third_party/llvm-10.0/llvm/lib/CodeGen/SwitchLoweringUtils.cpp - SwiftShader - Git at Google

 //===- SwitchLoweringUtils.cpp - Switch Lowering --------------------------===//
 //
 // 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 file contains switch inst lowering optimizations and utilities for
 // codegen, so that it can be used for both SelectionDAG and GlobalISel.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/SwitchLoweringUtils.h"

 using namespace llvm;
 using namespace SwitchCG;

 uint64_t SwitchCG::getJumpTableRange(const CaseClusterVector &Clusters,
                                      unsigned First, unsigned Last) {
   assert(Last >= First);
   const APInt &LowCase = Clusters[First].Low->getValue();
   const 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.
   return (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100) + 1;
 }

 uint64_t
 SwitchCG::getJumpTableNumCases(const SmallVectorImpl<unsigned> &TotalCases,
                                unsigned First, unsigned Last) {
   assert(Last >= First);
   assert(TotalCases[Last] >= TotalCases[First]);
   uint64_t NumCases =
       TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
   return NumCases;
 }

 void SwitchCG::SwitchLowering::findJumpTables(CaseClusterVector &Clusters,
                                               const SwitchInst *SI,
                                               MachineBasicBlock *DefaultMBB,
                                               ProfileSummaryInfo *PSI,
                                               BlockFrequencyInfo *BFI) {
 #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

   assert(TLI && "TLI not set!");
   if (!TLI->areJTsAllowed(SI->getParent()->getParent()))
     return;

   const unsigned MinJumpTableEntries = TLI->getMinimumJumpTableEntries();
   const unsigned SmallNumberOfEntries = MinJumpTableEntries / 2;

   // Bail if not enough cases.
   const int64_t N = Clusters.size();
   if (N < 2 || N < MinJumpTableEntries)
     return;

   // Accumulated number of cases in each cluster and those prior to it.
   SmallVector<unsigned, 8> TotalCases(N);
   for (unsigned i = 0; i < N; ++i) {
     const APInt &Hi = Clusters[i].High->getValue();
     const APInt &Lo = Clusters[i].Low->getValue();
     TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
     if (i != 0)
       TotalCases[i] += TotalCases[i - 1];
   }

   uint64_t Range = getJumpTableRange(Clusters,0, N - 1);
   uint64_t NumCases = getJumpTableNumCases(TotalCases, 0, N - 1);
   assert(NumCases < UINT64_MAX / 100);
   assert(Range >= NumCases);

   // Cheap case: the whole range may be suitable for jump table.
   if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
     CaseCluster JTCluster;
     if (buildJumpTable(Clusters, 0, N - 1, SI, DefaultMBB, JTCluster)) {
       Clusters[0] = JTCluster;
       Clusters.resize(1);
       return;
     }
   }

   // The algorithm below is not suitable for -O0.
   if (TM->getOptLevel() == CodeGenOpt::None)
     return;

   // 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);
   // PartitionsScore[i] is used to break ties when choosing between two
   // partitionings resulting in the same number of partitions.
   SmallVector<unsigned, 8> PartitionsScore(N);
   // For PartitionsScore, a small number of comparisons is considered as good as
   // a jump table and a single comparison is considered better than a jump
   // table.
   enum PartitionScores : unsigned {
     NoTable = 0,
     Table = 1,
     FewCases = 1,
     SingleCase = 2
   };

   // Base case: There is only one way to partition Clusters[N-1].
   MinPartitions[N - 1] = 1;
   LastElement[N - 1] = N - 1;
   PartitionsScore[N - 1] = PartitionScores::SingleCase;

   // 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;
     PartitionsScore[i] = PartitionsScore[i + 1] + PartitionScores::SingleCase;

     // 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].
       Range = getJumpTableRange(Clusters, i, j);
       NumCases = getJumpTableNumCases(TotalCases, i, j);
       assert(NumCases < UINT64_MAX / 100);
       assert(Range >= NumCases);

       if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
         unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
         unsigned Score = j == N - 1 ? 0 : PartitionsScore[j + 1];
         int64_t NumEntries = j - i + 1;

         if (NumEntries == 1)
           Score += PartitionScores::SingleCase;
         else if (NumEntries <= SmallNumberOfEntries)
           Score += PartitionScores::FewCases;
         else if (NumEntries >= MinJumpTableEntries)
           Score += PartitionScores::Table;

