lib/Transforms/Utils/LowerSwitch.cpp

   1 //===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // The LowerSwitch transformation rewrites switch instructions with a sequence
  10 // of branches, which allows targets to get away with not implementing the
  11 // switch instruction until it is convenient.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/ADT/DenseMap.h"
  16 #include "llvm/ADT/STLExtras.h"
  17 #include "llvm/ADT/SmallPtrSet.h"
  18 #include "llvm/ADT/SmallVector.h"
  19 #include "llvm/Analysis/AssumptionCache.h"
  20 #include "llvm/Analysis/LazyValueInfo.h"
  21 #include "llvm/Analysis/ValueTracking.h"
  22 #include "llvm/IR/BasicBlock.h"
  23 #include "llvm/IR/CFG.h"
  24 #include "llvm/IR/ConstantRange.h"
  25 #include "llvm/IR/Constants.h"
  26 #include "llvm/IR/Function.h"
  27 #include "llvm/IR/InstrTypes.h"
  28 #include "llvm/IR/Instructions.h"
  29 #include "llvm/IR/Value.h"
  30 #include "llvm/Pass.h"
  31 #include "llvm/Support/Casting.h"
  32 #include "llvm/Support/Compiler.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/Support/KnownBits.h"
  35 #include "llvm/Support/raw_ostream.h"
  36 #include "llvm/Transforms/Utils.h"
  37 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  38 #include <algorithm>
  39 #include <cassert>
  40 #include <cstdint>
  41 #include <iterator>
  42 #include <limits>
  43 #include <vector>
  44
  45 using namespace llvm;
  46
  47 #define DEBUG_TYPE "lower-switch"
  48
  49 namespace {
  50
  51   struct IntRange {
  52     int64_t Low, High;
  53   };
  54
  55 } // end anonymous namespace
  56
  57 // Return true iff R is covered by Ranges.
  58 static bool IsInRanges(const IntRange &R,
  59                        const std::vector<IntRange> &Ranges) {
  60   // Note: Ranges must be sorted, non-overlapping and non-adjacent.
  61
  62   // Find the first range whose High field is >= R.High,
  63   // then check if the Low field is <= R.Low. If so, we
  64   // have a Range that covers R.
  65   auto I = llvm::lower_bound(
  66       Ranges, R, [](IntRange A, IntRange B) { return A.High < B.High; });
  67   return I != Ranges.end() && I->Low <= R.Low;
  68 }
  69
  70 namespace {
  71
  72   /// Replace all SwitchInst instructions with chained branch instructions.
  73   class LowerSwitch : public FunctionPass {
  74   public:
  75     // Pass identification, replacement for typeid
  76     static char ID;
  77
  78     LowerSwitch() : FunctionPass(ID) {
  79       initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
  80     }
  81
  82     bool runOnFunction(Function &F) override;
  83
  84     void getAnalysisUsage(AnalysisUsage &AU) const override {
  85       AU.addRequired<LazyValueInfoWrapperPass>();
  86     }
  87
  88     struct CaseRange {
  89       ConstantInt* Low;
  90       ConstantInt* High;
  91       BasicBlock* BB;
  92
  93       CaseRange(ConstantInt *low, ConstantInt *high, BasicBlock *bb)
  94           : Low(low), High(high), BB(bb) {}
  95     };
  96
  97     using CaseVector = std::vector<CaseRange>;
  98     using CaseItr = std::vector<CaseRange>::iterator;
  99
 100   private:
 101     void processSwitchInst(SwitchInst *SI,
 102                            SmallPtrSetImpl<BasicBlock *> &DeleteList,
 103                            AssumptionCache *AC, LazyValueInfo *LVI);
 104
 105     BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
 106                               ConstantInt *LowerBound, ConstantInt *UpperBound,
 107                               Value *Val, BasicBlock *Predecessor,
 108                               BasicBlock *OrigBlock, BasicBlock *Default,
 109                               const std::vector<IntRange> &UnreachableRanges);
 110     BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val,
 111                              ConstantInt *LowerBound, ConstantInt *UpperBound,
 112                              BasicBlock *OrigBlock, BasicBlock *Default);
 113     unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
 114   };
 115
 116   /// The comparison function for sorting the switch case values in the vector.
 117   /// WARNING: Case ranges should be disjoint!
