Switch lowering: extract jump tables and bit tests before building binary tree (PR22262)

This is a re-commit of r235101, which also fixes the problems with the previous patch:

- Switches with only a default case and non-fallthrough were handled incorrectly

- The previous patch tickled a bug in PowerPC Early-Return Creation which is fixed here.

> This is a major rewrite of the SelectionDAG switch lowering. The previous code
> would lower switches as a binary tre, discovering clusters of cases
> suitable for lowering by jump tables or bit tests as it went along. To increase
> the likelihood of finding jump tables, the binary tree pivot was selected to
> maximize case density on both sides of the pivot.
>
> By not selecting the pivot in the middle, the binary trees would not always
> be balanced, leading to performance problems in the generated code.
>
> This patch rewrites the lowering to search for clusters of cases
> suitable for jump tables or bit tests first, and then builds the binary
> tree around those clusters. This way, the binary tree will always be balanced.
>
> This has the added benefit of decoupling the different aspects of the lowering:
> tree building and jump table or bit tests finding are now easier to tweak
> separately.
>
> For example, this will enable us to balance the tree based on profile info
> in the future.
>
> The algorithm for finding jump tables is quadratic, whereas the previous algorithm
> was O(n log n) for common cases, and quadratic only in the worst-case. This
> doesn't seem to be major problem in practice, e.g. compiling a file consisting
> of a 10k-case switch was only 30% slower, and such large switches should be rare
> in practice. Compiling e.g. gcc.c showed no compile-time difference.  If this
> does turn out to be a problem, we could limit the search space of the algorithm.
>
> This commit also disables all optimizations during switch lowering in -O0.
>
> Differential Revision: http://reviews.llvm.org/D8649

llvm-svn: 235560

1 files changed, 780 insertions, 663 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 2c813138f84..bc7d8fdb7ac 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1928,7 +1928,7 @@ void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B,
 
   // Avoid emitting unnecessary branches to the next block.
   if (MBB != NextBlock(SwitchBB))
-    BrRange = DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other, CopyTo,
+    BrRange = DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other, BrRange,
                           DAG.getBasicBlock(MBB));
 
   DAG.setRoot(BrRange);
@@ -2101,592 +2101,37 @@ SelectionDAGBuilder::visitLandingPadClauseBB(GlobalValue *ClauseGV,
   return VReg;
 }
 
