C++ MachineBasicBlock::isLayoutSuccessor方法代码示例

/// Return true if \p MBB has one successor immediately following, and is its
/// only predecessor
static bool hasTrivialSuccessor(const MachineBasicBlock &MBB) {
  if (MBB.succ_size() != 1)
    return false;

  const MachineBasicBlock *Succ = *MBB.succ_begin();
  return (Succ->pred_size() == 1) && MBB.isLayoutSuccessor(Succ);
}

bool PIC16InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                   MachineBasicBlock *&TBB,
                                   MachineBasicBlock *&FBB,
                                   SmallVectorImpl<MachineOperand> &Cond,
                                   bool AllowModify) const {
  MachineBasicBlock::iterator I = MBB.end();
  if (I == MBB.begin())
    return true;

  // Get the terminator instruction.
  --I;
  while (I->isDebugValue()) {
    if (I == MBB.begin())
      return true;
    --I;
  }
  // Handle unconditional branches. If the unconditional branch's target is
  // successor basic block then remove the unconditional branch. 
  if (I->getOpcode() == PIC16::br_uncond  && AllowModify) {
    if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
      TBB = 0;
      I->eraseFromParent();
    }
  }
  return true;
}

bool PTXInstrInfo::
IsAnySuccessorAlsoLayoutSuccessor(const MachineBasicBlock& MBB) {
  for (MachineBasicBlock::const_succ_iterator
      i = MBB.succ_begin(), e = MBB.succ_end(); i != e; ++i)
    if (MBB.isLayoutSuccessor((const MachineBasicBlock*) &*i))
      return true;
  return false;
}

bool OR1KInstrInfo::analyzeBranch(MachineBasicBlock &MBB,
                                  MachineBasicBlock *&TBB,
                                  MachineBasicBlock *&FBB,
                                  SmallVectorImpl<MachineOperand> &Cond,
                                  bool AllowModify) const {
  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  MachineBasicBlock::iterator I = MBB.end();
  while (I != MBB.begin()) {
    --I;
    if (I->isDebugValue())
      continue;

    // Working from the bottom, when we see a non-terminator
    // instruction, we're done.
    if (!isUnpredicatedTerminator(*I))
      break;

    // A terminator that isn't a branch can't easily be handled
    // by this analysis.
    if (!I->isBranch())
      return true;

    // Cannot handle indirect branches.
    if (I->getOpcode() == OR1K::JR)
      return true;

    // Handle unconditional branches.
    if (I->getOpcode() == OR1K::J) {
      if (!AllowModify) {
        TBB = I->getOperand(0).getMBB();
        continue;
      }

      // If the block has any instructions after a J, delete them.
      while (std::next(I) != MBB.end())
        std::next(I)->eraseFromParent();
      Cond.clear();
      FBB = 0;

      // Delete the J if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
        TBB = 0;
        I->eraseFromParent();
        I = MBB.end();
        continue;
      }

      // TBB is used to indicate the unconditinal destination.
      TBB = I->getOperand(0).getMBB();
      continue;
    }
    // Cannot handle conditional branches
    return true;
  }

  return false;
}

MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A,
                                                     MachineBasicBlock *B) {
  assert(A && B && "Missing MBB end point");

  MachineFunction *MF = A->getParent();

  // We may need to update A's terminator, but we can't do that if AnalyzeBranch
  // fails. If A uses a jump table, we won't touch it.
  const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
  MachineBasicBlock *TBB = 0, *FBB = 0;
  SmallVector<MachineOperand, 4> Cond;
  if (TII->AnalyzeBranch(*A, TBB, FBB, Cond))
    return NULL;

  ++NumSplits;

  MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
  MF->insert(llvm::next(MachineFunction::iterator(A)), NMBB);
  DEBUG(dbgs() << "PHIElimination splitting critical edge:"
        " BB#" << A->getNumber()
        << " -- BB#" << NMBB->getNumber()
        << " -- BB#" << B->getNumber() << '\n');

  A->ReplaceUsesOfBlockWith(B, NMBB);
  A->updateTerminator();

  // Insert unconditional "jump B" instruction in NMBB if necessary.
  NMBB->addSuccessor(B);
  if (!NMBB->isLayoutSuccessor(B)) {
    Cond.clear();
    MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond);
  }

