LLVM  8.0.0svn
LatencyPriorityQueue.cpp
Go to the documentation of this file.
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/Config/llvm-config.h"
18 #include "llvm/Support/Debug.h"
20 using namespace llvm;
21 
22 #define DEBUG_TYPE "scheduler"
23 
24 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
25  // The isScheduleHigh flag allows nodes with wraparound dependencies that
26  // cannot easily be modeled as edges with latencies to be scheduled as
27  // soon as possible in a top-down schedule.
28  if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
29  return false;
30  if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
31  return true;
32 
33  unsigned LHSNum = LHS->NodeNum;
34  unsigned RHSNum = RHS->NodeNum;
35 
36  // The most important heuristic is scheduling the critical path.
37  unsigned LHSLatency = PQ->getLatency(LHSNum);
38  unsigned RHSLatency = PQ->getLatency(RHSNum);
39  if (LHSLatency < RHSLatency) return true;
40  if (LHSLatency > RHSLatency) return false;
41 
42  // After that, if two nodes have identical latencies, look to see if one will
43  // unblock more other nodes than the other.
44  unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
45  unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
46  if (LHSBlocked < RHSBlocked) return true;
47  if (LHSBlocked > RHSBlocked) return false;
48 
49  // Finally, just to provide a stable ordering, use the node number as a
50  // deciding factor.
51  return RHSNum < LHSNum;
52 }
53 
54 
55 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
56 /// of SU, return it, otherwise return null.
57 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
58  SUnit *OnlyAvailablePred = nullptr;
59  for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
60  I != E; ++I) {
61  SUnit &Pred = *I->getSUnit();
62  if (!Pred.isScheduled) {
63  // We found an available, but not scheduled, predecessor. If it's the
64  // only one we have found, keep track of it... otherwise give up.
65  if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
66  return nullptr;
67  OnlyAvailablePred = &Pred;
68  }
69  }
70 
71  return OnlyAvailablePred;
72 }
73 
75  // Look at all of the successors of this node. Count the number of nodes that
76  // this node is the sole unscheduled node for.
77  unsigned NumNodesBlocking = 0;
78  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
79  I != E; ++I) {
80  if (getSingleUnscheduledPred(I->getSUnit()) == SU)
81  ++NumNodesBlocking;
82  }
83  NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
84 
85  Queue.push_back(SU);
86 }
87 
88 
89 // scheduledNode - As nodes are scheduled, we look to see if there are any
90 // successor nodes that have a single unscheduled predecessor. If so, that
91 // single predecessor has a higher priority, since scheduling it will make
92 // the node available.
94  for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
95  I != E; ++I) {
96  AdjustPriorityOfUnscheduledPreds(I->getSUnit());
97  }
98 }
99 
100 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
101 /// scheduled. If SU is not itself available, then there is at least one
102 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
103 /// unscheduled predecessor, we want to increase its priority: it getting
104 /// scheduled will make this node available, so it is better than some other
105 /// node of the same priority that will not make a node available.
106 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
107  if (SU->isAvailable) return; // All preds scheduled.
108 
109  SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
110  if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
111 
112  // Okay, we found a single predecessor that is available, but not scheduled.
113  // Since it is available, it must be in the priority queue. First remove it.
114  remove(OnlyAvailablePred);
115 
116  // Reinsert the node into the priority queue, which recomputes its
117  // NumNodesSolelyBlocking value.
118  push(OnlyAvailablePred);
119 }
120 
122  if (empty()) return nullptr;
123  std::vector<SUnit *>::iterator Best = Queue.begin();
124  for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
125  E = Queue.end(); I != E; ++I)
126  if (Picker(*Best, *I))
127  Best = I;
128  SUnit *V = *Best;
129  if (Best != std::prev(Queue.end()))
130  std::swap(*Best, Queue.back());
131  Queue.pop_back();
132  return V;
133 }
134 
136  assert(!Queue.empty() && "Queue is empty!");
137  std::vector<SUnit *>::iterator I = find(Queue, SU);
138  assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
139  if (I != std::prev(Queue.end()))
140  std::swap(*I, Queue.back());
141  Queue.pop_back();
142 }
143 
144 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
146  dbgs() << "Latency Priority Queue\n";
147  dbgs() << " Number of Queue Entries: " << Queue.size() << "\n";
148  for (const SUnit *SU : Queue) {
149  dbgs() << " ";
150  DAG->dumpNode(*SU);
151  }
152 }
153 #endif
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
virtual void dumpNode(const SUnit &SU) const =0
SmallVector< SDep, 4 > Preds
All sunit predecessors.
Definition: ScheduleDAG.h:260
bool isScheduled
True once scheduled.
Definition: ScheduleDAG.h:288
SmallVectorImpl< SDep >::const_iterator const_pred_iterator
Definition: ScheduleDAG.h:265
void scheduledNode(SUnit *SU) override
As each node is scheduled, this method is invoked.
#define LLVM_DUMP_METHOD
Definition: Compiler.h:74
void remove(SUnit *SU) override
bool operator()(const SUnit *LHS, const SUnit *RHS) const
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
unsigned getLatency(unsigned NodeNum) const
auto find(R &&Range, const T &Val) -> decltype(adl_begin(Range))
Provide wrappers to std::find which take ranges instead of having to pass begin/end explicitly...
Definition: STLExtras.h:1063
void push(SUnit *U) override
bool isScheduleHigh
True if preferable to schedule high.
Definition: ScheduleDAG.h:289
LatencyPriorityQueue * PQ
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:133
void swap(llvm::BitVector &LHS, llvm::BitVector &RHS)
Implement std::swap in terms of BitVector swap.
Definition: BitVector.h:941
LLVM_DUMP_METHOD void dump(ScheduleDAG *DAG) const override
SmallVectorImpl< SDep >::const_iterator const_succ_iterator
Definition: ScheduleDAG.h:266
bool isAvailable
True once available.
Definition: ScheduleDAG.h:287
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:268
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
SmallVector< SDep, 4 > Succs
All sunit successors.
Definition: ScheduleDAG.h:261
Scheduling unit. This is a node in the scheduling DAG.
Definition: ScheduleDAG.h:246