 118   struct CaseCmp {
 119     bool operator()(const LowerSwitch::CaseRange& C1,
 120                     const LowerSwitch::CaseRange& C2) {
 121       const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
 122       const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
 123       return CI1->getValue().slt(CI2->getValue());
 124     }
 125   };
 126
 127 } // end anonymous namespace
 128
 129 char LowerSwitch::ID = 0;
 130
 131 // Publicly exposed interface to pass...
 132 char &llvm::LowerSwitchID = LowerSwitch::ID;
 133
 134 INITIALIZE_PASS_BEGIN(LowerSwitch, "lowerswitch",
 135                       "Lower SwitchInst's to branches", false, false)
 136 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 137 INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
 138 INITIALIZE_PASS_END(LowerSwitch, "lowerswitch",
 139                     "Lower SwitchInst's to branches", false, false)
 140
 141 // createLowerSwitchPass - Interface to this file...
 142 FunctionPass *llvm::createLowerSwitchPass() {
 143   return new LowerSwitch();
 144 }
 145
 146 bool LowerSwitch::runOnFunction(Function &F) {
 147   LazyValueInfo *LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
 148   auto *ACT = getAnalysisIfAvailable<AssumptionCacheTracker>();
 149   AssumptionCache *AC = ACT ? &ACT->getAssumptionCache(F) : nullptr;
 150   // Prevent LazyValueInfo from using the DominatorTree as LowerSwitch does not
 151   // preserve it and it becomes stale (when available) pretty much immediately.
 152   // Currently the DominatorTree is only used by LowerSwitch indirectly via LVI
 153   // and computeKnownBits to refine isValidAssumeForContext's results. Given
 154   // that the latter can handle some of the simple cases w/o a DominatorTree,
 155   // it's easier to refrain from using the tree than to keep it up to date.
 156   LVI->disableDT();
 157
 158   bool Changed = false;
 159   SmallPtrSet<BasicBlock*, 8> DeleteList;
 160
 161   for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
 162     BasicBlock *Cur = &*I++; // Advance over block so we don't traverse new blocks
 163
 164     // If the block is a dead Default block that will be deleted later, don't
 165     // waste time processing it.
 166     if (DeleteList.count(Cur))
 167       continue;
 168
 169     if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
 170       Changed = true;
 171       processSwitchInst(SI, DeleteList, AC, LVI);
 172     }
 173   }
 174
 175   for (BasicBlock* BB: DeleteList) {
 176     LVI->eraseBlock(BB);
 177     DeleteDeadBlock(BB);
 178   }
 179
 180   return Changed;
 181 }
 182
 183 /// Used for debugging purposes.
 184 LLVM_ATTRIBUTE_USED
 185 static raw_ostream &operator<<(raw_ostream &O,
 186                                const LowerSwitch::CaseVector &C) {
 187   O << "[";
 188
 189   for (LowerSwitch::CaseVector::const_iterator B = C.begin(), E = C.end();
 190        B != E;) {
 191     O << "[" << B->Low->getValue() << ", " << B->High->getValue() << "]";
 192     if (++B != E)
 193       O << ", ";
 194   }
 195
 196   return O << "]";
 197 }
 198
 199 /// Update the first occurrence of the "switch statement" BB in the PHI
 200 /// node with the "new" BB. The other occurrences will:
 201 ///
 202 /// 1) Be updated by subsequent calls to this function.  Switch statements may
 203 /// have more than one outcoming edge into the same BB if they all have the same
 204 /// value. When the switch statement is converted these incoming edges are now
 205 /// coming from multiple BBs.
 206 /// 2) Removed if subsequent incoming values now share the same case, i.e.,
 207 /// multiple outcome edges are condensed into one. This is necessary to keep the
 208 /// number of phi values equal to the number of branches to SuccBB.
 209 static void
 210 fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB,
 211         const unsigned NumMergedCases = std::numeric_limits<unsigned>::max()) {
 212   for (BasicBlock::iterator I = SuccBB->begin(),
 213                             IE = SuccBB->getFirstNonPHI()->getIterator();
 214        I != IE; ++I) {
 215     PHINode *PN = cast<PHINode>(I);
 216
 217     // Only update the first occurrence.
 218     unsigned Idx = 0, E = PN->getNumIncomingValues();
 219     unsigned LocalNumMergedCases = NumMergedCases;
 220     for (; Idx != E; ++Idx) {
 221       if (PN->getIncomingBlock(Idx) == OrigBB) {
 222         PN->setIncomingBlock(Idx, NewBB);
 223         break;
 224       }
 225     }
 226
 227     // Remove additional occurrences coming from condensed cases and keep the
 228     // number of incoming values equal to the number of branches to SuccBB.