         // If this leads to fewer partitions, or to the same number of
         // partitions with better score, it is a better partitioning.
         if (NumPartitions < MinPartitions[i] ||
             (NumPartitions == MinPartitions[i] && Score > PartitionsScore[i])) {
           MinPartitions[i] = NumPartitions;
           LastElement[i] = j;
           PartitionsScore[i] = Score;
         }
       }
     }
   }

   // 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 >= MinJumpTableEntries &&
         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 SwitchCG::SwitchLowering::buildJumpTable(const CaseClusterVector &Clusters,
                                               unsigned First, unsigned Last,
                                               const SwitchInst *SI,
                                               MachineBasicBlock *DefaultMBB,
                                               CaseCluster &JTCluster) {
   assert(First <= Last);

   auto Prob = BranchProbability::getZero();
   unsigned NumCmps = 0;
   std::vector<MachineBasicBlock*> Table;
   DenseMap<MachineBasicBlock*, BranchProbability> JTProbs;

   // Initialize probabilities in JTProbs.
   for (unsigned I = First; I <= Last; ++I)
     JTProbs[Clusters[I].MBB] = BranchProbability::getZero();

   for (unsigned I = First; I <= Last; ++I) {
     assert(Clusters[I].Kind == CC_Range);
     Prob += Clusters[I].Prob;
     const APInt &Low = Clusters[I].Low->getValue();
     const APInt &High = Clusters[I].High->getValue();
     NumCmps += (Low == High) ? 1 : 2;
     if (I != First) {
       // Fill the gap between this and the previous cluster.
       const 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);
     }
     uint64_t ClusterSize = (High - Low).getLimitedValue() + 1;
     for (uint64_t J = 0; J < ClusterSize; ++J)
       Table.push_back(Clusters[I].MBB);
     JTProbs[Clusters[I].MBB] += Clusters[I].Prob;
   }

   unsigned NumDests = JTProbs.size();
   if (TLI->isSuitableForBitTests(NumDests, NumCmps,
                                  Clusters[First].Low->getValue(),
                                  Clusters[Last].High->getValue(), *DL)) {
     // 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;
     addSuccessorWithProb(JumpTableMBB, Succ, JTProbs[Succ]);
     Done.insert(Succ);
   }
   JumpTableMBB->normalizeSuccProbs();

   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.emplace_back(std::move(JTH), std::move(JT));

   JTCluster = CaseCluster::jumpTable(Clusters[First].Low, Clusters[Last].High,
                                      JTCases.size() - 1, Prob);
   return true;
 }

 void SwitchCG::SwitchLowering::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

   // The algorithm below is not suitable for -O0.
   if (TM->getOptLevel() == CodeGenOpt::None)
     return;

   // If target does not have legal shift left, do not emit bit tests at all.
   EVT PTy = TLI->getPointerTy(*DL);
   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 (!TLI->rangeFitsInWord(Clusters[i].Low->getValue(),
                                 Clusters[j].High->getValue(), *DL))
         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 {
       size_t NumClusters = Last - First + 1;
       std::memmove(&Clusters[DstIndex], &Clusters[First],
                    sizeof(Clusters[0]) * NumClusters);
       DstIndex += NumClusters;
     }
   }
   Clusters.resize(DstIndex);
 }

 bool SwitchCG::SwitchLowering::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 (!TLI->isSuitableForBitTests(NumDests, NumCmps, Low, High, *DL))
     return false;

   APInt LowBound;
   APInt CmpRange;

   const int BitWidth = TLI->getPointerTy(*DL).getSizeInBits();
   assert(TLI->rangeFitsInWord(Low, High, *DL) &&
          "Case range must fit in bit mask!");

   // Check if the clusters cover a contiguous range such that no value in the
   // range will jump to the default statement.
   bool ContiguousRange = true;
   for (int64_t I = First + 1; I <= Last; ++I) {
     if (Clusters[I].Low->getValue() != Clusters[I - 1].High->getValue() + 1) {
       ContiguousRange = false;
       break;
     }
   }

   if (Low.isStrictlyPositive() && 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;
     ContiguousRange = false;
   } else {
     LowBound = Low;
     CmpRange = High - Low;
   }

   CaseBitsVector CBV;
   auto TotalProb = BranchProbability::getZero();
   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, BranchProbability::getZero()));
     CaseBits *CB = &CBV[j];

