llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

//===-------------- PPCMIPeephole.cpp - MI Peephole Cleanups -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===---------------------------------------------------------------------===//
//
// This pass performs peephole optimizations to clean up ugly code
// sequences at the MachineInstruction layer.  It runs at the end of
// the SSA phases, following VSX swap removal.  A pass of dead code
// elimination follows this one for quick clean-up of any dead
// instructions introduced here.  Although we could do this as callbacks
// from the generic peephole pass, this would have a couple of bad
// effects:  it might remove optimization opportunities for VSX swap
// removal, and it would miss cleanups made possible following VSX
// swap removal.
//
//===---------------------------------------------------------------------===//

#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCInstrInfo.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "MCTargetDesc/PPCPredicates.h"

using namespace llvm;

#define DEBUG_TYPE "ppc-mi-peepholes"

STATISTIC(NumOptADDLIs, "Number of optimized ADD instruction fed by LI");

namespace llvm {
  void initializePPCMIPeepholePass(PassRegistry&);
}

namespace {

struct PPCMIPeephole : public MachineFunctionPass {

  static char ID;
  const PPCInstrInfo *TII;
  MachineFunction *MF;
  MachineRegisterInfo *MRI;

  PPCMIPeephole() : MachineFunctionPass(ID) {
    initializePPCMIPeepholePass(*PassRegistry::getPassRegistry());
  }

private:
  MachineDominatorTree *MDT;

  // Initialize class variables.
  void initialize(MachineFunction &MFParm);

  // Perform peepholes.
  bool simplifyCode(void);

  // Perform peepholes.
  bool eliminateRedundantCompare(void);

  // Find the "true" register represented by SrcReg (following chains
  // of copies and subreg_to_reg operations).
  unsigned lookThruCopyLike(unsigned SrcReg);

public:

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<MachineDominatorTree>();
    AU.addPreserved<MachineDominatorTree>();
    MachineFunctionPass::getAnalysisUsage(AU);
  }

  // Main entry point for this pass.
  bool runOnMachineFunction(MachineFunction &MF) override {
    if (skipFunction(*MF.getFunction()))
      return false;
    initialize(MF);
    return simplifyCode();
  }
};

// Initialize class variables.
void PPCMIPeephole::initialize(MachineFunction &MFParm) {
  MF = &MFParm;
  MRI = &MF->getRegInfo();
  MDT = &getAnalysis<MachineDominatorTree>();
  TII = MF->getSubtarget<PPCSubtarget>().getInstrInfo();
  DEBUG(dbgs() << "*** PowerPC MI peephole pass ***\n\n");
  DEBUG(MF->dump());
}

static MachineInstr *getVRegDefOrNull(MachineOperand *Op,
                                      MachineRegisterInfo *MRI) {
  assert(Op && "Invalid Operand!");
  if (!Op->isReg())
    return nullptr;

  unsigned Reg = Op->getReg();
  if (!TargetRegisterInfo::isVirtualRegister(Reg))
    return nullptr;

  return MRI->getVRegDef(Reg);
}

// Perform peephole optimizations.
bool PPCMIPeephole::simplifyCode(void) {
  bool Simplified = false;
  MachineInstr* ToErase = nullptr;

  for (MachineBasicBlock &MBB : *MF) {
    for (MachineInstr &MI : MBB) {

      // If the previous instruction was marked for elimination,
      // remove it now.
      if (ToErase) {
        ToErase->eraseFromParent();
        ToErase = nullptr;
      }

      // Ignore debug instructions.
      if (MI.isDebugValue())
        continue;

      // Per-opcode peepholes.
      switch (MI.getOpcode()) {

      default:
        break;

      case PPC::XXPERMDI: {
        // Perform simplifications of 2x64 vector swaps and splats.
        // A swap is identified by an immediate value of 2, and a splat
        // is identified by an immediate value of 0 or 3.
        int Immed = MI.getOperand(3).getImm();

        if (Immed != 1) {

          // For each of these simplifications, we need the two source
          // regs to match.  Unfortunately, MachineCSE ignores COPY and
          // SUBREG_TO_REG, so for example we can see
          //   XXPERMDI t, SUBREG_TO_REG(s), SUBREG_TO_REG(s), immed.
          // We have to look through chains of COPY and SUBREG_TO_REG
          // to find the real source values for comparison.
          unsigned TrueReg1 = lookThruCopyLike(MI.getOperand(1).getReg());
          unsigned TrueReg2 = lookThruCopyLike(MI.getOperand(2).getReg());

          if (TrueReg1 == TrueReg2
              && TargetRegisterInfo::isVirtualRegister(TrueReg1)) {
            MachineInstr *DefMI = MRI->getVRegDef(TrueReg1);
            unsigned DefOpc = DefMI ? DefMI->getOpcode() : 0;

