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 "llvm/ADT/Statistic.h"
13#include "llvm/CodeGen/MachineFunctionPass.h"
14using namespace llvm;
15
16#define DEBUG_TYPE "double barriers"
17
18STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
19
20namespace {
21class ARMOptimizeBarriersPass : public MachineFunctionPass {
22public:
23 static char ID;
24 ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
25
26 bool runOnMachineFunction(MachineFunction &Fn) override;
27
28 MachineFunctionProperties getRequiredProperties() const override {
29 return MachineFunctionProperties().setNoVRegs();
30 }
31
32 StringRef getPassName() const override { return "optimise barriers pass"; }
33};
34char ARMOptimizeBarriersPass::ID = 0;
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
40static bool CanMovePastDMB(const MachineInstr *MI) {
41 return !(MI->mayLoad() ||
42 MI->mayStore() ||
43 MI->hasUnmodeledSideEffects() ||
44 MI->isCall() ||
45 MI->isReturn());
46}
47
48bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
49 if (skipFunction(F: MF.getFunction()))
50 return false;
51
52 // Vector to store the DMBs we will remove after the first iteration
53 std::vector<MachineInstr *> ToRemove;
54 // DMBType is the Imm value of the first operand. It determines whether it's a
55 // DMB ish, dmb sy, dmb osh, etc
56 int64_t DMBType = -1;
57
58 // Find a dmb. If we can move it until the next dmb, tag the second one for
59 // removal
60 for (auto &MBB : MF) {
61 // Will be true when we have seen a DMB, and not seen any instruction since
62 // that cannot move past a DMB
63 bool IsRemovableNextDMB = false;
64 for (auto &MI : MBB) {
65 if (MI.getOpcode() == ARM::DMB) {
66 if (IsRemovableNextDMB) {
67 // If the Imm of this DMB is the same as that of the last DMB, we can
68 // tag this second DMB for removal
69 if (MI.getOperand(i: 0).getImm() == DMBType) {
70 ToRemove.push_back(x: &MI);
71 } else {
72 // If it has a different DMBType, we cannot remove it, but will scan
73 // for the next DMB, recording this DMB's type as last seen DMB type
74 DMBType = MI.getOperand(i: 0).getImm();
75 }
76 } else {
77 // After we see a DMB, a next one is removable
78 IsRemovableNextDMB = true;
79 DMBType = MI.getOperand(i: 0).getImm();
80 }
81 } else if (!CanMovePastDMB(MI: &MI)) {
82 // If we find an instruction unable to pass past a DMB, a next DMB is
83 // not removable
84 IsRemovableNextDMB = false;
85 }
86 }
87 }
88 bool Changed = false;
89 // Remove the tagged DMB
90 for (auto *MI : ToRemove) {
91 MI->eraseFromParent();
92 ++NumDMBsRemoved;
93 Changed = true;
94 }
95
96 return Changed;
97}
98
99/// createARMOptimizeBarriersPass - Returns an instance of the remove double
100/// barriers
101/// pass.
102FunctionPass *llvm::createARMOptimizeBarriersPass() {
103 return new ARMOptimizeBarriersPass();
104}
105