     // Update Mask, Bits and ExtraProb.
     uint64_t Lo = (Clusters[i].Low->getValue() - LowBound).getZExtValue();
     uint64_t Hi = (Clusters[i].High->getValue() - LowBound).getZExtValue();
     assert(Hi >= Lo && Hi < 64 && "Invalid bit case!");
     CB->Mask |= (-1ULL >> (63 - (Hi - Lo))) << Lo;
     CB->Bits += Hi - Lo + 1;
     CB->ExtraProb += Clusters[i].Prob;
     TotalProb += Clusters[i].Prob;
   }

   BitTestInfo BTI;
   llvm::sort(CBV, [](const CaseBits &a, const CaseBits &b) {
     // Sort by probability first, number of bits second, bit mask third.
     if (a.ExtraProb != b.ExtraProb)
       return a.ExtraProb > b.ExtraProb;
     if (a.Bits != b.Bits)
       return a.Bits > b.Bits;
     return a.Mask < b.Mask;
   });

   for (auto &CB : CBV) {
     MachineBasicBlock *BitTestBB =
         FuncInfo.MF->CreateMachineBasicBlock(SI->getParent());
     BTI.push_back(BitTestCase(CB.Mask, BitTestBB, CB.BB, CB.ExtraProb));
   }
   BitTestCases.emplace_back(std::move(LowBound), std::move(CmpRange),
                             SI->getCondition(), -1U, MVT::Other, false,
                             ContiguousRange, nullptr, nullptr, std::move(BTI),
                             TotalProb);

   BTCluster = CaseCluster::bitTests(Clusters[First].Low, Clusters[Last].High,
                                     BitTestCases.size() - 1, TotalProb);
   return true;
 }

 void SwitchCG::sortAndRangeify(CaseClusterVector &Clusters) {
 #ifndef NDEBUG
   for (const CaseCluster &CC : Clusters)
     assert(CC.Low == CC.High && "Input clusters must be single-case");
 #endif

   llvm::sort(Clusters, [](const CaseCluster &a, const CaseCluster &b) {
     return a.Low->getValue().slt(b.Low->getValue());
   });

   // 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.
       Clusters[DstIndex - 1].High = CaseVal;
       Clusters[DstIndex - 1].Prob += CC.Prob;
     } else {
       std::memmove(&Clusters[DstIndex++], &Clusters[SrcIndex],
                    sizeof(Clusters[SrcIndex]));
     }
   }
   Clusters.resize(DstIndex);
 }
	//===- SwitchLoweringUtils.cpp - Switch Lowering --------------------------===//
	//
	// 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 file contains switch inst lowering optimizations and utilities for
	// codegen, so that it can be used for both SelectionDAG and GlobalISel.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/MachineJumpTableInfo.h"
	#include "llvm/CodeGen/SwitchLoweringUtils.h"

	using namespace llvm;
	using namespace SwitchCG;

	uint64_t SwitchCG::getJumpTableRange(const CaseClusterVector &Clusters,
	unsigned First, unsigned Last) {
	assert(Last >= First);
	const APInt &LowCase = Clusters[First].Low->getValue();
	const 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.
	return (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100) + 1;
	}

	uint64_t
	SwitchCG::getJumpTableNumCases(const SmallVectorImpl<unsigned> &TotalCases,
	unsigned First, unsigned Last) {
	assert(Last >= First);
	assert(TotalCases[Last] >= TotalCases[First]);
	uint64_t NumCases =
	TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
	return NumCases;
	}

	void SwitchCG::SwitchLowering::findJumpTables(CaseClusterVector &Clusters,
	const SwitchInst *SI,
	MachineBasicBlock *DefaultMBB,
	ProfileSummaryInfo *PSI,
	BlockFrequencyInfo *BFI) {
	#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

	assert(TLI && "TLI not set!");
	if (!TLI->areJTsAllowed(SI->getParent()->getParent()))
	return;

	const unsigned MinJumpTableEntries = TLI->getMinimumJumpTableEntries();
	const unsigned SmallNumberOfEntries = MinJumpTableEntries / 2;

	// Bail if not enough cases.
	const int64_t N = Clusters.size();
	if (N < 2 \|\| N < MinJumpTableEntries)
	return;

	// Accumulated number of cases in each cluster and those prior to it.
	SmallVector<unsigned, 8> TotalCases(N);
	for (unsigned i = 0; i < N; ++i) {
	const APInt &Hi = Clusters[i].High->getValue();
	const APInt &Lo = Clusters[i].Low->getValue();
	TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
	if (i != 0)
	TotalCases[i] += TotalCases[i - 1];
	}

	uint64_t Range = getJumpTableRange(Clusters,0, N - 1);
	uint64_t NumCases = getJumpTableNumCases(TotalCases, 0, N - 1);
	assert(NumCases < UINT64_MAX / 100);
	assert(Range >= NumCases);

