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//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This is an extremely simple MachineInstr-level dead-code-elimination pass.
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "dead-mi-elimination"
STATISTIC(NumDeletes, "Number of dead instructions deleted");
namespace {
class DeadMachineInstructionElim : public MachineFunctionPass {
bool runOnMachineFunction(MachineFunction &MF) override;
const TargetRegisterInfo *TRI;
const MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
BitVector LivePhysRegs;
static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim() : MachineFunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool isDead(const MachineInstr *MI) const;
char DeadMachineInstructionElim::ID = 0;
char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
"Remove dead machine instructions", false, false)
bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
// Technically speaking inline asm without side effects and no defs can still
// be deleted. But there is so much bad inline asm code out there, we should
// let them be.
if (MI->isInlineAsm())
return false;
// Don't delete frame allocation labels.
if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
return false;
// Don't delete instructions with side effects.
bool SawStore = false;
if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
return false;
// Examine each operand.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
// Don't delete live physreg defs, or any reserved register defs.
if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg))
return false;
} else {
if (!MRI->use_nodbg_empty(Reg))
// This def has a non-debug use. Don't delete the instruction!
return false;
// If there are no defs with uses, the instruction is dead.
return true;
bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
bool AnyChanges = false;
MRI = &MF.getRegInfo();
TRI = MF.getSubtarget().getRegisterInfo();
TII = MF.getSubtarget().getInstrInfo();
// Loop over all instructions in all blocks, from bottom to top, so that it's
// more likely that chains of dependent but ultimately dead instructions will
// be cleaned up.
for (MachineBasicBlock &MBB : make_range(MF.rbegin(), MF.rend())) {
// Start out assuming that reserved registers are live out of this block.
LivePhysRegs = MRI->getReservedRegs();
// Add live-ins from successors to LivePhysRegs. Normally, physregs are not
// live across blocks, but some targets (x86) can have flags live out of a
// block.
for (MachineBasicBlock::succ_iterator S = MBB.succ_begin(),
E = MBB.succ_end(); S != E; S++)
for (const auto &LI : (*S)->liveins())
// Now scan the instructions and delete dead ones, tracking physreg
// liveness as we go.
for (MachineBasicBlock::reverse_iterator MII = MBB.rbegin(),
MIE = MBB.rend(); MII != MIE; ) {
MachineInstr *MI = &*MII++;
// If the instruction is dead, delete it!
if (isDead(MI)) {
LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI);
// It is possible that some DBG_VALUE instructions refer to this
// instruction. They get marked as undef and will be deleted
// in the live debug variable analysis.
AnyChanges = true;
// Record the physreg defs.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
// Check the subreg set, not the alias set, because a def
// of a super-register may still be partially live after
// this def.
for (MCSubRegIterator SR(Reg, TRI,/*IncludeSelf=*/true);
SR.isValid(); ++SR)
} else if (MO.isRegMask()) {
// Register mask of preserved registers. All clobbers are dead.
// Record the physreg uses, after the defs, in case a physreg is
// both defined and used in the same instruction.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isUse()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
return AnyChanges;