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//==-- AArch64DeadRegisterDefinitions.cpp - Replace dead defs w/ zero reg --==//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file When allowed by the instruction, replace a dead definition of a GPR
/// with the zero register. This makes the code a bit friendlier towards the
/// hardware's register renamer.
#include "AArch64.h"
#include "AArch64RegisterInfo.h"
#include "AArch64Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/ISDOpcodes.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "aarch64-dead-defs"
STATISTIC(NumDeadDefsReplaced, "Number of dead definitions replaced");
#define AARCH64_DEAD_REG_DEF_NAME "AArch64 Dead register definitions"
namespace {
class AArch64DeadRegisterDefinitions : public MachineFunctionPass {
const TargetRegisterInfo *TRI;
const MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
bool Changed;
void processMachineBasicBlock(MachineBasicBlock &MBB);
static char ID; // Pass identification, replacement for typeid.
AArch64DeadRegisterDefinitions() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &F) override;
StringRef getPassName() const override { return AARCH64_DEAD_REG_DEF_NAME; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
char AArch64DeadRegisterDefinitions::ID = 0;
} // end anonymous namespace
INITIALIZE_PASS(AArch64DeadRegisterDefinitions, "aarch64-dead-defs",
AARCH64_DEAD_REG_DEF_NAME, false, false)
static bool usesFrameIndex(const MachineInstr &MI) {
for (const MachineOperand &MO : MI.uses())
if (MO.isFI())
return true;
return false;
// Instructions that lose their 'read' operation for a subesquent fence acquire
// (DMB LD) once the zero register is used.
// WARNING: The aquire variants of the instructions are also affected, but they
// are split out into `atomicBarrierDroppedOnZero()` to support annotations on
// assembly.
static bool atomicReadDroppedOnZero(unsigned Opcode) {
switch (Opcode) {
case AArch64::LDADDB: case AArch64::LDADDH:
case AArch64::LDADDW: case AArch64::LDADDX:
case AArch64::LDADDLB: case AArch64::LDADDLH:
case AArch64::LDADDLW: case AArch64::LDADDLX:
case AArch64::LDCLRB: case AArch64::LDCLRH:
case AArch64::LDCLRW: case AArch64::LDCLRX:
case AArch64::LDCLRLB: case AArch64::LDCLRLH:
case AArch64::LDCLRLW: case AArch64::LDCLRLX:
case AArch64::LDEORB: case AArch64::LDEORH:
case AArch64::LDEORW: case AArch64::LDEORX:
case AArch64::LDEORLB: case AArch64::LDEORLH:
case AArch64::LDEORLW: case AArch64::LDEORLX:
case AArch64::LDSETB: case AArch64::LDSETH:
case AArch64::LDSETW: case AArch64::LDSETX:
case AArch64::LDSETLB: case AArch64::LDSETLH:
case AArch64::LDSETLW: case AArch64::LDSETLX:
case AArch64::LDSMAXB: case AArch64::LDSMAXH:
case AArch64::LDSMAXW: case AArch64::LDSMAXX:
case AArch64::LDSMAXLB: case AArch64::LDSMAXLH:
case AArch64::LDSMAXLW: case AArch64::LDSMAXLX:
case AArch64::LDSMINB: case AArch64::LDSMINH:
case AArch64::LDSMINW: case AArch64::LDSMINX:
case AArch64::LDSMINLB: case AArch64::LDSMINLH:
case AArch64::LDSMINLW: case AArch64::LDSMINLX:
case AArch64::LDUMAXB: case AArch64::LDUMAXH:
case AArch64::LDUMAXW: case AArch64::LDUMAXX:
case AArch64::LDUMAXLB: case AArch64::LDUMAXLH:
case AArch64::LDUMAXLW: case AArch64::LDUMAXLX:
case AArch64::LDUMINB: case AArch64::LDUMINH:
case AArch64::LDUMINW: case AArch64::LDUMINX:
case AArch64::LDUMINLB: case AArch64::LDUMINLH:
case AArch64::LDUMINLW: case AArch64::LDUMINLX:
return true;
return false;
void AArch64DeadRegisterDefinitions::processMachineBasicBlock(
MachineBasicBlock &MBB) {
const MachineFunction &MF = *MBB.getParent();
for (MachineInstr &MI : MBB) {
if (usesFrameIndex(MI)) {
// We need to skip this instruction because while it appears to have a
// dead def it uses a frame index which might expand into a multi
// instruction sequence during EPI.
LLVM_DEBUG(dbgs() << " Ignoring, operand is frame index\n");
if (MI.definesRegister(AArch64::XZR) || MI.definesRegister(AArch64::WZR)) {
// It is not allowed to write to the same register (not even the zero
// register) twice in a single instruction.
<< " Ignoring, XZR or WZR already used by the instruction\n");
if (atomicBarrierDroppedOnZero(MI.getOpcode()) || atomicReadDroppedOnZero(MI.getOpcode())) {
LLVM_DEBUG(dbgs() << " Ignoring, semantics change with xzr/wzr.\n");
const MCInstrDesc &Desc = MI.getDesc();
for (int I = 0, E = Desc.getNumDefs(); I != E; ++I) {
MachineOperand &MO = MI.getOperand(I);
if (!MO.isReg() || !MO.isDef())
// We should not have any relevant physreg defs that are replacable by
// zero before register allocation. So we just check for dead vreg defs.
Register Reg = MO.getReg();
if (!Reg.isVirtual() || (!MO.isDead() && !MRI->use_nodbg_empty(Reg)))
assert(!MO.isImplicit() && "Unexpected implicit def!");
LLVM_DEBUG(dbgs() << " Dead def operand #" << I << " in:\n ";
// Be careful not to change the register if it's a tied operand.
if (MI.isRegTiedToUseOperand(I)) {
LLVM_DEBUG(dbgs() << " Ignoring, def is tied operand.\n");
const TargetRegisterClass *RC = TII->getRegClass(Desc, I, TRI, MF);
unsigned NewReg;
if (RC == nullptr) {
LLVM_DEBUG(dbgs() << " Ignoring, register is not a GPR.\n");
} else if (RC->contains(AArch64::WZR))
NewReg = AArch64::WZR;
else if (RC->contains(AArch64::XZR))
NewReg = AArch64::XZR;
else {
LLVM_DEBUG(dbgs() << " Ignoring, register is not a GPR.\n");
LLVM_DEBUG(dbgs() << " Replacing with zero register. New:\n ");
Changed = true;
// Only replace one dead register, see check for zero register above.
// Scan the function for instructions that have a dead definition of a
// register. Replace that register with the zero register when possible.
bool AArch64DeadRegisterDefinitions::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
TRI = MF.getSubtarget().getRegisterInfo();
TII = MF.getSubtarget().getInstrInfo();
MRI = &MF.getRegInfo();
LLVM_DEBUG(dbgs() << "***** AArch64DeadRegisterDefinitions *****\n");
Changed = false;
for (auto &MBB : MF)
return Changed;
FunctionPass *llvm::createAArch64DeadRegisterDefinitions() {
return new AArch64DeadRegisterDefinitions();