  // Fix PHI nodes in B so they refer to NMBB instead of A
  for (MachineBasicBlock::iterator i = B->begin(), e = B->end();
       i != e && i->isPHI(); ++i)
    for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
      if (i->getOperand(ni+1).getMBB() == A)
        i->getOperand(ni+1).setMBB(NMBB);

  if (LiveVariables *LV=getAnalysisIfAvailable<LiveVariables>())
    LV->addNewBlock(NMBB, A, B);

  if (MachineDominatorTree *MDT=getAnalysisIfAvailable<MachineDominatorTree>())
    MDT->addNewBlock(NMBB, A);

  return NMBB;
}

// The AnalyzeBranch function is used to examine conditional instructions and
// remove unnecessary instructions. This method is used by BranchFolder and
// IfConverter machine function passes to improve the CFG.
// - TrueBlock is set to the destination if condition evaluates true (it is the
//   nullptr if the destination is the fall-through branch);
// - FalseBlock is set to the destination if condition evaluates to false (it
//   is the nullptr if the branch is unconditional);
// - condition is populated with machine operands needed to generate the branch
//   to insert in InsertBranch;
// Returns: false if branch could successfully be analyzed.
bool LanaiInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                   MachineBasicBlock *&TrueBlock,
                                   MachineBasicBlock *&FalseBlock,
                                   SmallVectorImpl<MachineOperand> &Condition,
                                   bool AllowModify) const {
  // Iterator to current instruction being considered.
  MachineBasicBlock::iterator Instruction = MBB.end();

  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  while (Instruction != MBB.begin()) {
    --Instruction;

    // Skip over debug values.
    if (Instruction->isDebugValue())
      continue;

    // Working from the bottom, when we see a non-terminator
    // instruction, we're done.
    if (!isUnpredicatedTerminator(*Instruction))
      break;

    // A terminator that isn't a branch can't easily be handled
    // by this analysis.
    if (!Instruction->isBranch())
      return true;

    // Handle unconditional branches.
    if (Instruction->getOpcode() == Lanai::BT) {
      if (!AllowModify) {
        TrueBlock = Instruction->getOperand(0).getMBB();
        continue;
      }

      // If the block has any instructions after a branch, delete them.
      while (std::next(Instruction) != MBB.end()) {
        std::next(Instruction)->eraseFromParent();
      }

      Condition.clear();
      FalseBlock = nullptr;

      // Delete the jump if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(Instruction->getOperand(0).getMBB())) {
        TrueBlock = nullptr;
        Instruction->eraseFromParent();
        Instruction = MBB.end();
        continue;
      }

      // TrueBlock is used to indicate the unconditional destination.
      TrueBlock = Instruction->getOperand(0).getMBB();
      continue;
    }

    // Handle conditional branches
    unsigned Opcode = Instruction->getOpcode();
    if (Opcode != Lanai::BRCC)
      return true; // Unknown opcode.

    // Multiple conditional branches are not handled here so only proceed if
    // there are no conditions enqueued.
    if (Condition.empty()) {
      LPCC::CondCode BranchCond =
          static_cast<LPCC::CondCode>(Instruction->getOperand(1).getImm());

      // TrueBlock is the target of the previously seen unconditional branch.
      FalseBlock = TrueBlock;
      TrueBlock = Instruction->getOperand(0).getMBB();
      Condition.push_back(MachineOperand::CreateImm(BranchCond));
      continue;
    }

    // Multiple conditional branches are not handled.
    return true;
  }

  // Return false indicating branch successfully analyzed.
  return false;
}

/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
bool
TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
                                 SmallVector<MachineInstr*, 16> &Copies) {
  if (!shouldTailDuplicate(MF, *TailBB))
    return false;

  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');

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

    // Clone the contents of TailBB into PredBB.
    DenseMap<unsigned, unsigned> LocalVRMap;
    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
    MachineBasicBlock::iterator I = TailBB->begin();
    while (I != TailBB->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);

    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 = prior(MachineFunction::iterator(TailBB));
  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
  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()) {
//.........这里部分代码省略.........