-/// handleSmallSwitchCaseRange - Emit a series of specific tests (suitable for
-/// small case ranges).
-bool SelectionDAGBuilder::handleSmallSwitchRange(CaseRec& CR,
-                                                 CaseRecVector& WorkList,
-                                                 const Value* SV,
-                                                 MachineBasicBlock *Default,
-                                                 MachineBasicBlock *SwitchBB) {
-  // Size is the number of Cases represented by this range.
-  size_t Size = CR.Range.second - CR.Range.first;
-  if (Size > 3)
-    return false;
-
-  // Get the MachineFunction which holds the current MBB.  This is used when
-  // inserting any additional MBBs necessary to represent the switch.
-  MachineFunction *CurMF = FuncInfo.MF;
-
-  // Figure out which block is immediately after the current one.
-  MachineBasicBlock *NextMBB = nullptr;
-  MachineFunction::iterator BBI = CR.CaseBB;
-  if (++BBI != FuncInfo.MF->end())
-    NextMBB = BBI;
-
-  BranchProbabilityInfo *BPI = FuncInfo.BPI;
-  // If any two of the cases has the same destination, and if one value
-  // is the same as the other, but has one bit unset that the other has set,
-  // use bit manipulation to do two compares at once.  For example:
-  // "if (X == 6 || X == 4)" -> "if ((X|2) == 6)"
-  // TODO: This could be extended to merge any 2 cases in switches with 3 cases.
-  // TODO: Handle cases where CR.CaseBB != SwitchBB.
-  if (Size == 2 && CR.CaseBB == SwitchBB) {
-    Case &Small = *CR.Range.first;
-    Case &Big = *(CR.Range.second-1);
-
-    if (Small.Low == Small.High && Big.Low == Big.High && Small.BB == Big.BB) {
-      const APInt& SmallValue = Small.Low->getValue();
-      const APInt& BigValue = Big.Low->getValue();
-
-      // Check that there is only one bit different.
-      if (BigValue.countPopulation() == SmallValue.countPopulation() + 1 &&
-          (SmallValue | BigValue) == BigValue) {
-        // Isolate the common bit.
-        APInt CommonBit = BigValue & ~SmallValue;
-        assert((SmallValue | CommonBit) == BigValue &&
-               CommonBit.countPopulation() == 1 && "Not a common bit?");
-
-        SDValue CondLHS = getValue(SV);
-        EVT VT = CondLHS.getValueType();
-        SDLoc DL = getCurSDLoc();
-
-        SDValue Or = DAG.getNode(ISD::OR, DL, VT, CondLHS,
-                                 DAG.getConstant(CommonBit, VT));
-        SDValue Cond = DAG.getSetCC(DL, MVT::i1,
-                                    Or, DAG.getConstant(BigValue, VT),
-                                    ISD::SETEQ);
-
-        // Update successor info.
-        // Both Small and Big will jump to Small.BB, so we sum up the weights.
-        addSuccessorWithWeight(SwitchBB, Small.BB,
-                               Small.ExtraWeight + Big.ExtraWeight);
-        addSuccessorWithWeight(SwitchBB, Default,
-          // The default destination is the first successor in IR.
-          BPI ? BPI->getEdgeWeight(SwitchBB->getBasicBlock(), (unsigned)0) : 0);
-
-        // Insert the true branch.
-        SDValue BrCond = DAG.getNode(ISD::BRCOND, DL, MVT::Other,
-                                     getControlRoot(), Cond,
-                                     DAG.getBasicBlock(Small.BB));
-
-        // Insert the false branch.
-        BrCond = DAG.getNode(ISD::BR, DL, MVT::Other, BrCond,
-                             DAG.getBasicBlock(Default));
-
-        DAG.setRoot(BrCond);
-        return true;
-      }
-    }
-  }
-
-  // Order cases by weight so the most likely case will be checked first.
-  uint32_t UnhandledWeights = 0;
-  if (BPI) {
-    for (CaseItr I = CR.Range.first, IE = CR.Range.second; I != IE; ++I) {
-      uint32_t IWeight = I->ExtraWeight;
-      UnhandledWeights += IWeight;
-      for (CaseItr J = CR.Range.first; J < I; ++J) {
-        uint32_t JWeight = J->ExtraWeight;
-        if (IWeight > JWeight)
-          std::swap(*I, *J);
-      }
-    }
-  }
-  // Rearrange the case blocks so that the last one falls through if possible.
-  Case &BackCase = *(CR.Range.second-1);
-  if (Size > 1 && NextMBB && Default != NextMBB && BackCase.BB != NextMBB) {
-    // The last case block won't fall through into 'NextMBB' if we emit the
-    // branches in this order.  See if rearranging a case value would help.
-    // We start at the bottom as it's the case with the least weight.
-    for (Case *I = &*(CR.Range.second-2), *E = &*CR.Range.first-1; I != E; --I)
-      if (I->BB == NextMBB) {
-        std::swap(*I, BackCase);
-        break;
-      }
-  }
-
-  // Create a CaseBlock record representing a conditional branch to
-  // the Case's target mbb if the value being switched on SV is equal
-  // to C.
-  MachineBasicBlock *CurBlock = CR.CaseBB;
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I) {
-    MachineBasicBlock *FallThrough;
-    if (I != E-1) {
-      FallThrough = CurMF->CreateMachineBasicBlock(CurBlock->getBasicBlock());
-      CurMF->insert(BBI, FallThrough);
-
-      // Put SV in a virtual register to make it available from the new blocks.
-      ExportFromCurrentBlock(SV);
-    } else {
-      // If the last case doesn't match, go to the default block.
-      FallThrough = Default;
-    }
-
-    const Value *RHS, *LHS, *MHS;
-    ISD::CondCode CC;
-    if (I->High == I->Low) {
-      // This is just small small case range :) containing exactly 1 case
-      CC = ISD::SETEQ;
-      LHS = SV; RHS = I->High; MHS = nullptr;
-    } else {
-      CC = ISD::SETLE;
-      LHS = I->Low; MHS = SV; RHS = I->High;
-    }
-
-    // The false weight should be sum of all un-handled cases.
-    UnhandledWeights -= I->ExtraWeight;
-    CaseBlock CB(CC, LHS, RHS, MHS, /* truebb */ I->BB, /* falsebb */ FallThrough,
-                 /* me */ CurBlock,
-                 /* trueweight */ I->ExtraWeight,
-                 /* falseweight */ UnhandledWeights);
-
-    // If emitting the first comparison, just call visitSwitchCase to emit the
-    // code into the current block.  Otherwise, push the CaseBlock onto the
-    // vector to be later processed by SDISel, and insert the node's MBB
-    // before the next MBB.
-    if (CurBlock == SwitchBB)
-      visitSwitchCase(CB, SwitchBB);
-    else
-      SwitchCases.push_back(CB);
-
-    CurBlock = FallThrough;
-  }
-
-  return true;
-}
-
-static inline bool areJTsAllowed(const TargetLowering &TLI) {
-  return TLI.isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
-         TLI.isOperationLegalOrCustom(ISD::BRIND, MVT::Other);
-}
-
-static APInt ComputeRange(const APInt &First, const APInt &Last) {
-  uint32_t BitWidth = std::max(Last.getBitWidth(), First.getBitWidth()) + 1;
-  APInt LastExt = Last.sext(BitWidth), FirstExt = First.sext(BitWidth);
-  return (LastExt - FirstExt + 1ULL);
-}
-
-/// handleJTSwitchCase - Emit jumptable for current switch case range
-bool SelectionDAGBuilder::handleJTSwitchCase(CaseRec &CR,
-                                             CaseRecVector &WorkList,
-                                             const Value *SV,
-                                             MachineBasicBlock *Default,
-                                             MachineBasicBlock *SwitchBB) {
-  Case& FrontCase = *CR.Range.first;
-  Case& BackCase  = *(CR.Range.second-1);
-
-  const APInt &First = FrontCase.Low->getValue();
-  const APInt &Last  = BackCase.High->getValue();
-
-  APInt TSize(First.getBitWidth(), 0);
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I)
-    TSize += I->size();
-
-  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-  if (!areJTsAllowed(TLI) || TSize.ult(TLI.getMinimumJumpTableEntries()))
-    return false;
-
-  APInt Range = ComputeRange(First, Last);
-  // The density is TSize / Range. Require at least 40%.
-  // It should not be possible for IntTSize to saturate for sane code, but make
-  // sure we handle Range saturation correctly.
-  uint64_t IntRange = Range.getLimitedValue(UINT64_MAX/10);
-  uint64_t IntTSize = TSize.getLimitedValue(UINT64_MAX/10);
-  if (IntTSize * 10 < IntRange * 4)
-    return false;
-
-  DEBUG(dbgs() << "Lowering jump table\n"
-               << "First entry: " << First << ". Last entry: " << Last << '\n'
-               << "Range: " << Range << ". Size: " << TSize << ".\n\n");
-
-  // Get the MachineFunction which holds the current MBB.  This is used when
-  // inserting any additional MBBs necessary to represent the switch.
-  MachineFunction *CurMF = FuncInfo.MF;
-
-  // Figure out which block is immediately after the current one.
-  MachineFunction::iterator BBI = CR.CaseBB;
-  ++BBI;
-
-  const BasicBlock *LLVMBB = CR.CaseBB->getBasicBlock();
-
-  // Create a new basic block to hold the code for loading the address
-  // of the jump table, and jumping to it.  Update successor information;
-  // we will either branch to the default case for the switch, or the jump
-  // table.
-  MachineBasicBlock *JumpTableBB = CurMF->CreateMachineBasicBlock(LLVMBB);
-  CurMF->insert(BBI, JumpTableBB);
-
-  addSuccessorWithWeight(CR.CaseBB, Default);
-  addSuccessorWithWeight(CR.CaseBB, JumpTableBB);
-
-  // Build a vector of destination BBs, corresponding to each target
-  // of the jump table. If the value of the jump table slot corresponds to
-  // a case statement, push the case's BB onto the vector, otherwise, push
-  // the default BB.
-  std::vector<MachineBasicBlock*> DestBBs;
-  APInt TEI = First;
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++TEI) {
-    const APInt &Low = I->Low->getValue();
-    const APInt &High = I->High->getValue();
-
-    if (Low.sle(TEI) && TEI.sle(High)) {
-      DestBBs.push_back(I->BB);
-      if (TEI==High)
-        ++I;
-    } else {
-      DestBBs.push_back(Default);
-    }
-  }
-
-  // Calculate weight for each unique destination in CR.
-  DenseMap<MachineBasicBlock*, uint32_t> DestWeights;
-  if (FuncInfo.BPI) {
-    for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I)
-      DestWeights[I->BB] += I->ExtraWeight;
-  }
-
-  // Update successor info. Add one edge to each unique successor.
-  BitVector SuccsHandled(CR.CaseBB->getParent()->getNumBlockIDs());
-  for (MachineBasicBlock *DestBB : DestBBs) {
-    if (!SuccsHandled[DestBB->getNumber()]) {
-      SuccsHandled[DestBB->getNumber()] = true;
-      auto I = DestWeights.find(DestBB);
-      addSuccessorWithWeight(JumpTableBB, DestBB,
-                             I != DestWeights.end() ? I->second : 0);
-    }
-  }
-
-  // Create a jump table index for this jump table.
-  unsigned JTEncoding = TLI.getJumpTableEncoding();
-  unsigned JTI = CurMF->getOrCreateJumpTableInfo(JTEncoding)
-                       ->createJumpTableIndex(DestBBs);
-
-  // Set the jump table information so that we can codegen it as a second
-  // MachineBasicBlock
-  JumpTable JT(-1U, JTI, JumpTableBB, Default);
-  JumpTableHeader JTH(First, Last, SV, CR.CaseBB, (CR.CaseBB == SwitchBB));
-  if (CR.CaseBB == SwitchBB)
-    visitJumpTableHeader(JT, JTH, SwitchBB);
