C++ MachineBasicBlock::empty方法代码示例

void SIWholeQuadMode::propagateBlock(MachineBasicBlock &MBB,
                                     std::vector<WorkItem>& Worklist) {
  BlockInfo BI = Blocks[&MBB]; // Make a copy to prevent dangling references.

  // Propagate through instructions
  if (!MBB.empty()) {
    MachineInstr *LastMI = &*MBB.rbegin();
    InstrInfo &LastII = Instructions[LastMI];
    if ((LastII.OutNeeds | BI.OutNeeds) != LastII.OutNeeds) {
      LastII.OutNeeds |= BI.OutNeeds;
      Worklist.push_back(LastMI);
    }
  }

  // Predecessor blocks must provide for our WQM/Exact needs.
  for (MachineBasicBlock *Pred : MBB.predecessors()) {
    BlockInfo &PredBI = Blocks[Pred];
    if ((PredBI.OutNeeds | BI.InNeeds) == PredBI.OutNeeds)
      continue;

    PredBI.OutNeeds |= BI.InNeeds;
    PredBI.InNeeds |= BI.InNeeds;
    Worklist.push_back(Pred);
  }

  // All successors must be prepared to accept the same set of WQM/Exact data.
  for (MachineBasicBlock *Succ : MBB.successors()) {
    BlockInfo &SuccBI = Blocks[Succ];
    if ((SuccBI.InNeeds | BI.OutNeeds) == SuccBI.InNeeds)
      continue;

    SuccBI.InNeeds |= BI.OutNeeds;
    Worklist.push_back(Succ);
  }
}

bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
  // Can't sink anything out of a block that has less than two successors.
  if (MBB.succ_size() <= 1 || MBB.empty()) return false;

  bool MadeChange = false;

  // Walk the basic block bottom-up.  Remember if we saw a store.
  MachineBasicBlock::iterator I = MBB.end();
  --I;
  bool ProcessedBegin, SawStore = false;
  do {
    MachineInstr *MI = I;  // The instruction to sink.
    
    // Predecrement I (if it's not begin) so that it isn't invalidated by
    // sinking.
    ProcessedBegin = I == MBB.begin();
    if (!ProcessedBegin)
      --I;

    if (MI->isDebugValue())
      continue;

    if (SinkInstruction(MI, SawStore))
      ++NumSunk, MadeChange = true;
    
    // If we just processed the first instruction in the block, we're done.
  } while (!ProcessedBegin);
  
  return MadeChange;
}

// Return true if \p MI dominates of uses of virtual register \p VReg
static bool dominatesAllUsesOf(const MachineInstr *MI, unsigned VReg,
                               MachineDominatorTree *MDT,
                               MachineRegisterInfo *MRI) {

  assert(TargetRegisterInfo::isVirtualRegister(VReg) &&
         "Expected virtual register!");

  for (auto it = MRI->use_nodbg_begin(VReg), end = MRI->use_nodbg_end();
       it != end; ++it) {
    MachineInstr *User = it->getParent();
    if (User->isPHI()) {
      unsigned BBOperandIdx = User->getOperandNo(&*it) + 1;
      MachineBasicBlock *MBB = User->getOperand(BBOperandIdx).getMBB();
      if (MBB->empty()) {
        const MachineBasicBlock *InstBB = MI->getParent();
        assert(InstBB != MBB && "Instruction found in empty MBB");
        if (!MDT->dominates(InstBB, MBB))
          return false;
        continue;
      }
      User = &*MBB->rbegin();
    }

    if (!MDT->dominates(MI, User))
      return false;
  }
  return true;
}

bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                   MachineBasicBlock &MBB,
                                   LiveVariables &LV,
                                   MachineLoopInfo *MLI) {
  if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
    return false;   // Quick exit for basic blocks without PHIs.

  bool Changed = false;
  for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
      // We break edges when registers are live out from the predecessor block
      // (not considering PHI nodes). If the register is live in to this block
      // anyway, we would gain nothing from splitting.
      // Avoid splitting backedges of loops. It would introduce small
      // out-of-line blocks into the loop which is very bad for code placement.
      if (PreMBB != &MBB &&
          !LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) {
        if (!MLI ||
            !(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) &&
              MLI->isLoopHeader(&MBB))) {
          if (PreMBB->SplitCriticalEdge(&MBB, this)) {
            Changed = true;
            ++NumCriticalEdgesSplit;
          }
        }
      }
    }
  }
  return Changed;
}

