lib/CodeGen/TailDuplicator.cpp

//===-- TailDuplicator.cpp - Duplicate blocks into predecessors' tails ---===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This utility class duplicates basic blocks ending in unconditional branches
// into the tails of their predecessors.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/TailDuplicator.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

#define DEBUG_TYPE "tailduplication"

STATISTIC(NumTails, "Number of tails duplicated");
STATISTIC(NumTailDups, "Number of tail duplicated blocks");
STATISTIC(NumInstrDups, "Additional instructions due to tail duplication");
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumAddedPHIs, "Number of phis added");

// Heuristic for tail duplication.
static cl::opt<unsigned> TailDuplicateSize(
    "tail-dup-size",
    cl::desc("Maximum instructions to consider tail duplicating"), cl::init(2),
    cl::Hidden);

static cl::opt<bool>
    TailDupVerify("tail-dup-verify",
                  cl::desc("Verify sanity of PHI instructions during taildup"),
                  cl::init(false), cl::Hidden);

static cl::opt<unsigned> TailDupLimit("tail-dup-limit", cl::init(~0U),
                                      cl::Hidden);

namespace llvm {

void TailDuplicator::initMF(MachineFunction &MF, const MachineModuleInfo *MMIin,
                            const MachineBranchProbabilityInfo *MBPIin) {
  TII = MF.getSubtarget().getInstrInfo();
  TRI = MF.getSubtarget().getRegisterInfo();
  MRI = &MF.getRegInfo();
  MMI = MMIin;
  MBPI = MBPIin;

  assert(MBPI != nullptr && "Machine Branch Probability Info required");

  PreRegAlloc = MRI->isSSA();
  RS.reset();

  if (MRI->tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
    RS.reset(new RegScavenger());
}

static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
    MachineBasicBlock *MBB = &*I;
    SmallSetVector<MachineBasicBlock *, 8> Preds(MBB->pred_begin(),
                                                 MBB->pred_end());
    MachineBasicBlock::iterator MI = MBB->begin();
    while (MI != MBB->end()) {
      if (!MI->isPHI())
        break;
      for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
                                                            PE = Preds.end();
           PI != PE; ++PI) {
        MachineBasicBlock *PredBB = *PI;
        bool Found = false;
        for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
          MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
          if (PHIBB == PredBB) {
            Found = true;
            break;
          }
        }
        if (!Found) {
          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
          dbgs() << "  missing input from predecessor BB#"
                 << PredBB->getNumber() << '\n';
          llvm_unreachable(nullptr);
        }
      }

      for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
        MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
        if (CheckExtra && !Preds.count(PHIBB)) {
          dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber() << ": "
                 << *MI;
          dbgs() << "  extra input from predecessor BB#" << PHIBB->getNumber()
                 << '\n';
          llvm_unreachable(nullptr);
        }
        if (PHIBB->getNumber() < 0) {
          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
          dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
          llvm_unreachable(nullptr);
        }
      }
      ++MI;
    }
  }
}

/// Tail duplicate the block and cleanup.
bool TailDuplicator::tailDuplicateAndUpdate(MachineFunction &MF, bool IsSimple,
                                            MachineBasicBlock *MBB) {
  // Save the successors list.
  SmallSetVector<MachineBasicBlock *, 8> Succs(MBB->succ_begin(),
                                               MBB->succ_end());

  SmallVector<MachineBasicBlock *, 8> TDBBs;
  SmallVector<MachineInstr *, 16> Copies;
  if (!tailDuplicate(MF, IsSimple, MBB, TDBBs, Copies))
    return false;

  ++NumTails;

  SmallVector<MachineInstr *, 8> NewPHIs;
  MachineSSAUpdater SSAUpdate(MF, &NewPHIs);

  // TailBB's immediate successors are now successors of those predecessors
  // which duplicated TailBB. Add the predecessors as sources to the PHI
  // instructions.
  bool isDead = MBB->pred_empty() && !MBB->hasAddressTaken();
  if (PreRegAlloc)
    updateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);

  // If it is dead, remove it.
  if (isDead) {
    NumInstrDups -= MBB->size();
    removeDeadBlock(MBB);
    ++NumDeadBlocks;
  }