 229     SmallVector<unsigned, 8> Indices;
 230     for (++Idx; LocalNumMergedCases > 0 && Idx < E; ++Idx)
 231       if (PN->getIncomingBlock(Idx) == OrigBB) {
 232         Indices.push_back(Idx);
 233         LocalNumMergedCases--;
 234       }
 235     // Remove incoming values in the reverse order to prevent invalidating
 236     // *successive* index.
 237     for (unsigned III : llvm::reverse(Indices))
 238       PN->removeIncomingValue(III);
 239   }
 240 }
 241
 242 /// Convert the switch statement into a binary lookup of the case values.
 243 /// The function recursively builds this tree. LowerBound and UpperBound are
 244 /// used to keep track of the bounds for Val that have already been checked by
 245 /// a block emitted by one of the previous calls to switchConvert in the call
 246 /// stack.
 247 BasicBlock *
 248 LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, ConstantInt *LowerBound,
 249                            ConstantInt *UpperBound, Value *Val,
 250                            BasicBlock *Predecessor, BasicBlock *OrigBlock,
 251                            BasicBlock *Default,
 252                            const std::vector<IntRange> &UnreachableRanges) {
 253   assert(LowerBound && UpperBound && "Bounds must be initialized");
 254   unsigned Size = End - Begin;
 255
 256   if (Size == 1) {
 257     // Check if the Case Range is perfectly squeezed in between
 258     // already checked Upper and Lower bounds. If it is then we can avoid
 259     // emitting the code that checks if the value actually falls in the range
 260     // because the bounds already tell us so.
 261     if (Begin->Low == LowerBound && Begin->High == UpperBound) {
 262       unsigned NumMergedCases = 0;
 263       NumMergedCases = UpperBound->getSExtValue() - LowerBound->getSExtValue();
 264       fixPhis(Begin->BB, OrigBlock, Predecessor, NumMergedCases);
 265       return Begin->BB;
 266     }
 267     return newLeafBlock(*Begin, Val, LowerBound, UpperBound, OrigBlock,
 268                         Default);
 269   }
 270
 271   unsigned Mid = Size / 2;
 272   std::vector<CaseRange> LHS(Begin, Begin + Mid);
 273   LLVM_DEBUG(dbgs() << "LHS: " << LHS << "\n");
 274   std::vector<CaseRange> RHS(Begin + Mid, End);
 275   LLVM_DEBUG(dbgs() << "RHS: " << RHS << "\n");
 276
 277   CaseRange &Pivot = *(Begin + Mid);
 278   LLVM_DEBUG(dbgs() << "Pivot ==> [" << Pivot.Low->getValue() << ", "
 279                     << Pivot.High->getValue() << "]\n");
 280
 281   // NewLowerBound here should never be the integer minimal value.
 282   // This is because it is computed from a case range that is never
 283   // the smallest, so there is always a case range that has at least
 284   // a smaller value.
 285   ConstantInt *NewLowerBound = Pivot.Low;
 286
 287   // Because NewLowerBound is never the smallest representable integer
 288   // it is safe here to subtract one.
 289   ConstantInt *NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
 290                                                 NewLowerBound->getValue() - 1);
 291
 292   if (!UnreachableRanges.empty()) {
 293     // Check if the gap between LHS's highest and NewLowerBound is unreachable.
 294     int64_t GapLow = LHS.back().High->getSExtValue() + 1;
 295     int64_t GapHigh = NewLowerBound->getSExtValue() - 1;
 296     IntRange Gap = { GapLow, GapHigh };
 297     if (GapHigh >= GapLow && IsInRanges(Gap, UnreachableRanges))
 298       NewUpperBound = LHS.back().High;
 299   }
 300
 301   LLVM_DEBUG(dbgs() << "LHS Bounds ==> [" << LowerBound->getSExtValue() << ", "
 302                     << NewUpperBound->getSExtValue() << "]\n"
 303                     << "RHS Bounds ==> [" << NewLowerBound->getSExtValue()
 304                     << ", " << UpperBound->getSExtValue() << "]\n");
 305
 306   // Create a new node that checks if the value is < pivot. Go to the
 307   // left branch if it is and right branch if not.
