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      1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file implements the Jump Threading pass.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "llvm/Transforms/Scalar.h"
     15 #include "llvm/ADT/DenseMap.h"
     16 #include "llvm/ADT/DenseSet.h"
     17 #include "llvm/ADT/STLExtras.h"
     18 #include "llvm/ADT/SmallPtrSet.h"
     19 #include "llvm/ADT/SmallSet.h"
     20 #include "llvm/ADT/Statistic.h"
     21 #include "llvm/Analysis/CFG.h"
     22 #include "llvm/Analysis/ConstantFolding.h"
     23 #include "llvm/Analysis/InstructionSimplify.h"
     24 #include "llvm/Analysis/LazyValueInfo.h"
     25 #include "llvm/Analysis/Loads.h"
     26 #include "llvm/Analysis/TargetLibraryInfo.h"
     27 #include "llvm/IR/DataLayout.h"
     28 #include "llvm/IR/IntrinsicInst.h"
     29 #include "llvm/IR/LLVMContext.h"
     30 #include "llvm/IR/Metadata.h"
     31 #include "llvm/IR/ValueHandle.h"
     32 #include "llvm/Pass.h"
     33 #include "llvm/Support/CommandLine.h"
     34 #include "llvm/Support/Debug.h"
     35 #include "llvm/Support/raw_ostream.h"
     36 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     37 #include "llvm/Transforms/Utils/Local.h"
     38 #include "llvm/Transforms/Utils/SSAUpdater.h"
     39 using namespace llvm;
     40 
     41 #define DEBUG_TYPE "jump-threading"
     42 
     43 STATISTIC(NumThreads, "Number of jumps threaded");
     44 STATISTIC(NumFolds,   "Number of terminators folded");
     45 STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
     46 
     47 static cl::opt<unsigned>
     48 BBDuplicateThreshold("jump-threading-threshold",
     49           cl::desc("Max block size to duplicate for jump threading"),
     50           cl::init(6), cl::Hidden);
     51 
     52 namespace {
     53   // These are at global scope so static functions can use them too.
     54   typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
     55   typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy;
     56 
     57   // This is used to keep track of what kind of constant we're currently hoping
     58   // to find.
     59   enum ConstantPreference {
     60     WantInteger,
     61     WantBlockAddress
     62   };
     63 
     64   /// This pass performs 'jump threading', which looks at blocks that have
     65   /// multiple predecessors and multiple successors.  If one or more of the
     66   /// predecessors of the block can be proven to always jump to one of the
     67   /// successors, we forward the edge from the predecessor to the successor by
     68   /// duplicating the contents of this block.
     69   ///
     70   /// An example of when this can occur is code like this:
     71   ///
     72   ///   if () { ...
     73   ///     X = 4;
     74   ///   }
     75   ///   if (X < 3) {
     76   ///
     77   /// In this case, the unconditional branch at the end of the first if can be
     78   /// revectored to the false side of the second if.
     79   ///
     80   class JumpThreading : public FunctionPass {
     81     TargetLibraryInfo *TLI;
     82     LazyValueInfo *LVI;
     83 #ifdef NDEBUG
     84     SmallPtrSet<BasicBlock*, 16> LoopHeaders;
     85 #else
     86     SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
     87 #endif
     88     DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet;
     89 
     90     unsigned BBDupThreshold;
     91 
     92     // RAII helper for updating the recursion stack.
