//===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between, //removed one -===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===------------------------------------------------------------------------------------------===// #include "ARM.h" #include "ARMMachineFunctionInfo.h" #include "ARMInstrInfo.h" #include "llvm/ADT/Statistic.h" #include "llvm/CodeGen/MachineFunctionPass.h" using namespace llvm; #define DEBUG_TYPE "double barriers" STATISTIC(NumDMBsRemoved, "Number of DMBs removed"); namespace { class ARMOptimizeBarriersPass : public MachineFunctionPass { public: static char ID; ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {} bool runOnMachineFunction(MachineFunction &Fn) override; const char *getPassName() const override { return "optimise barriers pass"; } private: }; char ARMOptimizeBarriersPass::ID = 0; } // Returns whether the instruction can safely move past a DMB instruction // The current implementation allows this iif MI does not have any possible // memory access static bool CanMovePastDMB(const MachineInstr *MI) { return !(MI->mayLoad() || MI->mayStore() || MI->hasUnmodeledSideEffects() || MI->isCall() || MI->isReturn()); } bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) { // Vector to store the DMBs we will remove after the first iteration std::vector<MachineInstr *> ToRemove; // DMBType is the Imm value of the first operand. It determines whether it's a // DMB ish, dmb sy, dmb osh, etc int64_t DMBType = -1; // Find a dmb. If we can move it until the next dmb, tag the second one for // removal for (auto &MBB : MF) { // Will be true when we have seen a DMB, and not seen any instruction since // that cannot move past a DMB bool IsRemovableNextDMB = false; for (auto &MI : MBB) { if (MI.getOpcode() == ARM::DMB) { if (IsRemovableNextDMB) { // If the Imm of this DMB is the same as that of the last DMB, we can // tag this second DMB for removal if (MI.getOperand(0).getImm() == DMBType) { ToRemove.push_back(&MI); } else { // If it has a different DMBType, we cannot remove it, but will scan // for the next DMB, recording this DMB's type as last seen DMB type DMBType = MI.getOperand(0).getImm(); } } else { // After we see a DMB, a next one is removable IsRemovableNextDMB = true; DMBType = MI.getOperand(0).getImm(); } } else if (!CanMovePastDMB(&MI)) { // If we find an instruction unable to pass past a DMB, a next DMB is // not removable IsRemovableNextDMB = false; } } } // Remove the tagged DMB for (auto MI : ToRemove) { MI->eraseFromParent(); ++NumDMBsRemoved; } return NumDMBsRemoved > 0; } /// createARMOptimizeBarriersPass - Returns an instance of the remove double /// barriers /// pass. FunctionPass *llvm::createARMOptimizeBarriersPass() { return new ARMOptimizeBarriersPass(); }