llvm-mirror/lib/Transforms/Utils/LowerSwitch.cpp

//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The LowerSwitch transformation rewrites switch instructions with a sequence
// of branches, which allows targets to get away with not implementing the
// switch instruction until it is convenient.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/LazyValueInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <limits>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "lower-switch"

namespace {

  struct IntRange {
    int64_t Low, High;
  };

} // end anonymous namespace

// Return true iff R is covered by Ranges.
static bool IsInRanges(const IntRange &R,
                       const std::vector<IntRange> &Ranges) {
  // Note: Ranges must be sorted, non-overlapping and non-adjacent.

  // Find the first range whose High field is >= R.High,
  // then check if the Low field is <= R.Low. If so, we
  // have a Range that covers R.
  auto I = std::lower_bound(
      Ranges.begin(), Ranges.end(), R,
      [](const IntRange &A, const IntRange &B) { return A.High < B.High; });
  return I != Ranges.end() && I->Low <= R.Low;
}

namespace {

  /// Replace all SwitchInst instructions with chained branch instructions.
  class LowerSwitch : public FunctionPass {
  public:
    // Pass identification, replacement for typeid
    static char ID;

    LowerSwitch() : FunctionPass(ID) {
      initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
    }

    bool runOnFunction(Function &F) override;

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addRequired<LazyValueInfoWrapperPass>();
    }

    struct CaseRange {
      ConstantInt* Low;
      ConstantInt* High;
      BasicBlock* BB;

      CaseRange(ConstantInt *low, ConstantInt *high, BasicBlock *bb)
          : Low(low), High(high), BB(bb) {}
    };

    using CaseVector = std::vector<CaseRange>;
    using CaseItr = std::vector<CaseRange>::iterator;

  private:
    void processSwitchInst(SwitchInst *SI,
                           SmallPtrSetImpl<BasicBlock *> &DeleteList,
                           AssumptionCache *AC, LazyValueInfo *LVI);

    BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
                              ConstantInt *LowerBound, ConstantInt *UpperBound,
                              Value *Val, BasicBlock *Predecessor,
                              BasicBlock *OrigBlock, BasicBlock *Default,
                              const std::vector<IntRange> &UnreachableRanges);
    BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val,
                             ConstantInt *LowerBound, ConstantInt *UpperBound,
                             BasicBlock *OrigBlock, BasicBlock *Default);
    unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
  };

  /// The comparison function for sorting the switch case values in the vector.
  /// WARNING: Case ranges should be disjoint!
  struct CaseCmp {
    bool operator()(const LowerSwitch::CaseRange& C1,
                    const LowerSwitch::CaseRange& C2) {
      const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
      const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
      return CI1->getValue().slt(CI2->getValue());
    }
  };

} // end anonymous namespace

char LowerSwitch::ID = 0;

// Publicly exposed interface to pass...
char &llvm::LowerSwitchID = LowerSwitch::ID;

INITIALIZE_PASS_BEGIN(LowerSwitch, "lowerswitch",
                      "Lower SwitchInst's to branches", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
INITIALIZE_PASS_END(LowerSwitch, "lowerswitch",
                    "Lower SwitchInst's to branches", false, false)

// createLowerSwitchPass - Interface to this file...
FunctionPass *llvm::createLowerSwitchPass() {
  return new LowerSwitch();
}

bool LowerSwitch::runOnFunction(Function &F) {
  LazyValueInfo *LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
  auto *ACT = getAnalysisIfAvailable<AssumptionCacheTracker>();
  AssumptionCache *AC = ACT ? &ACT->getAssumptionCache(F) : nullptr;
  // Prevent LazyValueInfo from using the DominatorTree as LowerSwitch does not
  // preserve it and it becomes stale (when available) pretty much immediately.
  // Currently the DominatorTree is only used by LowerSwitch indirectly via LVI
  // and computeKnownBits to refine isValidAssumeForContext's results. Given
  // that the latter can handle some of the simple cases w/o a DominatorTree,
  // it's easier to refrain from using the tree than to keep it up to date.
  LVI->disableDT();

  bool Changed = false;
  SmallPtrSet<BasicBlock*, 8> DeleteList;

  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
    BasicBlock *Cur = &*I++; // Advance over block so we don't traverse new blocks

