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//===--------- PPCPreEmitPeephole.cpp - Late peephole optimizations -------===//
// 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
// A pre-emit peephole for catching opportunities introduced by late passes such
// as MachineBlockPlacement.
#include "PPC.h"
#include "PPCInstrInfo.h"
#include "PPCSubtarget.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "ppc-pre-emit-peephole"
"Number of r+r instructions converted to r+i in pre-emit peephole");
"Number of instructions deleted in pre-emit peephole");
"Number of self copy instructions eliminated");
"Number of folding frame offset by using r+r in pre-emit peephole");
static cl::opt<bool>
EnablePCRelLinkerOpt("ppc-pcrel-linker-opt", cl::Hidden, cl::init(true),
cl::desc("enable PC Relative linker optimization"));
static cl::opt<bool>
RunPreEmitPeephole("ppc-late-peephole", cl::Hidden, cl::init(true),
cl::desc("Run pre-emit peephole optimizations."));
namespace {
static bool hasPCRelativeForm(MachineInstr &Use) {
switch (Use.getOpcode()) {
return false;
case PPC::LBZ:
case PPC::LBZ8:
case PPC::LHA:
case PPC::LHA8:
case PPC::LHZ:
case PPC::LHZ8:
case PPC::LWZ:
case PPC::LWZ8:
case PPC::STB:
case PPC::STB8:
case PPC::STH:
case PPC::STH8:
case PPC::STW:
case PPC::STW8:
case PPC::LD:
case PPC::STD:
case PPC::LWA:
case PPC::LXSD:
case PPC::LXSSP:
case PPC::LXV:
case PPC::STXSD:
case PPC::STXV:
case PPC::LFD:
case PPC::LFS:
case PPC::STFD:
case PPC::STFS:
case PPC::DFLOADf32:
case PPC::DFLOADf64:
case PPC::DFSTOREf32:
case PPC::DFSTOREf64:
return true;
class PPCPreEmitPeephole : public MachineFunctionPass {
static char ID;
PPCPreEmitPeephole() : MachineFunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
// This function removes any redundant load immediates. It has two level
// loops - The outer loop finds the load immediates BBI that could be used
// to replace following redundancy. The inner loop scans instructions that
// after BBI to find redundancy and update kill/dead flags accordingly. If
// AfterBBI is the same as BBI, it is redundant, otherwise any instructions
// that modify the def register of BBI would break the scanning.
// DeadOrKillToUnset is a pointer to the previous operand that had the
// kill/dead flag set. It keeps track of the def register of BBI, the use
// registers of AfterBBIs and the def registers of AfterBBIs.
bool removeRedundantLIs(MachineBasicBlock &MBB,
const TargetRegisterInfo *TRI) {
LLVM_DEBUG(dbgs() << "Remove redundant load immediates from MBB:\n";
MBB.dump(); dbgs() << "\n");
DenseSet<MachineInstr *> InstrsToErase;
for (auto BBI = MBB.instr_begin(); BBI != MBB.instr_end(); ++BBI) {
// Skip load immediate that is marked to be erased later because it
// cannot be used to replace any other instructions.
if (InstrsToErase.contains(&*BBI))
// Skip non-load immediate.
unsigned Opc = BBI->getOpcode();
if (Opc != PPC::LI && Opc != PPC::LI8 && Opc != PPC::LIS &&
Opc != PPC::LIS8)
// Skip load immediate, where the operand is a relocation (e.g., $r3 =
// LI target-flags(ppc-lo) %const.0).
if (!BBI->getOperand(1).isImm())
assert(BBI->getOperand(0).isReg() &&
"Expected a register for the first operand");
LLVM_DEBUG(dbgs() << "Scanning after load immediate: "; BBI->dump(););
Register Reg = BBI->getOperand(0).getReg();
int64_t Imm = BBI->getOperand(1).getImm();
MachineOperand *DeadOrKillToUnset = nullptr;
if (BBI->getOperand(0).isDead()) {
DeadOrKillToUnset = &BBI->getOperand(0);
LLVM_DEBUG(dbgs() << " Kill flag of " << *DeadOrKillToUnset
<< " from load immediate " << *BBI
<< " is a unsetting candidate\n");
// This loop scans instructions after BBI to see if there is any
// redundant load immediate.
for (auto AfterBBI = std::next(BBI); AfterBBI != MBB.instr_end();
++AfterBBI) {
// Track the operand that kill Reg. We would unset the kill flag of
// the operand if there is a following redundant load immediate.
int KillIdx = AfterBBI->findRegisterUseOperandIdx(Reg, true, TRI);
// We can't just clear implicit kills, so if we encounter one, stop
// looking further.