  // Update SSA form.
  if (!SSAUpdateVRs.empty()) {
    for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
      unsigned VReg = SSAUpdateVRs[i];
      SSAUpdate.Initialize(VReg);

      // If the original definition is still around, add it as an available
      // value.
      MachineInstr *DefMI = MRI->getVRegDef(VReg);
      MachineBasicBlock *DefBB = nullptr;
      if (DefMI) {
        DefBB = DefMI->getParent();
        SSAUpdate.AddAvailableValue(DefBB, VReg);
      }

      // Add the new vregs as available values.
      DenseMap<unsigned, AvailableValsTy>::iterator LI =
          SSAUpdateVals.find(VReg);
      for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
        MachineBasicBlock *SrcBB = LI->second[j].first;
        unsigned SrcReg = LI->second[j].second;
        SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
      }

      // Rewrite uses that are outside of the original def's block.
      MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
      while (UI != MRI->use_end()) {
        MachineOperand &UseMO = *UI;
        MachineInstr *UseMI = UseMO.getParent();
        ++UI;
        if (UseMI->isDebugValue()) {
          // SSAUpdate can replace the use with an undef. That creates
          // a debug instruction that is a kill.
          // FIXME: Should it SSAUpdate job to delete debug instructions
          // instead of replacing the use with undef?
          UseMI->eraseFromParent();
          continue;
        }
        if (UseMI->getParent() == DefBB && !UseMI->isPHI())
          continue;
        SSAUpdate.RewriteUse(UseMO);
      }
    }

    SSAUpdateVRs.clear();
    SSAUpdateVals.clear();
  }

  // Eliminate some of the copies inserted by tail duplication to maintain
  // SSA form.
  for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
    MachineInstr *Copy = Copies[i];
    if (!Copy->isCopy())
      continue;
    unsigned Dst = Copy->getOperand(0).getReg();
    unsigned Src = Copy->getOperand(1).getReg();
    if (MRI->hasOneNonDBGUse(Src) &&
        MRI->constrainRegClass(Src, MRI->getRegClass(Dst))) {
      // Copy is the only use. Do trivial copy propagation here.
      MRI->replaceRegWith(Dst, Src);
      Copy->eraseFromParent();
    }
  }

  if (NewPHIs.size())
    NumAddedPHIs += NewPHIs.size();

  return true;
}

/// Look for small blocks that are unconditionally branched to and do not fall
/// through. Tail-duplicate their instructions into their predecessors to
/// eliminate (dynamic) branches.
bool TailDuplicator::tailDuplicateBlocks(MachineFunction &MF) {
  bool MadeChange = false;

  if (PreRegAlloc && TailDupVerify) {
    DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
    VerifyPHIs(MF, true);
  }

  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E;) {
    MachineBasicBlock *MBB = &*I++;

    if (NumTails == TailDupLimit)
      break;

    bool IsSimple = isSimpleBB(MBB);

    if (!shouldTailDuplicate(MF, IsSimple, *MBB))
      continue;

    MadeChange |= tailDuplicateAndUpdate(MF, IsSimple, MBB);
  }

  if (PreRegAlloc && TailDupVerify)
    VerifyPHIs(MF, false);

  return MadeChange;
}

static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
                         const MachineRegisterInfo *MRI) {
  for (MachineInstr &UseMI : MRI->use_instructions(Reg)) {
    if (UseMI.isDebugValue())
      continue;
    if (UseMI.getParent() != BB)
      return true;
  }
  return false;
}

static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
    if (MI->getOperand(i + 1).getMBB() == SrcBB)
      return i;
  return 0;
}

// Remember which registers are used by phis in this block. This is
// used to determine which registers are liveout while modifying the
// block (which is why we need to copy the information).
static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
                              DenseSet<unsigned> *UsedByPhi) {
  for (const auto &MI : BB) {
    if (!MI.isPHI())
      break;
    for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
      unsigned SrcReg = MI.getOperand(i).getReg();
      UsedByPhi->insert(SrcReg);
    }
  }
}