    // If the block is a dead Default block that will be deleted later, don't
    // waste time processing it.
    if (DeleteList.count(Cur))
      continue;

    if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
      Changed = true;
      processSwitchInst(SI, DeleteList, AC, LVI);
    }
  }

  for (BasicBlock* BB: DeleteList) {
    LVI->eraseBlock(BB);
    DeleteDeadBlock(BB);
  }

  return Changed;
}

/// Used for debugging purposes.
LLVM_ATTRIBUTE_USED
static raw_ostream &operator<<(raw_ostream &O,
                               const LowerSwitch::CaseVector &C) {
  O << "[";

  for (LowerSwitch::CaseVector::const_iterator B = C.begin(), E = C.end();
       B != E;) {
    O << "[" << B->Low->getValue() << ", " << B->High->getValue() << "]";
    if (++B != E)
      O << ", ";
  }

  return O << "]";
}

/// Update the first occurrence of the "switch statement" BB in the PHI
/// node with the "new" BB. The other occurrences will:
///
/// 1) Be updated by subsequent calls to this function.  Switch statements may
/// have more than one outcoming edge into the same BB if they all have the same
/// value. When the switch statement is converted these incoming edges are now
/// coming from multiple BBs.
/// 2) Removed if subsequent incoming values now share the same case, i.e.,
/// multiple outcome edges are condensed into one. This is necessary to keep the
/// number of phi values equal to the number of branches to SuccBB.
static void
fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB,
        const unsigned NumMergedCases = std::numeric_limits<unsigned>::max()) {
  for (BasicBlock::iterator I = SuccBB->begin(),
                            IE = SuccBB->getFirstNonPHI()->getIterator();
       I != IE; ++I) {
    PHINode *PN = cast<PHINode>(I);

    // Only update the first occurrence.
    unsigned Idx = 0, E = PN->getNumIncomingValues();
    unsigned LocalNumMergedCases = NumMergedCases;
    for (; Idx != E; ++Idx) {
      if (PN->getIncomingBlock(Idx) == OrigBB) {
        PN->setIncomingBlock(Idx, NewBB);
        break;
      }
    }

    // Remove additional occurrences coming from condensed cases and keep the
    // number of incoming values equal to the number of branches to SuccBB.
    SmallVector<unsigned, 8> Indices;
    for (++Idx; LocalNumMergedCases > 0 && Idx < E; ++Idx)
      if (PN->getIncomingBlock(Idx) == OrigBB) {
        Indices.push_back(Idx);
        LocalNumMergedCases--;
      }
    // Remove incoming values in the reverse order to prevent invalidating
    // *successive* index.
    for (unsigned III : llvm::reverse(Indices))
      PN->removeIncomingValue(III);
  }
}

/// Convert the switch statement into a binary lookup of the case values.
/// The function recursively builds this tree. LowerBound and UpperBound are
/// used to keep track of the bounds for Val that have already been checked by
/// a block emitted by one of the previous calls to switchConvert in the call
/// stack.
BasicBlock *
LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, ConstantInt *LowerBound,
                           ConstantInt *UpperBound, Value *Val,
                           BasicBlock *Predecessor, BasicBlock *OrigBlock,
                           BasicBlock *Default,
                           const std::vector<IntRange> &UnreachableRanges) {
  assert(LowerBound && UpperBound && "Bounds must be initialized");
  unsigned Size = End - Begin;

  if (Size == 1) {
    // Check if the Case Range is perfectly squeezed in between
    // already checked Upper and Lower bounds. If it is then we can avoid
    // emitting the code that checks if the value actually falls in the range
    // because the bounds already tell us so.
    if (Begin->Low == LowerBound && Begin->High == UpperBound) {
      unsigned NumMergedCases = 0;
      NumMergedCases = UpperBound->getSExtValue() - LowerBound->getSExtValue();
      fixPhis(Begin->BB, OrigBlock, Predecessor, NumMergedCases);
      return Begin->BB;
    }
    return newLeafBlock(*Begin, Val, LowerBound, UpperBound, OrigBlock,
                        Default);
  }

  unsigned Mid = Size / 2;
  std::vector<CaseRange> LHS(Begin, Begin + Mid);
  LLVM_DEBUG(dbgs() << "LHS: " << LHS << "\n");
  std::vector<CaseRange> RHS(Begin + Mid, End);
  LLVM_DEBUG(dbgs() << "RHS: " << RHS << "\n");

  CaseRange &Pivot = *(Begin + Mid);
  LLVM_DEBUG(dbgs() << "Pivot ==> [" << Pivot.Low->getValue() << ", "
                    << Pivot.High->getValue() << "]\n");

