DeadMachineInstructionElim.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f
1//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This is an extremely simple MachineInstr-level dead-code-elimination pass. 11// 12//===----------------------------------------------------------------------===// 13 14#include "llvm/CodeGen/Passes.h" 15#include "llvm/ADT/Statistic.h" 16#include "llvm/CodeGen/MachineFunctionPass.h" 17#include "llvm/CodeGen/MachineRegisterInfo.h" 18#include "llvm/Pass.h" 19#include "llvm/Support/Debug.h" 20#include "llvm/Support/raw_ostream.h" 21#include "llvm/Target/TargetInstrInfo.h" 22#include "llvm/Target/TargetMachine.h" 23using namespace llvm; 24 25#define DEBUG_TYPE "codegen-dce" 26 27STATISTIC(NumDeletes, "Number of dead instructions deleted"); 28 29namespace { 30 class DeadMachineInstructionElim : public MachineFunctionPass { 31 bool runOnMachineFunction(MachineFunction &MF) override; 32 33 const TargetRegisterInfo *TRI; 34 const MachineRegisterInfo *MRI; 35 const TargetInstrInfo *TII; 36 BitVector LivePhysRegs; 37 38 public: 39 static char ID; // Pass identification, replacement for typeid 40 DeadMachineInstructionElim() : MachineFunctionPass(ID) { 41 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry()); 42 } 43 44 private: 45 bool isDead(const MachineInstr *MI) const; 46 }; 47} 48char DeadMachineInstructionElim::ID = 0; 49char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID; 50 51INITIALIZE_PASS(DeadMachineInstructionElim, "dead-mi-elimination", 52 "Remove dead machine instructions", false, false) 53 54bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const { 55 // Technically speaking inline asm without side effects and no defs can still 56 // be deleted. But there is so much bad inline asm code out there, we should 57 // let them be. 58 if (MI->isInlineAsm()) 59 return false; 60 61 // Don't delete instructions with side effects. 62 bool SawStore = false; 63 if (!MI->isSafeToMove(TII, nullptr, SawStore) && !MI->isPHI()) 64 return false; 65 66 // Examine each operand. 67 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 68 const MachineOperand &MO = MI->getOperand(i); 69 if (MO.isReg() && MO.isDef()) { 70 unsigned Reg = MO.getReg(); 71 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 72 // Don't delete live physreg defs, or any reserved register defs. 73 if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg)) 74 return false; 75 } else { 76 if (!MRI->use_nodbg_empty(Reg)) 77 // This def has a non-debug use. Don't delete the instruction! 78 return false; 79 } 80 } 81 } 82 83 // If there are no defs with uses, the instruction is dead. 84 return true; 85} 86 87bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) { 88 if (skipOptnoneFunction(*MF.getFunction())) 89 return false; 90 91 bool AnyChanges = false; 92 MRI = &MF.getRegInfo(); 93 TRI = MF.getTarget().getRegisterInfo(); 94 TII = MF.getTarget().getInstrInfo(); 95 96 // Loop over all instructions in all blocks, from bottom to top, so that it's 97 // more likely that chains of dependent but ultimately dead instructions will 98 // be cleaned up. 99 for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend(); 100 I != E; ++I) { 101 MachineBasicBlock *MBB = &*I; 102 103 // Start out assuming that reserved registers are live out of this block. 104 LivePhysRegs = MRI->getReservedRegs(); 105 106 // Add live-ins from sucessors to LivePhysRegs. Normally, physregs are not 107 // live across blocks, but some targets (x86) can have flags live out of a 108 // block. 109 for (MachineBasicBlock::succ_iterator S = MBB->succ_begin(), 110 E = MBB->succ_end(); S != E; S++) 111 for (MachineBasicBlock::livein_iterator LI = (*S)->livein_begin(); 112 LI != (*S)->livein_end(); LI++) 113 LivePhysRegs.set(*LI); 114 115 // Now scan the instructions and delete dead ones, tracking physreg 116 // liveness as we go. 117 for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(), 118 MIE = MBB->rend(); MII != MIE; ) { 119 MachineInstr *MI = &*MII; 120 121 // If the instruction is dead, delete it! 122 if (isDead(MI)) { 123 DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI); 124 // It is possible that some DBG_VALUE instructions refer to this 125 // instruction. Examine each def operand for such references; 126 // if found, mark the DBG_VALUE as undef (but don't delete it). 127 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 128 const MachineOperand &MO = MI->getOperand(i); 129 if (!MO.isReg() || !MO.isDef()) 130 continue; 131 unsigned Reg = MO.getReg(); 132 if (!TargetRegisterInfo::isVirtualRegister(Reg)) 133 continue; 134 MRI->markUsesInDebugValueAsUndef(Reg); 135 } 136 AnyChanges = true; 137 MI->eraseFromParent(); 138 ++NumDeletes; 139 MIE = MBB->rend(); 140 // MII is now pointing to the next instruction to process, 141 // so don't increment it. 142 continue; 143 } 144 145 // Record the physreg defs. 146 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 147 const MachineOperand &MO = MI->getOperand(i); 148 if (MO.isReg() && MO.isDef()) { 149 unsigned Reg = MO.getReg(); 150 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 151 // Check the subreg set, not the alias set, because a def 152 // of a super-register may still be partially live after 153 // this def. 154 for (MCSubRegIterator SR(Reg, TRI,/*IncludeSelf=*/true); 155 SR.isValid(); ++SR) 156 LivePhysRegs.reset(*SR); 157 } 158 } else if (MO.isRegMask()) { 159 // Register mask of preserved registers. All clobbers are dead. 160 LivePhysRegs.clearBitsNotInMask(MO.getRegMask()); 161 } 162 } 163 // Record the physreg uses, after the defs, in case a physreg is 164 // both defined and used in the same instruction. 165 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 166 const MachineOperand &MO = MI->getOperand(i); 167 if (MO.isReg() && MO.isUse()) { 168 unsigned Reg = MO.getReg(); 169 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 170 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) 171 LivePhysRegs.set(*AI); 172 } 173 } 174 } 175 176 // We didn't delete the current instruction, so increment MII to 177 // the next one. 178 ++MII; 179 } 180 } 181 182 LivePhysRegs.clear(); 183 return AnyChanges; 184} 185