/// Add a definition and source virtual registers pair for SSA update.
void TailDuplicator::addSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
                                       MachineBasicBlock *BB) {
  DenseMap<unsigned, AvailableValsTy>::iterator LI =
      SSAUpdateVals.find(OrigReg);
  if (LI != SSAUpdateVals.end())
    LI->second.push_back(std::make_pair(BB, NewReg));
  else {
    AvailableValsTy Vals;
    Vals.push_back(std::make_pair(BB, NewReg));
    SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
    SSAUpdateVRs.push_back(OrigReg);
  }
}

/// Process PHI node in TailBB by turning it into a copy in PredBB. Remember the
/// source register that's contributed by PredBB and update SSA update map.
void TailDuplicator::processPHI(
    MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
    DenseMap<unsigned, unsigned> &LocalVRMap,
    SmallVectorImpl<std::pair<unsigned, unsigned>> &Copies,
    const DenseSet<unsigned> &RegsUsedByPhi, bool Remove) {
  unsigned DefReg = MI->getOperand(0).getReg();
  unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
  assert(SrcOpIdx && "Unable to find matching PHI source?");
  unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
  const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
  LocalVRMap.insert(std::make_pair(DefReg, SrcReg));

  // Insert a copy from source to the end of the block. The def register is the
  // available value liveout of the block.
  unsigned NewDef = MRI->createVirtualRegister(RC);
  Copies.push_back(std::make_pair(NewDef, SrcReg));
  if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
    addSSAUpdateEntry(DefReg, NewDef, PredBB);

  if (!Remove)
    return;

  // Remove PredBB from the PHI node.
  MI->RemoveOperand(SrcOpIdx + 1);
  MI->RemoveOperand(SrcOpIdx);
  if (MI->getNumOperands() == 1)
    MI->eraseFromParent();
}

/// Duplicate a TailBB instruction to PredBB and update
/// the source operands due to earlier PHI translation.
void TailDuplicator::duplicateInstruction(
    MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
    MachineFunction &MF, DenseMap<unsigned, unsigned> &LocalVRMap,
    const DenseSet<unsigned> &UsedByPhi) {
  MachineInstr *NewMI = TII->duplicate(MI, MF);
  if (PreRegAlloc) {
    for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = NewMI->getOperand(i);
      if (!MO.isReg())
        continue;
      unsigned Reg = MO.getReg();
      if (!TargetRegisterInfo::isVirtualRegister(Reg))
        continue;
      if (MO.isDef()) {
        const TargetRegisterClass *RC = MRI->getRegClass(Reg);
        unsigned NewReg = MRI->createVirtualRegister(RC);
        MO.setReg(NewReg);
        LocalVRMap.insert(std::make_pair(Reg, NewReg));
        if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
          addSSAUpdateEntry(Reg, NewReg, PredBB);
      } else {
        DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
        if (VI != LocalVRMap.end()) {
          MO.setReg(VI->second);
          // Clear any kill flags from this operand.  The new register could
          // have uses after this one, so kills are not valid here.
          MO.setIsKill(false);
          MRI->constrainRegClass(VI->second, MRI->getRegClass(Reg));
        }
      }
    }
  }
  PredBB->insert(PredBB->instr_end(), NewMI);
}

/// After FromBB is tail duplicated into its predecessor blocks, the successors
/// have gained new predecessors. Update the PHI instructions in them
/// accordingly.
void TailDuplicator::updateSuccessorsPHIs(
    MachineBasicBlock *FromBB, bool isDead,
    SmallVectorImpl<MachineBasicBlock *> &TDBBs,
    SmallSetVector<MachineBasicBlock *, 8> &Succs) {
  for (SmallSetVector<MachineBasicBlock *, 8>::iterator SI = Succs.begin(),
                                                        SE = Succs.end();
       SI != SE; ++SI) {
    MachineBasicBlock *SuccBB = *SI;
    for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
         II != EE; ++II) {
      if (!II->isPHI())
        break;
      MachineInstrBuilder MIB(*FromBB->getParent(), II);
      unsigned Idx = 0;
      for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
        MachineOperand &MO = II->getOperand(i + 1);
        if (MO.getMBB() == FromBB) {
          Idx = i;
          break;
        }
      }

      assert(Idx != 0);
      MachineOperand &MO0 = II->getOperand(Idx);
      unsigned Reg = MO0.getReg();
      if (isDead) {
        // Folded into the previous BB.
        // There could be duplicate phi source entries. FIXME: Should sdisel
        // or earlier pass fixed this?
        for (unsigned i = II->getNumOperands() - 2; i != Idx; i -= 2) {
          MachineOperand &MO = II->getOperand(i + 1);
          if (MO.getMBB() == FromBB) {
            II->RemoveOperand(i + 1);
            II->RemoveOperand(i);
          }
        }
      } else
        Idx = 0;

