text: //===- BasicBlockUtils.cpp - BasicBlock Utilities --------------------------==//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  // This family of functions perform manipulations on basic blocks, and
  // instructions contained within basic blocks.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  #include "llvm/ADT/ArrayRef.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/Analysis/CFG.h"
  #include "llvm/Analysis/DomTreeUpdater.h"
  #include "llvm/Analysis/LoopInfo.h"
  #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  #include "llvm/Analysis/MemorySSAUpdater.h"
  #include "llvm/Analysis/PostDominators.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/CFG.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DebugInfoMetadata.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/InstrTypes.h"
  #include "llvm/IR/Instruction.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/LLVMContext.h"
  #include "llvm/IR/Type.h"
  #include "llvm/IR/User.h"
  #include "llvm/IR/Value.h"
  #include "llvm/IR/ValueHandle.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include <cassert>
  #include <cstdint>
  #include <string>
  #include <utility>
  #include <vector>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "basicblock-utils"
  
  void llvm::DetatchDeadBlocks(
      ArrayRef<BasicBlock *> BBs,
      SmallVectorImpl<DominatorTree::UpdateType> *Updates,
      bool KeepOneInputPHIs) {
    for (auto *BB : BBs) {
      // Loop through all of our successors and make sure they know that one
      // of their predecessors is going away.
      SmallPtrSet<BasicBlock *, 4> UniqueSuccessors;
      for (BasicBlock *Succ : successors(BB)) {
        Succ->removePredecessor(BB, KeepOneInputPHIs);
        if (Updates && UniqueSuccessors.insert(Succ).second)
          Updates->push_back({DominatorTree::Delete, BB, Succ});
      }
  
      // Zap all the instructions in the block.
      while (!BB->empty()) {
        Instruction &I = BB->back();
        // If this instruction is used, replace uses with an arbitrary value.
        // Because control flow can't get here, we don't care what we replace the
        // value with.  Note that since this block is unreachable, and all values
        // contained within it must dominate their uses, that all uses will
        // eventually be removed (they are themselves dead).
        if (!I.use_empty())
          I.replaceAllUsesWith(UndefValue::get(I.getType()));
        BB->getInstList().pop_back();
      }
      new UnreachableInst(BB->getContext(), BB);
      assert(BB->getInstList().size() == 1 &&
             isa<UnreachableInst>(BB->getTerminator()) &&
             "The successor list of BB isn't empty before "
             "applying corresponding DTU updates.");
    }
  }
  
  void llvm::DeleteDeadBlock(BasicBlock *BB, DomTreeUpdater *DTU,
                             bool KeepOneInputPHIs) {
    DeleteDeadBlocks({BB}, DTU, KeepOneInputPHIs);
  }
  
  void llvm::DeleteDeadBlocks(ArrayRef <BasicBlock *> BBs, DomTreeUpdater *DTU,
                              bool KeepOneInputPHIs) {
  #ifndef NDEBUG
    // Make sure that all predecessors of each dead block is also dead.
    SmallPtrSet<BasicBlock *, 4> Dead(BBs.begin(), BBs.end());
    assert(Dead.size() == BBs.size() && "Duplicating blocks?");
    for (auto *BB : Dead)
      for (BasicBlock *Pred : predecessors(BB))
        assert(Dead.count(Pred) && "All predecessors must be dead!");
  #endif
  
    SmallVector<DominatorTree::UpdateType, 4> Updates;
    DetatchDeadBlocks(BBs, DTU ? &Updates : nullptr, KeepOneInputPHIs);
  
    if (DTU)
      DTU->applyUpdatesPermissive(Updates);
  
    for (BasicBlock *BB : BBs)
      if (DTU)
        DTU->deleteBB(BB);
      else
        BB->eraseFromParent();
  }
  
  bool llvm::EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU,
                                        bool KeepOneInputPHIs) {
    df_iterator_default_set<BasicBlock*> Reachable;
  
    // Mark all reachable blocks.
    for (BasicBlock *BB : depth_first_ext(&F, Reachable))
      (void)BB/* Mark all reachable blocks */;
  
    // Collect all dead blocks.
    std::vector<BasicBlock*> DeadBlocks;
    for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
      if (!Reachable.count(&*I)) {
        BasicBlock *BB = &*I;
        DeadBlocks.push_back(BB);
      }
  
    // Delete the dead blocks.
    DeleteDeadBlocks(DeadBlocks, DTU, KeepOneInputPHIs);
  
    return !DeadBlocks.empty();
  }
  
  void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
                                     MemoryDependenceResults *MemDep) {
    if (!isa<PHINode>(BB->begin())) return;
  
    while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
      if (PN->getIncomingValue(0) != PN)
        PN->replaceAllUsesWith(PN->getIncomingValue(0));
      else
        PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
  
      if (MemDep)
        MemDep->removeInstruction(PN);  // Memdep updates AA itself.
  