if (KillIdx != -1 && AfterBBI->getOperand(KillIdx).isImplicit()) {
<< "Encountered an implicit kill, cannot proceed: ");
if (KillIdx != -1) {
assert(!DeadOrKillToUnset && "Shouldn't kill same register twice");
DeadOrKillToUnset = &AfterBBI->getOperand(KillIdx);
<< " Kill flag of " << *DeadOrKillToUnset << " from "
<< *AfterBBI << " is a unsetting candidate\n");
if (!AfterBBI->modifiesRegister(Reg, TRI))
// Finish scanning because Reg is overwritten by a non-load
// instruction.
if (AfterBBI->getOpcode() != Opc)
assert(AfterBBI->getOperand(0).isReg() &&
"Expected a register for the first operand");
// Finish scanning because Reg is overwritten by a relocation or a
// different value.
if (!AfterBBI->getOperand(1).isImm() ||
AfterBBI->getOperand(1).getImm() != Imm)
// It loads same immediate value to the same Reg, which is redundant.
// We would unset kill flag in previous Reg usage to extend live range
// of Reg first, then remove the redundancy.
if (DeadOrKillToUnset) {
<< " Unset dead/kill flag of " << *DeadOrKillToUnset
<< " from " << *DeadOrKillToUnset->getParent());
if (DeadOrKillToUnset->isDef())
DeadOrKillToUnset =
AfterBBI->findRegisterDefOperand(Reg, true, true, TRI);
if (DeadOrKillToUnset)
<< " Dead flag of " << *DeadOrKillToUnset << " from "
<< *AfterBBI << " is a unsetting candidate\n");
LLVM_DEBUG(dbgs() << " Remove redundant load immediate: ";
for (MachineInstr *MI : InstrsToErase) {
NumRemovedInPreEmit += InstrsToErase.size();
return !InstrsToErase.empty();
// Check if this instruction is a PLDpc that is part of a GOT indirect
// access.
bool isGOTPLDpc(MachineInstr &Instr) {
if (Instr.getOpcode() != PPC::PLDpc)
return false;
// The result must be a register.
const MachineOperand &LoadedAddressReg = Instr.getOperand(0);
if (!LoadedAddressReg.isReg())
return false;
// Make sure that this is a global symbol.
const MachineOperand &SymbolOp = Instr.getOperand(1);
if (!SymbolOp.isGlobal())
return false;
// Finally return true only if the GOT flag is present.
return (SymbolOp.getTargetFlags() & PPCII::MO_GOT_FLAG);
bool addLinkerOpt(MachineBasicBlock &MBB, const TargetRegisterInfo *TRI) {
MachineFunction *MF = MBB.getParent();
// If the linker opt is disabled then just return.
if (!EnablePCRelLinkerOpt)
return false;
// Add this linker opt only if we are using PC Relative memops.
if (!MF->getSubtarget<PPCSubtarget>().isUsingPCRelativeCalls())
return false;
// Struct to keep track of one def/use pair for a GOT indirect access.
struct GOTDefUsePair {
MachineBasicBlock::iterator DefInst;
MachineBasicBlock::iterator UseInst;
Register DefReg;
Register UseReg;
bool StillValid;
// Vector of def/ues pairs in this basic block.
SmallVector<GOTDefUsePair, 4> CandPairs;
SmallVector<GOTDefUsePair, 4> ValidPairs;
bool MadeChange = false;
// Run through all of the instructions in the basic block and try to
// collect potential pairs of GOT indirect access instructions.
for (auto BBI = MBB.instr_begin(); BBI != MBB.instr_end(); ++BBI) {
// Look for the initial GOT indirect load.
if (isGOTPLDpc(*BBI)) {
GOTDefUsePair CurrentPair{BBI, MachineBasicBlock::iterator(),
PPC::NoRegister, true};
// We haven't encountered any new PLD instructions, nothing to check.
if (CandPairs.empty())
// Run through the candidate pairs and see if any of the registers
// defined in the PLD instructions are used by this instruction.
// Note: the size of CandPairs can change in the loop.
for (unsigned Idx = 0; Idx < CandPairs.size(); Idx++) {
GOTDefUsePair &Pair = CandPairs[Idx];
// The instruction does not use or modify this PLD's def reg,
// ignore it.
if (!BBI->readsRegister(Pair.DefReg, TRI) &&
!BBI->modifiesRegister(Pair.DefReg, TRI))
// The use needs to be used in the address compuation and not
// as the register being stored for a store.
const MachineOperand *UseOp =
hasPCRelativeForm(*BBI) ? &BBI->getOperand(2) : nullptr;
// Check for a valid use.
if (UseOp && UseOp->isReg() && UseOp->getReg() == Pair.DefReg &&
UseOp->isUse() && UseOp->isKill()) {
Pair.UseInst = BBI;
Pair.UseReg = BBI->getOperand(0).getReg();
CandPairs.erase(CandPairs.begin() + Idx);
// Go through all of the pairs and check for any more valid uses.
for (auto Pair = ValidPairs.begin(); Pair != ValidPairs.end(); Pair++) {
// We shouldn't be here if we don't have a valid pair.
assert(Pair->UseInst.isValid() && Pair->StillValid &&
"Kept an invalid def/use pair for GOT PCRel opt");
// We have found a potential pair. Search through the instructions
// between the def and the use to see if it is valid to mark this as a
// linker opt.
