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"
14 #include "llvm/CodeGen/MachineFunctionPass.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;
ARMOptimizeBarriersPass()25 ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
26
27 bool runOnMachineFunction(MachineFunction &Fn) override;
28
getRequiredProperties() const29 MachineFunctionProperties getRequiredProperties() const override {
30 return MachineFunctionProperties().set(
31 MachineFunctionProperties::Property::NoVRegs);
32 }
33
getPassName() const34 StringRef getPassName() const override { return "optimise barriers pass"; }
35 };
36 char ARMOptimizeBarriersPass::ID = 0;
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
CanMovePastDMB(const MachineInstr * MI)42 static bool CanMovePastDMB(const MachineInstr *MI) {
43 return !(MI->mayLoad() ||
44 MI->mayStore() ||
45 MI->hasUnmodeledSideEffects() ||
46 MI->isCall() ||
47 MI->isReturn());
48 }
49
runOnMachineFunction(MachineFunction & MF)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.
createARMOptimizeBarriersPass()104 FunctionPass *llvm::createARMOptimizeBarriersPass() {
105 return new ARMOptimizeBarriersPass();
106 }
107