      PN->eraseFromParent();
    }
  }
  
  bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI) {
    // Recursively deleting a PHI may cause multiple PHIs to be deleted
    // or RAUW'd undef, so use an array of WeakTrackingVH for the PHIs to delete.
    SmallVector<WeakTrackingVH, 8> PHIs;
    for (PHINode &PN : BB->phis())
      PHIs.push_back(&PN);
  
    bool Changed = false;
    for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
      if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
        Changed |= RecursivelyDeleteDeadPHINode(PN, TLI);
  
    return Changed;
  }
  
  bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
                                       LoopInfo *LI, MemorySSAUpdater *MSSAU,
                                       MemoryDependenceResults *MemDep) {
    if (BB->hasAddressTaken())
      return false;
  
    // Can't merge if there are multiple predecessors, or no predecessors.
    BasicBlock *PredBB = BB->getUniquePredecessor();
    if (!PredBB) return false;
  
    // Don't break self-loops.
    if (PredBB == BB) return false;
    // Don't break unwinding instructions.
    if (PredBB->getTerminator()->isExceptionalTerminator())
      return false;
  
    // Can't merge if there are multiple distinct successors.
    if (PredBB->getUniqueSuccessor() != BB)
      return false;
  
    // Can't merge if there is PHI loop.
    for (PHINode &PN : BB->phis())
      for (Value *IncValue : PN.incoming_values())
        if (IncValue == &PN)
          return false;
  
    LLVM_DEBUG(dbgs() << "Merging: " << BB->getName() << " into "
                      << PredBB->getName() << "\n");
  
    // Begin by getting rid of unneeded PHIs.
    SmallVector<AssertingVH<Value>, 4> IncomingValues;
    if (isa<PHINode>(BB->front())) {
      for (PHINode &PN : BB->phis())
        if (!isa<PHINode>(PN.getIncomingValue(0)) ||
            cast<PHINode>(PN.getIncomingValue(0))->getParent() != BB)
          IncomingValues.push_back(PN.getIncomingValue(0));
      FoldSingleEntryPHINodes(BB, MemDep);
    }
  
    // DTU update: Collect all the edges that exit BB.
    // These dominator edges will be redirected from Pred.
    std::vector<DominatorTree::UpdateType> Updates;
    if (DTU) {
      Updates.reserve(1 + (2 * succ_size(BB)));
      // Add insert edges first. Experimentally, for the particular case of two
      // blocks that can be merged, with a single successor and single predecessor
      // respectively, it is beneficial to have all insert updates first. Deleting
      // edges first may lead to unreachable blocks, followed by inserting edges
      // making the blocks reachable again. Such DT updates lead to high compile
      // times. We add inserts before deletes here to reduce compile time.
      for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
        // This successor of BB may already have PredBB as a predecessor.
        if (llvm::find(successors(PredBB), *I) == succ_end(PredBB))
          Updates.push_back({DominatorTree::Insert, PredBB, *I});
      for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
        Updates.push_back({DominatorTree::Delete, BB, *I});
      Updates.push_back({DominatorTree::Delete, PredBB, BB});
    }
  
    if (MSSAU)
      MSSAU->moveAllAfterMergeBlocks(BB, PredBB, &*(BB->begin()));
  
    // Delete the unconditional branch from the predecessor...
    PredBB->getInstList().pop_back();
  
    // Make all PHI nodes that referred to BB now refer to Pred as their
    // source...
    BB->replaceAllUsesWith(PredBB);
  
    // Move all definitions in the successor to the predecessor...
    PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
    new UnreachableInst(BB->getContext(), BB);
  
    // Eliminate duplicate dbg.values describing the entry PHI node post-splice.
    for (auto Incoming : IncomingValues) {
      if (isa<Instruction>(*Incoming)) {
        SmallVector<DbgValueInst *, 2> DbgValues;
        SmallDenseSet<std::pair<DILocalVariable *, DIExpression *>, 2>
            DbgValueSet;
        llvm::findDbgValues(DbgValues, Incoming);
        for (auto &DVI : DbgValues) {
          auto R = DbgValueSet.insert({DVI->getVariable(), DVI->getExpression()});
          if (!R.second) {
            if (MSSAU)
              MSSAU->removeMemoryAccess(DVI);
            DVI->eraseFromParent();
          }
        }
      }
    }
  
