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//=- AArch64RedundantCopyElimination.cpp - Remove useless copy for AArch64 -=//
// 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
// This pass removes unnecessary copies/moves in BBs based on a dominating
// condition.
// We handle three cases:
// 1. For BBs that are targets of CBZ/CBNZ instructions, we know the value of
// the CBZ/CBNZ source register is zero on the taken/not-taken path. For
// instance, the copy instruction in the code below can be removed because
// the CBZW jumps to %bb.2 when w0 is zero.
// %bb.1:
// cbz w0, .LBB0_2
// .LBB0_2:
// mov w0, wzr ; <-- redundant
// 2. If the flag setting instruction defines a register other than WZR/XZR, we
// can remove a zero copy in some cases.
// %bb.0:
// subs w0, w1, w2
// str w0, [x1]
// .LBB0_2
// %bb.1:
// mov w0, wzr ; <-- redundant
// str w0, [x2]
// .LBB0_2
// 3. Finally, if the flag setting instruction is a comparison against a
// constant (i.e., ADDS[W|X]ri, SUBS[W|X]ri), we can remove a mov immediate
// in some cases.
// %bb.0:
// subs xzr, x0, #1
// b.eq .LBB0_1
// .LBB0_1:
// orr x0, xzr, #0x1 ; <-- redundant
// This pass should be run after register allocation.
// FIXME: This could also be extended to check the whole dominance subtree below
// the comparison if the compile time regression is acceptable.
// FIXME: Add support for handling CCMP instructions.
// FIXME: If the known register value is zero, we should be able to rewrite uses
// to use WZR/XZR directly in some cases.
#include "AArch64.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/CodeGen/LiveRegUnits.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "aarch64-copyelim"
STATISTIC(NumCopiesRemoved, "Number of copies removed.");
namespace {
class AArch64RedundantCopyElimination : public MachineFunctionPass {
const MachineRegisterInfo *MRI;
const TargetRegisterInfo *TRI;
// DomBBClobberedRegs is used when computing known values in the dominating
// BB.
LiveRegUnits DomBBClobberedRegs, DomBBUsedRegs;
// OptBBClobberedRegs is used when optimizing away redundant copies/moves.
LiveRegUnits OptBBClobberedRegs, OptBBUsedRegs;
static char ID;
AArch64RedundantCopyElimination() : MachineFunctionPass(ID) {
struct RegImm {
MCPhysReg Reg;
int32_t Imm;
RegImm(MCPhysReg Reg, int32_t Imm) : Reg(Reg), Imm(Imm) {}
bool knownRegValInBlock(MachineInstr &CondBr, MachineBasicBlock *MBB,
SmallVectorImpl<RegImm> &KnownRegs,
MachineBasicBlock::iterator &FirstUse);
bool optimizeBlock(MachineBasicBlock *MBB);
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
StringRef getPassName() const override {
return "AArch64 Redundant Copy Elimination";
char AArch64RedundantCopyElimination::ID = 0;
INITIALIZE_PASS(AArch64RedundantCopyElimination, "aarch64-copyelim",
"AArch64 redundant copy elimination pass", false, false)
/// It's possible to determine the value of a register based on a dominating
/// condition. To do so, this function checks to see if the basic block \p MBB
/// is the target of a conditional branch \p CondBr with an equality comparison.
/// If the branch is a CBZ/CBNZ, we know the value of its source operand is zero
/// in \p MBB for some cases. Otherwise, we find and inspect the NZCV setting
/// instruction (e.g., SUBS, ADDS). If this instruction defines a register
/// other than WZR/XZR, we know the value of the destination register is zero in
/// \p MMB for some cases. In addition, if the NZCV setting instruction is
/// comparing against a constant we know the other source register is equal to
/// the constant in \p MBB for some cases. If we find any constant values, push
/// a physical register and constant value pair onto the KnownRegs vector and
/// return true. Otherwise, return false if no known values were found.
bool AArch64RedundantCopyElimination::knownRegValInBlock(
MachineInstr &CondBr, MachineBasicBlock *MBB,
SmallVectorImpl<RegImm> &KnownRegs, MachineBasicBlock::iterator &FirstUse) {
unsigned Opc = CondBr.getOpcode();
// Check if the current basic block is the target block to which the
// CBZ/CBNZ instruction jumps when its Wt/Xt is zero.