 308   Function* F = OrigBlock->getParent();
 309   BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
 310
 311   ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
 312                                 Val, Pivot.Low, "Pivot");
 313
 314   BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
 315                                       NewUpperBound, Val, NewNode, OrigBlock,
 316                                       Default, UnreachableRanges);
 317   BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
 318                                       UpperBound, Val, NewNode, OrigBlock,
 319                                       Default, UnreachableRanges);
 320
 321   F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewNode);
 322   NewNode->getInstList().push_back(Comp);
 323
 324   BranchInst::Create(LBranch, RBranch, Comp, NewNode);
 325   return NewNode;
 326 }
 327
 328 /// Create a new leaf block for the binary lookup tree. It checks if the
 329 /// switch's value == the case's value. If not, then it jumps to the default
 330 /// branch. At this point in the tree, the value can't be another valid case
 331 /// value, so the jump to the "default" branch is warranted.
 332 BasicBlock *LowerSwitch::newLeafBlock(CaseRange &Leaf, Value *Val,
 333                                       ConstantInt *LowerBound,
 334                                       ConstantInt *UpperBound,
 335                                       BasicBlock *OrigBlock,
 336                                       BasicBlock *Default) {
 337   Function* F = OrigBlock->getParent();
 338   BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
 339   F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewLeaf);
 340
 341   // Emit comparison
 342   ICmpInst* Comp = nullptr;
 343   if (Leaf.Low == Leaf.High) {
 344     // Make the seteq instruction...
 345     Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
 346                         Leaf.Low, "SwitchLeaf");
 347   } else {
 348     // Make range comparison
 349     if (Leaf.Low == LowerBound) {
 350       // Val >= Min && Val <= Hi --> Val <= Hi
 351       Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
 352                           "SwitchLeaf");
 353     } else if (Leaf.High == UpperBound) {
 354       // Val <= Max && Val >= Lo --> Val >= Lo
 355       Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SGE, Val, Leaf.Low,
 356                           "SwitchLeaf");
 357     } else if (Leaf.Low->isZero()) {
 358       // Val >= 0 && Val <= Hi --> Val <=u Hi
 359       Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
 360                           "SwitchLeaf");
 361     } else {
 362       // Emit V-Lo <=u Hi-Lo
 363       Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
 364       Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
 365                                                    Val->getName()+".off",
 366                                                    NewLeaf);
 367       Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
 368       Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
 369                           "SwitchLeaf");
 370     }
 371   }
 372
 373   // Make the conditional branch...
 374   BasicBlock* Succ = Leaf.BB;
 375   BranchInst::Create(Succ, Default, Comp, NewLeaf);
 376
 377   // If there were any PHI nodes in this successor, rewrite one entry
 378   // from OrigBlock to come from NewLeaf.
 379   for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
 380     PHINode* PN = cast<PHINode>(I);
 381     // Remove all but one incoming entries from the cluster
 382     uint64_t Range = Leaf.High->getSExtValue() -
 383                      Leaf.Low->getSExtValue();
 384     for (uint64_t j = 0; j < Range; ++j) {
 385       PN->removeIncomingValue(OrigBlock);
 386     }
 387
 388     int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
 389     assert(BlockIdx != -1 && "Switch didn't go to this successor??");
 390     PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
 391   }
 392
 393   return NewLeaf;
 394 }
 395
 396 /// Transform simple list of \p SI's cases into list of CaseRange's \p Cases.
 397 /// \post \p Cases wouldn't contain references to \p SI's default BB.
 398 /// \returns Number of \p SI's cases that do not reference \p SI's default BB.
 399 unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
 400   unsigned NumSimpleCases = 0;
 401
 402   // Start with "simple" cases
 403   for (auto Case : SI->cases()) {
 404     if (Case.getCaseSuccessor() == SI->getDefaultDest())
 405       continue;
 406     Cases.push_back(CaseRange(Case.getCaseValue(), Case.getCaseValue(),
 407                               Case.getCaseSuccessor()));
 408     ++NumSimpleCases;
 409   }
 410
 411   llvm::sort(Cases, CaseCmp());
 412
 413   // Merge case into clusters
 414   if (Cases.size() >= 2) {
 415     CaseItr I = Cases.begin();
 416     for (CaseItr J = std::next(I), E = Cases.end(); J != E; ++J) {
 417       int64_t nextValue = J->Low->getSExtValue();
 418       int64_t currentValue = I->High->getSExtValue();
 419       BasicBlock* nextBB = J->BB;
 420       BasicBlock* currentBB = I->BB;
 421
 422       // If the two neighboring cases go to the same destination, merge them
 423       // into a single case.
 424       assert(nextValue > currentValue && "Cases should be strictly ascending");
 425       if ((nextValue == currentValue + 1) && (currentBB == nextBB)) {
 426         I->High = J->High;
 427         // FIXME: Combine branch weights.