    // Inherit predecessors name if it exists.
    if (!PredBB->hasName())
      PredBB->takeName(BB);
  
    if (LI)
      LI->removeBlock(BB);
  
    if (MemDep)
      MemDep->invalidateCachedPredecessors();
  
    // Finally, erase the old block and update dominator info.
    if (DTU) {
      assert(BB->getInstList().size() == 1 &&
             isa<UnreachableInst>(BB->getTerminator()) &&
             "The successor list of BB isn't empty before "
             "applying corresponding DTU updates.");
      DTU->applyUpdatesPermissive(Updates);
      DTU->deleteBB(BB);
    }
  
    else {
      BB->eraseFromParent(); // Nuke BB if DTU is nullptr.
    }
    return true;
  }
  
  void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
                                  BasicBlock::iterator &BI, Value *V) {
    Instruction &I = *BI;
    // Replaces all of the uses of the instruction with uses of the value
    I.replaceAllUsesWith(V);
  
    // Make sure to propagate a name if there is one already.
    if (I.hasName() && !V->hasName())
      V->takeName(&I);
  
    // Delete the unnecessary instruction now...
    BI = BIL.erase(BI);
  }
  
  void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
                                 BasicBlock::iterator &BI, Instruction *I) {
    assert(I->getParent() == nullptr &&
           "ReplaceInstWithInst: Instruction already inserted into basic block!");
  
    // Copy debug location to newly added instruction, if it wasn't already set
    // by the caller.
    if (!I->getDebugLoc())
      I->setDebugLoc(BI->getDebugLoc());
  
    // Insert the new instruction into the basic block...
    BasicBlock::iterator New = BIL.insert(BI, I);
  
    // Replace all uses of the old instruction, and delete it.
    ReplaceInstWithValue(BIL, BI, I);
  
    // Move BI back to point to the newly inserted instruction
    BI = New;
  }
  
  void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
    BasicBlock::iterator BI(From);
    ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
  }
  
  BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, DominatorTree *DT,
                              LoopInfo *LI, MemorySSAUpdater *MSSAU) {
    unsigned SuccNum = GetSuccessorNumber(BB, Succ);
  
    // If this is a critical edge, let SplitCriticalEdge do it.
    Instruction *LatchTerm = BB->getTerminator();
    if (SplitCriticalEdge(
            LatchTerm, SuccNum,
            CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA()))
      return LatchTerm->getSuccessor(SuccNum);
  
    // If the edge isn't critical, then BB has a single successor or Succ has a
    // single pred.  Split the block.
    if (BasicBlock *SP = Succ->getSinglePredecessor()) {
      // If the successor only has a single pred, split the top of the successor
      // block.
      assert(SP == BB && "CFG broken");
      SP = nullptr;
      return SplitBlock(Succ, &Succ->front(), DT, LI, MSSAU);
    }
  
    // Otherwise, if BB has a single successor, split it at the bottom of the
    // block.
    assert(BB->getTerminator()->getNumSuccessors() == 1 &&
           "Should have a single succ!");
    return SplitBlock(BB, BB->getTerminator(), DT, LI, MSSAU);
  }
  
  unsigned
  llvm::SplitAllCriticalEdges(Function &F,
                              const CriticalEdgeSplittingOptions &Options) {
    unsigned NumBroken = 0;
    for (BasicBlock &BB : F) {
      Instruction *TI = BB.getTerminator();
      if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
        for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
          if (SplitCriticalEdge(TI, i, Options))
            ++NumBroken;
    }
    return NumBroken;
  }
  
  BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
                               DominatorTree *DT, LoopInfo *LI,
                               MemorySSAUpdater *MSSAU) {
    BasicBlock::iterator SplitIt = SplitPt->getIterator();
    while (isa<PHINode>(SplitIt) || SplitIt->isEHPad())
      ++SplitIt;
    BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
  
    // The new block lives in whichever loop the old one did. This preserves
    // LCSSA as well, because we force the split point to be after any PHI nodes.
    if (LI)
      if (Loop *L = LI->getLoopFor(Old))
        L->addBasicBlockToLoop(New, *LI);
  
    if (DT)
      // Old dominates New. New node dominates all other nodes dominated by Old.
      if (DomTreeNode *OldNode = DT->getNode(Old)) {
        std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
  