      // If Idx is set, the operands at Idx and Idx+1 must be removed.
      // We reuse the location to avoid expensive RemoveOperand calls.

      DenseMap<unsigned, AvailableValsTy>::iterator LI =
          SSAUpdateVals.find(Reg);
      if (LI != SSAUpdateVals.end()) {
        // This register is defined in the tail block.
        for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
          MachineBasicBlock *SrcBB = LI->second[j].first;
          // If we didn't duplicate a bb into a particular predecessor, we
          // might still have added an entry to SSAUpdateVals to correcly
          // recompute SSA. If that case, avoid adding a dummy extra argument
          // this PHI.
          if (!SrcBB->isSuccessor(SuccBB))
            continue;

          unsigned SrcReg = LI->second[j].second;
          if (Idx != 0) {
            II->getOperand(Idx).setReg(SrcReg);
            II->getOperand(Idx + 1).setMBB(SrcBB);
            Idx = 0;
          } else {
            MIB.addReg(SrcReg).addMBB(SrcBB);
          }
        }
      } else {
        // Live in tail block, must also be live in predecessors.
        for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
          MachineBasicBlock *SrcBB = TDBBs[j];
          if (Idx != 0) {
            II->getOperand(Idx).setReg(Reg);
            II->getOperand(Idx + 1).setMBB(SrcBB);
            Idx = 0;
          } else {
            MIB.addReg(Reg).addMBB(SrcBB);
          }
        }
      }
      if (Idx != 0) {
        II->RemoveOperand(Idx + 1);
        II->RemoveOperand(Idx);
      }
    }
  }
}

/// Determine if it is profitable to duplicate this block.
bool TailDuplicator::shouldTailDuplicate(const MachineFunction &MF,
                                         bool IsSimple,
                                         MachineBasicBlock &TailBB) {
  // Only duplicate blocks that end with unconditional branches.
  if (TailBB.canFallThrough())
    return false;

  // Don't try to tail-duplicate single-block loops.
  if (TailBB.isSuccessor(&TailBB))
    return false;

  // Set the limit on the cost to duplicate. When optimizing for size,
  // duplicate only one, because one branch instruction can be eliminated to
  // compensate for the duplication.
  unsigned MaxDuplicateCount;
  if (TailDuplicateSize.getNumOccurrences() == 0 &&
      // FIXME: Use Function::optForSize().
      MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize))
    MaxDuplicateCount = 1;
  else
    MaxDuplicateCount = TailDuplicateSize;

  // If the target has hardware branch prediction that can handle indirect
  // branches, duplicating them can often make them predictable when there
  // are common paths through the code.  The limit needs to be high enough
  // to allow undoing the effects of tail merging and other optimizations
  // that rearrange the predecessors of the indirect branch.

  bool HasIndirectbr = false;
  if (!TailBB.empty())
    HasIndirectbr = TailBB.back().isIndirectBranch();

  if (HasIndirectbr && PreRegAlloc)
    MaxDuplicateCount = 20;

  // Check the instructions in the block to determine whether tail-duplication
  // is invalid or unlikely to be profitable.
  unsigned InstrCount = 0;
  for (MachineInstr &MI : TailBB) {
    // Non-duplicable things shouldn't be tail-duplicated.
    if (MI.isNotDuplicable())
      return false;

    // Convergent instructions can be duplicated only if doing so doesn't add
    // new control dependencies, which is what we're going to do here.
    if (MI.isConvergent())
      return false;

    // Do not duplicate 'return' instructions if this is a pre-regalloc run.
    // A return may expand into a lot more instructions (e.g. reload of callee
    // saved registers) after PEI.
    if (PreRegAlloc && MI.isReturn())
      return false;