//.........这里部分代码省略.........
      }
    }
  }

  ReplaceUsesOfBlockWith(Succ, NMBB);

  // If updateTerminator() removes instructions, we need to remove them from
  // SlotIndexes.
  SmallVector<MachineInstr*, 4> Terminators;
  if (Indexes) {
    for (instr_iterator I = getFirstInstrTerminator(), E = instr_end();
         I != E; ++I)
      Terminators.push_back(I);
  }

  updateTerminator();

  if (Indexes) {
    SmallVector<MachineInstr*, 4> NewTerminators;
    for (instr_iterator I = getFirstInstrTerminator(), E = instr_end();
         I != E; ++I)
      NewTerminators.push_back(I);

    for (SmallVectorImpl<MachineInstr*>::iterator I = Terminators.begin(),
        E = Terminators.end(); I != E; ++I) {
      if (std::find(NewTerminators.begin(), NewTerminators.end(), *I) ==
          NewTerminators.end())
       Indexes->removeMachineInstrFromMaps(*I);
    }
  }

  // Insert unconditional "jump Succ" instruction in NMBB if necessary.
  NMBB->addSuccessor(Succ);
  if (!NMBB->isLayoutSuccessor(Succ)) {
    Cond.clear();
    MF->getSubtarget().getInstrInfo()->InsertBranch(*NMBB, Succ, nullptr, Cond,
                                                    dl);

    if (Indexes) {
      for (instr_iterator I = NMBB->instr_begin(), E = NMBB->instr_end();
           I != E; ++I) {
        // Some instructions may have been moved to NMBB by updateTerminator(),
        // so we first remove any instruction that already has an index.
        if (Indexes->hasIndex(I))
          Indexes->removeMachineInstrFromMaps(I);
        Indexes->insertMachineInstrInMaps(I);
      }
    }
  }

  // Fix PHI nodes in Succ so they refer to NMBB instead of this
  for (MachineBasicBlock::instr_iterator
         i = Succ->instr_begin(),e = Succ->instr_end();
       i != e && i->isPHI(); ++i)
    for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
      if (i->getOperand(ni+1).getMBB() == this)
        i->getOperand(ni+1).setMBB(NMBB);

  // Inherit live-ins from the successor
  for (MachineBasicBlock::livein_iterator I = Succ->livein_begin(),
         E = Succ->livein_end(); I != E; ++I)
    NMBB->addLiveIn(*I);

  // Update LiveVariables.
  const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
  if (LV) {

bool MSP430InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                    MachineBasicBlock *&TBB,
                                    MachineBasicBlock *&FBB,
                                    SmallVectorImpl<MachineOperand> &Cond,
                                    bool AllowModify) const {
  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  MachineBasicBlock::iterator I = MBB.end();
  while (I != MBB.begin()) {
    --I;
    if (I->isDebugValue())
      continue;

    // Working from the bottom, when we see a non-terminator
    // instruction, we're done.
    if (!isUnpredicatedTerminator(I))
      break;

    // A terminator that isn't a branch can't easily be handled
    // by this analysis.
    if (!I->isBranch())
      return true;

    // Cannot handle indirect branches.
    if (I->getOpcode() == MSP430::Br ||
        I->getOpcode() == MSP430::Bm)
      return true;

    // Handle unconditional branches.
    if (I->getOpcode() == MSP430::JMP) {
      if (!AllowModify) {
        TBB = I->getOperand(0).getMBB();
        continue;
      }

      // If the block has any instructions after a JMP, delete them.
      while (std::next(I) != MBB.end())
        std::next(I)->eraseFromParent();
      Cond.clear();
      FBB = nullptr;

      // Delete the JMP if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
        TBB = nullptr;
        I->eraseFromParent();
        I = MBB.end();
        continue;
      }

      // TBB is used to indicate the unconditinal destination.
      TBB = I->getOperand(0).getMBB();
      continue;
    }

    // Handle conditional branches.
    assert(I->getOpcode() == MSP430::JCC && "Invalid conditional branch");
    MSP430CC::CondCodes BranchCode =
      static_cast<MSP430CC::CondCodes>(I->getOperand(1).getImm());
    if (BranchCode == MSP430CC::COND_INVALID)
      return true;  // Can't handle weird stuff.

