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"
15using namespace llvm;
16
17#define DEBUG_TYPE "double barriers"
18
19STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
20
21namespace {
22class ARMOptimizeBarriersPass : public MachineFunctionPass {
23public:
24  static char ID;
25  ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
26
27  bool runOnMachineFunction(MachineFunction &Fn) override;
28
29  MachineFunctionProperties getRequiredProperties() const override {
30    return MachineFunctionProperties().set(
31        MachineFunctionProperties::Property::NoVRegs);
32  }
33
34  StringRef getPassName() const override { return "optimise barriers pass"; }
35};
36char 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
42static bool CanMovePastDMB(const MachineInstr *MI) {
43  return !(MI->mayLoad() ||
44          MI->mayStore() ||
45          MI->hasUnmodeledSideEffects() ||
46          MI->isCall() ||
47          MI->isReturn());
48}
49
50bool 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.
104FunctionPass *llvm::createARMOptimizeBarriersPass() {
105  return new ARMOptimizeBarriersPass();
106}
107