    // Avoid duplicating calls before register allocation. Calls presents a
    // barrier to register allocation so duplicating them may end up increasing
    // spills.
    if (PreRegAlloc && MI.isCall())
      return false;

    if (!MI.isPHI() && !MI.isDebugValue())
      InstrCount += 1;

    if (InstrCount > MaxDuplicateCount)
      return false;
  }

  // Check if any of the successors of TailBB has a PHI node in which the
  // value corresponding to TailBB uses a subregister.
  // If a phi node uses a register paired with a subregister, the actual
  // "value type" of the phi may differ from the type of the register without
  // any subregisters. Due to a bug, tail duplication may add a new operand
  // without a necessary subregister, producing an invalid code. This is
  // demonstrated by test/CodeGen/Hexagon/tail-dup-subreg-abort.ll.
  // Disable tail duplication for this case for now, until the problem is
  // fixed.
  for (auto SB : TailBB.successors()) {
    for (auto &I : *SB) {
      if (!I.isPHI())
        break;
      unsigned Idx = getPHISrcRegOpIdx(&I, &TailBB);
      assert(Idx != 0);
      MachineOperand &PU = I.getOperand(Idx);
      if (PU.getSubReg() != 0)
        return false;
    }
  }

  if (HasIndirectbr && PreRegAlloc)
    return true;

  if (IsSimple)
    return true;

  if (!PreRegAlloc)
    return true;

  return canCompletelyDuplicateBB(TailBB);
}

/// True if this BB has only one unconditional jump.
bool TailDuplicator::isSimpleBB(MachineBasicBlock *TailBB) {
  if (TailBB->succ_size() != 1)
    return false;
  if (TailBB->pred_empty())
    return false;
  MachineBasicBlock::iterator I = TailBB->getFirstNonDebugInstr();
  if (I == TailBB->end())
    return true;
  return I->isUnconditionalBranch();
}

static bool bothUsedInPHI(const MachineBasicBlock &A,
                          SmallPtrSet<MachineBasicBlock *, 8> SuccsB) {
  for (MachineBasicBlock *BB : A.successors())
    if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
      return true;

  return false;
}

bool TailDuplicator::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
  for (MachineBasicBlock *PredBB : BB.predecessors()) {
    if (PredBB->succ_size() > 1)
      return false;

    MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
    SmallVector<MachineOperand, 4> PredCond;
    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
      return false;

    if (!PredCond.empty())
      return false;
  }
  return true;
}

bool TailDuplicator::duplicateSimpleBB(
    MachineBasicBlock *TailBB, SmallVectorImpl<MachineBasicBlock *> &TDBBs,
    const DenseSet<unsigned> &UsedByPhi,
    SmallVectorImpl<MachineInstr *> &Copies) {
  SmallPtrSet<MachineBasicBlock *, 8> Succs(TailBB->succ_begin(),
                                            TailBB->succ_end());
  SmallVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
                                            TailBB->pred_end());
  bool Changed = false;
  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
                                                        PE = Preds.end();
       PI != PE; ++PI) {
    MachineBasicBlock *PredBB = *PI;

    if (PredBB->hasEHPadSuccessor())
      continue;

    if (bothUsedInPHI(*PredBB, Succs))
      continue;

    MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
    SmallVector<MachineOperand, 4> PredCond;
    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
      continue;

    Changed = true;
    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
                 << "From simple Succ: " << *TailBB);

    MachineBasicBlock *NewTarget = *TailBB->succ_begin();
    MachineBasicBlock *NextBB = &*std::next(PredBB->getIterator());

    // Make PredFBB explicit.
    if (PredCond.empty())
      PredFBB = PredTBB;

    // Make fall through explicit.
    if (!PredTBB)
      PredTBB = NextBB;
    if (!PredFBB)
      PredFBB = NextBB;

    // Redirect
    if (PredFBB == TailBB)
      PredFBB = NewTarget;
    if (PredTBB == TailBB)
      PredTBB = NewTarget;

    // Make the branch unconditional if possible
    if (PredTBB == PredFBB) {
      PredCond.clear();
      PredFBB = nullptr;
    }