     93     struct RecursionSetRemover {
     94       DenseSet<std::pair<Value*, BasicBlock*> > &TheSet;
     95       std::pair<Value*, BasicBlock*> ThePair;
     96 
     97       RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S,
     98                           std::pair<Value*, BasicBlock*> P)
     99         : TheSet(S), ThePair(P) { }
    100 
    101       ~RecursionSetRemover() {
    102         TheSet.erase(ThePair);
    103       }
    104     };
    105   public:
    106     static char ID; // Pass identification
    107     JumpThreading(int T = -1) : FunctionPass(ID) {
    108       BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
    109       initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
    110     }
    111 
    112     bool runOnFunction(Function &F) override;
    113 
    114     void getAnalysisUsage(AnalysisUsage &AU) const override {
    115       AU.addRequired<LazyValueInfo>();
    116       AU.addPreserved<LazyValueInfo>();
    117       AU.addRequired<TargetLibraryInfoWrapperPass>();
    118     }
    119 
    120     void FindLoopHeaders(Function &F);
    121     bool ProcessBlock(BasicBlock *BB);
    122     bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
    123                     BasicBlock *SuccBB);
    124     bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
    125                                   const SmallVectorImpl<BasicBlock *> &PredBBs);
    126 
    127     bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
    128                                          PredValueInfo &Result,
    129                                          ConstantPreference Preference,
    130                                          Instruction *CxtI = nullptr);
    131     bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
    132                                 ConstantPreference Preference,
    133                                 Instruction *CxtI = nullptr);
    134 
    135     bool ProcessBranchOnPHI(PHINode *PN);
    136     bool ProcessBranchOnXOR(BinaryOperator *BO);
    137 
    138     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
    139     bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
    140   };
    141 }
    142 
    143 char JumpThreading::ID = 0;
    144 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
    145                 "Jump Threading", false, false)
    146 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
    147 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
    148 INITIALIZE_PASS_END(JumpThreading, "jump-threading",
    149                 "Jump Threading", false, false)
    150 
    151 // Public interface to the Jump Threading pass
    152 FunctionPass *llvm::createJumpThreadingPass(int Threshold) { return new JumpThreading(Threshold); }
    153 
    154 /// runOnFunction - Top level algorithm.
    155 ///
    156 bool JumpThreading::runOnFunction(Function &F) {
    157   if (skipOptnoneFunction(F))
    158     return false;
    159 
    160   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
    161   TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
    162   LVI = &getAnalysis<LazyValueInfo>();
    163 
    164   // Remove unreachable blocks from function as they may result in infinite
    165   // loop. We do threading if we found something profitable. Jump threading a
    166   // branch can create other opportunities. If these opportunities form a cycle
    167   // i.e. if any jump treading is undoing previous threading in the path, then
    168   // we will loop forever. We take care of this issue by not jump threading for
    169   // back edges. This works for normal cases but not for unreachable blocks as
    170   // they may have cycle with no back edge.
    171   removeUnreachableBlocks(F);
    172 
    173   FindLoopHeaders(F);
    174 
    175   bool Changed, EverChanged = false;
    176   do {
    177     Changed = false;
    178     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
    179       BasicBlock *BB = I;
    180       // Thread all of the branches we can over this block.
    181       while (ProcessBlock(BB))
    182         Changed = true;
    183 
    184       ++I;
    185 
    186       // If the block is trivially dead, zap it.  This eliminates the successor
    187       // edges which simplifies the CFG.
    188       if (pred_empty(BB) &&
    189           BB != &BB->getParent()->getEntryBlock()) {
    190         DEBUG(dbgs() << "  JT: Deleting dead block '" << BB->getName()
    191               << "' with terminator: " << *BB->getTerminator() << '\n');
    192         LoopHeaders.erase(BB);
    193         LVI->eraseBlock(BB);
    194         DeleteDeadBlock(BB);
    195         Changed = true;
    196         continue;
    197       }
    198 
    199       BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
    200 
    201       // Can't thread an unconditional jump, but if the block is "almost
    202       // empty", we can replace uses of it with uses of the successor and make
    203       // this dead.
    204       if (BI && BI->isUnconditional() &&
    205           BB != &BB->getParent()->getEntryBlock() &&
    206           // If the terminator is the only non-phi instruction, try to nuke it.
