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"
15 #include "llvm/CodeGen/MachineFunctionPass.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;
ARMOptimizeBarriersPass()26 ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
27
28 bool runOnMachineFunction(MachineFunction &Fn) override;
29
getRequiredProperties() const30 MachineFunctionProperties getRequiredProperties() const override {
31 return MachineFunctionProperties().set(
32 MachineFunctionProperties::Property::AllVRegsAllocated);
33 }
34
getPassName() const35 const char *getPassName() const override {
36 return "optimise barriers pass";
37 }
38 };
39 char ARMOptimizeBarriersPass::ID = 0;
40 }
41
42 // Returns whether the instruction can safely move past a DMB instruction
43 // The current implementation allows this iif MI does not have any possible
44 // memory access
CanMovePastDMB(const MachineInstr * MI)45 static bool CanMovePastDMB(const MachineInstr *MI) {
46 return !(MI->mayLoad() ||
47 MI->mayStore() ||
48 MI->hasUnmodeledSideEffects() ||
49 MI->isCall() ||
50 MI->isReturn());
51 }
52
runOnMachineFunction(MachineFunction & MF)53 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
54 if (skipFunction(*MF.getFunction()))
55 return false;
56
57 // Vector to store the DMBs we will remove after the first iteration
58 std::vector<MachineInstr *> ToRemove;
59 // DMBType is the Imm value of the first operand. It determines whether it's a
60 // DMB ish, dmb sy, dmb osh, etc
61 int64_t DMBType = -1;
62
63 // Find a dmb. If we can move it until the next dmb, tag the second one for
64 // removal
65 for (auto &MBB : MF) {
66 // Will be true when we have seen a DMB, and not seen any instruction since
67 // that cannot move past a DMB
68 bool IsRemovableNextDMB = false;
69 for (auto &MI : MBB) {
70 if (MI.getOpcode() == ARM::DMB) {
71 if (IsRemovableNextDMB) {
72 // If the Imm of this DMB is the same as that of the last DMB, we can
73 // tag this second DMB for removal
74 if (MI.getOperand(0).getImm() == DMBType) {
75 ToRemove.push_back(&MI);
76 } else {
77 // If it has a different DMBType, we cannot remove it, but will scan
78 // for the next DMB, recording this DMB's type as last seen DMB type
79 DMBType = MI.getOperand(0).getImm();
80 }
81 } else {
82 // After we see a DMB, a next one is removable
83 IsRemovableNextDMB = true;
84 DMBType = MI.getOperand(0).getImm();
85 }
86 } else if (!CanMovePastDMB(&MI)) {
87 // If we find an instruction unable to pass past a DMB, a next DMB is
88 // not removable
89 IsRemovableNextDMB = false;
90 }
91 }
92 }
93 // Remove the tagged DMB
94 for (auto MI : ToRemove) {
95 MI->eraseFromParent();
96 ++NumDMBsRemoved;
97 }
98
99 return NumDMBsRemoved > 0;
100 }
101
102 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
103 /// barriers
104 /// pass.
createARMOptimizeBarriersPass()105 FunctionPass *llvm::createARMOptimizeBarriersPass() {
106 return new ARMOptimizeBarriersPass();
107 }
108