            // If this is a splat fed by a splatting load, the splat is
            // redundant. Replace with a copy. This doesn't happen directly due
            // to code in PPCDAGToDAGISel.cpp, but it can happen when converting
            // a load of a double to a vector of 64-bit integers.
            auto isConversionOfLoadAndSplat = [=]() -> bool {
              if (DefOpc != PPC::XVCVDPSXDS && DefOpc != PPC::XVCVDPUXDS)
                return false;
              unsigned DefReg = lookThruCopyLike(DefMI->getOperand(1).getReg());
              if (TargetRegisterInfo::isVirtualRegister(DefReg)) {
                MachineInstr *LoadMI = MRI->getVRegDef(DefReg);
                if (LoadMI && LoadMI->getOpcode() == PPC::LXVDSX)
                  return true;
              }
              return false;
            };
            if (DefMI && (Immed == 0 || Immed == 3)) {
              if (DefOpc == PPC::LXVDSX || isConversionOfLoadAndSplat()) {
                DEBUG(dbgs()
                      << "Optimizing load-and-splat/splat "
                      "to load-and-splat/copy: ");
                DEBUG(MI.dump());
                BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                        MI.getOperand(0).getReg())
                    .add(MI.getOperand(1));
                ToErase = &MI;
                Simplified = true;
              }
            }

            // If this is a splat or a swap fed by another splat, we
            // can replace it with a copy.
            if (DefOpc == PPC::XXPERMDI) {
              unsigned FeedImmed = DefMI->getOperand(3).getImm();
              unsigned FeedReg1
                = lookThruCopyLike(DefMI->getOperand(1).getReg());
              unsigned FeedReg2
                = lookThruCopyLike(DefMI->getOperand(2).getReg());

              if ((FeedImmed == 0 || FeedImmed == 3) && FeedReg1 == FeedReg2) {
                DEBUG(dbgs()
                      << "Optimizing splat/swap or splat/splat "
                      "to splat/copy: ");
                DEBUG(MI.dump());
                BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                        MI.getOperand(0).getReg())
                    .add(MI.getOperand(1));
                ToErase = &MI;
                Simplified = true;
              }

              // If this is a splat fed by a swap, we can simplify modify
              // the splat to splat the other value from the swap's input
              // parameter.
              else if ((Immed == 0 || Immed == 3)
                       && FeedImmed == 2 && FeedReg1 == FeedReg2) {
                DEBUG(dbgs() << "Optimizing swap/splat => splat: ");
                DEBUG(MI.dump());
                MI.getOperand(1).setReg(DefMI->getOperand(1).getReg());
                MI.getOperand(2).setReg(DefMI->getOperand(2).getReg());
                MI.getOperand(3).setImm(3 - Immed);
                Simplified = true;
              }