    207           BB->getFirstNonPHIOrDbg()->isTerminator()) {
    208         // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
    209         // block, we have to make sure it isn't in the LoopHeaders set.  We
    210         // reinsert afterward if needed.
    211         bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
    212         BasicBlock *Succ = BI->getSuccessor(0);
    213 
    214         // FIXME: It is always conservatively correct to drop the info
    215         // for a block even if it doesn't get erased.  This isn't totally
    216         // awesome, but it allows us to use AssertingVH to prevent nasty
    217         // dangling pointer issues within LazyValueInfo.
    218         LVI->eraseBlock(BB);
    219         if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
    220           Changed = true;
    221           // If we deleted BB and BB was the header of a loop, then the
    222           // successor is now the header of the loop.
    223           BB = Succ;
    224         }
    225 
    226         if (ErasedFromLoopHeaders)
    227           LoopHeaders.insert(BB);
    228       }
    229     }
    230     EverChanged |= Changed;
    231   } while (Changed);
    232 
    233   LoopHeaders.clear();
    234   return EverChanged;
    235 }
    236 
    237 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
    238 /// thread across it. Stop scanning the block when passing the threshold.
    239 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
    240                                              unsigned Threshold) {
    241   /// Ignore PHI nodes, these will be flattened when duplication happens.
    242   BasicBlock::const_iterator I = BB->getFirstNonPHI();
    243 
    244   // FIXME: THREADING will delete values that are just used to compute the
    245   // branch, so they shouldn't count against the duplication cost.
    246 
    247   // Sum up the cost of each instruction until we get to the terminator.  Don't
    248   // include the terminator because the copy won't include it.
    249   unsigned Size = 0;
    250   for (; !isa<TerminatorInst>(I); ++I) {
    251 
    252     // Stop scanning the block if we've reached the threshold.
    253     if (Size > Threshold)
    254       return Size;
    255 
    256     // Debugger intrinsics don't incur code size.
    257     if (isa<DbgInfoIntrinsic>(I)) continue;
    258 
    259     // If this is a pointer->pointer bitcast, it is free.
    260     if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
    261       continue;
    262 
    263     // All other instructions count for at least one unit.
    264     ++Size;
    265 
    266     // Calls are more expensive.  If they are non-intrinsic calls, we model them
    267     // as having cost of 4.  If they are a non-vector intrinsic, we model them
    268     // as having cost of 2 total, and if they are a vector intrinsic, we model
    269     // them as having cost 1.
    270     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
    271       if (CI->cannotDuplicate())
    272         // Blocks with NoDuplicate are modelled as having infinite cost, so they
    273         // are never duplicated.
    274         return ~0U;
    275       else if (!isa<IntrinsicInst>(CI))
    276         Size += 3;
    277       else if (!CI->getType()->isVectorTy())
    278         Size += 1;
    279     }
    280   }
    281 
    282   // Threading through a switch statement is particularly profitable.  If this
    283   // block ends in a switch, decrease its cost to make it more likely to happen.
    284   if (isa<SwitchInst>(I))
    285     Size = Size > 6 ? Size-6 : 0;
    286 
    287   // The same holds for indirect branches, but slightly more so.
    288   if (isa<IndirectBrInst>(I))
    289     Size = Size > 8 ? Size-8 : 0;
    290 
    291   return Size;
    292 }
    293 
    294 /// FindLoopHeaders - We do not want jump threading to turn proper loop
    295 /// structures into irreducible loops.  Doing this breaks up the loop nesting
    296 /// hierarchy and pessimizes later transformations.  To prevent this from
    297 /// happening, we first have to find the loop headers.  Here we approximate this
    298 /// by finding targets of backedges in the CFG.
    299 ///
    300 /// Note that there definitely are cases when we want to allow threading of
    301 /// edges across a loop header.  For example, threading a jump from outside the
    302 /// loop (the preheader) to an exit block of the loop is definitely profitable.
