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" 14 using namespace llvm; 15 16 #define DEBUG_TYPE "double barriers" 17 18 STATISTIC(NumDMBsRemoved, "Number of DMBs removed"); 19 20 namespace { 21 class ARMOptimizeBarriersPass : public MachineFunctionPass { 22 public: 23 static char ID; 24 ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {} 25 26 bool runOnMachineFunction(MachineFunction &Fn) override; 27 28 MachineFunctionProperties getRequiredProperties() const override { 29 return MachineFunctionProperties().set( 30 MachineFunctionProperties::Property::NoVRegs); 31 } 32 33 StringRef getPassName() const override { return "optimise barriers pass"; } 34 }; 35 char ARMOptimizeBarriersPass::ID = 0; 36 } 37 38 // Returns whether the instruction can safely move past a DMB instruction 39 // The current implementation allows this iif MI does not have any possible 40 // memory access 41 static bool CanMovePastDMB(const MachineInstr *MI) { 42 return !(MI->mayLoad() || 43 MI->mayStore() || 44 MI->hasUnmodeledSideEffects() || 45 MI->isCall() || 46 MI->isReturn()); 47 } 48 49 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) { 50 if (skipFunction(MF.getFunction())) 51 return false; 52 53 // Vector to store the DMBs we will remove after the first iteration 54 std::vector<MachineInstr *> ToRemove; 55 // DMBType is the Imm value of the first operand. It determines whether it's a 56 // DMB ish, dmb sy, dmb osh, etc 57 int64_t DMBType = -1; 58 59 // Find a dmb. If we can move it until the next dmb, tag the second one for 60 // removal 61 for (auto &MBB : MF) { 62 // Will be true when we have seen a DMB, and not seen any instruction since 63 // that cannot move past a DMB 64 bool IsRemovableNextDMB = false; 65 for (auto &MI : MBB) { 66 if (MI.getOpcode() == ARM::DMB) { 67 if (IsRemovableNextDMB) { 68 // If the Imm of this DMB is the same as that of the last DMB, we can 69 // tag this second DMB for removal 70 if (MI.getOperand(0).getImm() == DMBType) { 71 ToRemove.push_back(&MI); 72 } else { 73 // If it has a different DMBType, we cannot remove it, but will scan 74 // for the next DMB, recording this DMB's type as last seen DMB type 75 DMBType = MI.getOperand(0).getImm(); 76 } 77 } else { 78 // After we see a DMB, a next one is removable 79 IsRemovableNextDMB = true; 80 DMBType = MI.getOperand(0).getImm(); 81 } 82 } else if (!CanMovePastDMB(&MI)) { 83 // If we find an instruction unable to pass past a DMB, a next DMB is 84 // not removable 85 IsRemovableNextDMB = false; 86 } 87 } 88 } 89 bool Changed = false; 90 // Remove the tagged DMB 91 for (auto *MI : ToRemove) { 92 MI->eraseFromParent(); 93 ++NumDMBsRemoved; 94 Changed = true; 95 } 96 97 return Changed; 98 } 99 100 /// createARMOptimizeBarriersPass - Returns an instance of the remove double 101 /// barriers 102 /// pass. 103 FunctionPass *llvm::createARMOptimizeBarriersPass() { 104 return new ARMOptimizeBarriersPass(); 105 } 106