              // If this is a swap fed by a swap, we can replace it
              // with a copy from the first swap's input.
              else if (Immed == 2 && FeedImmed == 2 && FeedReg1 == FeedReg2) {
                DEBUG(dbgs() << "Optimizing swap/swap => copy: ");
                DEBUG(MI.dump());
                BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                        MI.getOperand(0).getReg())
                    .add(DefMI->getOperand(1));
                ToErase = &MI;
                Simplified = true;
              }
            } else if ((Immed == 0 || Immed == 3) && DefOpc == PPC::XXPERMDIs &&
                       (DefMI->getOperand(2).getImm() == 0 ||
                        DefMI->getOperand(2).getImm() == 3)) {
              // Splat fed by another splat - switch the output of the first
              // and remove the second.
              DefMI->getOperand(0).setReg(MI.getOperand(0).getReg());
              ToErase = &MI;
              Simplified = true;
              DEBUG(dbgs() << "Removing redundant splat: ");
              DEBUG(MI.dump());
            }
          }
        }
        break;
      }
      case PPC::VSPLTB:
      case PPC::VSPLTH:
      case PPC::XXSPLTW: {
        unsigned MyOpcode = MI.getOpcode();
        unsigned OpNo = MyOpcode == PPC::XXSPLTW ? 1 : 2;
        unsigned TrueReg = lookThruCopyLike(MI.getOperand(OpNo).getReg());
        if (!TargetRegisterInfo::isVirtualRegister(TrueReg))
          break;
        MachineInstr *DefMI = MRI->getVRegDef(TrueReg);
        if (!DefMI)
          break;
        unsigned DefOpcode = DefMI->getOpcode();
        auto isConvertOfSplat = [=]() -> bool {
          if (DefOpcode != PPC::XVCVSPSXWS && DefOpcode != PPC::XVCVSPUXWS)
            return false;
          unsigned ConvReg = DefMI->getOperand(1).getReg();
          if (!TargetRegisterInfo::isVirtualRegister(ConvReg))
            return false;
          MachineInstr *Splt = MRI->getVRegDef(ConvReg);
          return Splt && (Splt->getOpcode() == PPC::LXVWSX ||
            Splt->getOpcode() == PPC::XXSPLTW);
        };
        bool AlreadySplat = (MyOpcode == DefOpcode) ||
          (MyOpcode == PPC::VSPLTB && DefOpcode == PPC::VSPLTBs) ||
          (MyOpcode == PPC::VSPLTH && DefOpcode == PPC::VSPLTHs) ||
          (MyOpcode == PPC::XXSPLTW && DefOpcode == PPC::XXSPLTWs) ||
          (MyOpcode == PPC::XXSPLTW && DefOpcode == PPC::LXVWSX) ||
          (MyOpcode == PPC::XXSPLTW && DefOpcode == PPC::MTVSRWS)||
          (MyOpcode == PPC::XXSPLTW && isConvertOfSplat());
        // If the instruction[s] that feed this splat have already splat
        // the value, this splat is redundant.
        if (AlreadySplat) {
          DEBUG(dbgs() << "Changing redundant splat to a copy: ");
          DEBUG(MI.dump());
          BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                  MI.getOperand(0).getReg())
              .add(MI.getOperand(OpNo));
          ToErase = &MI;
          Simplified = true;
        }
        // Splat fed by a shift. Usually when we align value to splat into
        // vector element zero.
        if (DefOpcode == PPC::XXSLDWI) {
          unsigned ShiftRes = DefMI->getOperand(0).getReg();
          unsigned ShiftOp1 = DefMI->getOperand(1).getReg();
          unsigned ShiftOp2 = DefMI->getOperand(2).getReg();
          unsigned ShiftImm = DefMI->getOperand(3).getImm();
          unsigned SplatImm = MI.getOperand(2).getImm();
          if (ShiftOp1 == ShiftOp2) {
            unsigned NewElem = (SplatImm + ShiftImm) & 0x3;
            if (MRI->hasOneNonDBGUse(ShiftRes)) {
              DEBUG(dbgs() << "Removing redundant shift: ");
              DEBUG(DefMI->dump());
              ToErase = DefMI;
            }
            Simplified = true;
            DEBUG(dbgs() << "Changing splat immediate from " << SplatImm <<
                  " to " << NewElem << " in instruction: ");
            DEBUG(MI.dump());
            MI.getOperand(1).setReg(ShiftOp1);
            MI.getOperand(2).setImm(NewElem);
          }
        }
        break;
      }
      case PPC::XVCVDPSP: {
        // If this is a DP->SP conversion fed by an FRSP, the FRSP is redundant.
        unsigned TrueReg = lookThruCopyLike(MI.getOperand(1).getReg());
        if (!TargetRegisterInfo::isVirtualRegister(TrueReg))
          break;
        MachineInstr *DefMI = MRI->getVRegDef(TrueReg);