    303 /// It is also almost always profitable to thread backedges from within the loop
    304 /// to exit blocks, and is often profitable to thread backedges to other blocks
    305 /// within the loop (forming a nested loop).  This simple analysis is not rich
    306 /// enough to track all of these properties and keep it up-to-date as the CFG
    307 /// mutates, so we don't allow any of these transformations.
    308 ///
    309 void JumpThreading::FindLoopHeaders(Function &F) {
    310   SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
    311   FindFunctionBackedges(F, Edges);
    312 
    313   for (unsigned i = 0, e = Edges.size(); i != e; ++i)
    314     LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
    315 }
    316 
    317 /// getKnownConstant - Helper method to determine if we can thread over a
    318 /// terminator with the given value as its condition, and if so what value to
    319 /// use for that. What kind of value this is depends on whether we want an
    320 /// integer or a block address, but an undef is always accepted.
    321 /// Returns null if Val is null or not an appropriate constant.
    322 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
    323   if (!Val)
    324     return nullptr;
    325 
    326   // Undef is "known" enough.
    327   if (UndefValue *U = dyn_cast<UndefValue>(Val))
    328     return U;
    329 
    330   if (Preference == WantBlockAddress)
    331     return dyn_cast<BlockAddress>(Val->stripPointerCasts());
    332 
    333   return dyn_cast<ConstantInt>(Val);
    334 }
    335 
    336 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
    337 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
    338 /// in any of our predecessors.  If so, return the known list of value and pred
    339 /// BB in the result vector.
    340 ///
    341 /// This returns true if there were any known values.
    342 ///
    343 bool JumpThreading::
    344 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
    345                                 ConstantPreference Preference,
    346                                 Instruction *CxtI) {
    347   // This method walks up use-def chains recursively.  Because of this, we could
    348   // get into an infinite loop going around loops in the use-def chain.  To
    349   // prevent this, keep track of what (value, block) pairs we've already visited
    350   // and terminate the search if we loop back to them
    351   if (!RecursionSet.insert(std::make_pair(V, BB)).second)
    352     return false;
    353 
    354   // An RAII help to remove this pair from the recursion set once the recursion
    355   // stack pops back out again.
    356   RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
    357 
    358   // If V is a constant, then it is known in all predecessors.
    359   if (Constant *KC = getKnownConstant(V, Preference)) {
    360     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
    361       Result.push_back(std::make_pair(KC, *PI));
    362 
    363     return true;
    364   }
    365 
    366   // If V is a non-instruction value, or an instruction in a different block,
    367   // then it can't be derived from a PHI.
    368   Instruction *I = dyn_cast<Instruction>(V);
    369   if (!I || I->getParent() != BB) {
    370 
    371     // Okay, if this is a live-in value, see if it has a known value at the end
    372     // of any of our predecessors.
    373     //
    374     // FIXME: This should be an edge property, not a block end property.
    375     /// TODO: Per PR2563, we could infer value range information about a
    376     /// predecessor based on its terminator.
    377     //
    378     // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
    379     // "I" is a non-local compare-with-a-constant instruction.  This would be
    380     // able to handle value inequalities better, for example if the compare is
    381     // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
    382     // Perhaps getConstantOnEdge should be smart enough to do this?
    383 
    384     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
    385       BasicBlock *P = *PI;
    386       // If the value is known by LazyValueInfo to be a constant in a
    387       // predecessor, use that information to try to thread this block.
    388       Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
    389       if (Constant *KC = getKnownConstant(PredCst, Preference))
    390         Result.push_back(std::make_pair(KC, P));
    391     }
    392 
    393     return !Result.empty();
    394   }
    395 
    396   /// If I is a PHI node, then we know the incoming values for any constants.