MachineBasicBlock::iterator BBI = Pair->DefInst;
for (; BBI != Pair->UseInst; ++BBI) {
if (BBI->readsRegister(Pair->UseReg, TRI) ||
BBI->modifiesRegister(Pair->UseReg, TRI)) {
Pair->StillValid = false;
if (!Pair->StillValid)
// The load/store instruction that uses the address from the PLD will
// either use a register (for a store) or define a register (for the
// load). That register will be added as an implicit def to the PLD
// and as an implicit use on the second memory op. This is a precaution
// to prevent future passes from using that register between the two
// instructions.
MachineOperand ImplDef =
MachineOperand::CreateReg(Pair->UseReg, true, true);
MachineOperand ImplUse =
MachineOperand::CreateReg(Pair->UseReg, false, true);
// Create the symbol.
MCContext &Context = MF->getContext();
MCSymbol *Symbol = Context.createNamedTempSymbol("pcrel");
MachineOperand PCRelLabel =
MachineOperand::CreateMCSymbol(Symbol, PPCII::MO_PCREL_OPT_FLAG);
Pair->DefInst->addOperand(*MF, PCRelLabel);
Pair->UseInst->addOperand(*MF, PCRelLabel);
MadeChange |= true;
return MadeChange;
// This function removes redundant pairs of accumulator prime/unprime
// instructions. In some situations, it's possible the compiler inserts an
// accumulator prime instruction followed by an unprime instruction (e.g.
// when we store an accumulator after restoring it from a spill). If the
// accumulator is not used between the two, they can be removed. This
// function removes these redundant pairs from basic blocks.
// The algorithm is quite straightforward - every time we encounter a prime
// instruction, the primed register is added to a candidate set. Any use
// other than a prime removes the candidate from the set and any de-prime
// of a current candidate marks both the prime and de-prime for removal.
// This way we ensure we only remove prime/de-prime *pairs* with no
// intervening uses.
bool removeAccPrimeUnprime(MachineBasicBlock &MBB) {
DenseSet<MachineInstr *> InstrsToErase;
// Initially, none of the acc registers are candidates.
SmallVector<MachineInstr *, 8> Candidates(
PPC::UACCRCRegClass.getNumRegs(), nullptr);
for (MachineInstr &BBI : MBB.instrs()) {
unsigned Opc = BBI.getOpcode();
// If we are visiting a xxmtacc instruction, we add it and its operand
// register to the candidate set.
if (Opc == PPC::XXMTACC) {
Register Acc = BBI.getOperand(0).getReg();
assert(PPC::ACCRCRegClass.contains(Acc) &&
"Unexpected register for XXMTACC");
Candidates[Acc - PPC::ACC0] = &BBI;
// If we are visiting a xxmfacc instruction and its operand register is
// in the candidate set, we mark the two instructions for removal.
else if (Opc == PPC::XXMFACC) {
Register Acc = BBI.getOperand(0).getReg();
assert(PPC::ACCRCRegClass.contains(Acc) &&
"Unexpected register for XXMFACC");
if (!Candidates[Acc - PPC::ACC0])
InstrsToErase.insert(Candidates[Acc - PPC::ACC0]);
// If we are visiting an instruction using an accumulator register
// as operand, we remove it from the candidate set.
else {
for (MachineOperand &Operand : BBI.operands()) {
if (!Operand.isReg())
Register Reg = Operand.getReg();
if (PPC::ACCRCRegClass.contains(Reg))
Candidates[Reg - PPC::ACC0] = nullptr;
for (MachineInstr *MI : InstrsToErase)
NumRemovedInPreEmit += InstrsToErase.size();
return !InstrsToErase.empty();
bool runOnMachineFunction(MachineFunction &MF) override {
if (skipFunction(MF.getFunction()) || !RunPreEmitPeephole) {
// Remove UNENCODED_NOP even when this pass is disabled.
// This needs to be done unconditionally so we don't emit zeros
// in the instruction stream.
