LLVM  3.7.0
ARMOptimizeBarriersPass.cpp
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1 //===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between,
2 //removed one -===//
3 //
4 // The LLVM Compiler Infrastructure
5 //
6 // This file is distributed under the University of Illinois Open Source
7 // License. See LICENSE.TXT for details.
8 //
9 //===------------------------------------------------------------------------------------------===//
10 
11 #include "ARM.h"
12 #include "ARMInstrInfo.h"
13 #include "ARMMachineFunctionInfo.h"
14 #include "llvm/ADT/Statistic.h"
16 using namespace llvm;
17 
18 #define DEBUG_TYPE "double barriers"
19 
20 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
21 
22 namespace {
23 class ARMOptimizeBarriersPass : public MachineFunctionPass {
24 public:
25  static char ID;
26  ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
27 
28  bool runOnMachineFunction(MachineFunction &Fn) override;
29 
30  const char *getPassName() const override {
31  return "optimise barriers pass";
32  }
33 };
35 }
36 
37 // Returns whether the instruction can safely move past a DMB instruction
38 // The current implementation allows this iif MI does not have any possible
39 // memory access
40 static bool CanMovePastDMB(const MachineInstr *MI) {
41  return !(MI->mayLoad() ||
42  MI->mayStore() ||
44  MI->isCall() ||
45  MI->isReturn());
46 }
47 
48 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
49  // Vector to store the DMBs we will remove after the first iteration
50  std::vector<MachineInstr *> ToRemove;
51  // DMBType is the Imm value of the first operand. It determines whether it's a
52  // DMB ish, dmb sy, dmb osh, etc
53  int64_t DMBType = -1;
54 
55  // Find a dmb. If we can move it until the next dmb, tag the second one for
56  // removal
57  for (auto &MBB : MF) {
58  // Will be true when we have seen a DMB, and not seen any instruction since
59  // that cannot move past a DMB
60  bool IsRemovableNextDMB = false;
61  for (auto &MI : MBB) {
62  if (MI.getOpcode() == ARM::DMB) {
63  if (IsRemovableNextDMB) {
64  // If the Imm of this DMB is the same as that of the last DMB, we can
65  // tag this second DMB for removal
66  if (MI.getOperand(0).getImm() == DMBType) {
67  ToRemove.push_back(&MI);
68  } else {
69  // If it has a different DMBType, we cannot remove it, but will scan
70  // for the next DMB, recording this DMB's type as last seen DMB type
71  DMBType = MI.getOperand(0).getImm();
72  }
73  } else {
74  // After we see a DMB, a next one is removable
75  IsRemovableNextDMB = true;
76  DMBType = MI.getOperand(0).getImm();
77  }
78  } else if (!CanMovePastDMB(&MI)) {
79  // If we find an instruction unable to pass past a DMB, a next DMB is
80  // not removable
81  IsRemovableNextDMB = false;
82  }
83  }
84  }
85  // Remove the tagged DMB
86  for (auto MI : ToRemove) {
87  MI->eraseFromParent();
88  ++NumDMBsRemoved;
89  }
90 
91  return NumDMBsRemoved > 0;
92 }
93 
94 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
95 /// barriers
96 /// pass.
98  return new ARMOptimizeBarriersPass();
99 }
STATISTIC(NumFunctions,"Total number of functions")
bool mayStore(QueryType Type=AnyInBundle) const
Return true if this instruction could possibly modify memory.
Definition: MachineInstr.h:579
FunctionPass * createARMOptimizeBarriersPass()
createARMOptimizeBarriersPass - Returns an instance of the remove double barriers pass...
static bool CanMovePastDMB(const MachineInstr *MI)
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
bool mayLoad(QueryType Type=AnyInBundle) const
Return true if this instruction could possibly read memory.
Definition: MachineInstr.h:566
bool isReturn(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:399
bool hasUnmodeledSideEffects() const
Return true if this instruction has side effects that are not modeled by mayLoad / mayStore...
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:294
Representation of each machine instruction.
Definition: MachineInstr.h:51
bool isCall(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:403