        // This can occur when building a vector of single precision or integer
        // values.
        if (DefMI && DefMI->getOpcode() == PPC::XXPERMDI) {
          unsigned DefsReg1 = lookThruCopyLike(DefMI->getOperand(1).getReg());
          unsigned DefsReg2 = lookThruCopyLike(DefMI->getOperand(2).getReg());
          if (!TargetRegisterInfo::isVirtualRegister(DefsReg1) ||
              !TargetRegisterInfo::isVirtualRegister(DefsReg2))
            break;
          MachineInstr *P1 = MRI->getVRegDef(DefsReg1);
          MachineInstr *P2 = MRI->getVRegDef(DefsReg2);

          if (!P1 || !P2)
            break;

          // Remove the passed FRSP instruction if it only feeds this MI and
          // set any uses of that FRSP (in this MI) to the source of the FRSP.
          auto removeFRSPIfPossible = [&](MachineInstr *RoundInstr) {
            if (RoundInstr->getOpcode() == PPC::FRSP &&
                MRI->hasOneNonDBGUse(RoundInstr->getOperand(0).getReg())) {
              Simplified = true;
              unsigned ConvReg1 = RoundInstr->getOperand(1).getReg();
              unsigned FRSPDefines = RoundInstr->getOperand(0).getReg();
              MachineInstr &Use = *(MRI->use_instr_begin(FRSPDefines));
              for (int i = 0, e = Use.getNumOperands(); i < e; ++i)
                if (Use.getOperand(i).isReg() &&
                    Use.getOperand(i).getReg() == FRSPDefines)
                  Use.getOperand(i).setReg(ConvReg1);
              DEBUG(dbgs() << "Removing redundant FRSP:\n");
              DEBUG(RoundInstr->dump());
              DEBUG(dbgs() << "As it feeds instruction:\n");
              DEBUG(MI.dump());
              DEBUG(dbgs() << "Through instruction:\n");
              DEBUG(DefMI->dump());
              RoundInstr->eraseFromParent();
            }
          };

          // If the input to XVCVDPSP is a vector that was built (even
          // partially) out of FRSP's, the FRSP(s) can safely be removed
          // since this instruction performs the same operation.
          if (P1 != P2) {
            removeFRSPIfPossible(P1);
            removeFRSPIfPossible(P2);
            break;
          }
          removeFRSPIfPossible(P1);
        }
        break;
      }

      // TODO: Any instruction that has an immediate form fed only by a PHI
      // whose operands are all load immediate can be folded away. We currently
      // do this for ADD instructions, but should expand it to arithmetic and
      // binary instructions with immediate forms in the future.
      case PPC::ADD4:
      case PPC::ADD8: {
        auto isSingleUsePHI = [&](MachineOperand *PhiOp) {
          assert(PhiOp && "Invalid Operand!");
          MachineInstr *DefPhiMI = getVRegDefOrNull(PhiOp, MRI);

          return DefPhiMI && (DefPhiMI->getOpcode() == PPC::PHI) &&
                 MRI->hasOneNonDBGUse(DefPhiMI->getOperand(0).getReg());
        };

        auto dominatesAllSingleUseLIs = [&](MachineOperand *DominatorOp,
                                            MachineOperand *PhiOp) {
          assert(PhiOp && "Invalid Operand!");
          assert(DominatorOp && "Invalid Operand!");
          MachineInstr *DefPhiMI = getVRegDefOrNull(PhiOp, MRI);
          MachineInstr *DefDomMI = getVRegDefOrNull(DominatorOp, MRI);

          // Note: the vregs only show up at odd indices position of PHI Node,
          // the even indices position save the BB info.
          for (unsigned i = 1; i < DefPhiMI->getNumOperands(); i += 2) {
            MachineInstr *LiMI =
                getVRegDefOrNull(&DefPhiMI->getOperand(i), MRI);
            if (!LiMI || !MRI->hasOneNonDBGUse(LiMI->getOperand(0).getReg()) ||
                !MDT->dominates(DefDomMI, LiMI) ||
                (LiMI->getOpcode() != PPC::LI && LiMI->getOpcode() != PPC::LI8))
              return false;
          }

          return true;
        };

        MachineOperand Op1 = MI.getOperand(1);
        MachineOperand Op2 = MI.getOperand(2);
        if (isSingleUsePHI(&Op2) && dominatesAllSingleUseLIs(&Op1, &Op2))
          std::swap(Op1, Op2);
        else if (!isSingleUsePHI(&Op1) || !dominatesAllSingleUseLIs(&Op2, &Op1))
          break; // We don't have an ADD fed by LI's that can be transformed