	// Cheap case: the whole range may be suitable for jump table.
	if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
	CaseCluster JTCluster;
	if (buildJumpTable(Clusters, 0, N - 1, SI, DefaultMBB, JTCluster)) {
	Clusters[0] = JTCluster;
	Clusters.resize(1);
	return;
	}
	}

	// The algorithm below is not suitable for -O0.
	if (TM->getOptLevel() == CodeGenOpt::None)
	return;

	// 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);
	// PartitionsScore[i] is used to break ties when choosing between two
	// partitionings resulting in the same number of partitions.
	SmallVector<unsigned, 8> PartitionsScore(N);
	// For PartitionsScore, a small number of comparisons is considered as good as
	// a jump table and a single comparison is considered better than a jump
	// table.
	enum PartitionScores : unsigned {
	NoTable = 0,
	Table = 1,
	FewCases = 1,
	SingleCase = 2
	};

	// Base case: There is only one way to partition Clusters[N-1].
	MinPartitions[N - 1] = 1;
	LastElement[N - 1] = N - 1;
	PartitionsScore[N - 1] = PartitionScores::SingleCase;

	// 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;
	PartitionsScore[i] = PartitionsScore[i + 1] + PartitionScores::SingleCase;

	// 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].
	Range = getJumpTableRange(Clusters, i, j);
	NumCases = getJumpTableNumCases(TotalCases, i, j);
	assert(NumCases < UINT64_MAX / 100);
	assert(Range >= NumCases);

	if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
	unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
	unsigned Score = j == N - 1 ? 0 : PartitionsScore[j + 1];
	int64_t NumEntries = j - i + 1;

	if (NumEntries == 1)
	Score += PartitionScores::SingleCase;
	else if (NumEntries <= SmallNumberOfEntries)
	Score += PartitionScores::FewCases;
	else if (NumEntries >= MinJumpTableEntries)
	Score += PartitionScores::Table;

	// If this leads to fewer partitions, or to the same number of
	// partitions with better score, it is a better partitioning.
	if (NumPartitions < MinPartitions[i] \|\|
	(NumPartitions == MinPartitions[i] && Score > PartitionsScore[i])) {
	MinPartitions[i] = NumPartitions;
	LastElement[i] = j;
	PartitionsScore[i] = Score;
	}
	}
	}
	}

	// 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 >= MinJumpTableEntries &&
	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 SwitchCG::SwitchLowering::buildJumpTable(const CaseClusterVector &Clusters,
	unsigned First, unsigned Last,
	const SwitchInst *SI,
	MachineBasicBlock *DefaultMBB,
	CaseCluster &JTCluster) {
	assert(First <= Last);

	auto Prob = BranchProbability::getZero();
	unsigned NumCmps = 0;
	std::vector<MachineBasicBlock*> Table;
	DenseMap<MachineBasicBlock*, BranchProbability> JTProbs;

	// Initialize probabilities in JTProbs.
	for (unsigned I = First; I <= Last; ++I)
	JTProbs[Clusters[I].MBB] = BranchProbability::getZero();

	for (unsigned I = First; I <= Last; ++I) {
	assert(Clusters[I].Kind == CC_Range);
	Prob += Clusters[I].Prob;
	const APInt &Low = Clusters[I].Low->getValue();
	const APInt &High = Clusters[I].High->getValue();
	NumCmps += (Low == High) ? 1 : 2;
	if (I != First) {
	// Fill the gap between this and the previous cluster.
	const 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);
	}
	uint64_t ClusterSize = (High - Low).getLimitedValue() + 1;
	for (uint64_t J = 0; J < ClusterSize; ++J)
	Table.push_back(Clusters[I].MBB);
	JTProbs[Clusters[I].MBB] += Clusters[I].Prob;
	}

	unsigned NumDests = JTProbs.size();
	if (TLI->isSuitableForBitTests(NumDests, NumCmps,
	Clusters[First].Low->getValue(),
	Clusters[Last].High->getValue(), *DL)) {
	// 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;
	addSuccessorWithProb(JumpTableMBB, Succ, JTProbs[Succ]);
	Done.insert(Succ);
	}
	JumpTableMBB->normalizeSuccProbs();

	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.emplace_back(std::move(JTH), std::move(JT));

