
59  I != E; ++I) {
60  SUnit &Pred = *I->getSUnit();
61  if (!Pred.isScheduled) {
62  // We found an available, but not scheduled, predecessor. If it's the
63  // only one we have found, keep track of it... otherwise give up.
64  if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
65  return nullptr;
66  OnlyAvailablePred = &Pred;
67  }
68  }
69 
70  return OnlyAvailablePred;
71 }
72 
74  // Look at all of the successors of this node. Count the number of nodes that
75  // this node is the sole unscheduled node for.
76  unsigned NumNodesBlocking = 0;
77  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
78  I != E; ++I) {
79  if (getSingleUnscheduledPred(I->getSUnit()) == SU)
80  ++NumNodesBlocking;
81  }
82  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
83 
84  Queue.push_back(SU);
85 }
86 
87 
88 // scheduledNode - As nodes are scheduled, we look to see if there are any
89 // successor nodes that have a single unscheduled predecessor. If so, that
90 // single predecessor has a higher priority, since scheduling it will make
91 // the node available.
93  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
94  I != E; ++I) {
95  AdjustPriorityOfUnscheduledPreds(I->getSUnit());
96  }
97 }
98 
99 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
100 /// scheduled. If SU is not itself available, then there is at least one
101 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
102 /// unscheduled predecessor, we want to increase its priority: it getting
103 /// scheduled will make this node available, so it is better than some other
104 /// node of the same priority that will not make a node available.
105 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
106  if (SU->isAvailable) return; // All preds scheduled.
107 
108  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
109  if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
110 
111  // Okay, we found a single predecessor that is available, but not scheduled.
112  // Since it is available, it must be in the priority queue. First remove it.
113  remove(OnlyAvailablePred);
114 
115  // Reinsert the node into the priority queue, which recomputes its
116  // NumNodesSolelyBlocking value.
117  push(OnlyAvailablePred);
118 }
119 
121  if (empty()) return nullptr;
122  std::vector<SUnit *>::iterator Best = Queue.begin();
123  for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
124  E = Queue.end(); I != E; ++I)
125  if (Picker(*Best, *I))
126  Best = I;
127  SUnit *V = *Best;
128  if (Best != std::prev(Queue.end()))
129  std::swap(*Best, Queue.back());
130  Queue.pop_back();
131  return V;
132 }
133 
135  assert(!Queue.empty() && "Queue is empty!");
136  std::vector<SUnit *>::iterator I = find(Queue, SU);
137  assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
138  if (I != std::prev(Queue.end()))
139  std::swap(*I, Queue.back());
140  Queue.pop_back();
141 }
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
SmallVector< SDep, 4 > Preds
All sunit predecessors.
Definition: ScheduleDAG.h:261
bool isScheduled
True once scheduled.
Definition: ScheduleDAG.h:289
SmallVectorImpl< SDep >::const_iterator const_pred_iterator
Definition: ScheduleDAG.h:266
void remove(SUnit *SU) override
void scheduledNode(SUnit *Node) override
As each node is scheduled, this method is invoked.
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
unsigned getLatency(unsigned NodeNum) const
void push(SUnit *U) override
bool isScheduleHigh
True if preferable to schedule high.
Definition: ScheduleDAG.h:290
auto find(R &&Range, const T &Val) -> decltype(std::begin(Range))
Provide wrappers to std::find which take ranges instead of having to pass begin/end explicitly...
Definition: STLExtras.h:788
LatencyPriorityQueue * PQ
void swap(llvm::BitVector &LHS, llvm::BitVector &RHS)
Implement std::swap in terms of BitVector swap.
Definition: BitVector.h:923
SmallVectorImpl< SDep >::const_iterator const_succ_iterator
Definition: ScheduleDAG.h:267
bool isAvailable
True once available.
Definition: ScheduleDAG.h:288
bool operator()(const SUnit *left, const SUnit *right) const
unsigned getNumSolelyBlockNodes(unsigned NodeNum) const
#define I(x, y, z)
Definition: MD5.cpp:58
unsigned NodeNum
Entry # of node in the node vector.
Definition: ScheduleDAG.h:269
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
SmallVector< SDep, 4 > Succs
All sunit successors.
Definition: ScheduleDAG.h:262
Scheduling unit. This is a node in the scheduling DAG.
Definition: ScheduleDAG.h:247