        // Now we know that Op1 is the PHI node and Op2 is the dominator
        unsigned DominatorReg = Op2.getReg();

        const TargetRegisterClass *TRC = MI.getOpcode() == PPC::ADD8
                                             ? &PPC::G8RC_and_G8RC_NOX0RegClass
                                             : &PPC::GPRC_and_GPRC_NOR0RegClass;
        MRI->setRegClass(DominatorReg, TRC);

        // replace LIs with ADDIs
        MachineInstr *DefPhiMI = getVRegDefOrNull(&Op1, MRI);
        for (unsigned i = 1; i < DefPhiMI->getNumOperands(); i += 2) {
          MachineInstr *LiMI = getVRegDefOrNull(&DefPhiMI->getOperand(i), MRI);
          DEBUG(dbgs() << "Optimizing LI to ADDI: ");
          DEBUG(LiMI->dump());

          // There could be repeated registers in the PHI, e.g: %vreg1<def> =
          // PHI %vreg6, <BB#2>, %vreg8, <BB#3>, %vreg8, <BB#6>; So if we've
          // already replaced the def instruction, skip.
          if (LiMI->getOpcode() == PPC::ADDI || LiMI->getOpcode() == PPC::ADDI8)
            continue;

          assert((LiMI->getOpcode() == PPC::LI ||
                  LiMI->getOpcode() == PPC::LI8) &&
                 "Invalid Opcode!");
          auto LiImm = LiMI->getOperand(1).getImm(); // save the imm of LI
          LiMI->RemoveOperand(1);                    // remove the imm of LI
          LiMI->setDesc(TII->get(LiMI->getOpcode() == PPC::LI ? PPC::ADDI
                                                              : PPC::ADDI8));
          MachineInstrBuilder(*LiMI->getParent()->getParent(), *LiMI)
              .addReg(DominatorReg)
              .addImm(LiImm); // restore the imm of LI
          DEBUG(LiMI->dump());
        }

        // Replace ADD with COPY
        DEBUG(dbgs() << "Optimizing ADD to COPY: ");
        DEBUG(MI.dump());
        BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                MI.getOperand(0).getReg())
            .add(Op1);
        ToErase = &MI;
        Simplified = true;
        NumOptADDLIs++;
        break;
      }
      }
    }

    // If the last instruction was marked for elimination,
    // remove it now.
    if (ToErase) {
      ToErase->eraseFromParent();
      ToErase = nullptr;
    }
  }

  // We try to eliminate redundant compare instruction.
  Simplified |= eliminateRedundantCompare();

  return Simplified;
}

// helper functions for eliminateRedundantCompare
static bool isEqOrNe(MachineInstr *BI) {
  PPC::Predicate Pred = (PPC::Predicate)BI->getOperand(0).getImm();
  unsigned PredCond = PPC::getPredicateCondition(Pred);
  return (PredCond == PPC::PRED_EQ || PredCond == PPC::PRED_NE);
}

static bool isSupportedCmpOp(unsigned opCode) {
  return (opCode == PPC::CMPLD  || opCode == PPC::CMPD  ||
          opCode == PPC::CMPLW  || opCode == PPC::CMPW  ||
          opCode == PPC::CMPLDI || opCode == PPC::CMPDI ||
          opCode == PPC::CMPLWI || opCode == PPC::CMPWI);
}

static bool is64bitCmpOp(unsigned opCode) {
  return (opCode == PPC::CMPLD  || opCode == PPC::CMPD ||
          opCode == PPC::CMPLDI || opCode == PPC::CMPDI);
}

static bool isSignedCmpOp(unsigned opCode) {
  return (opCode == PPC::CMPD  || opCode == PPC::CMPW ||
          opCode == PPC::CMPDI || opCode == PPC::CMPWI);
}

static unsigned getSignedCmpOpCode(unsigned opCode) {
  if (opCode == PPC::CMPLD)  return PPC::CMPD;
  if (opCode == PPC::CMPLW)  return PPC::CMPW;
  if (opCode == PPC::CMPLDI) return PPC::CMPDI;
  if (opCode == PPC::CMPLWI) return PPC::CMPWI;
  return opCode;
}