	JTCluster = CaseCluster::jumpTable(Clusters[First].Low, Clusters[Last].High,
	JTCases.size() - 1, Prob);
	return true;
	}

	void SwitchCG::SwitchLowering::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

	// The algorithm below is not suitable for -O0.
	if (TM->getOptLevel() == CodeGenOpt::None)
	return;

	// If target does not have legal shift left, do not emit bit tests at all.
	EVT PTy = TLI->getPointerTy(*DL);
	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 (!TLI->rangeFitsInWord(Clusters[i].Low->getValue(),
	Clusters[j].High->getValue(), *DL))
	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 {
	size_t NumClusters = Last - First + 1;
	std::memmove(&Clusters[DstIndex], &Clusters[First],
	sizeof(Clusters[0]) * NumClusters);
	DstIndex += NumClusters;
	}
	}
	Clusters.resize(DstIndex);
	}

	bool SwitchCG::SwitchLowering::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 (!TLI->isSuitableForBitTests(NumDests, NumCmps, Low, High, *DL))
	return false;

	APInt LowBound;
	APInt CmpRange;

	const int BitWidth = TLI->getPointerTy(*DL).getSizeInBits();
	assert(TLI->rangeFitsInWord(Low, High, *DL) &&
	"Case range must fit in bit mask!");

	// Check if the clusters cover a contiguous range such that no value in the
	// range will jump to the default statement.
	bool ContiguousRange = true;
	for (int64_t I = First + 1; I <= Last; ++I) {
	if (Clusters[I].Low->getValue() != Clusters[I - 1].High->getValue() + 1) {
	ContiguousRange = false;
	break;
	}
	}

	if (Low.isStrictlyPositive() && 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;
	ContiguousRange = false;
	} else {
	LowBound = Low;
	CmpRange = High - Low;
	}

	CaseBitsVector CBV;
	auto TotalProb = BranchProbability::getZero();
	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, BranchProbability::getZero()));
	CaseBits *CB = &CBV[j];

	// Update Mask, Bits and ExtraProb.
	uint64_t Lo = (Clusters[i].Low->getValue() - LowBound).getZExtValue();
	uint64_t Hi = (Clusters[i].High->getValue() - LowBound).getZExtValue();
	assert(Hi >= Lo && Hi < 64 && "Invalid bit case!");
	CB->Mask \|= (-1ULL >> (63 - (Hi - Lo))) << Lo;
	CB->Bits += Hi - Lo + 1;
	CB->ExtraProb += Clusters[i].Prob;
	TotalProb += Clusters[i].Prob;
	}

	BitTestInfo BTI;
	llvm::sort(CBV, [](const CaseBits &a, const CaseBits &b) {
	// Sort by probability first, number of bits second, bit mask third.
	if (a.ExtraProb != b.ExtraProb)
	return a.ExtraProb > b.ExtraProb;
	if (a.Bits != b.Bits)
	return a.Bits > b.Bits;
	return a.Mask < b.Mask;
	});

	for (auto &CB : CBV) {
	MachineBasicBlock *BitTestBB =
	FuncInfo.MF->CreateMachineBasicBlock(SI->getParent());
	BTI.push_back(BitTestCase(CB.Mask, BitTestBB, CB.BB, CB.ExtraProb));
	}
	BitTestCases.emplace_back(std::move(LowBound), std::move(CmpRange),
	SI->getCondition(), -1U, MVT::Other, false,
	ContiguousRange, nullptr, nullptr, std::move(BTI),
	TotalProb);

	BTCluster = CaseCluster::bitTests(Clusters[First].Low, Clusters[Last].High,
	BitTestCases.size() - 1, TotalProb);
	return true;
	}

	void SwitchCG::sortAndRangeify(CaseClusterVector &Clusters) {
	#ifndef NDEBUG
	for (const CaseCluster &CC : Clusters)
	assert(CC.Low == CC.High && "Input clusters must be single-case");
	#endif

	llvm::sort(Clusters, [](const CaseCluster &a, const CaseCluster &b) {
	return a.Low->getValue().slt(b.Low->getValue());
	});

	// 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.
	Clusters[DstIndex - 1].High = CaseVal;
	Clusters[DstIndex - 1].Prob += CC.Prob;
	} else {
	std::memmove(&Clusters[DstIndex++], &Clusters[SrcIndex],
	sizeof(Clusters[SrcIndex]));
	}
	}
	Clusters.resize(DstIndex);
	}