SmallVector<MachineInstr *, 4> InstrsToErase;
for (MachineBasicBlock &MBB : MF)
for (MachineInstr &MI : MBB)
if (MI.getOpcode() == PPC::UNENCODED_NOP)
for (MachineInstr *MI : InstrsToErase)
return false;
bool Changed = false;
const PPCInstrInfo *TII = MF.getSubtarget<PPCSubtarget>().getInstrInfo();
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
SmallVector<MachineInstr *, 4> InstrsToErase;
for (MachineBasicBlock &MBB : MF) {
Changed |= removeRedundantLIs(MBB, TRI);
Changed |= addLinkerOpt(MBB, TRI);
Changed |= removeAccPrimeUnprime(MBB);
for (MachineInstr &MI : MBB) {
unsigned Opc = MI.getOpcode();
if (Opc == PPC::UNENCODED_NOP) {
// Detect self copies - these can result from running AADB.
if (PPCInstrInfo::isSameClassPhysRegCopy(Opc)) {
const MCInstrDesc &MCID = TII->get(Opc);
if (MCID.getNumOperands() == 3 &&
MI.getOperand(0).getReg() == MI.getOperand(1).getReg() &&
MI.getOperand(0).getReg() == MI.getOperand(2).getReg()) {
LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: ");
else if (MCID.getNumOperands() == 2 &&
MI.getOperand(0).getReg() == MI.getOperand(1).getReg()) {
LLVM_DEBUG(dbgs() << "Deleting self-copy instruction: ");
MachineInstr *DefMIToErase = nullptr;
if (TII->convertToImmediateForm(MI, &DefMIToErase)) {
Changed = true;
LLVM_DEBUG(dbgs() << "Converted instruction to imm form: ");
if (DefMIToErase) {
if (TII->foldFrameOffset(MI)) {
Changed = true;
LLVM_DEBUG(dbgs() << "Frame offset folding by using index form: ");
// Eliminate conditional branch based on a constant CR bit by
// CRSET or CRUNSET. We eliminate the conditional branch or
// convert it into an unconditional branch. Also, if the CR bit
// is not used by other instructions, we eliminate CRSET as well.
auto I = MBB.getFirstInstrTerminator();
if (I == MBB.instr_end())
MachineInstr *Br = &*I;
if (Br->getOpcode() != PPC::BC && Br->getOpcode() != PPC::BCn)
MachineInstr *CRSetMI = nullptr;
Register CRBit = Br->getOperand(0).getReg();
unsigned CRReg = getCRFromCRBit(CRBit);
bool SeenUse = false;
MachineBasicBlock::reverse_iterator It = Br, Er = MBB.rend();
for (It++; It != Er; It++) {
if (It->modifiesRegister(CRBit, TRI)) {
if ((It->getOpcode() == PPC::CRUNSET ||
It->getOpcode() == PPC::CRSET) &&
It->getOperand(0).getReg() == CRBit)
CRSetMI = &*It;
if (It->readsRegister(CRBit, TRI))
SeenUse = true;
if (!CRSetMI) continue;
unsigned CRSetOp = CRSetMI->getOpcode();
if ((Br->getOpcode() == PPC::BCn && CRSetOp == PPC::CRSET) ||
(Br->getOpcode() == PPC::BC && CRSetOp == PPC::CRUNSET)) {
// Remove this branch since it cannot be taken.
else {
// This conditional branch is always taken. So, remove all branches
// and insert an unconditional branch to the destination of this.
MachineBasicBlock::iterator It = Br, Er = MBB.end();
for (; It != Er; It++) {
if (It->isDebugInstr()) continue;
assert(It->isTerminator() && "Non-terminator after a terminator");
if (!MBB.isLayoutSuccessor(Br->getOperand(1).getMBB())) {
ArrayRef<MachineOperand> NoCond;
TII->insertBranch(MBB, Br->getOperand(1).getMBB(), nullptr,
NoCond, Br->getDebugLoc());
for (auto &Succ : MBB.successors())
if (Succ != Br->getOperand(1).getMBB()) {
// If the CRBit is not used by another instruction, we can eliminate
// CRSET/CRUNSET instruction.
if (!SeenUse) {
// We need to check use of the CRBit in successors.
for (auto &SuccMBB : MBB.successors())
if (SuccMBB->isLiveIn(CRBit) || SuccMBB->isLiveIn(CRReg)) {
SeenUse = true;
if (!SeenUse)
for (MachineInstr *MI : InstrsToErase) {
LLVM_DEBUG(dbgs() << "PPC pre-emit peephole: erasing instruction: ");
return Changed;
INITIALIZE_PASS(PPCPreEmitPeephole, DEBUG_TYPE, "PowerPC Pre-Emit Peephole",
false, false)
char PPCPreEmitPeephole::ID = 0;
FunctionPass *llvm::createPPCPreEmitPeepholePass() {
return new PPCPreEmitPeephole();