// We can decrement immediate x in (GE x) by changing it to (GT x-1) or
// (LT x) to (LE x-1)
static unsigned getPredicateToDecImm(MachineInstr *BI, MachineInstr *CMPI) {
  uint64_t Imm = CMPI->getOperand(2).getImm();
  bool SignedCmp = isSignedCmpOp(CMPI->getOpcode());
  if ((!SignedCmp && Imm == 0) || (SignedCmp && Imm == 0x8000))
    return 0;

  PPC::Predicate Pred = (PPC::Predicate)BI->getOperand(0).getImm();
  unsigned PredCond = PPC::getPredicateCondition(Pred);
  unsigned PredHint = PPC::getPredicateHint(Pred);
  if (PredCond == PPC::PRED_GE)
    return PPC::getPredicate(PPC::PRED_GT, PredHint);
  if (PredCond == PPC::PRED_LT)
    return PPC::getPredicate(PPC::PRED_LE, PredHint);

  return 0;
}

// We can increment immediate x in (GT x) by changing it to (GE x+1) or
// (LE x) to (LT x+1)
static unsigned getPredicateToIncImm(MachineInstr *BI, MachineInstr *CMPI) {
  uint64_t Imm = CMPI->getOperand(2).getImm();
  bool SignedCmp = isSignedCmpOp(CMPI->getOpcode());
  if ((!SignedCmp && Imm == 0xFFFF) || (SignedCmp && Imm == 0x7FFF))
    return 0;

  PPC::Predicate Pred = (PPC::Predicate)BI->getOperand(0).getImm();
  unsigned PredCond = PPC::getPredicateCondition(Pred);
  unsigned PredHint = PPC::getPredicateHint(Pred);
  if (PredCond == PPC::PRED_GT)
    return PPC::getPredicate(PPC::PRED_GE, PredHint);
  if (PredCond == PPC::PRED_LE)
    return PPC::getPredicate(PPC::PRED_LT, PredHint);

  return 0;
}

static bool eligibleForCompareElimination(MachineBasicBlock &MBB,
                                          MachineRegisterInfo *MRI) {

  auto isEligibleBB = [&](MachineBasicBlock &BB) {
    auto BII = BB.getFirstInstrTerminator();
    // We optimize BBs ending with a conditional branch.
    // We check only for BCC here, not BCCLR, because BCCLR
    // will be formed only later in the pipeline. 
    if (BB.succ_size() == 2 &&
        BII != BB.instr_end() &&
        (*BII).getOpcode() == PPC::BCC &&
        (*BII).getOperand(1).isReg()) {
      // We optimize only if the condition code is used only by one BCC.
      unsigned CndReg = (*BII).getOperand(1).getReg();
      if (!TargetRegisterInfo::isVirtualRegister(CndReg) ||
          !MRI->hasOneNonDBGUse(CndReg))
        return false;

      // We skip this BB if a physical register is used in comparison.
      MachineInstr *CMPI = MRI->getVRegDef(CndReg);
      for (MachineOperand &MO : CMPI->operands())
        if (MO.isReg() && !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
          return false;

      return true;
    }
    return false;
  };

  if (MBB.pred_size() != 1)
    return false;

  MachineBasicBlock *PredMBB = *MBB.pred_begin();
  if (isEligibleBB(MBB) && isEligibleBB(*PredMBB))
    return true;

  return false;
}

// If multiple conditional branches are executed based on the (essentially)
// same comparison, we merge compare instructions into one and make multiple
// conditional branches on this comparison.
// For example,
//   if (a == 0) { ... }
//   else if (a < 0) { ... }
// can be executed by one compare and two conditional branches instead of
// two pairs of a compare and a conditional branch.
//
// This method merges two compare instructions in two MBBs and modifies the
// compare and conditional branch instructions if needed.
// For the above example, the input for this pass looks like:
//   cmplwi r3, 0
//   beq    0, .LBB0_3
//   cmpwi  r3, -1
//   bgt    0, .LBB0_4
// So, before merging two compares, we need to modify these instructions as
//   cmpwi  r3, 0       ; cmplwi and cmpwi yield same result for beq
//   beq    0, .LBB0_3
//   cmpwi  r3, 0       ; greather than -1 means greater or equal to 0
//   bge    0, .LBB0_4

bool PPCMIPeephole::eliminateRedundantCompare(void) {
  bool Simplified = false;

  for (MachineBasicBlock &MBB2 : *MF) {
    // We only consider two basic blocks MBB1 and MBB2 if
    // - both MBBs end with a conditional branch,
    // - MBB1 is the only predecessor of MBB2, and
    // - compare does not take a physical register as a operand in both MBBs.
    if (!eligibleForCompareElimination(MBB2, MRI))
      continue;