    // Working from the bottom, handle the first conditional branch.
    if (Cond.empty()) {
      FBB = TBB;
      TBB = I->getOperand(0).getMBB();
      Cond.push_back(MachineOperand::CreateImm(BranchCode));
      continue;
    }

    // Handle subsequent conditional branches. Only handle the case where all
    // conditional branches branch to the same destination.
    assert(Cond.size() == 1);
    assert(TBB);

    // Only handle the case where all conditional branches branch to
    // the same destination.
    if (TBB != I->getOperand(0).getMBB())
      return true;

    MSP430CC::CondCodes OldBranchCode = (MSP430CC::CondCodes)Cond[0].getImm();
    // If the conditions are the same, we can leave them alone.
    if (OldBranchCode == BranchCode)
      continue;

    return true;
  }

  return false;
}

MachineBasicBlock *
MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ, Pass *P) {
  MachineFunction *MF = getParent();
  DebugLoc dl;  // FIXME: this is nowhere

  // We may need to update this's terminator, but we can't do that if
  // AnalyzeBranch fails. If this uses a jump table, we won't touch it.
  const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
  MachineBasicBlock *TBB = 0, *FBB = 0;
  SmallVector<MachineOperand, 4> Cond;
  if (TII->AnalyzeBranch(*this, TBB, FBB, Cond))
    return NULL;

  // Avoid bugpoint weirdness: A block may end with a conditional branch but
  // jumps to the same MBB is either case. We have duplicate CFG edges in that
  // case that we can't handle. Since this never happens in properly optimized
  // code, just skip those edges.
  if (TBB && TBB == FBB) {
    DEBUG(dbgs() << "Won't split critical edge after degenerate BB#"
                 << getNumber() << '\n');
    return NULL;
  }

  MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
  MF->insert(llvm::next(MachineFunction::iterator(this)), NMBB);
  DEBUG(dbgs() << "Splitting critical edge:"
        " BB#" << getNumber()
        << " -- BB#" << NMBB->getNumber()
        << " -- BB#" << Succ->getNumber() << '\n');

  // On some targets like Mips, branches may kill virtual registers. Make sure
  // that LiveVariables is properly updated after updateTerminator replaces the
  // terminators.
  LiveVariables *LV = P->getAnalysisIfAvailable<LiveVariables>();

  // Collect a list of virtual registers killed by the terminators.
  SmallVector<unsigned, 4> KilledRegs;
  if (LV)
    for (iterator I = getFirstTerminator(), E = end(); I != E; ++I) {
      MachineInstr *MI = I;
      for (MachineInstr::mop_iterator OI = MI->operands_begin(),
           OE = MI->operands_end(); OI != OE; ++OI) {
        if (!OI->isReg() || !OI->isUse() || !OI->isKill() || OI->isUndef())
          continue;
        unsigned Reg = OI->getReg();
        if (TargetRegisterInfo::isVirtualRegister(Reg) &&
            LV->getVarInfo(Reg).removeKill(MI)) {
          KilledRegs.push_back(Reg);
          DEBUG(dbgs() << "Removing terminator kill: " << *MI);
          OI->setIsKill(false);
        }
      }
    }

  ReplaceUsesOfBlockWith(Succ, NMBB);
  updateTerminator();

  // Insert unconditional "jump Succ" instruction in NMBB if necessary.
  NMBB->addSuccessor(Succ);
  if (!NMBB->isLayoutSuccessor(Succ)) {
    Cond.clear();
    MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, Succ, NULL, Cond, dl);
  }

  // Fix PHI nodes in Succ so they refer to NMBB instead of this
  for (MachineBasicBlock::iterator i = Succ->begin(), e = Succ->end();
       i != e && i->isPHI(); ++i)
    for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
      if (i->getOperand(ni+1).getMBB() == this)
        i->getOperand(ni+1).setMBB(NMBB);

  // Inherit live-ins from the successor
  for (MachineBasicBlock::livein_iterator I = Succ->livein_begin(),
	 E = Succ->livein_end(); I != E; ++I)
    NMBB->addLiveIn(*I);

  // Update LiveVariables.
  if (LV) {
    // Restore kills of virtual registers that were killed by the terminators.
    while (!KilledRegs.empty()) {
      unsigned Reg = KilledRegs.pop_back_val();
      for (iterator I = end(), E = begin(); I != E;) {
        if (!(--I)->addRegisterKilled(Reg, NULL, /* addIfNotFound= */ false))
          continue;
        LV->getVarInfo(Reg).Kills.push_back(I);
        DEBUG(dbgs() << "Restored terminator kill: " << *I);
        break;
      }
    }
    // Update relevant live-through information.
    LV->addNewBlock(NMBB, this, Succ);
  }

  if (MachineDominatorTree *MDT =
      P->getAnalysisIfAvailable<MachineDominatorTree>()) {
    // Update dominator information.
    MachineDomTreeNode *SucccDTNode = MDT->getNode(Succ);

    bool IsNewIDom = true;
    for (const_pred_iterator PI = Succ->pred_begin(), E = Succ->pred_end();
//.........这里部分代码省略.........