        DomTreeNode *NewNode = DT->addNewBlock(New, Old);
        for (DomTreeNode *I : Children)
          DT->changeImmediateDominator(I, NewNode);
      }
  
    // Move MemoryAccesses still tracked in Old, but part of New now.
    // Update accesses in successor blocks accordingly.
    if (MSSAU)
      MSSAU->moveAllAfterSpliceBlocks(Old, New, &*(New->begin()));
  
    return New;
  }
  
  /// Update DominatorTree, LoopInfo, and LCCSA analysis information.
  static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
                                        ArrayRef<BasicBlock *> Preds,
                                        DominatorTree *DT, LoopInfo *LI,
                                        MemorySSAUpdater *MSSAU,
                                        bool PreserveLCSSA, bool &HasLoopExit) {
    // Update dominator tree if available.
    if (DT) {
      if (OldBB == DT->getRootNode()->getBlock()) {
        assert(NewBB == &NewBB->getParent()->getEntryBlock());
        DT->setNewRoot(NewBB);
      } else {
        // Split block expects NewBB to have a non-empty set of predecessors.
        DT->splitBlock(NewBB);
      }
    }
  
    // Update MemoryPhis after split if MemorySSA is available
    if (MSSAU)
      MSSAU->wireOldPredecessorsToNewImmediatePredecessor(OldBB, NewBB, Preds);
  
    // The rest of the logic is only relevant for updating the loop structures.
    if (!LI)
      return;
  
    assert(DT && "DT should be available to update LoopInfo!");
    Loop *L = LI->getLoopFor(OldBB);
  
    // If we need to preserve loop analyses, collect some information about how
    // this split will affect loops.
    bool IsLoopEntry = !!L;
    bool SplitMakesNewLoopHeader = false;
    for (BasicBlock *Pred : Preds) {
      // Preds that are not reachable from entry should not be used to identify if
      // OldBB is a loop entry or if SplitMakesNewLoopHeader. Unreachable blocks
      // are not within any loops, so we incorrectly mark SplitMakesNewLoopHeader
      // as true and make the NewBB the header of some loop. This breaks LI.
      if (!DT->isReachableFromEntry(Pred))
        continue;
      // If we need to preserve LCSSA, determine if any of the preds is a loop
      // exit.
      if (PreserveLCSSA)
        if (Loop *PL = LI->getLoopFor(Pred))
          if (!PL->contains(OldBB))
            HasLoopExit = true;
  
      // If we need to preserve LoopInfo, note whether any of the preds crosses
      // an interesting loop boundary.
      if (!L)
        continue;
      if (L->contains(Pred))
        IsLoopEntry = false;
      else
        SplitMakesNewLoopHeader = true;
    }
  
    // Unless we have a loop for OldBB, nothing else to do here.
    if (!L)
      return;
  
    if (IsLoopEntry) {
      // Add the new block to the nearest enclosing loop (and not an adjacent
      // loop). To find this, examine each of the predecessors and determine which
      // loops enclose them, and select the most-nested loop which contains the
      // loop containing the block being split.
      Loop *InnermostPredLoop = nullptr;
      for (BasicBlock *Pred : Preds) {
        if (Loop *PredLoop = LI->getLoopFor(Pred)) {
          // Seek a loop which actually contains the block being split (to avoid
          // adjacent loops).
          while (PredLoop && !PredLoop->contains(OldBB))
            PredLoop = PredLoop->getParentLoop();
  
          // Select the most-nested of these loops which contains the block.
          if (PredLoop && PredLoop->contains(OldBB) &&
              (!InnermostPredLoop ||
               InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
            InnermostPredLoop = PredLoop;
        }
      }
  
      if (InnermostPredLoop)
        InnermostPredLoop->addBasicBlockToLoop(NewBB, *LI);
    } else {
      L->addBasicBlockToLoop(NewBB, *LI);
      if (SplitMakesNewLoopHeader)
        L->moveToHeader(NewBB);
    }
  }
  
  /// Update the PHI nodes in OrigBB to include the values coming from NewBB.
  /// This also updates AliasAnalysis, if available.
  static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,
                             ArrayRef<BasicBlock *> Preds, BranchInst *BI,
                             bool HasLoopExit) {
    // Otherwise, create a new PHI node in NewBB for each PHI node in OrigBB.
    SmallPtrSet<BasicBlock *, 16> PredSet(Preds.begin(), Preds.end());
    for (BasicBlock::iterator I = OrigBB->begin(); isa<PHINode>(I); ) {
      PHINode *PN = cast<PHINode>(I++);
  