  // NewLowerBound here should never be the integer minimal value.
  // This is because it is computed from a case range that is never
  // the smallest, so there is always a case range that has at least
  // a smaller value.
  ConstantInt *NewLowerBound = Pivot.Low;

  // Because NewLowerBound is never the smallest representable integer
  // it is safe here to subtract one.
  ConstantInt *NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
                                                NewLowerBound->getValue() - 1);

  if (!UnreachableRanges.empty()) {
    // Check if the gap between LHS's highest and NewLowerBound is unreachable.
    int64_t GapLow = LHS.back().High->getSExtValue() + 1;
    int64_t GapHigh = NewLowerBound->getSExtValue() - 1;
    IntRange Gap = { GapLow, GapHigh };
    if (GapHigh >= GapLow && IsInRanges(Gap, UnreachableRanges))
      NewUpperBound = LHS.back().High;
  }

  LLVM_DEBUG(dbgs() << "LHS Bounds ==> [" << LowerBound->getSExtValue() << ", "
                    << NewUpperBound->getSExtValue() << "]\n"
                    << "RHS Bounds ==> [" << NewLowerBound->getSExtValue()
                    << ", " << UpperBound->getSExtValue() << "]\n");

  // Create a new node that checks if the value is < pivot. Go to the
  // left branch if it is and right branch if not.
  Function* F = OrigBlock->getParent();
  BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");

  ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
                                Val, Pivot.Low, "Pivot");

  BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
                                      NewUpperBound, Val, NewNode, OrigBlock,
                                      Default, UnreachableRanges);
  BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
                                      UpperBound, Val, NewNode, OrigBlock,
                                      Default, UnreachableRanges);

  F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewNode);
  NewNode->getInstList().push_back(Comp);

  BranchInst::Create(LBranch, RBranch, Comp, NewNode);
  return NewNode;
}

/// Create a new leaf block for the binary lookup tree. It checks if the
/// switch's value == the case's value. If not, then it jumps to the default
/// branch. At this point in the tree, the value can't be another valid case
/// value, so the jump to the "default" branch is warranted.
BasicBlock *LowerSwitch::newLeafBlock(CaseRange &Leaf, Value *Val,
                                      ConstantInt *LowerBound,
                                      ConstantInt *UpperBound,
                                      BasicBlock *OrigBlock,
                                      BasicBlock *Default) {
  Function* F = OrigBlock->getParent();
  BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
  F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewLeaf);

  // Emit comparison
  ICmpInst* Comp = nullptr;
  if (Leaf.Low == Leaf.High) {
    // Make the seteq instruction...
    Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
                        Leaf.Low, "SwitchLeaf");
  } else {
    // Make range comparison
    if (Leaf.Low == LowerBound) {
      // Val >= Min && Val <= Hi --> Val <= Hi
      Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
                          "SwitchLeaf");
    } else if (Leaf.High == UpperBound) {
      // Val <= Max && Val >= Lo --> Val >= Lo
      Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SGE, Val, Leaf.Low,
                          "SwitchLeaf");
    } else if (Leaf.Low->isZero()) {
      // Val >= 0 && Val <= Hi --> Val <=u Hi
      Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
                          "SwitchLeaf");
    } else {
      // Emit V-Lo <=u Hi-Lo
      Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
      Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
                                                   Val->getName()+".off",
                                                   NewLeaf);
      Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
      Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
                          "SwitchLeaf");
    }
  }

  // Make the conditional branch...
  BasicBlock* Succ = Leaf.BB;
  BranchInst::Create(Succ, Default, Comp, NewLeaf);

  // If there were any PHI nodes in this successor, rewrite one entry
  // from OrigBlock to come from NewLeaf.
  for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
    PHINode* PN = cast<PHINode>(I);
    // Remove all but one incoming entries from the cluster
    uint64_t Range = Leaf.High->getSExtValue() -
                     Leaf.Low->getSExtValue();
    for (uint64_t j = 0; j < Range; ++j) {
      PN->removeIncomingValue(OrigBlock);
    }

    int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
    assert(BlockIdx != -1 && "Switch didn't go to this successor??");
    PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
  }

  return NewLeaf;
}

/// Transform simple list of \p SI's cases into list of CaseRange's \p Cases.
/// \post \p Cases wouldn't contain references to \p SI's default BB.
/// \returns Number of \p SI's cases that do not reference \p SI's default BB.
unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
  unsigned NumSimpleCases = 0;