if (((Opc == AArch64::CBZW || Opc == AArch64::CBZX) &&
MBB == CondBr.getOperand(1).getMBB()) ||
((Opc == AArch64::CBNZW || Opc == AArch64::CBNZX) &&
MBB != CondBr.getOperand(1).getMBB())) {
FirstUse = CondBr;
KnownRegs.push_back(RegImm(CondBr.getOperand(0).getReg(), 0));
return true;
// Otherwise, must be a conditional branch.
if (Opc != AArch64::Bcc)
return false;
// Must be an equality check (i.e., == or !=).
AArch64CC::CondCode CC = (AArch64CC::CondCode)CondBr.getOperand(0).getImm();
if (CC != AArch64CC::EQ && CC != AArch64CC::NE)
return false;
MachineBasicBlock *BrTarget = CondBr.getOperand(1).getMBB();
if ((CC == AArch64CC::EQ && BrTarget != MBB) ||
(CC == AArch64CC::NE && BrTarget == MBB))
return false;
// Stop if we get to the beginning of PredMBB.
MachineBasicBlock *PredMBB = *MBB->pred_begin();
assert(PredMBB == CondBr.getParent() &&
"Conditional branch not in predecessor block!");
if (CondBr == PredMBB->begin())
return false;
// Registers clobbered in PredMBB between CondBr instruction and current
// instruction being checked in loop.
// Find compare instruction that sets NZCV used by CondBr.
MachineBasicBlock::reverse_iterator RIt = CondBr.getReverseIterator();
for (MachineInstr &PredI : make_range(std::next(RIt), PredMBB->rend())) {
bool IsCMN = false;
switch (PredI.getOpcode()) {
// CMN is an alias for ADDS with a dead destination register.
case AArch64::ADDSWri:
case AArch64::ADDSXri:
IsCMN = true;
// CMP is an alias for SUBS with a dead destination register.
case AArch64::SUBSWri:
case AArch64::SUBSXri: {
// Sometimes the first operand is a FrameIndex. Bail if tht happens.
if (!PredI.getOperand(1).isReg())
return false;
MCPhysReg DstReg = PredI.getOperand(0).getReg();
MCPhysReg SrcReg = PredI.getOperand(1).getReg();
bool Res = false;
// If we're comparing against a non-symbolic immediate and the source
// register of the compare is not modified (including a self-clobbering
// compare) between the compare and conditional branch we known the value
// of the 1st source operand.
if (PredI.getOperand(2).isImm() && DomBBClobberedRegs.available(SrcReg) &&
SrcReg != DstReg) {
// We've found the instruction that sets NZCV.
int32_t KnownImm = PredI.getOperand(2).getImm();
int32_t Shift = PredI.getOperand(3).getImm();
KnownImm <<= Shift;
if (IsCMN)
KnownImm = -KnownImm;
FirstUse = PredI;
KnownRegs.push_back(RegImm(SrcReg, KnownImm));
Res = true;
// If this instructions defines something other than WZR/XZR, we know it's
// result is zero in some cases.
if (DstReg == AArch64::WZR || DstReg == AArch64::XZR)
return Res;
// The destination register must not be modified between the NZCV setting
// instruction and the conditional branch.
if (!DomBBClobberedRegs.available(DstReg))
return Res;
FirstUse = PredI;
KnownRegs.push_back(RegImm(DstReg, 0));
return true;
// Look for NZCV setting instructions that define something other than
case AArch64::ADCSWr:
case AArch64::ADCSXr:
case AArch64::ADDSWrr:
case AArch64::ADDSWrs:
case AArch64::ADDSWrx:
case AArch64::ADDSXrr:
case AArch64::ADDSXrs:
case AArch64::ADDSXrx:
case AArch64::ADDSXrx64:
case AArch64::ANDSWri:
case AArch64::ANDSWrr:
case AArch64::ANDSWrs:
case AArch64::ANDSXri:
case AArch64::ANDSXrr:
case AArch64::ANDSXrs:
case AArch64::BICSWrr:
case AArch64::BICSWrs:
case AArch64::BICSXrs:
case AArch64::BICSXrr:
case AArch64::SBCSWr:
case AArch64::SBCSXr:
case AArch64::SUBSWrr:
case AArch64::SUBSWrs:
case AArch64::SUBSWrx:
case AArch64::SUBSXrr:
case AArch64::SUBSXrs:
case AArch64::SUBSXrx:
case AArch64::SUBSXrx64: {
MCPhysReg DstReg = PredI.getOperand(0).getReg();
if (DstReg == AArch64::WZR || DstReg == AArch64::XZR)
return false;
// The destination register of the NZCV setting instruction must not be
// modified before the conditional branch.
