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