/// Sink an instruction and its associated debug instructions.
static void performSink(MachineInstr &MI, MachineBasicBlock &SuccToSinkTo,
                        MachineBasicBlock::iterator InsertPos) {
  // Collect matching debug values.
  SmallVector<MachineInstr *, 2> DbgValuesToSink;
  collectDebugValues(MI, DbgValuesToSink);

  // If we cannot find a location to use (merge with), then we erase the debug
  // location to prevent debug-info driven tools from potentially reporting
  // wrong location information.
  if (!SuccToSinkTo.empty() && InsertPos != SuccToSinkTo.end())
    MI.setDebugLoc(DILocation::getMergedLocation(MI.getDebugLoc(),
                                                 InsertPos->getDebugLoc()));
  else
    MI.setDebugLoc(DebugLoc());

  // Move the instruction.
  MachineBasicBlock *ParentBlock = MI.getParent();
  SuccToSinkTo.splice(InsertPos, ParentBlock, MI,
                      ++MachineBasicBlock::iterator(MI));

  // Move previously adjacent debug value instructions to the insert position.
  for (SmallVectorImpl<MachineInstr *>::iterator DBI = DbgValuesToSink.begin(),
                                                 DBE = DbgValuesToSink.end();
       DBI != DBE; ++DBI) {
    MachineInstr *DbgMI = *DBI;
    SuccToSinkTo.splice(InsertPos, ParentBlock, DbgMI,
                        ++MachineBasicBlock::iterator(DbgMI));
  }
}

bool PPCInstrInfo::BlockHasNoFallThrough(MachineBasicBlock &MBB) const {
  if (MBB.empty()) return false;
  
  switch (MBB.back().getOpcode()) {
  case PPC::B:     // Uncond branch.
  case PPC::BCTR:  // Indirect branch.
    return true;
  default: return false;
  }
}

static bool
bothUsedInPHI(const MachineBasicBlock &A,
              SmallPtrSet<MachineBasicBlock*, 8> SuccsB) {
  for (MachineBasicBlock::const_succ_iterator SI = A.succ_begin(),
         SE = A.succ_end(); SI != SE; ++SI) {
    MachineBasicBlock *BB = *SI;
    if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
      return true;
  }

  return false;
}

unsigned PTXInstrInfo::RemoveBranch(MachineBasicBlock &MBB) const {
  unsigned count = 0;
  while (!MBB.empty())
    if (IsAnyKindOfBranch(MBB.back())) {
      MBB.pop_back();
      ++count;
    } else
      break;
  DEBUG(dbgs() << "RemoveBranch: MBB:   " << MBB.getName().str() << "\n");
  DEBUG(dbgs() << "RemoveBranch: remove " << count << " branch inst\n");
  return count;
}

bool AlphaInstrInfo::BlockHasNoFallThrough(const MachineBasicBlock &MBB) const {
  if (MBB.empty()) return false;
  
  switch (MBB.back().getOpcode()) {
  case Alpha::RETDAG: // Return.
  case Alpha::RETDAGp:
  case Alpha::BR:     // Uncond branch.
  case Alpha::JMP:  // Indirect branch.
    return true;
  default: return false;
  }
}

HexagonBlockRanges::InstrIndexMap::InstrIndexMap(MachineBasicBlock &B)
    : Block(B) {
  IndexType Idx = IndexType::First;
  First = Idx;
  for (auto &In : B) {
    if (In.isDebugInstr())
      continue;
    assert(getIndex(&In) == IndexType::None && "Instruction already in map");
    Map.insert(std::make_pair(Idx, &In));
    ++Idx;
  }
  Last = B.empty() ? IndexType::None : unsigned(Idx)-1;
}

/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
/// predecessor basic blocks.
///
bool PHIElimination::EliminatePHINodes(MachineFunction &MF,
                                             MachineBasicBlock &MBB) {
  if (MBB.empty() || !MBB.front().isPHI())
    return false;   // Quick exit for basic blocks without PHIs.

