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().setNoVRegs(); 30 } 31 32 StringRef getPassName() const override { return "optimise barriers pass"; } 33 }; 34 char 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 40 static bool CanMovePastDMB(const MachineInstr *MI) { 41 return !(MI->mayLoad() || 42 MI->mayStore() || 43 MI->hasUnmodeledSideEffects() || 44 MI->isCall() || 45 MI->isReturn()); 46 } 47 48 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) { 49 if (skipFunction(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(0).getImm() == DMBType) { 70 ToRemove.push_back(&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(0).getImm(); 75 } 76 } else { 77 // After we see a DMB, a next one is removable 78 IsRemovableNextDMB = true; 79 DMBType = MI.getOperand(0).getImm(); 80 } 81 } else if (!CanMovePastDMB(&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. 102 FunctionPass *llvm::createARMOptimizeBarriersPass() { 103 return new ARMOptimizeBarriersPass(); 104 } 105