 428       } else if (++I != J) {
 429         *I = *J;
 430       }
 431     }
 432     Cases.erase(std::next(I), Cases.end());
 433   }
 434
 435   return NumSimpleCases;
 436 }
 437
 438 /// Replace the specified switch instruction with a sequence of chained if-then
 439 /// insts in a balanced binary search.
 440 void LowerSwitch::processSwitchInst(SwitchInst *SI,
 441                                     SmallPtrSetImpl<BasicBlock *> &DeleteList,
 442                                     AssumptionCache *AC, LazyValueInfo *LVI) {
 443   BasicBlock *OrigBlock = SI->getParent();
 444   Function *F = OrigBlock->getParent();
 445   Value *Val = SI->getCondition();  // The value we are switching on...
 446   BasicBlock* Default = SI->getDefaultDest();
 447
 448   // Don't handle unreachable blocks. If there are successors with phis, this
 449   // would leave them behind with missing predecessors.
 450   if ((OrigBlock != &F->getEntryBlock() && pred_empty(OrigBlock)) ||
 451       OrigBlock->getSinglePredecessor() == OrigBlock) {
 452     DeleteList.insert(OrigBlock);
 453     return;
 454   }
 455
 456   // Prepare cases vector.
 457   CaseVector Cases;
 458   const unsigned NumSimpleCases = Clusterify(Cases, SI);
 459   LLVM_DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
 460                     << ". Total non-default cases: " << NumSimpleCases
 461                     << "\nCase clusters: " << Cases << "\n");
 462
 463   // If there is only the default destination, just branch.
 464   if (Cases.empty()) {
 465     BranchInst::Create(Default, OrigBlock);
 466     // Remove all the references from Default's PHIs to OrigBlock, but one.
 467     fixPhis(Default, OrigBlock, OrigBlock);
 468     SI->eraseFromParent();
 469     return;
 470   }
 471
 472   ConstantInt *LowerBound = nullptr;
 473   ConstantInt *UpperBound = nullptr;
 474   bool DefaultIsUnreachableFromSwitch = false;
 475
 476   if (isa<UnreachableInst>(Default->getFirstNonPHIOrDbg())) {
 477     // Make the bounds tightly fitted around the case value range, because we
 478     // know that the value passed to the switch must be exactly one of the case
 479     // values.
 480     LowerBound = Cases.front().Low;
 481     UpperBound = Cases.back().High;
 482     DefaultIsUnreachableFromSwitch = true;
 483   } else {
 484     // Constraining the range of the value being switched over helps eliminating
 485     // unreachable BBs and minimizing the number of `add` instructions
 486     // newLeafBlock ends up emitting. Running CorrelatedValuePropagation after
 487     // LowerSwitch isn't as good, and also much more expensive in terms of
 488     // compile time for the following reasons:
 489     // 1. it processes many kinds of instructions, not just switches;
 490     // 2. even if limited to icmp instructions only, it will have to process
 491     //    roughly C icmp's per switch, where C is the number of cases in the
 492     //    switch, while LowerSwitch only needs to call LVI once per switch.
 493     const DataLayout &DL = F->getParent()->getDataLayout();
 494     KnownBits Known = computeKnownBits(Val, DL, /*Depth=*/0, AC, SI);
 495     // TODO Shouldn't this create a signed range?
 496     ConstantRange KnownBitsRange =
 497         ConstantRange::fromKnownBits(Known, /*IsSigned=*/false);
 498     const ConstantRange LVIRange = LVI->getConstantRange(Val, OrigBlock, SI);
 499     ConstantRange ValRange = KnownBitsRange.intersectWith(LVIRange);
 500     // We delegate removal of unreachable non-default cases to other passes. In
 501     // the unlikely event that some of them survived, we just conservatively
 502     // maintain the invariant that all the cases lie between the bounds. This
 503     // may, however, still render the default case effectively unreachable.
