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 "ARMMachineFunctionInfo.h" 13 #include "ARMInstrInfo.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 private: 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 // Vector to store the DMBs we will remove after the first iteration 52 std::vector<MachineInstr *> ToRemove; 53 // DMBType is the Imm value of the first operand. It determines whether it's a 54 // DMB ish, dmb sy, dmb osh, etc 55 int64_t DMBType = -1; 56 57 // Find a dmb. If we can move it until the next dmb, tag the second one for 58 // removal 59 for (auto &MBB : MF) { 60 // Will be true when we have seen a DMB, and not seen any instruction since 61 // that cannot move past a DMB 62 bool IsRemovableNextDMB = false; 63 for (auto &MI : MBB) { 64 if (MI.getOpcode() == ARM::DMB) { 65 if (IsRemovableNextDMB) { 66 // If the Imm of this DMB is the same as that of the last DMB, we can 67 // tag this second DMB for removal 68 if (MI.getOperand(0).getImm() == DMBType) { 69 ToRemove.push_back(&MI); 70 } else { 71 // If it has a different DMBType, we cannot remove it, but will scan 72 // for the next DMB, recording this DMB's type as last seen DMB type 73 DMBType = MI.getOperand(0).getImm(); 74 } 75 } else { 76 // After we see a DMB, a next one is removable 77 IsRemovableNextDMB = true; 78 DMBType = MI.getOperand(0).getImm(); 79 } 80 } else if (!CanMovePastDMB(&MI)) { 81 // If we find an instruction unable to pass past a DMB, a next DMB is 82 // not removable 83 IsRemovableNextDMB = false; 84 } 85 } 86 } 87 // Remove the tagged DMB 88 for (auto MI : ToRemove) { 89 MI->eraseFromParent(); 90 ++NumDMBsRemoved; 91 } 92 93 return NumDMBsRemoved > 0; 94 } 95 96 /// createARMOptimizeBarriersPass - Returns an instance of the remove double 97 /// barriers 98 /// pass. 99 FunctionPass *llvm::createARMOptimizeBarriersPass() { 100 return new ARMOptimizeBarriersPass(); 101 } 102