      // Check to see if all of the values coming in are the same.  If so, we
      // don't need to create a new PHI node, unless it's needed for LCSSA.
      Value *InVal = nullptr;
      if (!HasLoopExit) {
        InVal = PN->getIncomingValueForBlock(Preds[0]);
        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
          if (!PredSet.count(PN->getIncomingBlock(i)))
            continue;
          if (!InVal)
            InVal = PN->getIncomingValue(i);
          else if (InVal != PN->getIncomingValue(i)) {
            InVal = nullptr;
            break;
          }
        }
      }
  
      if (InVal) {
        // If all incoming values for the new PHI would be the same, just don't
        // make a new PHI.  Instead, just remove the incoming values from the old
        // PHI.
  
        // NOTE! This loop walks backwards for a reason! First off, this minimizes
        // the cost of removal if we end up removing a large number of values, and
        // second off, this ensures that the indices for the incoming values
        // aren't invalidated when we remove one.
        for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i)
          if (PredSet.count(PN->getIncomingBlock(i)))
            PN->removeIncomingValue(i, false);
  
        // Add an incoming value to the PHI node in the loop for the preheader
        // edge.
        PN->addIncoming(InVal, NewBB);
        continue;
      }
  
      // If the values coming into the block are not the same, we need a new
      // PHI.
      // Create the new PHI node, insert it into NewBB at the end of the block
      PHINode *NewPHI =
          PHINode::Create(PN->getType(), Preds.size(), PN->getName() + ".ph", BI);
  
      // NOTE! This loop walks backwards for a reason! First off, this minimizes
      // the cost of removal if we end up removing a large number of values, and
      // second off, this ensures that the indices for the incoming values aren't
      // invalidated when we remove one.
      for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i) {
        BasicBlock *IncomingBB = PN->getIncomingBlock(i);
        if (PredSet.count(IncomingBB)) {
          Value *V = PN->removeIncomingValue(i, false);
          NewPHI->addIncoming(V, IncomingBB);
        }
      }
  
      PN->addIncoming(NewPHI, NewBB);
    }
  }
  
  BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
                                           ArrayRef<BasicBlock *> Preds,
                                           const char *Suffix, DominatorTree *DT,
                                           LoopInfo *LI, MemorySSAUpdater *MSSAU,
                                           bool PreserveLCSSA) {
    // Do not attempt to split that which cannot be split.
    if (!BB->canSplitPredecessors())
      return nullptr;
  
    // For the landingpads we need to act a bit differently.
    // Delegate this work to the SplitLandingPadPredecessors.
    if (BB->isLandingPad()) {
      SmallVector<BasicBlock*, 2> NewBBs;
      std::string NewName = std::string(Suffix) + ".split-lp";
  
      SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs, DT,
                                  LI, MSSAU, PreserveLCSSA);
      return NewBBs[0];
    }
  
    // Create new basic block, insert right before the original block.
    BasicBlock *NewBB = BasicBlock::Create(
        BB->getContext(), BB->getName() + Suffix, BB->getParent(), BB);
  
    // The new block unconditionally branches to the old block.
    BranchInst *BI = BranchInst::Create(BB, NewBB);
    // Splitting the predecessors of a loop header creates a preheader block.
    if (LI && LI->isLoopHeader(BB))
      // Using the loop start line number prevents debuggers stepping into the
      // loop body for this instruction.
      BI->setDebugLoc(LI->getLoopFor(BB)->getStartLoc());
    else
      BI->setDebugLoc(BB->getFirstNonPHIOrDbg()->getDebugLoc());
  
    // Move the edges from Preds to point to NewBB instead of BB.
    for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
      // This is slightly more strict than necessary; the minimum requirement
      // is that there be no more than one indirectbr branching to BB. And
      // all BlockAddress uses would need to be updated.
      assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
             "Cannot split an edge from an IndirectBrInst");
      assert(!isa<CallBrInst>(Preds[i]->getTerminator()) &&
             "Cannot split an edge from a CallBrInst");
      Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
    }
  
    // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
    // node becomes an incoming value for BB's phi node.  However, if the Preds
    // list is empty, we need to insert dummy entries into the PHI nodes in BB to
    // account for the newly created predecessor.
    if (Preds.empty()) {
      // Insert dummy values as the incoming value.
      for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
        cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
    }
  