    397   if (PHINode *PN = dyn_cast<PHINode>(I)) {
    398     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
    399       Value *InVal = PN->getIncomingValue(i);
    400       if (Constant *KC = getKnownConstant(InVal, Preference)) {
    401         Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
    402       } else {
    403         Constant *CI = LVI->getConstantOnEdge(InVal,
    404                                               PN->getIncomingBlock(i),
    405                                               BB, CxtI);
    406         if (Constant *KC = getKnownConstant(CI, Preference))
    407           Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
    408       }
    409     }
    410 
    411     return !Result.empty();
    412   }
    413 
    414   PredValueInfoTy LHSVals, RHSVals;
    415 
    416   // Handle some boolean conditions.
    417   if (I->getType()->getPrimitiveSizeInBits() == 1) {
    418     assert(Preference == WantInteger && "One-bit non-integer type?");
    419     // X | true -> true
    420     // X & false -> false
    421     if (I->getOpcode() == Instruction::Or ||
    422         I->getOpcode() == Instruction::And) {
    423       ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
    424                                       WantInteger, CxtI);
    425       ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
    426                                       WantInteger, CxtI);
    427 
    428       if (LHSVals.empty() && RHSVals.empty())
    429         return false;
    430 
    431       ConstantInt *InterestingVal;
    432       if (I->getOpcode() == Instruction::Or)
    433         InterestingVal = ConstantInt::getTrue(I->getContext());
    434       else
    435         InterestingVal = ConstantInt::getFalse(I->getContext());
    436 
    437       SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
    438 
    439       // Scan for the sentinel.  If we find an undef, force it to the
    440       // interesting value: x|undef -> true and x&undef -> false.
    441       for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
    442         if (LHSVals[i].first == InterestingVal ||
    443             isa<UndefValue>(LHSVals[i].first)) {
    444           Result.push_back(LHSVals[i]);
    445           Result.back().first = InterestingVal;
    446           LHSKnownBBs.insert(LHSVals[i].second);
    447         }
    448       for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
    449         if (RHSVals[i].first == InterestingVal ||
    450             isa<UndefValue>(RHSVals[i].first)) {
    451           // If we already inferred a value for this block on the LHS, don't
    452           // re-add it.
    453           if (!LHSKnownBBs.count(RHSVals[i].second)) {
    454             Result.push_back(RHSVals[i]);
    455             Result.back().first = InterestingVal;
    456           }
    457         }
    458 
    459       return !Result.empty();
    460     }
    461 
    462     // Handle the NOT form of XOR.
    463     if (I->getOpcode() == Instruction::Xor &&
    464         isa<ConstantInt>(I->getOperand(1)) &&
    465         cast<ConstantInt>(I->getOperand(1))->isOne()) {
    466       ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
    467                                       WantInteger, CxtI);
    468       if (Result.empty())
    469         return false;
    470 
    471       // Invert the known values.
    472       for (unsigned i = 0, e = Result.size(); i != e; ++i)
    473         Result[i].first = ConstantExpr::getNot(Result[i].first);
    474 
    475       return true;
    476     }
    477 
    478   // Try to simplify some other binary operator values.
    479   } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
    480     assert(Preference != WantBlockAddress
    481             && "A binary operator creating a block address?");
    482     if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
    483       PredValueInfoTy LHSVals;
    484       ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
    485                                       WantInteger, CxtI);
    486 
    487       // Try to use constant folding to simplify the binary operator.
    488       for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
    489         Constant *V = LHSVals[i].first;
    490         Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
    491 
    492         if (Constant *KC = getKnownConstant(Folded, WantInteger))
    493           Result.push_back(std::make_pair(KC, LHSVals[i].second));
    494       }
    495     }
    496 
    497     return !Result.empty();
    498   }
    499 
    500   // Handle compare with phi operand, where the PHI is defined in this block.
    501   if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
    502     assert(Preference == WantInteger && "Compares only produce integers");
    503     PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
    504     if (PN && PN->getParent() == BB) {
    505       const DataLayout &DL = PN->getModule()->getDataLayout();
    506       // We can do this simplification if any comparisons fold to true or false.
    507       // See if any do.