-
-  JTCases.push_back(JumpTableBlock(JTH, JT));
-  return true;
-}
-
-/// handleBTSplitSwitchCase - emit comparison and split binary search tree into
-/// 2 subtrees.
-bool SelectionDAGBuilder::handleBTSplitSwitchCase(CaseRec& CR,
-                                                  CaseRecVector& WorkList,
-                                                  const Value* SV,
-                                                  MachineBasicBlock* SwitchBB) {
-  Case& FrontCase = *CR.Range.first;
-  Case& BackCase  = *(CR.Range.second-1);
-
-  // Size is the number of Cases represented by this range.
-  unsigned Size = CR.Range.second - CR.Range.first;
-
-  const APInt &First = FrontCase.Low->getValue();
-  const APInt &Last  = BackCase.High->getValue();
-  double FMetric = 0;
-  CaseItr Pivot = CR.Range.first + Size/2;
-
-  // Select optimal pivot, maximizing sum density of LHS and RHS. This will
-  // (heuristically) allow us to emit JumpTable's later.
-  APInt TSize(First.getBitWidth(), 0);
-  for (CaseItr I = CR.Range.first, E = CR.Range.second;
-       I!=E; ++I)
-    TSize += I->size();
-
-  APInt LSize = FrontCase.size();
-  APInt RSize = TSize-LSize;
-  DEBUG(dbgs() << "Selecting best pivot: \n"
-               << "First: " << First << ", Last: " << Last <<'\n'
-               << "LSize: " << LSize << ", RSize: " << RSize << '\n');
-  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-  for (CaseItr I = CR.Range.first, J=I+1, E = CR.Range.second;
-       J!=E; ++I, ++J) {
-    const APInt &LEnd = I->High->getValue();
-    const APInt &RBegin = J->Low->getValue();
-    APInt Range = ComputeRange(LEnd, RBegin);
-    assert((Range - 2ULL).isNonNegative() &&
-           "Invalid case distance");
-    // Use volatile double here to avoid excess precision issues on some hosts,
-    // e.g. that use 80-bit X87 registers.
-    // Only consider the density of sub-ranges that actually have sufficient
-    // entries to be lowered as a jump table.
-    volatile double LDensity =
-        LSize.ult(TLI.getMinimumJumpTableEntries())
-            ? 0.0
-            : LSize.roundToDouble() / (LEnd - First + 1ULL).roundToDouble();
-    volatile double RDensity =
-        RSize.ult(TLI.getMinimumJumpTableEntries())
-            ? 0.0
-            : RSize.roundToDouble() / (Last - RBegin + 1ULL).roundToDouble();
-    volatile double Metric = Range.logBase2() * (LDensity + RDensity);
-    // Should always split in some non-trivial place
-    DEBUG(dbgs() <<"=>Step\n"
-                 << "LEnd: " << LEnd << ", RBegin: " << RBegin << '\n'
-                 << "LDensity: " << LDensity
-                 << ", RDensity: " << RDensity << '\n'
-                 << "Metric: " << Metric << '\n');
-    if (FMetric < Metric) {
-      Pivot = J;
-      FMetric = Metric;
-      DEBUG(dbgs() << "Current metric set to: " << FMetric << '\n');
-    }
-
-    LSize += J->size();
-    RSize -= J->size();
-  }
-
-  if (FMetric == 0 || !areJTsAllowed(TLI))
-    Pivot = CR.Range.first + Size/2;
-  splitSwitchCase(CR, Pivot, WorkList, SV, SwitchBB);
-  return true;
-}
-
-void SelectionDAGBuilder::splitSwitchCase(CaseRec &CR, CaseItr Pivot,
-                                          CaseRecVector &WorkList,
-                                          const Value *SV,
-                                          MachineBasicBlock *SwitchBB) {
-  // Get the MachineFunction which holds the current MBB.  This is used when
-  // inserting any additional MBBs necessary to represent the switch.
-  MachineFunction *CurMF = FuncInfo.MF;
-
-  // Figure out which block is immediately after the current one.
-  MachineFunction::iterator BBI = CR.CaseBB;
-  ++BBI;
-
-  const BasicBlock *LLVMBB = CR.CaseBB->getBasicBlock();
-
-  CaseRange LHSR(CR.Range.first, Pivot);
-  CaseRange RHSR(Pivot, CR.Range.second);
-  const ConstantInt *C = Pivot->Low;
-  MachineBasicBlock *FalseBB = nullptr, *TrueBB = nullptr;
-
-  // We know that we branch to the LHS if the Value being switched on is
-  // less than the Pivot value, C.  We use this to optimize our binary
-  // tree a bit, by recognizing that if SV is greater than or equal to the
-  // LHS's Case Value, and that Case Value is exactly one less than the
-  // Pivot's Value, then we can branch directly to the LHS's Target,
-  // rather than creating a leaf node for it.
-  if ((LHSR.second - LHSR.first) == 1 && LHSR.first->High == CR.GE &&
-      C->getValue() == (CR.GE->getValue() + 1LL)) {
-    TrueBB = LHSR.first->BB;
-  } else {
-    TrueBB = CurMF->CreateMachineBasicBlock(LLVMBB);
-    CurMF->insert(BBI, TrueBB);
-    WorkList.push_back(CaseRec(TrueBB, C, CR.GE, LHSR));
-
-    // Put SV in a virtual register to make it available from the new blocks.
-    ExportFromCurrentBlock(SV);
-  }
-
-  // Similar to the optimization above, if the Value being switched on is
-  // known to be less than the Constant CR.LT, and the current Case Value
-  // is CR.LT - 1, then we can branch directly to the target block for
-  // the current Case Value, rather than emitting a RHS leaf node for it.
-  if ((RHSR.second - RHSR.first) == 1 && CR.LT &&
-      RHSR.first->Low->getValue() == (CR.LT->getValue() - 1LL)) {
-    FalseBB = RHSR.first->BB;
-  } else {
-    FalseBB = CurMF->CreateMachineBasicBlock(LLVMBB);
-    CurMF->insert(BBI, FalseBB);
-    WorkList.push_back(CaseRec(FalseBB, CR.LT, C, RHSR));
-
-    // Put SV in a virtual register to make it available from the new blocks.
-    ExportFromCurrentBlock(SV);
-  }
-
-  // Create a CaseBlock record representing a conditional branch to
-  // the LHS node if the value being switched on SV is less than C.
-  // Otherwise, branch to LHS.
-  CaseBlock CB(ISD::SETLT, SV, C, nullptr, TrueBB, FalseBB, CR.CaseBB);
-
-  if (CR.CaseBB == SwitchBB)
-    visitSwitchCase(CB, SwitchBB);
-  else
-    SwitchCases.push_back(CB);
-}
-
-/// handleBitTestsSwitchCase - if current case range has few destination and
-/// range span less, than machine word bitwidth, encode case range into series
-/// of masks and emit bit tests with these masks.
-bool SelectionDAGBuilder::handleBitTestsSwitchCase(CaseRec& CR,
-                                                   CaseRecVector& WorkList,
-                                                   const Value* SV,
-                                                   MachineBasicBlock* Default,
-                                                   MachineBasicBlock* SwitchBB) {
-  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-  EVT PTy = TLI.getPointerTy();
-  unsigned IntPtrBits = PTy.getSizeInBits();
-
-  Case& FrontCase = *CR.Range.first;
-  Case& BackCase  = *(CR.Range.second-1);
-
-  // Get the MachineFunction which holds the current MBB.  This is used when
-  // inserting any additional MBBs necessary to represent the switch.
-  MachineFunction *CurMF = FuncInfo.MF;
-
-  // If target does not have legal shift left, do not emit bit tests at all.
-  if (!TLI.isOperationLegal(ISD::SHL, PTy))
-    return false;
-
-  size_t numCmps = 0;
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I) {
-    // Single case counts one, case range - two.
-    numCmps += (I->Low == I->High ? 1 : 2);
-  }
-
-  // Count unique destinations
-  SmallSet<MachineBasicBlock*, 4> Dests;
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I) {
-    Dests.insert(I->BB);
-    if (Dests.size() > 3)
-      // Don't bother the code below, if there are too much unique destinations
-      return false;
-  }
-  DEBUG(dbgs() << "Total number of unique destinations: "
-        << Dests.size() << '\n'
-        << "Total number of comparisons: " << numCmps << '\n');
-
-  // Compute span of values.
-  const APInt& minValue = FrontCase.Low->getValue();
-  const APInt& maxValue = BackCase.High->getValue();
-  APInt cmpRange = maxValue - minValue;
-
-  DEBUG(dbgs() << "Compare range: " << cmpRange << '\n'
-               << "Low bound: " << minValue << '\n'
-               << "High bound: " << maxValue << '\n');
-
-  if (cmpRange.uge(IntPtrBits) ||
-      (!(Dests.size() == 1 && numCmps >= 3) &&
-       !(Dests.size() == 2 && numCmps >= 5) &&
-       !(Dests.size() >= 3 && numCmps >= 6)))
-    return false;
-
-  DEBUG(dbgs() << "Emitting bit tests\n");
-  APInt lowBound = APInt::getNullValue(cmpRange.getBitWidth());
-
-  // Optimize the case where all the case values fit in a
-  // word without having to subtract minValue. In this case,
-  // we can optimize away the subtraction.
-  if (minValue.isNonNegative() && maxValue.slt(IntPtrBits)) {
-    cmpRange = maxValue;
-  } else {
-    lowBound = minValue;
-  }
-
-  CaseBitsVector CasesBits;
-  unsigned i, count = 0;
-
-  for (CaseItr I = CR.Range.first, E = CR.Range.second; I!=E; ++I) {
-    MachineBasicBlock* Dest = I->BB;
-    for (i = 0; i < count; ++i)
-      if (Dest == CasesBits[i].BB)
-        break;
-
-    if (i == count) {
-      assert((count < 3) && "Too much destinations to test!");
-      CasesBits.push_back(CaseBits(0, Dest, 0, 0/*Weight*/));
-      count++;
-    }
-
-    const APInt& lowValue = I->Low->getValue();
-    const APInt& highValue = I->High->getValue();
-
-    uint64_t lo = (lowValue - lowBound).getZExtValue();
-    uint64_t hi = (highValue - lowBound).getZExtValue();
-    CasesBits[i].ExtraWeight += I->ExtraWeight;
-
-    for (uint64_t j = lo; j <= hi; j++) {
-      CasesBits[i].Mask |=  1ULL << j;
-      CasesBits[i].Bits++;
-    }
-
-  }
-  std::sort(CasesBits.begin(), CasesBits.end(), CaseBitsCmp());
-
-  BitTestInfo BTC;
-
-  // Figure out which block is immediately after the current one.
-  MachineFunction::iterator BBI = CR.CaseBB;
-  ++BBI;
-
-  const BasicBlock *LLVMBB = CR.CaseBB->getBasicBlock();
-
-  DEBUG(dbgs() << "Cases:\n");
-  for (unsigned i = 0, e = CasesBits.size(); i!=e; ++i) {
-    DEBUG(dbgs() << "Mask: " << CasesBits[i].Mask
-                 << ", Bits: " << CasesBits[i].Bits
-                 << ", BB: " << CasesBits[i].BB << '\n');
-
-    MachineBasicBlock *CaseBB = CurMF->CreateMachineBasicBlock(LLVMBB);
-    CurMF->insert(BBI, CaseBB);
-    BTC.push_back(BitTestCase(CasesBits[i].Mask,
-                              CaseBB,
-                              CasesBits[i].BB, CasesBits[i].ExtraWeight));
-
-    // Put SV in a virtual register to make it available from the new blocks.
-    ExportFromCurrentBlock(SV);
-  }
-
-  BitTestBlock BTB(lowBound, cmpRange, SV,
-                   -1U, MVT::Other, (CR.CaseBB == SwitchBB),
-                   CR.CaseBB, Default, std::move(BTC));
-
-  if (CR.CaseBB == SwitchBB)
-    visitBitTestHeader(BTB, SwitchBB);
-
-  BitTestCases.push_back(std::move(BTB));
-
-  return true;
-}
-
-void SelectionDAGBuilder::Clusterify(CaseVector &Cases, const SwitchInst *SI) {
-  BranchProbabilityInfo *BPI = FuncInfo.BPI;
+void SelectionDAGBuilder::sortAndRangeify(CaseClusterVector &Clusters) {
+#ifndef NDEBUG
+  for (const CaseCluster &CC : Clusters)
+    assert(CC.Low == CC.High && "Input clusters must be single-case");
+#endif
 
