LLVM  9.0.0svn
ARMOptimizeBarriersPass.cpp
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1 //===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between,
2 //removed one -===//
3 //
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //
8 //===------------------------------------------------------------------------------------------===//
9 
10 #include "ARM.h"
11 #include "ARMInstrInfo.h"
12 #include "ARMMachineFunctionInfo.h"
13 #include "llvm/ADT/Statistic.h"
15 using namespace llvm;
16 
17 #define DEBUG_TYPE "double barriers"
18 
19 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
20 
21 namespace {
22 class ARMOptimizeBarriersPass : public MachineFunctionPass {
23 public:
24  static char ID;
25  ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
26 
27  bool runOnMachineFunction(MachineFunction &Fn) override;
28 
29  MachineFunctionProperties getRequiredProperties() const override {
32  }
33 
34  StringRef getPassName() const override { return "optimise barriers pass"; }
35 };
37 }
38 
39 // Returns whether the instruction can safely move past a DMB instruction
40 // The current implementation allows this iif MI does not have any possible
41 // memory access
42 static bool CanMovePastDMB(const MachineInstr *MI) {
43  return !(MI->mayLoad() ||
44  MI->mayStore() ||
46  MI->isCall() ||
47  MI->isReturn());
48 }
49 
50 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
51  if (skipFunction(MF.getFunction()))
52  return false;
53 
54  // Vector to store the DMBs we will remove after the first iteration
55  std::vector<MachineInstr *> ToRemove;
56  // DMBType is the Imm value of the first operand. It determines whether it's a
57  // DMB ish, dmb sy, dmb osh, etc
58  int64_t DMBType = -1;
59 
60  // Find a dmb. If we can move it until the next dmb, tag the second one for
61  // removal
62  for (auto &MBB : MF) {
63  // Will be true when we have seen a DMB, and not seen any instruction since
64  // that cannot move past a DMB
65  bool IsRemovableNextDMB = false;
66  for (auto &MI : MBB) {
67  if (MI.getOpcode() == ARM::DMB) {
68  if (IsRemovableNextDMB) {
69  // If the Imm of this DMB is the same as that of the last DMB, we can
70  // tag this second DMB for removal
71  if (MI.getOperand(0).getImm() == DMBType) {
72  ToRemove.push_back(&MI);
73  } else {
74  // If it has a different DMBType, we cannot remove it, but will scan
75  // for the next DMB, recording this DMB's type as last seen DMB type
76  DMBType = MI.getOperand(0).getImm();
77  }
78  } else {
79  // After we see a DMB, a next one is removable
80  IsRemovableNextDMB = true;
81  DMBType = MI.getOperand(0).getImm();
82  }
83  } else if (!CanMovePastDMB(&MI)) {
84  // If we find an instruction unable to pass past a DMB, a next DMB is
85  // not removable
86  IsRemovableNextDMB = false;
87  }
88  }
89  }
90  bool Changed = false;
91  // Remove the tagged DMB
92  for (auto MI : ToRemove) {
93  MI->eraseFromParent();
94  ++NumDMBsRemoved;
95  Changed = true;
96  }
97 
98  return Changed;
99 }
100 
101 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
102 /// barriers
103 /// pass.
105  return new ARMOptimizeBarriersPass();
106 }
bool isCall(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:634
This class represents lattice values for constants.
Definition: AllocatorList.h:23
FunctionPass * createARMOptimizeBarriersPass()
createARMOptimizeBarriersPass - Returns an instance of the remove double barriers pass...
STATISTIC(NumFunctions, "Total number of functions")
static bool CanMovePastDMB(const MachineInstr *MI)
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
bool isReturn(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:624
bool mayStore(QueryType Type=AnyInBundle) const
Return true if this instruction could possibly modify memory.
Definition: MachineInstr.h:821
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:284
const Function & getFunction() const
Return the LLVM function that this machine code represents.
MachineFunctionProperties & set(Property P)
Representation of each machine instruction.
Definition: MachineInstr.h:63
bool mayLoad(QueryType Type=AnyInBundle) const
Return true if this instruction could possibly read memory.
Definition: MachineInstr.h:808
bool hasUnmodeledSideEffects() const
Return true if this instruction has side effects that are not modeled by mayLoad / mayStore...
IRTranslator LLVM IR MI
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:48
Properties which a MachineFunction may have at a given point in time.