    // Update DominatorTree, LoopInfo, and LCCSA analysis information.
    bool HasLoopExit = false;
    UpdateAnalysisInformation(BB, NewBB, Preds, DT, LI, MSSAU, PreserveLCSSA,
                              HasLoopExit);
  
    if (!Preds.empty()) {
      // Update the PHI nodes in BB with the values coming from NewBB.
      UpdatePHINodes(BB, NewBB, Preds, BI, HasLoopExit);
    }
  
    return NewBB;
  }
  
  void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
                                         ArrayRef<BasicBlock *> Preds,
                                         const char *Suffix1, const char *Suffix2,
                                         SmallVectorImpl<BasicBlock *> &NewBBs,
                                         DominatorTree *DT, LoopInfo *LI,
                                         MemorySSAUpdater *MSSAU,
                                         bool PreserveLCSSA) {
    assert(OrigBB->isLandingPad() && "Trying to split a non-landing pad!");
  
    // Create a new basic block for OrigBB's predecessors listed in Preds. Insert
    // it right before the original block.
    BasicBlock *NewBB1 = BasicBlock::Create(OrigBB->getContext(),
                                            OrigBB->getName() + Suffix1,
                                            OrigBB->getParent(), OrigBB);
    NewBBs.push_back(NewBB1);
  
    // The new block unconditionally branches to the old block.
    BranchInst *BI1 = BranchInst::Create(OrigBB, NewBB1);
    BI1->setDebugLoc(OrigBB->getFirstNonPHI()->getDebugLoc());
  
    // Move the edges from Preds to point to NewBB1 instead of OrigBB.
    for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
      // This is slightly more strict than necessary; the minimum requirement
      // is that there be no more than one indirectbr branching to BB. And
      // all BlockAddress uses would need to be updated.
      assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
             "Cannot split an edge from an IndirectBrInst");
      Preds[i]->getTerminator()->replaceUsesOfWith(OrigBB, NewBB1);
    }
  
    bool HasLoopExit = false;
    UpdateAnalysisInformation(OrigBB, NewBB1, Preds, DT, LI, MSSAU, PreserveLCSSA,
                              HasLoopExit);
  
    // Update the PHI nodes in OrigBB with the values coming from NewBB1.
    UpdatePHINodes(OrigBB, NewBB1, Preds, BI1, HasLoopExit);
  
    // Move the remaining edges from OrigBB to point to NewBB2.
    SmallVector<BasicBlock*, 8> NewBB2Preds;
    for (pred_iterator i = pred_begin(OrigBB), e = pred_end(OrigBB);
         i != e; ) {
      BasicBlock *Pred = *i++;
      if (Pred == NewBB1) continue;
      assert(!isa<IndirectBrInst>(Pred->getTerminator()) &&
             "Cannot split an edge from an IndirectBrInst");
      NewBB2Preds.push_back(Pred);
      e = pred_end(OrigBB);
    }
  
    BasicBlock *NewBB2 = nullptr;
    if (!NewBB2Preds.empty()) {
      // Create another basic block for the rest of OrigBB's predecessors.
      NewBB2 = BasicBlock::Create(OrigBB->getContext(),
                                  OrigBB->getName() + Suffix2,
                                  OrigBB->getParent(), OrigBB);
      NewBBs.push_back(NewBB2);
  
      // The new block unconditionally branches to the old block.
      BranchInst *BI2 = BranchInst::Create(OrigBB, NewBB2);
      BI2->setDebugLoc(OrigBB->getFirstNonPHI()->getDebugLoc());
  
      // Move the remaining edges from OrigBB to point to NewBB2.
      for (BasicBlock *NewBB2Pred : NewBB2Preds)
        NewBB2Pred->getTerminator()->replaceUsesOfWith(OrigBB, NewBB2);
  
      // Update DominatorTree, LoopInfo, and LCCSA analysis information.
      HasLoopExit = false;
      UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, DT, LI, MSSAU,
                                PreserveLCSSA, HasLoopExit);
  
      // Update the PHI nodes in OrigBB with the values coming from NewBB2.
      UpdatePHINodes(OrigBB, NewBB2, NewBB2Preds, BI2, HasLoopExit);
    }
  
