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; 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 }; 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 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 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. 104 FunctionPass *llvm::createARMOptimizeBarriersPass() { 105 return new ARMOptimizeBarriersPass(); 106 } 107