LLVM  9.0.0svn
ScheduleHazardRecognizer.h
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1 //=- llvm/CodeGen/ScheduleHazardRecognizer.h - Scheduling Support -*- C++ -*-=//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements the ScheduleHazardRecognizer class, which implements
10 // hazard-avoidance heuristics for scheduling.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_CODEGEN_SCHEDULEHAZARDRECOGNIZER_H
15 #define LLVM_CODEGEN_SCHEDULEHAZARDRECOGNIZER_H
16 
17 namespace llvm {
18 
19 class MachineInstr;
20 class SUnit;
21 
22 /// HazardRecognizer - This determines whether or not an instruction can be
23 /// issued this cycle, and whether or not a noop needs to be inserted to handle
24 /// the hazard.
26 protected:
27  /// MaxLookAhead - Indicate the number of cycles in the scoreboard
28  /// state. Important to restore the state after backtracking. Additionally,
29  /// MaxLookAhead=0 identifies a fake recognizer, allowing the client to
30  /// bypass virtual calls. Currently the PostRA scheduler ignores it.
31  unsigned MaxLookAhead = 0;
32 
33 public:
34  ScheduleHazardRecognizer() = default;
35  virtual ~ScheduleHazardRecognizer();
36 
37  enum HazardType {
38  NoHazard, // This instruction can be emitted at this cycle.
39  Hazard, // This instruction can't be emitted at this cycle.
40  NoopHazard // This instruction can't be emitted, and needs noops.
41  };
42 
43  unsigned getMaxLookAhead() const { return MaxLookAhead; }
44 
45  bool isEnabled() const { return MaxLookAhead != 0; }
46 
47  /// atIssueLimit - Return true if no more instructions may be issued in this
48  /// cycle.
49  ///
50  /// FIXME: remove this once MachineScheduler is the only client.
51  virtual bool atIssueLimit() const { return false; }
52 
53  /// getHazardType - Return the hazard type of emitting this node. There are
54  /// three possible results. Either:
55  /// * NoHazard: it is legal to issue this instruction on this cycle.
56  /// * Hazard: issuing this instruction would stall the machine. If some
57  /// other instruction is available, issue it first.
58  /// * NoopHazard: issuing this instruction would break the program. If
59  /// some other instruction can be issued, do so, otherwise issue a noop.
60  virtual HazardType getHazardType(SUnit *m, int Stalls = 0) {
61  return NoHazard;
62  }
63 
64  /// Reset - This callback is invoked when a new block of
65  /// instructions is about to be schedule. The hazard state should be
66  /// set to an initialized state.
67  virtual void Reset() {}
68 
69  /// EmitInstruction - This callback is invoked when an instruction is
70  /// emitted, to advance the hazard state.
71  virtual void EmitInstruction(SUnit *) {}
72 
73  /// This overload will be used when the hazard recognizer is being used
74  /// by a non-scheduling pass, which does not use SUnits.
75  virtual void EmitInstruction(MachineInstr *) {}
76 
77  /// PreEmitNoops - This callback is invoked prior to emitting an instruction.
78  /// It should return the number of noops to emit prior to the provided
79  /// instruction.
80  /// Note: This is only used during PostRA scheduling. EmitNoop is not called
81  /// for these noops.
82  virtual unsigned PreEmitNoops(SUnit *) {
83  return 0;
84  }
85 
86  /// This overload will be used when the hazard recognizer is being used
87  /// by a non-scheduling pass, which does not use SUnits.
88  virtual unsigned PreEmitNoops(MachineInstr *) {
89  return 0;
90  }
91 
92  /// ShouldPreferAnother - This callback may be invoked if getHazardType
93  /// returns NoHazard. If, even though there is no hazard, it would be better to
94  /// schedule another available instruction, this callback should return true.
95  virtual bool ShouldPreferAnother(SUnit *) {
96  return false;
97  }
98 
99  /// AdvanceCycle - This callback is invoked whenever the next top-down
100  /// instruction to be scheduled cannot issue in the current cycle, either
101  /// because of latency or resource conflicts. This should increment the
102  /// internal state of the hazard recognizer so that previously "Hazard"
103  /// instructions will now not be hazards.
104  virtual void AdvanceCycle() {}
105 
106  /// RecedeCycle - This callback is invoked whenever the next bottom-up
107  /// instruction to be scheduled cannot issue in the current cycle, either
108  /// because of latency or resource conflicts.
109  virtual void RecedeCycle() {}
110 
111  /// EmitNoop - This callback is invoked when a noop was added to the
112  /// instruction stream.
113  virtual void EmitNoop() {
114  // Default implementation: count it as a cycle.
115  AdvanceCycle();
116  }
117 };
118 
119 } // end namespace llvm
120 
121 #endif // LLVM_CODEGEN_SCHEDULEHAZARDRECOGNIZER_H
virtual void EmitInstruction(MachineInstr *)
This overload will be used when the hazard recognizer is being used by a non-scheduling pass...
This class represents lattice values for constants.
Definition: AllocatorList.h:23
virtual void AdvanceCycle()
AdvanceCycle - This callback is invoked whenever the next top-down instruction to be scheduled cannot...
virtual void RecedeCycle()
RecedeCycle - This callback is invoked whenever the next bottom-up instruction to be scheduled cannot...
unsigned MaxLookAhead
MaxLookAhead - Indicate the number of cycles in the scoreboard state.
HazardRecognizer - This determines whether or not an instruction can be issued this cycle...
virtual unsigned PreEmitNoops(MachineInstr *)
This overload will be used when the hazard recognizer is being used by a non-scheduling pass...
virtual bool ShouldPreferAnother(SUnit *)
ShouldPreferAnother - This callback may be invoked if getHazardType returns NoHazard.
virtual void EmitNoop()
EmitNoop - This callback is invoked when a noop was added to the instruction stream.
virtual void EmitInstruction(SUnit *)
EmitInstruction - This callback is invoked when an instruction is emitted, to advance the hazard stat...
virtual void Reset()
Reset - This callback is invoked when a new block of instructions is about to be schedule.
virtual unsigned PreEmitNoops(SUnit *)
PreEmitNoops - This callback is invoked prior to emitting an instruction.
Representation of each machine instruction.
Definition: MachineInstr.h:63
virtual HazardType getHazardType(SUnit *m, int Stalls=0)
getHazardType - Return the hazard type of emitting this node.
Scheduling unit. This is a node in the scheduling DAG.
Definition: ScheduleDAG.h:242
virtual bool atIssueLimit() const
atIssueLimit - Return true if no more instructions may be issued in this cycle.