    LandingPadInst *LPad = OrigBB->getLandingPadInst();
    Instruction *Clone1 = LPad->clone();
    Clone1->setName(Twine("lpad") + Suffix1);
    NewBB1->getInstList().insert(NewBB1->getFirstInsertionPt(), Clone1);
  
    if (NewBB2) {
      Instruction *Clone2 = LPad->clone();
      Clone2->setName(Twine("lpad") + Suffix2);
      NewBB2->getInstList().insert(NewBB2->getFirstInsertionPt(), Clone2);
  
      // Create a PHI node for the two cloned landingpad instructions only
      // if the original landingpad instruction has some uses.
      if (!LPad->use_empty()) {
        assert(!LPad->getType()->isTokenTy() &&
               "Split cannot be applied if LPad is token type. Otherwise an "
               "invalid PHINode of token type would be created.");
        PHINode *PN = PHINode::Create(LPad->getType(), 2, "lpad.phi", LPad);
        PN->addIncoming(Clone1, NewBB1);
        PN->addIncoming(Clone2, NewBB2);
        LPad->replaceAllUsesWith(PN);
      }
      LPad->eraseFromParent();
    } else {
      // There is no second clone. Just replace the landing pad with the first
      // clone.
      LPad->replaceAllUsesWith(Clone1);
      LPad->eraseFromParent();
    }
  }
  
  ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
                                               BasicBlock *Pred,
                                               DomTreeUpdater *DTU) {
    Instruction *UncondBranch = Pred->getTerminator();
    // Clone the return and add it to the end of the predecessor.
    Instruction *NewRet = RI->clone();
    Pred->getInstList().push_back(NewRet);
  
    // If the return instruction returns a value, and if the value was a
    // PHI node in "BB", propagate the right value into the return.
    for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
         i != e; ++i) {
      Value *V = *i;
      Instruction *NewBC = nullptr;
      if (BitCastInst *BCI = dyn_cast<BitCastInst>(V)) {
        // Return value might be bitcasted. Clone and insert it before the
        // return instruction.
        V = BCI->getOperand(0);
        NewBC = BCI->clone();
        Pred->getInstList().insert(NewRet->getIterator(), NewBC);
        *i = NewBC;
      }
      if (PHINode *PN = dyn_cast<PHINode>(V)) {
        if (PN->getParent() == BB) {
          if (NewBC)
            NewBC->setOperand(0, PN->getIncomingValueForBlock(Pred));
          else
            *i = PN->getIncomingValueForBlock(Pred);
        }
      }
    }
  
    // Update any PHI nodes in the returning block to realize that we no
    // longer branch to them.
    BB->removePredecessor(Pred);
    UncondBranch->eraseFromParent();
  
    if (DTU)
      DTU->applyUpdates({{DominatorTree::Delete, Pred, BB}});
  
    return cast<ReturnInst>(NewRet);
  }
  
  Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
                                               Instruction *SplitBefore,
                                               bool Unreachable,
                                               MDNode *BranchWeights,
                                               DominatorTree *DT, LoopInfo *LI,
                                               BasicBlock *ThenBlock) {
    BasicBlock *Head = SplitBefore->getParent();
    BasicBlock *Tail = Head->splitBasicBlock(SplitBefore->getIterator());
    Instruction *HeadOldTerm = Head->getTerminator();
    LLVMContext &C = Head->getContext();
    Instruction *CheckTerm;
    bool CreateThenBlock = (ThenBlock == nullptr);
    if (CreateThenBlock) {
      ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
      if (Unreachable)
        CheckTerm = new UnreachableInst(C, ThenBlock);
      else
        CheckTerm = BranchInst::Create(Tail, ThenBlock);
      CheckTerm->setDebugLoc(SplitBefore->getDebugLoc());
    } else
      CheckTerm = ThenBlock->getTerminator();
    BranchInst *HeadNewTerm =
      BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/Tail, Cond);
    HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
    ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
  
    if (DT) {
      if (DomTreeNode *OldNode = DT->getNode(Head)) {
        std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
  
        DomTreeNode *NewNode = DT->addNewBlock(Tail, Head);
        for (DomTreeNode *Child : Children)
          DT->changeImmediateDominator(Child, NewNode);
  