-  // Extract cases from the switch and sort them.
-  typedef std::pair<const ConstantInt*, unsigned> CasePair;
-  std::vector<CasePair> Sorted;
-  Sorted.reserve(SI->getNumCases());
-  for (auto I : SI->cases())
-    Sorted.push_back(std::make_pair(I.getCaseValue(), I.getSuccessorIndex()));
-  std::sort(Sorted.begin(), Sorted.end(), [](CasePair a, CasePair b) {
-    return a.first->getValue().slt(b.first->getValue());
+  std::sort(Clusters.begin(), Clusters.end(),
+            [](const CaseCluster &a, const CaseCluster &b) {
+    return a.Low->getValue().slt(b.Low->getValue());
   });
 
-  // Merge adjacent cases with the same destination, build Cases vector.
-  assert(Cases.empty() && "Cases should be empty before Clusterify;");
-  Cases.reserve(SI->getNumCases());
-  MachineBasicBlock *PreviousSucc = nullptr;
-  for (CasePair &CP : Sorted) {
-    const ConstantInt *CaseVal = CP.first;
-    unsigned SuccIndex = CP.second;
-    MachineBasicBlock *Succ = FuncInfo.MBBMap[SI->getSuccessor(SuccIndex)];
-    uint32_t Weight = BPI ? BPI->getEdgeWeight(SI->getParent(), SuccIndex) : 0;
-
-    if (PreviousSucc == Succ &&
-        (CaseVal->getValue() - Cases.back().High->getValue()) == 1) {
+  // Merge adjacent clusters with the same destination.
+  const unsigned N = Clusters.size();
+  unsigned DstIndex = 0;
+  for (unsigned SrcIndex = 0; SrcIndex < N; ++SrcIndex) {
+    CaseCluster &CC = Clusters[SrcIndex];
+    const ConstantInt *CaseVal = CC.Low;
+    MachineBasicBlock *Succ = CC.MBB;
+
+    if (DstIndex != 0 && Clusters[DstIndex - 1].MBB == Succ &&
+        (CaseVal->getValue() - Clusters[DstIndex - 1].High->getValue()) == 1) {
       // If this case has the same successor and is a neighbour, merge it into
       // the previous cluster.
-      Cases.back().High = CaseVal;
-      Cases.back().ExtraWeight += Weight;
+      Clusters[DstIndex - 1].High = CaseVal;
+      Clusters[DstIndex - 1].Weight += CC.Weight;
     } else {
-      Cases.push_back(Case(CaseVal, CaseVal, Succ, Weight));
+      std::memmove(&Clusters[DstIndex++], &Clusters[SrcIndex],
+                   sizeof(Clusters[SrcIndex]));
     }
-
-    PreviousSucc = Succ;
   }
-
-  DEBUG({
-      size_t numCmps = 0;
-      for (auto &I : Cases)
-        // A range counts double, since it requires two compares.
-        numCmps += I.Low != I.High ? 2 : 1;
-
-      dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
-             << ". Total compares: " << numCmps << '\n';
-    });
+  Clusters.resize(DstIndex);
 }
 
 void SelectionDAGBuilder::UpdateSplitBlock(MachineBasicBlock *First,
@@ -2702,90 +2147,6 @@ void SelectionDAGBuilder::UpdateSplitBlock(MachineBasicBlock *First,
       BitTestCases[i].Parent = Last;
 }
 
-void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
-  MachineBasicBlock *SwitchMBB = FuncInfo.MBB;
-
-  // Create a vector of Cases, sorted so that we can efficiently create a binary
-  // search tree from them.
-  CaseVector Cases;
-  Clusterify(Cases, &SI);
-
-  // Get the default destination MBB.
-  MachineBasicBlock *Default = FuncInfo.MBBMap[SI.getDefaultDest()];
-
-  if (isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg()) &&
-      !Cases.empty()) {
-    // Replace an unreachable default destination with the most popular case
-    // destination.
-    DenseMap<const BasicBlock *, unsigned> Popularity;
-    unsigned MaxPop = 0;
-    const BasicBlock *MaxBB = nullptr;
-    for (auto I : SI.cases()) {
-      const BasicBlock *BB = I.getCaseSuccessor();
-      if (++Popularity[BB] > MaxPop) {
-        MaxPop = Popularity[BB];
-        MaxBB = BB;
-      }
-    }
-
-    // Set new default.
-    assert(MaxPop > 0);
-    assert(MaxBB);
-    Default = FuncInfo.MBBMap[MaxBB];
-
-    // Remove cases that were pointing to the destination that is now the default.
-    Cases.erase(std::remove_if(Cases.begin(), Cases.end(),
-                               [&](const Case &C) { return C.BB == Default; }),
-                Cases.end());
-  }
-
-  // If there is only the default destination, go there directly.
-  if (Cases.empty()) {
-    // Update machine-CFG edges.
-    SwitchMBB->addSuccessor(Default);
-
-    // If this is not a fall-through branch, emit the branch.
-    if (Default != NextBlock(SwitchMBB)) {
-      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other,
-                              getControlRoot(), DAG.getBasicBlock(Default)));
-    }
-    return;
-  }
-
-  // Get the Value to be switched on.
-  const Value *SV = SI.getCondition();
-
-  // Push the initial CaseRec onto the worklist
-  CaseRecVector WorkList;
-  WorkList.push_back(CaseRec(SwitchMBB,nullptr,nullptr,
-                             CaseRange(Cases.begin(),Cases.end())));
-
-  while (!WorkList.empty()) {
-    // Grab a record representing a case range to process off the worklist
-    CaseRec CR = WorkList.back();
-    WorkList.pop_back();
-
-    if (handleBitTestsSwitchCase(CR, WorkList, SV, Default, SwitchMBB))
-      continue;
-
-    // If the range has few cases (two or less) emit a series of specific
-    // tests.
-    if (handleSmallSwitchRange(CR, WorkList, SV, Default, SwitchMBB))
-      continue;
-
-    // If the switch has more than N blocks, and is at least 40% dense, and the
-    // target supports indirect branches, then emit a jump table rather than
-    // lowering the switch to a binary tree of conditional branches.
-    // N defaults to 4 and is controlled via TLS.getMinimumJumpTableEntries().
-    if (handleJTSwitchCase(CR, WorkList, SV, Default, SwitchMBB))
-      continue;
-
-    // Emit binary tree. We need to pick a pivot, and push left and right ranges
-    // onto the worklist. Leafs are handled via handleSmallSwitchRange() call.
-    handleBTSplitSwitchCase(CR, WorkList, SV, SwitchMBB);
-  }
-}
-
 void SelectionDAGBuilder::visitIndirectBr(const IndirectBrInst &I) {
   MachineBasicBlock *IndirectBrMBB = FuncInfo.MBB;
 