bool Mos6502InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                   MachineBasicBlock *&TBB,
                                   MachineBasicBlock *&FBB,
                                   SmallVectorImpl<MachineOperand> &Cond,
                                   bool AllowModify) const
{

  MachineBasicBlock::iterator I = MBB.end();
  MachineBasicBlock::iterator UnCondBrIter = MBB.end();
  while (I != MBB.begin()) {
    --I;

    if (I->isDebugValue())
      continue;

    // When we see a non-terminator, we are done.
    if (!isUnpredicatedTerminator(I))
      break;

    // Terminator is not a branch.
    if (!I->isBranch())
      return true;

    // Handle Unconditional branches.
    if (I->getOpcode() == M6502::BA) {
      UnCondBrIter = I;

      if (!AllowModify) {
        TBB = I->getOperand(0).getMBB();
        continue;
      }

      while (std::next(I) != MBB.end())
        std::next(I)->eraseFromParent();

      Cond.clear();
      FBB = nullptr;

      if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
        TBB = nullptr;
        I->eraseFromParent();
        I = MBB.end();
        UnCondBrIter = MBB.end();
        continue;
      }

      TBB = I->getOperand(0).getMBB();
      continue;
    }

    unsigned Opcode = I->getOpcode();
    if (Opcode != M6502::BCOND && Opcode != M6502::FBCOND)
      return true; // Unknown Opcode.

    SPCC::CondCodes BranchCode = (SPCC::CondCodes)I->getOperand(1).getImm();

    if (Cond.empty()) {
      MachineBasicBlock *TargetBB = I->getOperand(0).getMBB();
      if (AllowModify && UnCondBrIter != MBB.end() &&
          MBB.isLayoutSuccessor(TargetBB)) {

        // Transform the code
        //
        //    brCC L1
        //    ba L2
        // L1:
        //    ..
        // L2:
        //
        // into
        //
        //   brnCC L2
        // L1:
        //   ...
        // L2:
        //
        BranchCode = GetOppositeBranchCondition(BranchCode);
        MachineBasicBlock::iterator OldInst = I;
        BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(Opcode))
          .addMBB(UnCondBrIter->getOperand(0).getMBB()).addImm(BranchCode);
        BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(M6502::BA))
          .addMBB(TargetBB);

        OldInst->eraseFromParent();
        UnCondBrIter->eraseFromParent();

        UnCondBrIter = MBB.end();
        I = MBB.end();
        continue;
      }
      FBB = TBB;
      TBB = I->getOperand(0).getMBB();
      Cond.push_back(MachineOperand::CreateImm(BranchCode));
      continue;
    }
    // FIXME: Handle subsequent conditional branches.
    // For now, we can't handle multiple conditional branches.
    return true;
  }
  return false;
//.........这里部分代码省略.........

/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
bool
TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
                                 SmallVector<MachineInstr*, 16> &Copies) {
    // Set the limit on the number of instructions to duplicate, with a default
    // of one less than the tail-merge threshold. 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 &&
            MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
        MaxDuplicateCount = 1;
    else
        MaxDuplicateCount = TailDuplicateSize;

    if (PreRegAlloc) {
        if (TailBB->empty())
            return false;
        const TargetInstrDesc &TID = TailBB->back().getDesc();
        // Pre-regalloc tail duplication hurts compile time and doesn't help
        // much except for indirect branches and returns.
        if (!TID.isIndirectBranch() && !TID.isReturn())
            return false;
        // 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.
        MaxDuplicateCount = 20;
    }

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

    // Check the instructions in the block to determine whether tail-duplication
    // is invalid or unlikely to be profitable.
    unsigned InstrCount = 0;
    bool HasCall = false;
    for (MachineBasicBlock::iterator I = TailBB->begin();
            I != TailBB->end(); ++I) {
        // Non-duplicable things shouldn't be tail-duplicated.
        if (I->getDesc().isNotDuplicable()) 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 && I->getDesc().isReturn()) return false;
        // Don't duplicate more than the threshold.
        if (InstrCount == MaxDuplicateCount) return false;
        // Remember if we saw a call.
        if (I->getDesc().isCall()) HasCall = true;
        if (!I->isPHI() && !I->isDebugValue())
            InstrCount += 1;
    }
    // Don't tail-duplicate calls before register allocation. Calls presents a
    // barrier to register allocation so duplicating them may end up increasing
    // spills.
    if (InstrCount > 1 && (PreRegAlloc && HasCall))
        return false;

    DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');

    // 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!");
        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;
        // EH edges are ignored by AnalyzeBranch.
        if (PredBB->succ_size() != 1)
            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);

        // Clone the contents of TailBB into PredBB.
//.........这里部分代码省略.........

bool SystemZInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                     MachineBasicBlock *&TBB,
                                     MachineBasicBlock *&FBB,
                                     SmallVectorImpl<MachineOperand> &Cond,
                                     bool AllowModify) const {
  // Most of the code and comments here are boilerplate.

  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  MachineBasicBlock::iterator I = MBB.end();
  while (I != MBB.begin()) {
    --I;
    if (I->isDebugValue())
      continue;

    // Working from the bottom, when we see a non-terminator instruction, we're
    // done.
    if (!isUnpredicatedTerminator(I))
      break;

    // A terminator that isn't a branch can't easily be handled by this
    // analysis.
    unsigned ThisCond;
    const MachineOperand *ThisTarget;
    if (!isBranch(I, ThisCond, ThisTarget))
      return true;

    // Can't handle indirect branches.
    if (!ThisTarget->isMBB())
      return true;

    if (ThisCond == SystemZ::CCMASK_ANY) {
      // Handle unconditional branches.
      if (!AllowModify) {
        TBB = ThisTarget->getMBB();
        continue;
      }

      // If the block has any instructions after a JMP, delete them.
      while (llvm::next(I) != MBB.end())
        llvm::next(I)->eraseFromParent();

      Cond.clear();
      FBB = 0;

      // Delete the JMP if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(ThisTarget->getMBB())) {
        TBB = 0;
        I->eraseFromParent();
        I = MBB.end();
        continue;
      }

      // TBB is used to indicate the unconditinal destination.
      TBB = ThisTarget->getMBB();
      continue;
    }

    // Working from the bottom, handle the first conditional branch.
    if (Cond.empty()) {
      // FIXME: add X86-style branch swap
      FBB = TBB;
      TBB = ThisTarget->getMBB();
      Cond.push_back(MachineOperand::CreateImm(ThisCond));
      continue;
    }

    // Handle subsequent conditional branches.
    assert(Cond.size() == 1);
    assert(TBB);

    // Only handle the case where all conditional branches branch to the same
    // destination.
    if (TBB != ThisTarget->getMBB())
      return true;

    // If the conditions are the same, we can leave them alone.
    unsigned OldCond = Cond[0].getImm();
    if (OldCond == ThisCond)
      continue;

    // FIXME: Try combining conditions like X86 does.  Should be easy on Z!
  }

  return false;
}

bool AVRInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                 MachineBasicBlock *&TBB,
                                 MachineBasicBlock *&FBB,
                                 SmallVectorImpl<MachineOperand> &Cond,
                                 bool AllowModify) const {
  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  MachineBasicBlock::iterator I = MBB.end();
  MachineBasicBlock::iterator UnCondBrIter = MBB.end();

  while (I != MBB.begin()) {
    --I;
    if (I->isDebugValue()) {
      continue;
    }

    // Working from the bottom, when we see a non-terminator
    // instruction, we're done.
    if (!isUnpredicatedTerminator(*I)) {
      break;
    }

    // A terminator that isn't a branch can't easily be handled
    // by this analysis.
    if (!I->getDesc().isBranch()) {
      return true;
    }

    // Handle unconditional branches.
    //:TODO: add here jmp
    if (I->getOpcode() == AVR::RJMPk) {
      UnCondBrIter = I;

      if (!AllowModify) {
        TBB = I->getOperand(0).getMBB();
        continue;
      }

      // If the block has any instructions after a JMP, delete them.
      while (std::next(I) != MBB.end()) {
        std::next(I)->eraseFromParent();
      }

      Cond.clear();
      FBB = 0;

      // Delete the JMP if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
        TBB = 0;
        I->eraseFromParent();
        I = MBB.end();
        UnCondBrIter = MBB.end();
        continue;
      }

      // TBB is used to indicate the unconditinal destination.
      TBB = I->getOperand(0).getMBB();
      continue;
    }

    // Handle conditional branches.
    AVRCC::CondCodes BranchCode = getCondFromBranchOpc(I->getOpcode());
    if (BranchCode == AVRCC::COND_INVALID) {
      return true; // Can't handle indirect branch.
    }

