ARMOptimizeBarriersPass.cpp revision 6948897e478cbd66626159776a8017b3c18579b9 
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"
16using namespace llvm;
17
18#define DEBUG_TYPE "double barriers"
19
20STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
21
22namespace {
23class ARMOptimizeBarriersPass : public MachineFunctionPass {
24public:
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};
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  // 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.
97FunctionPass *llvm::createARMOptimizeBarriersPass() {
98  return new ARMOptimizeBarriersPass();
99}
100