  // Get an iterator to the first instruction after the last PHI node (this may
  // also be the end of the basic block).
  MachineBasicBlock::iterator AfterPHIsIt = MBB.SkipPHIsAndLabels(MBB.begin());

  while (MBB.front().isPHI())
    LowerAtomicPHINode(MBB, AfterPHIsIt);

  return true;
}

bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
  // Can't sink anything out of a block that has less than two successors.
  if (MBB.succ_size() <= 1 || MBB.empty()) return false;

  // Don't bother sinking code out of unreachable blocks. In addition to being
  // unprofitable, it can also lead to infinite looping, because in an
  // unreachable loop there may be nowhere to stop.
  if (!DT->isReachableFromEntry(&MBB)) return false;

  bool MadeChange = false;

  // Cache all successors, sorted by frequency info and loop depth.
  AllSuccsCache AllSuccessors;

  // Walk the basic block bottom-up.  Remember if we saw a store.
  MachineBasicBlock::iterator I = MBB.end();
  --I;
  bool ProcessedBegin, SawStore = false;
  do {
    MachineInstr &MI = *I;  // The instruction to sink.

    // Predecrement I (if it's not begin) so that it isn't invalidated by
    // sinking.
    ProcessedBegin = I == MBB.begin();
    if (!ProcessedBegin)
      --I;

    if (MI.isDebugInstr())
      continue;

    bool Joined = PerformTrivialForwardCoalescing(MI, &MBB);
    if (Joined) {
      MadeChange = true;
      continue;
    }

    if (SinkInstruction(MI, SawStore, AllSuccessors)) {
      ++NumSunk;
      MadeChange = true;
    }

    // If we just processed the first instruction in the block, we're done.
  } while (!ProcessedBegin);

  return MadeChange;
}

/// BlockHasNoFallThrough - Analyse if MachineBasicBlock does not
/// fall-through into its successor block.
bool XCoreInstrInfo::
BlockHasNoFallThrough(const MachineBasicBlock &MBB) const 
{
  if (MBB.empty()) return false;
  
  switch (MBB.back().getOpcode()) {
  case XCore::RETSP_u6:     // Return.
  case XCore::RETSP_lu6:
  case XCore::BAU_1r:       // Indirect branch.
  case XCore::BRFU_u6:      // Uncond branch.
  case XCore::BRFU_lu6:
  case XCore::BRBU_u6:
  case XCore::BRBU_lu6:
    return true;
  default: return false;
  }
}

bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                         MachineBasicBlock &MBB,
                                         LiveVariables &LV) {
  if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
    return false;   // Quick exit for basic blocks without PHIs.

  for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
      // We break edges when registers are live out from the predecessor block
      // (not considering PHI nodes). If the register is live in to this block
      // anyway, we would gain nothing from splitting.
      if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB))
        SplitCriticalEdge(PreMBB, &MBB);
    }
  }
  return true;
}

// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg
// when following the CFG edge to SuccMBB. This needs to be after any def of
// SrcReg, but before any subsequent point where control flow might jump out of
// the basic block.
MachineBasicBlock::iterator
llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
                                          MachineBasicBlock &SuccMBB,
                                          unsigned SrcReg) {
  // Handle the trivial case trivially.
  if (MBB.empty())
    return MBB.begin();

  // Usually, we just want to insert the copy before the first terminator
  // instruction. However, for the edge going to a landing pad, we must insert
  // the copy before the call/invoke instruction.
  if (!SuccMBB.isLandingPad())
    return MBB.getFirstTerminator();

  // Discover any defs/uses in this basic block.
  SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
  for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
         RE = MRI->reg_end(); RI != RE; ++RI) {
    MachineInstr *DefUseMI = &*RI;
    if (DefUseMI->getParent() == &MBB)
      DefUsesInMBB.insert(DefUseMI);
  }

  MachineBasicBlock::iterator InsertPoint;
  if (DefUsesInMBB.empty()) {
    // No defs.  Insert the copy at the start of the basic block.
    InsertPoint = MBB.begin();
  } else if (DefUsesInMBB.size() == 1) {
    // Insert the copy immediately after the def/use.
    InsertPoint = *DefUsesInMBB.begin();
    ++InsertPoint;
  } else {
    // Insert the copy immediately after the last def/use.
    InsertPoint = MBB.end();
    while (!DefUsesInMBB.count(&*--InsertPoint)) {}
    ++InsertPoint;
  }

  // Make sure the copy goes after any phi nodes however.
  return SkipPHIsAndLabels(MBB, InsertPoint);
}