@@ -7819,3 +7180,759 @@ void SelectionDAGBuilder::updateDAGForMaybeTailCall(SDValue MaybeTC) {
     HasTailCall = true;
 }
 
+bool SelectionDAGBuilder::isDense(const CaseClusterVector &Clusters,
+                                  unsigned *TotalCases, unsigned First,
+                                  unsigned Last) {
+  assert(Last >= First);
+  assert(TotalCases[Last] >= TotalCases[First]);
+
+  APInt LowCase = Clusters[First].Low->getValue();
+  APInt HighCase = Clusters[Last].High->getValue();
+  assert(LowCase.getBitWidth() == HighCase.getBitWidth());
+
+  // FIXME: A range of consecutive cases has 100% density, but only requires one
+  // comparison to lower. We should discriminate against such consecutive ranges
+  // in jump tables.
+
+  uint64_t Diff = (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100);
+  uint64_t Range = Diff + 1;
+
+  uint64_t NumCases =
+      TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
+
+  assert(NumCases < UINT64_MAX / 100);
+  assert(Range >= NumCases);
+
+  return NumCases * 100 >= Range * MinJumpTableDensity;
+}
+
+static inline bool areJTsAllowed(const TargetLowering &TLI) {
+  return TLI.isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
+         TLI.isOperationLegalOrCustom(ISD::BRIND, MVT::Other);
+}
+
+bool SelectionDAGBuilder::buildJumpTable(CaseClusterVector &Clusters,
+                                         unsigned First, unsigned Last,
+                                         const SwitchInst *SI,
+                                         MachineBasicBlock *DefaultMBB,
+                                         CaseCluster &JTCluster) {
+  assert(First <= Last);
+
+  uint64_t Weight = 0;
+  unsigned NumCmps = 0;
+  std::vector<MachineBasicBlock*> Table;
+  DenseMap<MachineBasicBlock*, uint32_t> JTWeights;
+  for (unsigned I = First; I <= Last; ++I) {
+    assert(Clusters[I].Kind == CC_Range);
+    Weight += Clusters[I].Weight;
+    APInt Low = Clusters[I].Low->getValue();
+    APInt High = Clusters[I].High->getValue();
+    NumCmps += (Low == High) ? 1 : 2;
+    if (I != First) {
+      // Fill the gap between this and the previous cluster.
+      APInt PreviousHigh = Clusters[I - 1].High->getValue();
+      assert(PreviousHigh.slt(Low));
+      uint64_t Gap = (Low - PreviousHigh).getLimitedValue() - 1;
+      for (uint64_t J = 0; J < Gap; J++)
+        Table.push_back(DefaultMBB);
+    }
+    for (APInt X = Low; X.sle(High); ++X)
+      Table.push_back(Clusters[I].MBB);
+    JTWeights[Clusters[I].MBB] += Clusters[I].Weight;
+  }
+
+  unsigned NumDests = JTWeights.size();
+  if (isSuitableForBitTests(NumDests, NumCmps,
+                            Clusters[First].Low->getValue(),
+                            Clusters[Last].High->getValue())) {
+    // Clusters[First..Last] should be lowered as bit tests instead.
+    return false;
+  }
+
+  // Create the MBB that will load from and jump through the table.
+  // Note: We create it here, but it's not inserted into the function yet.
+  MachineFunction *CurMF = FuncInfo.MF;
+  MachineBasicBlock *JumpTableMBB =
+      CurMF->CreateMachineBasicBlock(SI->getParent());
+
+  // Add successors. Note: use table order for determinism.
+  SmallPtrSet<MachineBasicBlock *, 8> Done;
+  for (MachineBasicBlock *Succ : Table) {
+    if (Done.count(Succ))
+      continue;
+    addSuccessorWithWeight(JumpTableMBB, Succ, JTWeights[Succ]);
+    Done.insert(Succ);
+  }
+
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  unsigned JTI = CurMF->getOrCreateJumpTableInfo(TLI.getJumpTableEncoding())
+                     ->createJumpTableIndex(Table);
+
+  // Set up the jump table info.
+  JumpTable JT(-1U, JTI, JumpTableMBB, nullptr);
+  JumpTableHeader JTH(Clusters[First].Low->getValue(),
+                      Clusters[Last].High->getValue(), SI->getCondition(),
+                      nullptr, false);
+  JTCases.push_back(JumpTableBlock(JTH, JT));
+
+  JTCluster = CaseCluster::jumpTable(Clusters[First].Low, Clusters[Last].High,
+                                     JTCases.size() - 1, Weight);
+  return true;
+}
+
+void SelectionDAGBuilder::findJumpTables(CaseClusterVector &Clusters,
+                                         const SwitchInst *SI,
+                                         MachineBasicBlock *DefaultMBB) {
+#ifndef NDEBUG
+  // Clusters must be non-empty, sorted, and only contain Range clusters.
+  assert(!Clusters.empty());
+  for (CaseCluster &C : Clusters)
+    assert(C.Kind == CC_Range);
+  for (unsigned i = 1, e = Clusters.size(); i < e; ++i)
+    assert(Clusters[i - 1].High->getValue().slt(Clusters[i].Low->getValue()));
+#endif
+
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  if (!areJTsAllowed(TLI))
+    return;
+
+  const int64_t N = Clusters.size();
+  const unsigned MinJumpTableSize = TLI.getMinimumJumpTableEntries();
+
+  // Split Clusters into minimum number of dense partitions. The algorithm uses
+  // the same idea as Kannan & Proebsting "Correction to 'Producing Good Code
+  // for the Case Statement'" (1994), but builds the MinPartitions array in
+  // reverse order to make it easier to reconstruct the partitions in ascending
+  // order. In the choice between two optimal partitionings, it picks the one
+  // which yields more jump tables.
+
+  // MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
+  SmallVector<unsigned, 8> MinPartitions(N);
+  // LastElement[i] is the last element of the partition starting at i.
+  SmallVector<unsigned, 8> LastElement(N);
+  // NumTables[i]: nbr of >= MinJumpTableSize partitions from Clusters[i..N-1].
+  SmallVector<unsigned, 8> NumTables(N);
+  // TotalCases[i]: Total nbr of cases in Clusters[0..i].
+  SmallVector<unsigned, 8> TotalCases(N);
+
+  for (unsigned i = 0; i < N; ++i) {
+    APInt Hi = Clusters[i].High->getValue();
+    APInt Lo = Clusters[i].Low->getValue();
+    TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
+    if (i != 0)
+      TotalCases[i] += TotalCases[i - 1];
+  }
+
+  // Base case: There is only one way to partition Clusters[N-1].
+  MinPartitions[N - 1] = 1;
+  LastElement[N - 1] = N - 1;
+  assert(MinJumpTableSize > 1);
+  NumTables[N - 1] = 0;
+
+  // Note: loop indexes are signed to avoid underflow.
+  for (int64_t i = N - 2; i >= 0; i--) {
+    // Find optimal partitioning of Clusters[i..N-1].
+    // Baseline: Put Clusters[i] into a partition on its own.
+    MinPartitions[i] = MinPartitions[i + 1] + 1;
+    LastElement[i] = i;
+    NumTables[i] = NumTables[i + 1];
+
+    // Search for a solution that results in fewer partitions.
+    for (int64_t j = N - 1; j > i; j--) {
+      // Try building a partition from Clusters[i..j].
+      if (isDense(Clusters, &TotalCases[0], i, j)) {
+        unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
+        bool IsTable = j - i + 1 >= MinJumpTableSize;
+        unsigned Tables = IsTable + (j == N - 1 ? 0 : NumTables[j + 1]);
+
+        // If this j leads to fewer partitions, or same number of partitions
+        // with more lookup tables, it is a better partitioning.
+        if (NumPartitions < MinPartitions[i] ||
+            (NumPartitions == MinPartitions[i] && Tables > NumTables[i])) {
+          MinPartitions[i] = NumPartitions;
+          LastElement[i] = j;
+          NumTables[i] = Tables;