    // Working from the bottom, handle the first conditional branch.
    if (Cond.empty()) {
      MachineBasicBlock *TargetBB = I->getOperand(0).getMBB();
      if (AllowModify && UnCondBrIter != MBB.end() &&
          MBB.isLayoutSuccessor(TargetBB)) {
        // If we can modify the code and it ends in something like:
        //
        //     jCC L1
        //     jmp L2
        //   L1:
        //     ...
        //   L2:
        //
        // Then we can change this to:
        //
        //     jnCC L2
        //   L1:
        //     ...
        //   L2:
        //
        // Which is a bit more efficient.
        // We conditionally jump to the fall-through block.
        BranchCode = getOppositeCondition(BranchCode);
        unsigned JNCC = getBrCond(BranchCode).getOpcode();
        MachineBasicBlock::iterator OldInst = I;

        BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(JNCC))
            .addMBB(UnCondBrIter->getOperand(0).getMBB());
        BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(AVR::RJMPk))
            .addMBB(TargetBB);

        OldInst->eraseFromParent();
        UnCondBrIter->eraseFromParent();

//.........这里部分代码省略.........

bool DCPU16InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                    MachineBasicBlock *&TBB,
                                    MachineBasicBlock *&FBB,
                                    SmallVectorImpl<MachineOperand> &Cond,
                                    bool AllowModify) const {
  // Start from the bottom of the block and work up, examining the
  // terminator instructions.
  MachineBasicBlock::iterator I = MBB.end();
  while (I != MBB.begin()) {
    --I;
    if (I->isDebugValue())
      continue;

    // Working from the bottom, when we see a non-terminator
    // instruction, we're done.
    if (!isUnpredicatedTerminator(I))
      break;

    // A terminator that isn't a branch can't easily be handled
    // by this analysis.
    if (!I->isBranch())
      return true;

    // Cannot handle indirect branches.
    if (I->getOpcode() == DCPU16::Br ||
        I->getOpcode() == DCPU16::Bm)
      return true;

    // Handle unconditional branches.
    if (I->getOpcode() == DCPU16::JMP) {
      if (!AllowModify) {
        TBB = I->getOperand(0).getMBB();
        continue;
      }

      // If the block has any instructions after a JMP, delete them.
      while (llvm::next(I) != MBB.end())
        llvm::next(I)->eraseFromParent();
      Cond.clear();
      FBB = 0;

      // Delete the JMP if it's equivalent to a fall-through.
      if (MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
        TBB = 0;
        I->eraseFromParent();
        I = MBB.end();
        continue;
      }

      // TBB is used to indicate the unconditinal destination.
      TBB = I->getOperand(0).getMBB();
      continue;
    }

    // Handle conditional branches.
    assert(isBR_CC(I->getOpcode()) && "Invalid conditional branch");
    DCPU16CC::CondCodes BranchCode =
      static_cast<DCPU16CC::CondCodes>(I->getOperand(0).getImm());
    if (BranchCode == DCPU16CC::COND_INVALID)
      return true;  // Can't handle weird stuff.

    MachineOperand LHS = I->getOperand(1);
    MachineOperand RHS = I->getOperand(2);

    // Working from the bottom, handle the first conditional branch.
    if (Cond.empty()) {
      FBB = TBB;
      TBB = I->getOperand(3).getMBB();
      Cond.push_back(MachineOperand::CreateImm(I->getOpcode()));
      Cond.push_back(MachineOperand::CreateImm(BranchCode));
      Cond.push_back(LHS);
      Cond.push_back(RHS);
      continue;
    }

    assert(Cond.size() == 4);
    assert(TBB);

    // Is it a complex CC?
    DCPU16CC::CondCodes complexCC;
    if ((BranchCode == DCPU16CC::COND_E)
        && AcceptsAdditionalEqualityCheck((DCPU16CC::CondCodes) Cond[1].getImm(), &complexCC)
        && (TBB == I->getOperand(3).getMBB())
        // This should actually check for equality but that's just too much code...
        && (((Cond[2].getType() == LHS.getType()) && (Cond[3].getType() == RHS.getType()))
          || ((Cond[2].getType() == RHS.getType()) && (Cond[3].getType() == LHS.getType())))) {

      Cond[1] = MachineOperand::CreateImm(complexCC);
    }
    break;
  }

  return false;
}