        // Head dominates ThenBlock.
        if (CreateThenBlock)
          DT->addNewBlock(ThenBlock, Head);
        else
          DT->changeImmediateDominator(ThenBlock, Head);
      }
    }
  
    if (LI) {
      if (Loop *L = LI->getLoopFor(Head)) {
        L->addBasicBlockToLoop(ThenBlock, *LI);
        L->addBasicBlockToLoop(Tail, *LI);
      }
    }
  
    return CheckTerm;
  }
  
  void llvm::SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
                                           Instruction **ThenTerm,
                                           Instruction **ElseTerm,
                                           MDNode *BranchWeights) {
    BasicBlock *Head = SplitBefore->getParent();
    BasicBlock *Tail = Head->splitBasicBlock(SplitBefore->getIterator());
    Instruction *HeadOldTerm = Head->getTerminator();
    LLVMContext &C = Head->getContext();
    BasicBlock *ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
    BasicBlock *ElseBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
    *ThenTerm = BranchInst::Create(Tail, ThenBlock);
    (*ThenTerm)->setDebugLoc(SplitBefore->getDebugLoc());
    *ElseTerm = BranchInst::Create(Tail, ElseBlock);
    (*ElseTerm)->setDebugLoc(SplitBefore->getDebugLoc());
    BranchInst *HeadNewTerm =
      BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/ElseBlock, Cond);
    HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
    ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
  }
  
  Value *llvm::GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
                               BasicBlock *&IfFalse) {
    PHINode *SomePHI = dyn_cast<PHINode>(BB->begin());
    BasicBlock *Pred1 = nullptr;
    BasicBlock *Pred2 = nullptr;
  
    if (SomePHI) {
      if (SomePHI->getNumIncomingValues() != 2)
        return nullptr;
      Pred1 = SomePHI->getIncomingBlock(0);
      Pred2 = SomePHI->getIncomingBlock(1);
    } else {
      pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
      if (PI == PE) // No predecessor
        return nullptr;
      Pred1 = *PI++;
      if (PI == PE) // Only one predecessor
        return nullptr;
      Pred2 = *PI++;
      if (PI != PE) // More than two predecessors
        return nullptr;
    }
  
    // We can only handle branches.  Other control flow will be lowered to
    // branches if possible anyway.
    BranchInst *Pred1Br = dyn_cast<BranchInst>(Pred1->getTerminator());
    BranchInst *Pred2Br = dyn_cast<BranchInst>(Pred2->getTerminator());
    if (!Pred1Br || !Pred2Br)
      return nullptr;
  
    // Eliminate code duplication by ensuring that Pred1Br is conditional if
    // either are.
    if (Pred2Br->isConditional()) {
      // If both branches are conditional, we don't have an "if statement".  In
      // reality, we could transform this case, but since the condition will be
      // required anyway, we stand no chance of eliminating it, so the xform is
      // probably not profitable.
      if (Pred1Br->isConditional())
        return nullptr;
  
      std::swap(Pred1, Pred2);
      std::swap(Pred1Br, Pred2Br);
    }
  
    if (Pred1Br->isConditional()) {
      // The only thing we have to watch out for here is to make sure that Pred2
      // doesn't have incoming edges from other blocks.  If it does, the condition
      // doesn't dominate BB.
      if (!Pred2->getSinglePredecessor())
        return nullptr;
  
      // If we found a conditional branch predecessor, make sure that it branches
      // to BB and Pred2Br.  If it doesn't, this isn't an "if statement".
      if (Pred1Br->getSuccessor(0) == BB &&
          Pred1Br->getSuccessor(1) == Pred2) {
        IfTrue = Pred1;
        IfFalse = Pred2;
      } else if (Pred1Br->getSuccessor(0) == Pred2 &&
                 Pred1Br->getSuccessor(1) == BB) {
        IfTrue = Pred2;
        IfFalse = Pred1;
      } else {
        // We know that one arm of the conditional goes to BB, so the other must
        // go somewhere unrelated, and this must not be an "if statement".
        return nullptr;
      }
  
      return Pred1Br->getCondition();
    }
  
    // Ok, if we got here, both predecessors end with an unconditional branch to
    // BB.  Don't panic!  If both blocks only have a single (identical)
    // predecessor, and THAT is a conditional branch, then we're all ok!
    BasicBlock *CommonPred = Pred1->getSinglePredecessor();
    if (CommonPred == nullptr || CommonPred != Pred2->getSinglePredecessor())
      return nullptr;
  
    // Otherwise, if this is a conditional branch, then we can use it!
    BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator());
    if (!BI) return nullptr;
  
    assert(BI->isConditional() && "Two successors but not conditional?");
    if (BI->getSuccessor(0) == Pred1) {
      IfTrue = Pred1;
      IfFalse = Pred2;
    } else {
      IfTrue = Pred2;
      IfFalse = Pred1;
    }
    return BI->getCondition();
  }
  