  // Start with "simple" cases
  for (auto Case : SI->cases()) {
    if (Case.getCaseSuccessor() == SI->getDefaultDest())
      continue;
    Cases.push_back(CaseRange(Case.getCaseValue(), Case.getCaseValue(),
                              Case.getCaseSuccessor()));
    ++NumSimpleCases;
  }

  llvm::sort(Cases, CaseCmp());

  // Merge case into clusters
  if (Cases.size() >= 2) {
    CaseItr I = Cases.begin();
    for (CaseItr J = std::next(I), E = Cases.end(); J != E; ++J) {
      int64_t nextValue = J->Low->getSExtValue();
      int64_t currentValue = I->High->getSExtValue();
      BasicBlock* nextBB = J->BB;
      BasicBlock* currentBB = I->BB;

      // If the two neighboring cases go to the same destination, merge them
      // into a single case.
      assert(nextValue > currentValue && "Cases should be strictly ascending");
      if ((nextValue == currentValue + 1) && (currentBB == nextBB)) {
        I->High = J->High;
        // FIXME: Combine branch weights.
      } else if (++I != J) {
        *I = *J;
      }
    }
    Cases.erase(std::next(I), Cases.end());
  }

  return NumSimpleCases;
}

static ConstantRange getConstantRangeFromKnownBits(const KnownBits &Known) {
  APInt Lower = Known.One;
  APInt Upper = ~Known.Zero + 1;
  if (Upper == Lower)
    return ConstantRange(Known.getBitWidth(), /*isFullSet=*/true);
  return ConstantRange(Lower, Upper);
}

/// Replace the specified switch instruction with a sequence of chained if-then
/// insts in a balanced binary search.
void LowerSwitch::processSwitchInst(SwitchInst *SI,
                                    SmallPtrSetImpl<BasicBlock *> &DeleteList,
                                    AssumptionCache *AC, LazyValueInfo *LVI) {
  BasicBlock *OrigBlock = SI->getParent();
  Function *F = OrigBlock->getParent();
  Value *Val = SI->getCondition();  // The value we are switching on...
  BasicBlock* Default = SI->getDefaultDest();

  // Don't handle unreachable blocks. If there are successors with phis, this
  // would leave them behind with missing predecessors.
  if ((OrigBlock != &F->getEntryBlock() && pred_empty(OrigBlock)) ||
      OrigBlock->getSinglePredecessor() == OrigBlock) {
    DeleteList.insert(OrigBlock);
    return;
  }

  // Prepare cases vector.
  CaseVector Cases;
  const unsigned NumSimpleCases = Clusterify(Cases, SI);
  LLVM_DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
                    << ". Total non-default cases: " << NumSimpleCases
                    << "\nCase clusters: " << Cases << "\n");

  // If there is only the default destination, just branch.
  if (Cases.empty()) {
    BranchInst::Create(Default, OrigBlock);
    // Remove all the references from Default's PHIs to OrigBlock, but one.
    fixPhis(Default, OrigBlock, OrigBlock);
    SI->eraseFromParent();
    return;
  }

  ConstantInt *LowerBound = nullptr;
  ConstantInt *UpperBound = nullptr;
  bool DefaultIsUnreachableFromSwitch = false;

  if (isa<UnreachableInst>(Default->getFirstNonPHIOrDbg())) {
    // Make the bounds tightly fitted around the case value range, because we
    // know that the value passed to the switch must be exactly one of the case
    // values.
    LowerBound = Cases.front().Low;
    UpperBound = Cases.back().High;
    DefaultIsUnreachableFromSwitch = true;
  } else {
    // Constraining the range of the value being switched over helps eliminating
    // unreachable BBs and minimizing the number of `add` instructions
    // newLeafBlock ends up emitting. Running CorrelatedValuePropagation after
    // LowerSwitch isn't as good, and also much more expensive in terms of
    // compile time for the following reasons:
    // 1. it processes many kinds of instructions, not just switches;
    // 2. even if limited to icmp instructions only, it will have to process
    //    roughly C icmp's per switch, where C is the number of cases in the
    //    switch, while LowerSwitch only needs to call LVI once per switch.
    const DataLayout &DL = F->getParent()->getDataLayout();
    KnownBits Known = computeKnownBits(Val, DL, /*Depth=*/0, AC, SI);
    ConstantRange KnownBitsRange = getConstantRangeFromKnownBits(Known);
    const ConstantRange LVIRange = LVI->getConstantRange(Val, OrigBlock, SI);
    ConstantRange ValRange = KnownBitsRange.intersectWith(LVIRange);
    // We delegate removal of unreachable non-default cases to other passes. In
    // the unlikely event that some of them survived, we just conservatively
    // maintain the invariant that all the cases lie between the bounds. This
    // may, however, still render the default case effectively unreachable.
    APInt Low = Cases.front().Low->getValue();
    APInt High = Cases.back().High->getValue();
    APInt Min = APIntOps::smin(ValRange.getSignedMin(), Low);
    APInt Max = APIntOps::smax(ValRange.getSignedMax(), High);