+        }
+      }
+    }
+  }
+
+  // Iterate over the partitions, replacing some with jump tables in-place.
+  unsigned DstIndex = 0;
+  for (unsigned First = 0, Last; First < N; First = Last + 1) {
+    Last = LastElement[First];
+    assert(Last >= First);
+    assert(DstIndex <= First);
+    unsigned NumClusters = Last - First + 1;
+
+    CaseCluster JTCluster;
+    if (NumClusters >= MinJumpTableSize &&
+        buildJumpTable(Clusters, First, Last, SI, DefaultMBB, JTCluster)) {
+      Clusters[DstIndex++] = JTCluster;
+    } else {
+      for (unsigned I = First; I <= Last; ++I)
+        std::memmove(&Clusters[DstIndex++], &Clusters[I], sizeof(Clusters[I]));
+    }
+  }
+  Clusters.resize(DstIndex);
+}
+
+bool SelectionDAGBuilder::rangeFitsInWord(const APInt &Low, const APInt &High) {
+  // FIXME: Using the pointer type doesn't seem ideal.
+  uint64_t BW = DAG.getTargetLoweringInfo().getPointerTy().getSizeInBits();
+  uint64_t Range = (High - Low).getLimitedValue(UINT64_MAX - 1) + 1;
+  return Range <= BW;
+}
+
+bool SelectionDAGBuilder::isSuitableForBitTests(unsigned NumDests,
+                                                unsigned NumCmps,
+                                                const APInt &Low,
+                                                const APInt &High) {
+  // FIXME: I don't think NumCmps is the correct metric: a single case and a
+  // range of cases both require only one branch to lower. Just looking at the
+  // number of clusters and destinations should be enough to decide whether to
+  // build bit tests.
+
+  // To lower a range with bit tests, the range must fit the bitwidth of a
+  // machine word.
+  if (!rangeFitsInWord(Low, High))
+    return false;
+
+  // Decide whether it's profitable to lower this range with bit tests. Each
+  // destination requires a bit test and branch, and there is an overall range
+  // check branch. For a small number of clusters, separate comparisons might be
+  // cheaper, and for many destinations, splitting the range might be better.
+  return (NumDests == 1 && NumCmps >= 3) ||
+         (NumDests == 2 && NumCmps >= 5) ||
+         (NumDests == 3 && NumCmps >= 6);
+}
+
+bool SelectionDAGBuilder::buildBitTests(CaseClusterVector &Clusters,
+                                        unsigned First, unsigned Last,
+                                        const SwitchInst *SI,
+                                        CaseCluster &BTCluster) {
+  assert(First <= Last);
+  if (First == Last)
+    return false;
+
+  BitVector Dests(FuncInfo.MF->getNumBlockIDs());
+  unsigned NumCmps = 0;
+  for (int64_t I = First; I <= Last; ++I) {
+    assert(Clusters[I].Kind == CC_Range);
+    Dests.set(Clusters[I].MBB->getNumber());
+    NumCmps += (Clusters[I].Low == Clusters[I].High) ? 1 : 2;
+  }
+  unsigned NumDests = Dests.count();
+
+  APInt Low = Clusters[First].Low->getValue();
+  APInt High = Clusters[Last].High->getValue();
+  assert(Low.slt(High));
+
+  if (!isSuitableForBitTests(NumDests, NumCmps, Low, High))
+    return false;
+
+  APInt LowBound;
+  APInt CmpRange;
+
+  const int BitWidth =
+      DAG.getTargetLoweringInfo().getPointerTy().getSizeInBits();
+  assert((High - Low + 1).sle(BitWidth) && "Case range must fit in bit mask!");
+
+  if (Low.isNonNegative() && High.slt(BitWidth)) {
+    // Optimize the case where all the case values fit in a
+    // word without having to subtract minValue. In this case,
+    // we can optimize away the subtraction.
+    LowBound = APInt::getNullValue(Low.getBitWidth());
+    CmpRange = High;
+  } else {
+    LowBound = Low;
+    CmpRange = High - Low;
+  }
+
+  CaseBitsVector CBV;
+  uint64_t TotalWeight = 0;
+  for (unsigned i = First; i <= Last; ++i) {
+    // Find the CaseBits for this destination.
+    unsigned j;
+    for (j = 0; j < CBV.size(); ++j)
+      if (CBV[j].BB == Clusters[i].MBB)
+        break;
+    if (j == CBV.size())
+      CBV.push_back(CaseBits(0, Clusters[i].MBB, 0, 0));
+    CaseBits *CB = &CBV[j];
+
+    // Update Mask, Bits and ExtraWeight.
+    uint64_t Lo = (Clusters[i].Low->getValue() - LowBound).getZExtValue();
+    uint64_t Hi = (Clusters[i].High->getValue() - LowBound).getZExtValue();
+    for (uint64_t j = Lo; j <= Hi; ++j) {
+      CB->Mask |= 1ULL << j;
+      CB->Bits++;
+    }
+    CB->ExtraWeight += Clusters[i].Weight;
+    TotalWeight += Clusters[i].Weight;
+  }
+
+  BitTestInfo BTI;
+  std::sort(CBV.begin(), CBV.end(), [](const CaseBits &a, const CaseBits &b) {
+    // FIXME: Sort by weight.
+    return a.Bits > b.Bits;
+  });
+
+  for (auto &CB : CBV) {
+    MachineBasicBlock *BitTestBB =
+        FuncInfo.MF->CreateMachineBasicBlock(SI->getParent());
+    BTI.push_back(BitTestCase(CB.Mask, BitTestBB, CB.BB, CB.ExtraWeight));
+  }
+  BitTestCases.push_back(BitTestBlock(LowBound, CmpRange, SI->getCondition(),
+                                      -1U, MVT::Other, false, nullptr,
+                                      nullptr, std::move(BTI)));
+
+  BTCluster = CaseCluster::bitTests(Clusters[First].Low, Clusters[Last].High,
+                                    BitTestCases.size() - 1, TotalWeight);
+  return true;
+}
+
+void SelectionDAGBuilder::findBitTestClusters(CaseClusterVector &Clusters,
+                                              const SwitchInst *SI) {
+// Partition Clusters into as few subsets as possible, where each subset has a
+// range that fits in a machine word and has <= 3 unique destinations.
+
+#ifndef NDEBUG
+  // Clusters must be sorted and contain Range or JumpTable clusters.
+  assert(!Clusters.empty());
+  assert(Clusters[0].Kind == CC_Range || Clusters[0].Kind == CC_JumpTable);
+  for (const CaseCluster &C : Clusters)
+    assert(C.Kind == CC_Range || C.Kind == CC_JumpTable);
+  for (unsigned i = 1; i < Clusters.size(); ++i)
+    assert(Clusters[i-1].High->getValue().slt(Clusters[i].Low->getValue()));
+#endif
+
+  // If target does not have legal shift left, do not emit bit tests at all.
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  EVT PTy = TLI.getPointerTy();
+  if (!TLI.isOperationLegal(ISD::SHL, PTy))
+    return;
+
+  int BitWidth = PTy.getSizeInBits();
+  const int64_t N = Clusters.size();
+
+  // MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
+  SmallVector<unsigned, 8> MinPartitions(N);
+  // LastElement[i] is the last element of the partition starting at i.
+  SmallVector<unsigned, 8> LastElement(N);
+
+  // FIXME: This might not be the best algorithm for finding bit test clusters.
+
+  // Base case: There is only one way to partition Clusters[N-1].
+  MinPartitions[N - 1] = 1;
+  LastElement[N - 1] = N - 1;
+
+  // Note: loop indexes are signed to avoid underflow.
+  for (int64_t i = N - 2; i >= 0; --i) {
+    // Find optimal partitioning of Clusters[i..N-1].
+    // Baseline: Put Clusters[i] into a partition on its own.
+    MinPartitions[i] = MinPartitions[i + 1] + 1;
+    LastElement[i] = i;
+
+    // Search for a solution that results in fewer partitions.
+    // Note: the search is limited by BitWidth, reducing time complexity.
+    for (int64_t j = std::min(N - 1, i + BitWidth - 1); j > i; --j) {
+      // Try building a partition from Clusters[i..j].
+
+      // Check the range.
+      if (!rangeFitsInWord(Clusters[i].Low->getValue(),