if (!DomBBClobberedRegs.available(DstReg))
return false;
// We've found the instruction that sets NZCV whose DstReg == 0.
FirstUse = PredI;
KnownRegs.push_back(RegImm(DstReg, 0));
return true;
// Bail if we see an instruction that defines NZCV that we don't handle.
if (PredI.definesRegister(AArch64::NZCV))
return false;
// Track clobbered and used registers.
LiveRegUnits::accumulateUsedDefed(PredI, DomBBClobberedRegs, DomBBUsedRegs,
return false;
bool AArch64RedundantCopyElimination::optimizeBlock(MachineBasicBlock *MBB) {
// Check if the current basic block has a single predecessor.
if (MBB->pred_size() != 1)
return false;
// Check if the predecessor has two successors, implying the block ends in a
// conditional branch.
MachineBasicBlock *PredMBB = *MBB->pred_begin();
if (PredMBB->succ_size() != 2)
return false;
MachineBasicBlock::iterator CondBr = PredMBB->getLastNonDebugInstr();
if (CondBr == PredMBB->end())
return false;
// Keep track of the earliest point in the PredMBB block where kill markers
// need to be removed if a COPY is removed.
MachineBasicBlock::iterator FirstUse;
// After calling knownRegValInBlock, FirstUse will either point to a CBZ/CBNZ
// or a compare (i.e., SUBS). In the latter case, we must take care when
// updating FirstUse when scanning for COPY instructions. In particular, if
// there's a COPY in between the compare and branch the COPY should not
// update FirstUse.
bool SeenFirstUse = false;
// Registers that contain a known value at the start of MBB.
SmallVector<RegImm, 4> KnownRegs;
MachineBasicBlock::iterator Itr = std::next(CondBr);
do {
if (!knownRegValInBlock(*Itr, MBB, KnownRegs, FirstUse))
// Reset the clobbered and used register units.
// Look backward in PredMBB for COPYs from the known reg to find other
// registers that are known to be a constant value.
for (auto PredI = Itr;; --PredI) {
if (FirstUse == PredI)
SeenFirstUse = true;
if (PredI->isCopy()) {
MCPhysReg CopyDstReg = PredI->getOperand(0).getReg();
MCPhysReg CopySrcReg = PredI->getOperand(1).getReg();
for (auto &KnownReg : KnownRegs) {
if (!OptBBClobberedRegs.available(KnownReg.Reg))
// If we have X = COPY Y, and Y is known to be zero, then now X is
// known to be zero.
if (CopySrcReg == KnownReg.Reg &&
OptBBClobberedRegs.available(CopyDstReg)) {
KnownRegs.push_back(RegImm(CopyDstReg, KnownReg.Imm));
if (SeenFirstUse)
FirstUse = PredI;
// If we have X = COPY Y, and X is known to be zero, then now Y is
// known to be zero.
if (CopyDstReg == KnownReg.Reg &&
OptBBClobberedRegs.available(CopySrcReg)) {
KnownRegs.push_back(RegImm(CopySrcReg, KnownReg.Imm));
if (SeenFirstUse)
FirstUse = PredI;
// Stop if we get to the beginning of PredMBB.
if (PredI == PredMBB->begin())
LiveRegUnits::accumulateUsedDefed(*PredI, OptBBClobberedRegs,
OptBBUsedRegs, TRI);
// Stop if all of the known-zero regs have been clobbered.
if (all_of(KnownRegs, [&](RegImm KnownReg) {
return !OptBBClobberedRegs.available(KnownReg.Reg);
} while (Itr != PredMBB->begin() && Itr->isTerminator());
// We've not found a registers with a known value, time to bail out.
if (KnownRegs.empty())
return false;
bool Changed = false;
// UsedKnownRegs is the set of KnownRegs that have had uses added to MBB.