    LowerBound = ConstantInt::get(SI->getContext(), Min);
    UpperBound = ConstantInt::get(SI->getContext(), Max);
    DefaultIsUnreachableFromSwitch = (Min + (NumSimpleCases - 1) == Max);
  }

  std::vector<IntRange> UnreachableRanges;

  if (DefaultIsUnreachableFromSwitch) {
    DenseMap<BasicBlock *, unsigned> Popularity;
    unsigned MaxPop = 0;
    BasicBlock *PopSucc = nullptr;

    IntRange R = {std::numeric_limits<int64_t>::min(),
                  std::numeric_limits<int64_t>::max()};
    UnreachableRanges.push_back(R);
    for (const auto &I : Cases) {
      int64_t Low = I.Low->getSExtValue();
      int64_t High = I.High->getSExtValue();

      IntRange &LastRange = UnreachableRanges.back();
      if (LastRange.Low == Low) {
        // There is nothing left of the previous range.
        UnreachableRanges.pop_back();
      } else {
        // Terminate the previous range.
        assert(Low > LastRange.Low);
        LastRange.High = Low - 1;
      }
      if (High != std::numeric_limits<int64_t>::max()) {
        IntRange R = { High + 1, std::numeric_limits<int64_t>::max() };
        UnreachableRanges.push_back(R);
      }

      // Count popularity.
      int64_t N = High - Low + 1;
      unsigned &Pop = Popularity[I.BB];
      if ((Pop += N) > MaxPop) {
        MaxPop = Pop;
        PopSucc = I.BB;
      }
    }
#ifndef NDEBUG
    /* UnreachableRanges should be sorted and the ranges non-adjacent. */
    for (auto I = UnreachableRanges.begin(), E = UnreachableRanges.end();
         I != E; ++I) {
      assert(I->Low <= I->High);
      auto Next = I + 1;
      if (Next != E) {
        assert(Next->Low > I->High);
      }
    }
#endif

    // As the default block in the switch is unreachable, update the PHI nodes
    // (remove all of the references to the default block) to reflect this.
    const unsigned NumDefaultEdges = SI->getNumCases() + 1 - NumSimpleCases;
    for (unsigned I = 0; I < NumDefaultEdges; ++I)
      Default->removePredecessor(OrigBlock);

    // Use the most popular block as the new default, reducing the number of
    // cases.
    assert(MaxPop > 0 && PopSucc);
    Default = PopSucc;
    Cases.erase(
        llvm::remove_if(
            Cases, [PopSucc](const CaseRange &R) { return R.BB == PopSucc; }),
        Cases.end());

    // If there are no cases left, just branch.
    if (Cases.empty()) {
      BranchInst::Create(Default, OrigBlock);
      SI->eraseFromParent();
      // As all the cases have been replaced with a single branch, only keep
      // one entry in the PHI nodes.
      for (unsigned I = 0 ; I < (MaxPop - 1) ; ++I)
        PopSucc->removePredecessor(OrigBlock);
      return;
    }
  }

  // Create a new, empty default block so that the new hierarchy of
  // if-then statements go to this and the PHI nodes are happy.
  BasicBlock *NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
  F->getBasicBlockList().insert(Default->getIterator(), NewDefault);
  BranchInst::Create(Default, NewDefault);

  BasicBlock *SwitchBlock =
      switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
                    OrigBlock, OrigBlock, NewDefault, UnreachableRanges);

  // If there are entries in any PHI nodes for the default edge, make sure
  // to update them as well.
  fixPhis(Default, OrigBlock, NewDefault);

  // Branch to our shiny new if-then stuff...
  BranchInst::Create(SwitchBlock, OrigBlock);

  // We are now done with the switch instruction, delete it.
  BasicBlock *OldDefault = SI->getDefaultDest();
  OrigBlock->getInstList().erase(SI);

  // If the Default block has no more predecessors just add it to DeleteList.
  if (pred_begin(OldDefault) == pred_end(OldDefault))
    DeleteList.insert(OldDefault);
}