+                           Clusters[j].High->getValue()))
+        continue;
+
+      // Check nbr of destinations and cluster types.
+      // FIXME: This works, but doesn't seem very efficient.
+      bool RangesOnly = true;
+      BitVector Dests(FuncInfo.MF->getNumBlockIDs());
+      for (int64_t k = i; k <= j; k++) {
+        if (Clusters[k].Kind != CC_Range) {
+          RangesOnly = false;
+          break;
+        }
+        Dests.set(Clusters[k].MBB->getNumber());
+      }
+      if (!RangesOnly || Dests.count() > 3)
+        break;
+
+      // Check if it's a better partition.
+      unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
+      if (NumPartitions < MinPartitions[i]) {
+        // Found a better partition.
+        MinPartitions[i] = NumPartitions;
+        LastElement[i] = j;
+      }
+    }
+  }
+
+  // Iterate over the partitions, replacing with bit-test clusters in-place.
+  unsigned DstIndex = 0;
+  for (unsigned First = 0, Last; First < N; First = Last + 1) {
+    Last = LastElement[First];
+    assert(First <= Last);
+    assert(DstIndex <= First);
+
+    CaseCluster BitTestCluster;
+    if (buildBitTests(Clusters, First, Last, SI, BitTestCluster)) {
+      Clusters[DstIndex++] = BitTestCluster;
+    } else {
+      for (unsigned I = First; I <= Last; ++I)
+        std::memmove(&Clusters[DstIndex++], &Clusters[I], sizeof(Clusters[I]));
+    }
+  }
+  Clusters.resize(DstIndex);
+}
+
+void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond,
+                                        MachineBasicBlock *SwitchMBB,
+                                        MachineBasicBlock *DefaultMBB) {
+  MachineFunction *CurMF = FuncInfo.MF;
+  MachineBasicBlock *NextMBB = nullptr;
+  MachineFunction::iterator BBI = W.MBB;
+  if (++BBI != FuncInfo.MF->end())
+    NextMBB = BBI;
+
+  unsigned Size = W.LastCluster - W.FirstCluster + 1;
+
+  BranchProbabilityInfo *BPI = FuncInfo.BPI;
+
+  if (Size == 2 && W.MBB == SwitchMBB) {
+    // If any two of the cases has the same destination, and if one value
+    // is the same as the other, but has one bit unset that the other has set,
+    // use bit manipulation to do two compares at once.  For example:
+    // "if (X == 6 || X == 4)" -> "if ((X|2) == 6)"
+    // TODO: This could be extended to merge any 2 cases in switches with 3
+    // cases.
+    // TODO: Handle cases where W.CaseBB != SwitchBB.
+    CaseCluster &Small = *W.FirstCluster;
+    CaseCluster &Big = *W.LastCluster;
+
+    if (Small.Low == Small.High && Big.Low == Big.High &&
+        Small.MBB == Big.MBB) {
+      const APInt &SmallValue = Small.Low->getValue();
+      const APInt &BigValue = Big.Low->getValue();
+
+      // Check that there is only one bit different.
+      if (BigValue.countPopulation() == SmallValue.countPopulation() + 1 &&
+          (SmallValue | BigValue) == BigValue) {
+        // Isolate the common bit.
+        APInt CommonBit = BigValue & ~SmallValue;
+        assert((SmallValue | CommonBit) == BigValue &&
+               CommonBit.countPopulation() == 1 && "Not a common bit?");
+
+        SDValue CondLHS = getValue(Cond);
+        EVT VT = CondLHS.getValueType();
+        SDLoc DL = getCurSDLoc();
+
+        SDValue Or = DAG.getNode(ISD::OR, DL, VT, CondLHS,
+                                 DAG.getConstant(CommonBit, VT));
+        SDValue Cond = DAG.getSetCC(DL, MVT::i1, Or,
+                                    DAG.getConstant(BigValue, VT), ISD::SETEQ);
+
+        // Update successor info.
+        // Both Small and Big will jump to Small.BB, so we sum up the weights.
+        addSuccessorWithWeight(SwitchMBB, Small.MBB, Small.Weight + Big.Weight);
+        addSuccessorWithWeight(
+            SwitchMBB, DefaultMBB,
+            // The default destination is the first successor in IR.
+            BPI ? BPI->getEdgeWeight(SwitchMBB->getBasicBlock(), (unsigned)0)
+                : 0);
+
+        // Insert the true branch.
+        SDValue BrCond =
+            DAG.getNode(ISD::BRCOND, DL, MVT::Other, getControlRoot(), Cond,
+                        DAG.getBasicBlock(Small.MBB));
+        // Insert the false branch.
+        BrCond = DAG.getNode(ISD::BR, DL, MVT::Other, BrCond,
+                             DAG.getBasicBlock(DefaultMBB));
+
+        DAG.setRoot(BrCond);
+        return;
+      }
+    }
+  }
+
+  if (TM.getOptLevel() != CodeGenOpt::None) {
+    // Order cases by weight so the most likely case will be checked first.
+    std::sort(W.FirstCluster, W.LastCluster + 1,
+              [](const CaseCluster &a, const CaseCluster &b) {
+      return a.Weight > b.Weight;
+    });
+
+    // Rearrange the case blocks so that the last one falls through if possible.
+    // Start at the bottom as that's the case with the lowest weight.
+    // FIXME: Take branch probability into account.
+    for (CaseClusterIt I = W.LastCluster - 1; I >= W.FirstCluster; --I) {
+      if (I->Kind == CC_Range && I->MBB == NextMBB) {
+        std::swap(*I, *W.LastCluster);
+        break;
+      }
+    }
+  }
+
+  // Compute total weight.
+  uint32_t UnhandledWeights = 0;
+  for (CaseClusterIt I = W.FirstCluster; I <= W.LastCluster; ++I)
+    UnhandledWeights += I->Weight;
+
+  MachineBasicBlock *CurMBB = W.MBB;
+  for (CaseClusterIt I = W.FirstCluster, E = W.LastCluster; I <= E; ++I) {
+    MachineBasicBlock *Fallthrough;
+    if (I == W.LastCluster) {
+      // For the last cluster, fall through to the default destination.
+      Fallthrough = DefaultMBB;
+    } else {
+      Fallthrough = CurMF->CreateMachineBasicBlock(CurMBB->getBasicBlock());
+      CurMF->insert(BBI, Fallthrough);
+      // Put Cond in a virtual register to make it available from the new blocks.
+      ExportFromCurrentBlock(Cond);
+    }
+
+    switch (I->Kind) {
+      case CC_JumpTable: {
+        // FIXME: Optimize away range check based on pivot comparisons.
+        JumpTableHeader *JTH = &JTCases[I->JTCasesIndex].first;
+        JumpTable *JT = &JTCases[I->JTCasesIndex].second;
+
+        // The jump block hasn't been inserted yet; insert it here.
+        MachineBasicBlock *JumpMBB = JT->MBB;
+        CurMF->insert(BBI, JumpMBB);
+        addSuccessorWithWeight(CurMBB, Fallthrough);
+        addSuccessorWithWeight(CurMBB, JumpMBB);
+
+        // The jump table header will be inserted in our current block, do the
+        // range check, and fall through to our fallthrough block.
+        JTH->HeaderBB = CurMBB;
+        JT->Default = Fallthrough; // FIXME: Move Default to JumpTableHeader.
+
+        // If we're in the right place, emit the jump table header right now.
+        if (CurMBB == SwitchMBB) {
+          visitJumpTableHeader(*JT, *JTH, SwitchMBB);
+          JTH->Emitted = true;
+        }
+        break;
+      }
+      case CC_BitTests: {
+        // FIXME: Optimize away range check based on pivot comparisons.
+        BitTestBlock *BTB = &BitTestCases[I->BTCasesIndex];
+
+        // The bit test blocks haven't been inserted yet; insert them here.
+        for (BitTestCase &BTC : BTB->Cases)
+          CurMF->insert(BBI, BTC.ThisBB);
+
+        // Fill in fields of the BitTestBlock.
+        BTB->Parent = CurMBB;
+        BTB->Default = Fallthrough;
+
+        // If we're in the right place, emit the bit test header header right now.
+        if (CurMBB ==SwitchMBB) {