 504     APInt Low = Cases.front().Low->getValue();
 505     APInt High = Cases.back().High->getValue();
 506     APInt Min = APIntOps::smin(ValRange.getSignedMin(), Low);
 507     APInt Max = APIntOps::smax(ValRange.getSignedMax(), High);
 508
 509     LowerBound = ConstantInt::get(SI->getContext(), Min);
 510     UpperBound = ConstantInt::get(SI->getContext(), Max);
 511     DefaultIsUnreachableFromSwitch = (Min + (NumSimpleCases - 1) == Max);
 512   }
 513
 514   std::vector<IntRange> UnreachableRanges;
 515
 516   if (DefaultIsUnreachableFromSwitch) {
 517     DenseMap<BasicBlock *, unsigned> Popularity;
 518     unsigned MaxPop = 0;
 519     BasicBlock *PopSucc = nullptr;
 520
 521     IntRange R = {std::numeric_limits<int64_t>::min(),
 522                   std::numeric_limits<int64_t>::max()};
 523     UnreachableRanges.push_back(R);
 524     for (const auto &I : Cases) {
 525       int64_t Low = I.Low->getSExtValue();
 526       int64_t High = I.High->getSExtValue();
 527
 528       IntRange &LastRange = UnreachableRanges.back();
 529       if (LastRange.Low == Low) {
 530         // There is nothing left of the previous range.
 531         UnreachableRanges.pop_back();
 532       } else {
 533         // Terminate the previous range.
 534         assert(Low > LastRange.Low);
 535         LastRange.High = Low - 1;
 536       }
 537       if (High != std::numeric_limits<int64_t>::max()) {
 538         IntRange R = { High + 1, std::numeric_limits<int64_t>::max() };
 539         UnreachableRanges.push_back(R);
 540       }
 541
 542       // Count popularity.
 543       int64_t N = High - Low + 1;
 544       unsigned &Pop = Popularity[I.BB];
 545       if ((Pop += N) > MaxPop) {
 546         MaxPop = Pop;
 547         PopSucc = I.BB;
 548       }
 549     }
 550 #ifndef NDEBUG
 551     /* UnreachableRanges should be sorted and the ranges non-adjacent. */
 552     for (auto I = UnreachableRanges.begin(), E = UnreachableRanges.end();
 553          I != E; ++I) {
 554       assert(I->Low <= I->High);
 555       auto Next = I + 1;
 556       if (Next != E) {
 557         assert(Next->Low > I->High);
 558       }
 559     }
 560 #endif
 561
 562     // As the default block in the switch is unreachable, update the PHI nodes
 563     // (remove all of the references to the default block) to reflect this.
 564     const unsigned NumDefaultEdges = SI->getNumCases() + 1 - NumSimpleCases;
 565     for (unsigned I = 0; I < NumDefaultEdges; ++I)
 566       Default->removePredecessor(OrigBlock);
 567
 568     // Use the most popular block as the new default, reducing the number of
 569     // cases.
 570     assert(MaxPop > 0 && PopSucc);
 571     Default = PopSucc;
 572     Cases.erase(
 573         llvm::remove_if(
 574             Cases, [PopSucc](const CaseRange &R) { return R.BB == PopSucc; }),
 575         Cases.end());
 576
 577     // If there are no cases left, just branch.
 578     if (Cases.empty()) {
 579       BranchInst::Create(Default, OrigBlock);
 580       SI->eraseFromParent();
 581       // As all the cases have been replaced with a single branch, only keep
 582       // one entry in the PHI nodes.
 583       for (unsigned I = 0 ; I < (MaxPop - 1) ; ++I)
 584         PopSucc->removePredecessor(OrigBlock);
 585       return;
 586     }
 587
 588     // If the condition was a PHI node with the switch block as a predecessor
 589     // removing predecessors may have caused the condition to be erased.
 590     // Getting the condition value again here protects against that.
 591     Val = SI->getCondition();
 592   }
 593
 594   // Create a new, empty default block so that the new hierarchy of
 595   // if-then statements go to this and the PHI nodes are happy.
 596   BasicBlock *NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
 597   F->getBasicBlockList().insert(Default->getIterator(), NewDefault);
 598   BranchInst::Create(Default, NewDefault);
 599
 600   BasicBlock *SwitchBlock =
 601       switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
 602                     OrigBlock, OrigBlock, NewDefault, UnreachableRanges);
 603
 604   // If there are entries in any PHI nodes for the default edge, make sure
 605   // to update them as well.
 606   fixPhis(Default, OrigBlock, NewDefault);
 607
 608   // Branch to our shiny new if-then stuff...
 609   BranchInst::Create(SwitchBlock, OrigBlock);
 610
 611   // We are now done with the switch instruction, delete it.
 612   BasicBlock *OldDefault = SI->getDefaultDest();
 613   OrigBlock->getInstList().erase(SI);
 614
 615   // If the Default block has no more predecessors just add it to DeleteList.
 616   if (pred_begin(OldDefault) == pred_end(OldDefault))
 617     DeleteList.insert(OldDefault);
 618 }