SmallSetVector<unsigned, 4> UsedKnownRegs;
MachineBasicBlock::iterator LastChange = MBB->begin();
// Remove redundant copy/move instructions unless KnownReg is modified.
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;) {
MachineInstr *MI = &*I;
bool RemovedMI = false;
bool IsCopy = MI->isCopy();
bool IsMoveImm = MI->isMoveImmediate();
if (IsCopy || IsMoveImm) {
Register DefReg = MI->getOperand(0).getReg();
Register SrcReg = IsCopy ? MI->getOperand(1).getReg() : Register();
int64_t SrcImm = IsMoveImm ? MI->getOperand(1).getImm() : 0;
if (!MRI->isReserved(DefReg) &&
((IsCopy && (SrcReg == AArch64::XZR || SrcReg == AArch64::WZR)) ||
IsMoveImm)) {
for (RegImm &KnownReg : KnownRegs) {
if (KnownReg.Reg != DefReg &&
!TRI->isSuperRegister(DefReg, KnownReg.Reg))
// For a copy, the known value must be a zero.
if (IsCopy && KnownReg.Imm != 0)
if (IsMoveImm) {
// For a move immediate, the known immediate must match the source
// immediate.
if (KnownReg.Imm != SrcImm)
// Don't remove a move immediate that implicitly defines the upper
// bits when only the lower 32 bits are known.
MCPhysReg CmpReg = KnownReg.Reg;
if (any_of(MI->implicit_operands(), [CmpReg](MachineOperand &O) {
return !O.isDead() && O.isReg() && O.isDef() &&
O.getReg() != CmpReg;
// Don't remove a move immediate that implicitly defines the upper
// bits as different.
if (TRI->isSuperRegister(DefReg, KnownReg.Reg) && KnownReg.Imm < 0)
if (IsCopy)
LLVM_DEBUG(dbgs() << "Remove redundant Copy : " << *MI);
LLVM_DEBUG(dbgs() << "Remove redundant Move : " << *MI);
Changed = true;
LastChange = I;
RemovedMI = true;
// Skip to the next instruction if we removed the COPY/MovImm.
if (RemovedMI)
// Remove any regs the MI clobbers from the KnownConstRegs set.
for (unsigned RI = 0; RI < KnownRegs.size();)
if (MI->modifiesRegister(KnownRegs[RI].Reg, TRI)) {
std::swap(KnownRegs[RI], KnownRegs[KnownRegs.size() - 1]);
// Don't increment RI since we need to now check the swapped-in
// KnownRegs[RI].
} else {
// Continue until the KnownRegs set is empty.
if (KnownRegs.empty())
if (!Changed)
return false;
// Add newly used regs to the block's live-in list if they aren't there
// already.
for (MCPhysReg KnownReg : UsedKnownRegs)
if (!MBB->isLiveIn(KnownReg))
// Clear kills in the range where changes were made. This is conservative,
// but should be okay since kill markers are being phased out.
LLVM_DEBUG(dbgs() << "Clearing kill flags.\n\tFirstUse: " << *FirstUse
<< "\tLastChange: ";
if (LastChange == MBB->end()) dbgs() << "<end>\n";
else dbgs() << *LastChange);
for (MachineInstr &MMI : make_range(FirstUse, PredMBB->end()))
for (MachineInstr &MMI : make_range(MBB->begin(), LastChange))
return true;
bool AArch64RedundantCopyElimination::runOnMachineFunction(
MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
// Resize the clobbered and used register unit trackers. We do this once per
// function.
bool Changed = false;
for (MachineBasicBlock &MBB : MF)
Changed |= optimizeBlock(&MBB);
return Changed;
FunctionPass *llvm::createAArch64RedundantCopyEliminationPass() {
return new AArch64RedundantCopyElimination();