    // Avoid adding fall through branches.
    if (PredFBB == NextBB)
      PredFBB = nullptr;
    if (PredTBB == NextBB && PredFBB == nullptr)
      PredTBB = nullptr;

    TII->RemoveBranch(*PredBB);

    if (!PredBB->isSuccessor(NewTarget))
      PredBB->replaceSuccessor(TailBB, NewTarget);
    else {
      PredBB->removeSuccessor(TailBB, true);
      assert(PredBB->succ_size() <= 1);
    }

    if (PredTBB)
      TII->InsertBranch(*PredBB, PredTBB, PredFBB, PredCond, DebugLoc());

    TDBBs.push_back(PredBB);
  }
  return Changed;
}

/// If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
bool TailDuplicator::tailDuplicate(MachineFunction &MF, bool IsSimple,
                                   MachineBasicBlock *TailBB,
                                   SmallVectorImpl<MachineBasicBlock *> &TDBBs,
                                   SmallVectorImpl<MachineInstr *> &Copies) {
  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');

  DenseSet<unsigned> UsedByPhi;
  getRegsUsedByPHIs(*TailBB, &UsedByPhi);

  if (IsSimple)
    return duplicateSimpleBB(TailBB, TDBBs, UsedByPhi, Copies);

  // Iterate through all the unique predecessors and tail-duplicate this
  // block into them, if possible. Copying the list ahead of time also
  // avoids trouble with the predecessor list reallocating.
  bool Changed = false;
  SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
                                               TailBB->pred_end());
  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
                                                        PE = Preds.end();
       PI != PE; ++PI) {
    MachineBasicBlock *PredBB = *PI;

    assert(TailBB != PredBB &&
           "Single-block loop should have been rejected earlier!");
    // EH edges are ignored by AnalyzeBranch.
    if (PredBB->succ_size() > 1)
      continue;

    MachineBasicBlock *PredTBB, *PredFBB;
    SmallVector<MachineOperand, 4> PredCond;
    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
      continue;
    if (!PredCond.empty())
      continue;
    // Don't duplicate into a fall-through predecessor (at least for now).
    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
      continue;

    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
                 << "From Succ: " << *TailBB);

    TDBBs.push_back(PredBB);

    // Remove PredBB's unconditional branch.
    TII->RemoveBranch(*PredBB);

    if (RS && !TailBB->livein_empty()) {
      // Update PredBB livein.
      RS->enterBasicBlock(*PredBB);
      if (!PredBB->empty())
        RS->forward(std::prev(PredBB->end()));
      for (const auto &LI : TailBB->liveins()) {
        if (!RS->isRegUsed(LI.PhysReg, false))
          // If a register is previously livein to the tail but it's not live
          // at the end of predecessor BB, then it should be added to its
          // livein list.
          PredBB->addLiveIn(LI);
      }
    }

    // Clone the contents of TailBB into PredBB.
    DenseMap<unsigned, unsigned> LocalVRMap;
    SmallVector<std::pair<unsigned, unsigned>, 4> CopyInfos;
    // Use instr_iterator here to properly handle bundles, e.g.
    // ARM Thumb2 IT block.
    MachineBasicBlock::instr_iterator I = TailBB->instr_begin();
    while (I != TailBB->instr_end()) {
      MachineInstr *MI = &*I;
      ++I;
      if (MI->isPHI()) {
        // Replace the uses of the def of the PHI with the register coming
        // from PredBB.
        processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
      } else {
        // Replace def of virtual registers with new registers, and update
        // uses with PHI source register or the new registers.
        duplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap, UsedByPhi);
      }
    }
    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
                               TII->get(TargetOpcode::COPY), CopyInfos[i].first)
                           .addReg(CopyInfos[i].second));
    }

    // Simplify
    TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);

    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch

    // Update the CFG.
    PredBB->removeSuccessor(PredBB->succ_begin());
    assert(PredBB->succ_empty() &&
           "TailDuplicate called on block with multiple successors!");
    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
                                          E = TailBB->succ_end();
         I != E; ++I)
      PredBB->addSuccessor(*I, MBPI->getEdgeProbability(TailBB, I));