    MachineBasicBlock *MBB1 = *MBB2.pred_begin();
    MachineInstr *BI1   = &*MBB1->getFirstInstrTerminator();
    MachineInstr *CMPI1 = MRI->getVRegDef(BI1->getOperand(1).getReg());

    MachineInstr *BI2   = &*MBB2.getFirstInstrTerminator();
    MachineInstr *CMPI2 = MRI->getVRegDef(BI2->getOperand(1).getReg());

    // We cannot optimize an unsupported compare opcode or
    // a mix of 32-bit and 64-bit comaprisons
    if (!isSupportedCmpOp(CMPI1->getOpcode()) ||
        !isSupportedCmpOp(CMPI2->getOpcode()) ||
        is64bitCmpOp(CMPI1->getOpcode()) != is64bitCmpOp(CMPI2->getOpcode()))
      continue;

    unsigned NewOpCode = 0;
    unsigned NewPredicate1 = 0, NewPredicate2 = 0;
    int16_t Imm1 = 0, NewImm1 = 0, Imm2 = 0, NewImm2 = 0;

    if (CMPI1->getOpcode() != CMPI2->getOpcode()) {
      // Typically, unsigned comparison is used for equality check, but
      // we replace it with a signed comparison if the comparison
      // to be merged is a signed comparison.
      // In other cases of opcode mismatch, we cannot optimize this.
      if (isEqOrNe(BI2) &&
          CMPI1->getOpcode() == getSignedCmpOpCode(CMPI2->getOpcode()))
        NewOpCode = CMPI1->getOpcode();
      else if (isEqOrNe(BI1) &&
               getSignedCmpOpCode(CMPI1->getOpcode()) == CMPI2->getOpcode())
        NewOpCode = CMPI2->getOpcode();
      else continue;
    }

    if (CMPI1->getOperand(2).isReg() && CMPI2->getOperand(2).isReg()) {
      // In case of comparisons between two registers, these two registers
      // must be same to merge two comparisons.
      unsigned Cmp1Operand1 = CMPI1->getOperand(1).getReg();
      unsigned Cmp1Operand2 = CMPI1->getOperand(2).getReg();
      unsigned Cmp2Operand1 = CMPI2->getOperand(1).getReg();
      unsigned Cmp2Operand2 = CMPI2->getOperand(2).getReg();
      if (Cmp1Operand1 == Cmp2Operand1 && Cmp1Operand2 == Cmp2Operand2) {
        // Same pair of registers in the same order; ready to merge as is.
      }
      else if (Cmp1Operand1 == Cmp2Operand2 && Cmp1Operand2 == Cmp2Operand1) {
        // Same pair of registers in different order.
        // We reverse the predicate to merge compare instructions.
        PPC::Predicate Pred = (PPC::Predicate)BI2->getOperand(0).getImm();
        NewPredicate2 = (unsigned)PPC::getSwappedPredicate(Pred);
      }
      else continue;
    }
    else if (CMPI1->getOperand(2).isImm() && CMPI2->getOperand(2).isImm()){
      // In case of comparisons between a register and an immediate,
      // the operand register must be same for two compare instructions.
      if (CMPI1->getOperand(1).getReg() != CMPI2->getOperand(1).getReg())
        continue;

      NewImm1 = Imm1 = (int16_t)CMPI1->getOperand(2).getImm();
      NewImm2 = Imm2 = (int16_t)CMPI2->getOperand(2).getImm();