+          visitBitTestHeader(*BTB, SwitchMBB);
+          BTB->Emitted = true;
+        }
+        break;
+      }
+      case CC_Range: {
+        const Value *RHS, *LHS, *MHS;
+        ISD::CondCode CC;
+        if (I->Low == I->High) {
+          // Check Cond == I->Low.
+          CC = ISD::SETEQ;
+          LHS = Cond;
+          RHS=I->Low;
+          MHS = nullptr;
+        } else {
+          // Check I->Low <= Cond <= I->High.
+          CC = ISD::SETLE;
+          LHS = I->Low;
+          MHS = Cond;
+          RHS = I->High;
+        }
+
+        // The false weight is the sum of all unhandled cases.
+        UnhandledWeights -= I->Weight;
+        CaseBlock CB(CC, LHS, RHS, MHS, I->MBB, Fallthrough, CurMBB, I->Weight,
+                     UnhandledWeights);
+
+        if (CurMBB == SwitchMBB)
+          visitSwitchCase(CB, SwitchMBB);
+        else
+          SwitchCases.push_back(CB);
+
+        break;
+      }
+    }
+    CurMBB = Fallthrough;
+  }
+}
+
+void SelectionDAGBuilder::splitWorkItem(SwitchWorkList &WorkList,
+                                        const SwitchWorkListItem &W,
+                                        Value *Cond,
+                                        MachineBasicBlock *SwitchMBB) {
+  assert(W.FirstCluster->Low->getValue().slt(W.LastCluster->Low->getValue()) &&
+         "Clusters not sorted?");
+
+  unsigned NumClusters = W.LastCluster - W.FirstCluster + 1;
+  assert(NumClusters >= 2 && "Too small to split!");
+
+  // FIXME: When we have profile info, we might want to balance the tree based
+  // on weights instead of node count.
+
+  CaseClusterIt PivotCluster = W.FirstCluster + NumClusters / 2;
+  CaseClusterIt FirstLeft = W.FirstCluster;
+  CaseClusterIt LastLeft = PivotCluster - 1;
+  CaseClusterIt FirstRight = PivotCluster;
+  CaseClusterIt LastRight = W.LastCluster;
+  const ConstantInt *Pivot = PivotCluster->Low;
+
+  // New blocks will be inserted immediately after the current one.
+  MachineFunction::iterator BBI = W.MBB;
+  ++BBI;
+
+  // We will branch to the LHS if Value < Pivot. If LHS is a single cluster,
+  // we can branch to its destination directly if it's squeezed exactly in
+  // between the known lower bound and Pivot - 1.
+  MachineBasicBlock *LeftMBB;
+  if (FirstLeft == LastLeft && FirstLeft->Kind == CC_Range &&
+      FirstLeft->Low == W.GE &&
+      (FirstLeft->High->getValue() + 1LL) == Pivot->getValue()) {
+    LeftMBB = FirstLeft->MBB;
+  } else {
+    LeftMBB = FuncInfo.MF->CreateMachineBasicBlock(W.MBB->getBasicBlock());
+    FuncInfo.MF->insert(BBI, LeftMBB);
+    WorkList.push_back({LeftMBB, FirstLeft, LastLeft, W.GE, Pivot});
+    // Put Cond in a virtual register to make it available from the new blocks.
+    ExportFromCurrentBlock(Cond);
+  }
+
+  // Similarly, we will branch to the RHS if Value >= Pivot. If RHS is a
+  // single cluster, RHS.Low == Pivot, and we can branch to its destination
+  // directly if RHS.High equals the current upper bound.
+  MachineBasicBlock *RightMBB;
+  if (FirstRight == LastRight && FirstRight->Kind == CC_Range &&
+      W.LT && (FirstRight->High->getValue() + 1ULL) == W.LT->getValue()) {
+    RightMBB = FirstRight->MBB;
+  } else {
+    RightMBB = FuncInfo.MF->CreateMachineBasicBlock(W.MBB->getBasicBlock());
+    FuncInfo.MF->insert(BBI, RightMBB);
+    WorkList.push_back({RightMBB, FirstRight, LastRight, Pivot, W.LT});
+    // Put Cond in a virtual register to make it available from the new blocks.
+    ExportFromCurrentBlock(Cond);
+  }
+
+  // Create the CaseBlock record that will be used to lower the branch.
+  CaseBlock CB(ISD::SETLT, Cond, Pivot, nullptr, LeftMBB, RightMBB, W.MBB);
+
+  if (W.MBB == SwitchMBB)
+    visitSwitchCase(CB, SwitchMBB);
+  else
+    SwitchCases.push_back(CB);
+}
+
+void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
+  // Extract cases from the switch.
+  BranchProbabilityInfo *BPI = FuncInfo.BPI;
+  CaseClusterVector Clusters;
+  Clusters.reserve(SI.getNumCases());
+  for (auto I : SI.cases()) {
+    MachineBasicBlock *Succ = FuncInfo.MBBMap[I.getCaseSuccessor()];
+    const ConstantInt *CaseVal = I.getCaseValue();
+    uint32_t Weight = 0; // FIXME: Use 1 instead?
+    if (BPI)
+      Weight = BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex());
+    Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));
+  }
+
+  MachineBasicBlock *DefaultMBB = FuncInfo.MBBMap[SI.getDefaultDest()];
+
+  if (TM.getOptLevel() != CodeGenOpt::None) {
+    // Cluster adjacent cases with the same destination.
+    sortAndRangeify(Clusters);
+
+    // Replace an unreachable default with the most popular destination.
+    // FIXME: Exploit unreachable default more aggressively.
+    bool UnreachableDefault =
+        isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg());
+    if (UnreachableDefault && !Clusters.empty()) {
+      DenseMap<const BasicBlock *, unsigned> Popularity;
+      unsigned MaxPop = 0;
+      const BasicBlock *MaxBB = nullptr;
+      for (auto I : SI.cases()) {
+        const BasicBlock *BB = I.getCaseSuccessor();
+        if (++Popularity[BB] > MaxPop) {
+          MaxPop = Popularity[BB];
+          MaxBB = BB;
+        }
+      }
+      // Set new default.
+      assert(MaxPop > 0 && MaxBB);
+      DefaultMBB = FuncInfo.MBBMap[MaxBB];
+
+      // Remove cases that were pointing to the destination that is now the
+      // default.
+      CaseClusterVector New;
+      New.reserve(Clusters.size());
+      for (CaseCluster &CC : Clusters) {
+        if (CC.MBB != DefaultMBB)
+          New.push_back(CC);
+      }
+      Clusters = std::move(New);
+    }
+  }
+
+  // If there is only the default destination, jump there directly.
+  MachineBasicBlock *SwitchMBB = FuncInfo.MBB;
+  if (Clusters.empty()) {
+    SwitchMBB->addSuccessor(DefaultMBB);
+    if (DefaultMBB != NextBlock(SwitchMBB)) {
+      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other,
+                              getControlRoot(), DAG.getBasicBlock(DefaultMBB)));
+    }
+    return;
+  }
+
+  if (TM.getOptLevel() != CodeGenOpt::None) {
+    findJumpTables(Clusters, &SI, DefaultMBB);
+    findBitTestClusters(Clusters, &SI);
+  }
+
+
+  DEBUG({
+    dbgs() << "Case clusters: ";
+    for (const CaseCluster &C : Clusters) {
+      if (C.Kind == CC_JumpTable) dbgs() << "JT:";
+      if (C.Kind == CC_BitTests) dbgs() << "BT:";
+
+      C.Low->getValue().print(dbgs(), true);
+      if (C.Low != C.High) {
+        dbgs() << '-';
+        C.High->getValue().print(dbgs(), true);
+      }
+      dbgs() << ' ';
+    }
+    dbgs() << '\n';
+  });
+
+  assert(!Clusters.empty());
+  SwitchWorkList WorkList;
+  CaseClusterIt First = Clusters.begin();
+  CaseClusterIt Last = Clusters.end() - 1;
+  WorkList.push_back({SwitchMBB, First, Last, nullptr, nullptr});
+
+  while (!WorkList.empty()) {
+    SwitchWorkListItem W = WorkList.back();
+    WorkList.pop_back();
+    unsigned NumClusters = W.LastCluster - W.FirstCluster + 1;
+
+    if (NumClusters > 3 && TM.getOptLevel() != CodeGenOpt::None) {
+      // For optimized builds, lower large range as a balanced binary tree.
+      splitWorkItem(WorkList, W, SI.getCondition(), SwitchMBB);
+      continue;
+    }
+
+    lowerWorkItem(W, SI.getCondition(), SwitchMBB, DefaultMBB);
+  }
+}