123 lines · cpp
1//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This is an extremely simple MachineInstr-level dead-code-elimination pass.10//11//===----------------------------------------------------------------------===//12 13#include "llvm/CodeGen/DeadMachineInstructionElim.h"14#include "llvm/ADT/PostOrderIterator.h"15#include "llvm/ADT/Statistic.h"16#include "llvm/CodeGen/LiveRegUnits.h"17#include "llvm/CodeGen/MachineFunctionPass.h"18#include "llvm/CodeGen/MachineRegisterInfo.h"19#include "llvm/CodeGen/TargetSubtargetInfo.h"20#include "llvm/InitializePasses.h"21#include "llvm/Pass.h"22#include "llvm/Support/Debug.h"23#include "llvm/Support/raw_ostream.h"24 25using namespace llvm;26 27#define DEBUG_TYPE "dead-mi-elimination"28 29STATISTIC(NumDeletes, "Number of dead instructions deleted");30 31namespace {32class DeadMachineInstructionElimImpl {33 const MachineRegisterInfo *MRI = nullptr;34 const TargetInstrInfo *TII = nullptr;35 LiveRegUnits LivePhysRegs;36 37public:38 bool runImpl(MachineFunction &MF);39 40private:41 bool eliminateDeadMI(MachineFunction &MF);42};43 44class DeadMachineInstructionElim : public MachineFunctionPass {45public:46 static char ID; // Pass identification, replacement for typeid47 48 DeadMachineInstructionElim() : MachineFunctionPass(ID) {49 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());50 }51 52 bool runOnMachineFunction(MachineFunction &MF) override {53 if (skipFunction(MF.getFunction()))54 return false;55 return DeadMachineInstructionElimImpl().runImpl(MF);56 }57 58 void getAnalysisUsage(AnalysisUsage &AU) const override {59 AU.setPreservesCFG();60 MachineFunctionPass::getAnalysisUsage(AU);61 }62};63} // namespace64 65PreservedAnalyses66DeadMachineInstructionElimPass::run(MachineFunction &MF,67 MachineFunctionAnalysisManager &) {68 if (!DeadMachineInstructionElimImpl().runImpl(MF))69 return PreservedAnalyses::all();70 PreservedAnalyses PA = getMachineFunctionPassPreservedAnalyses();71 PA.preserveSet<CFGAnalyses>();72 return PA;73}74 75char DeadMachineInstructionElim::ID = 0;76char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;77 78INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,79 "Remove dead machine instructions", false, false)80 81bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {82 MRI = &MF.getRegInfo();83 84 const TargetSubtargetInfo &ST = MF.getSubtarget();85 TII = ST.getInstrInfo();86 LivePhysRegs.init(*ST.getRegisterInfo());87 88 bool AnyChanges = eliminateDeadMI(MF);89 while (AnyChanges && eliminateDeadMI(MF))90 ;91 return AnyChanges;92}93 94bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {95 bool AnyChanges = false;96 97 // Loop over all instructions in all blocks, from bottom to top, so that it's98 // more likely that chains of dependent but ultimately dead instructions will99 // be cleaned up.100 for (MachineBasicBlock *MBB : post_order(&MF)) {101 LivePhysRegs.addLiveOuts(*MBB);102 103 // Now scan the instructions and delete dead ones, tracking physreg104 // liveness as we go.105 for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {106 // If the instruction is dead, delete it!107 if (MI.isDead(*MRI, &LivePhysRegs)) {108 LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);109 // It is possible that some DBG_VALUE instructions refer to this110 // instruction. They will be deleted in the live debug variable111 // analysis.112 MI.eraseFromParent();113 AnyChanges = true;114 ++NumDeletes;115 continue;116 }117 LivePhysRegs.stepBackward(MI);118 }119 }120 LivePhysRegs.clear();121 return AnyChanges;122}123