      // If immediate are not same, we try to adjust by changing predicate;
      // e.g. GT imm means GE (imm+1).
      if (Imm1 != Imm2 && (!isEqOrNe(BI2) || !isEqOrNe(BI1))) {
        int Diff = Imm1 - Imm2;
        if (Diff < -2 || Diff > 2)
          continue;

        unsigned PredToInc1 = getPredicateToIncImm(BI1, CMPI1);
        unsigned PredToDec1 = getPredicateToDecImm(BI1, CMPI1);
        unsigned PredToInc2 = getPredicateToIncImm(BI2, CMPI2);
        unsigned PredToDec2 = getPredicateToDecImm(BI2, CMPI2);
        if (Diff == 2) {
          if (PredToInc2 && PredToDec1) {
            NewPredicate2 = PredToInc2;
            NewPredicate1 = PredToDec1;
            NewImm2++;
            NewImm1--;
          }
        }
        else if (Diff == 1) {
          if (PredToInc2) {
            NewImm2++;
            NewPredicate2 = PredToInc2;
          }
          else if (PredToDec1) {
            NewImm1--;
            NewPredicate1 = PredToDec1;
          }
        }
        else if (Diff == -1) {
          if (PredToDec2) {
            NewImm2--;
            NewPredicate2 = PredToDec2;
          }
          else if (PredToInc1) {
            NewImm1++;
            NewPredicate1 = PredToInc1;
          }
        }
        else if (Diff == -2) {
          if (PredToDec2 && PredToInc1) {
            NewPredicate2 = PredToDec2;
            NewPredicate1 = PredToInc1;
            NewImm2--;
            NewImm1++;
          }
        }
      }

      // We cannnot merge two compares if the immediates are not same.
      if (NewImm2 != NewImm1)
        continue;
    }

    DEBUG(dbgs() << "Optimize two pairs of compare and branch:\n");
    DEBUG(CMPI1->dump());
    DEBUG(BI1->dump());
    DEBUG(CMPI2->dump());
    DEBUG(BI2->dump());

    // We adjust opcode, predicates and immediate as we determined above.
    if (NewOpCode != 0 && NewOpCode != CMPI1->getOpcode()) {
      CMPI1->setDesc(TII->get(NewOpCode));
    }
    if (NewPredicate1) {
      BI1->getOperand(0).setImm(NewPredicate1);
    }
    if (NewPredicate2) {
      BI2->getOperand(0).setImm(NewPredicate2);
    }
    if (NewImm1 != Imm1) {
      CMPI1->getOperand(2).setImm(NewImm1);
    }

    // We finally eliminate compare instruction in MBB2.
    BI2->getOperand(1).setReg(BI1->getOperand(1).getReg());
    BI2->getOperand(1).setIsKill(true);
    BI1->getOperand(1).setIsKill(false);
    CMPI2->eraseFromParent();

    DEBUG(dbgs() << "into a compare and two branches:\n");
    DEBUG(CMPI1->dump());
    DEBUG(BI1->dump());
    DEBUG(BI2->dump());

    Simplified = true;
  }

  return Simplified;
}

// This is used to find the "true" source register for an
// XXPERMDI instruction, since MachineCSE does not handle the
// "copy-like" operations (Copy and SubregToReg).  Returns
// the original SrcReg unless it is the target of a copy-like
// operation, in which case we chain backwards through all
// such operations to the ultimate source register.  If a
// physical register is encountered, we stop the search.
unsigned PPCMIPeephole::lookThruCopyLike(unsigned SrcReg) {

  while (true) {

    MachineInstr *MI = MRI->getVRegDef(SrcReg);
    if (!MI->isCopyLike())
      return SrcReg;

    unsigned CopySrcReg;
    if (MI->isCopy())
      CopySrcReg = MI->getOperand(1).getReg();
    else {
      assert(MI->isSubregToReg() && "bad opcode for lookThruCopyLike");
      CopySrcReg = MI->getOperand(2).getReg();
    }

    if (!TargetRegisterInfo::isVirtualRegister(CopySrcReg))
      return CopySrcReg;

    SrcReg = CopySrcReg;
  }
}

} // end default namespace

INITIALIZE_PASS_BEGIN(PPCMIPeephole, DEBUG_TYPE,
                      "PowerPC MI Peephole Optimization", false, false)
INITIALIZE_PASS_END(PPCMIPeephole, DEBUG_TYPE,
                    "PowerPC MI Peephole Optimization", false, false)

char PPCMIPeephole::ID = 0;
FunctionPass*
llvm::createPPCMIPeepholePass() { return new PPCMIPeephole(); }