    Changed = true;
    ++NumTailDups;
  }

  // If TailBB was duplicated into all its predecessors except for the prior
  // block, which falls through unconditionally, move the contents of this
  // block into the prior block.
  MachineBasicBlock *PrevBB = &*std::prev(TailBB->getIterator());
  MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
  SmallVector<MachineOperand, 4> PriorCond;
  // This has to check PrevBB->succ_size() because EH edges are ignored by
  // AnalyzeBranch.
  if (PrevBB->succ_size() == 1 &&
      !TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
      PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 &&
      !TailBB->hasAddressTaken()) {
    DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
                 << "From MBB: " << *TailBB);
    if (PreRegAlloc) {
      DenseMap<unsigned, unsigned> LocalVRMap;
      SmallVector<std::pair<unsigned, unsigned>, 4> CopyInfos;
      MachineBasicBlock::iterator I = TailBB->begin();
      // Process PHI instructions first.
      while (I != TailBB->end() && I->isPHI()) {
        // Replace the uses of the def of the PHI with the register coming
        // from PredBB.
        MachineInstr *MI = &*I++;
        processPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
      }

      // Now copy the non-PHI instructions.
      while (I != TailBB->end()) {
        // Replace def of virtual registers with new registers, and update
        // uses with PHI source register or the new registers.
        MachineInstr *MI = &*I++;
        assert(!MI->isBundle() && "Not expecting bundles before regalloc!");
        duplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap, UsedByPhi);
        MI->eraseFromParent();
      }
      MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
      for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
        Copies.push_back(BuildMI(*PrevBB, Loc, DebugLoc(),
                                 TII->get(TargetOpcode::COPY),
                                 CopyInfos[i].first)
                             .addReg(CopyInfos[i].second));
      }
    } else {
      // No PHIs to worry about, just splice the instructions over.
      PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
    }
    PrevBB->removeSuccessor(PrevBB->succ_begin());
    assert(PrevBB->succ_empty());
    PrevBB->transferSuccessors(TailBB);
    TDBBs.push_back(PrevBB);
    Changed = true;
  }

  // If this is after register allocation, there are no phis to fix.
  if (!PreRegAlloc)
    return Changed;

  // If we made no changes so far, we are safe.
  if (!Changed)
    return Changed;

  // Handle the nasty case in that we duplicated a block that is part of a loop
  // into some but not all of its predecessors. For example:
  //    1 -> 2 <-> 3                 |
  //          \                      |
  //           \---> rest            |
  // if we duplicate 2 into 1 but not into 3, we end up with
  // 12 -> 3 <-> 2 -> rest           |
  //   \             /               |
  //    \----->-----/                |
  // If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
  // with a phi in 3 (which now dominates 2).
  // What we do here is introduce a copy in 3 of the register defined by the
  // phi, just like when we are duplicating 2 into 3, but we don't copy any
  // real instructions or remove the 3 -> 2 edge from the phi in 2.
  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
                                                        PE = Preds.end();
       PI != PE; ++PI) {
    MachineBasicBlock *PredBB = *PI;
    if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end())
      continue;

    // EH edges
    if (PredBB->succ_size() != 1)
      continue;

    DenseMap<unsigned, unsigned> LocalVRMap;
    SmallVector<std::pair<unsigned, unsigned>, 4> CopyInfos;
    MachineBasicBlock::iterator I = TailBB->begin();
    // Process PHI instructions first.
    while (I != TailBB->end() && I->isPHI()) {
      // Replace the uses of the def of the PHI with the register coming
      // from PredBB.
      MachineInstr *MI = &*I++;
      processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
    }
    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
                               TII->get(TargetOpcode::COPY), CopyInfos[i].first)
                           .addReg(CopyInfos[i].second));
    }
  }

  return Changed;
}

/// Remove the specified dead machine basic block from the function, updating
/// the CFG.
void TailDuplicator::removeDeadBlock(MachineBasicBlock *MBB) {
  assert(MBB->pred_empty() && "MBB must be dead!");
  DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);

  // Remove all successors.
  while (!MBB->succ_empty())
    MBB->removeSuccessor(MBB->succ_end() - 1);

  // Remove the block.
  MBB->eraseFromParent();
}

} // End llvm namespace