    508       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
    509         BasicBlock *PredBB = PN->getIncomingBlock(i);
    510         Value *LHS = PN->getIncomingValue(i);
    511         Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
    512 
    513         Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, DL);
    514         if (!Res) {
    515           if (!isa<Constant>(RHS))
    516             continue;
    517 
    518           LazyValueInfo::Tristate
    519             ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
    520                                            cast<Constant>(RHS), PredBB, BB,
    521                                            CxtI ? CxtI : Cmp);
    522           if (ResT == LazyValueInfo::Unknown)
    523             continue;
    524           Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
    525         }
    526 
    527         if (Constant *KC = getKnownConstant(Res, WantInteger))
    528           Result.push_back(std::make_pair(KC, PredBB));
    529       }
    530 
    531       return !Result.empty();
    532     }
    533 
    534     // If comparing a live-in value against a constant, see if we know the
    535     // live-in value on any predecessors.
    536     if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
    537       if (!isa<Instruction>(Cmp->getOperand(0)) ||
    538           cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
    539         Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
    540 
    541         for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
    542           BasicBlock *P = *PI;
    543           // If the value is known by LazyValueInfo to be a constant in a
    544           // predecessor, use that information to try to thread this block.
    545           LazyValueInfo::Tristate Res =
    546             LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
    547                                     RHSCst, P, BB, CxtI ? CxtI : Cmp);
    548           if (Res == LazyValueInfo::Unknown)
    549             continue;
    550 
    551           Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
    552           Result.push_back(std::make_pair(ResC, P));
    553         }
    554 
    555         return !Result.empty();
    556       }
    557 
    558       // Try to find a constant value for the LHS of a comparison,
    559       // and evaluate it statically if we can.
    560       if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
    561         PredValueInfoTy LHSVals;
    562         ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
    563                                         WantInteger, CxtI);
    564 
    565         for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
    566           Constant *V = LHSVals[i].first;
    567           Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
    568                                                       V, CmpConst);
    569           if (Constant *KC = getKnownConstant(Folded, WantInteger))
    570             Result.push_back(std::make_pair(KC, LHSVals[i].second));
    571         }
    572 
    573         return !Result.empty();
    574       }
    575     }
    576   }
    577 
    578   if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
    579     // Handle select instructions where at least one operand is a known constant
    580     // and we can figure out the condition value for any predecessor block.
    581     Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
    582     Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
    583     PredValueInfoTy Conds;
    584     if ((TrueVal || FalseVal) &&
    585         ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
    586                                         WantInteger, CxtI)) {
    587       for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
    588         Constant *Cond = Conds[i].first;
    589 
    590         // Figure out what value to use for the condition.
    591         bool KnownCond;
    592         if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
    593           // A known boolean.
    594           KnownCond = CI->isOne();
    595         } else {
    596           assert(isa<UndefValue>(Cond) && "Unexpected condition value");
    597           // Either operand will do, so be sure to pick the one that's a known
    598           // constant.
    599           // FIXME: Do this more cleverly if both values are known constants?
    600           KnownCond = (TrueVal != nullptr);
    601         }
    602 
    603         // See if the select has a known constant value for this predecessor.
    604         if (Constant *Val = KnownCond ? TrueVal : FalseVal)
    605           Result.push_back(std::make_pair(Val, Conds[i].second));
    606       }
    607 
    608       return !Result.empty();
    609     }
    610   }
    611 
    612   // If all else fails, see if LVI can figure out a constant value for us.
    613   Constant *CI = LVI->getConstant(V, BB, CxtI);
    614   if (Constant *KC = getKnownConstant(CI, Preference)) {
    615     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
    616       Result.push_back(std::make_pair(KC, *PI));
    617   }
    618 
    619   return !Result.empty();
    620 }
    621 
    622 
    623 
    624 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
    625 /// in an undefined jump, decide which block is best to revector to.
    626 ///
    627 /// Since we can pick an arbitrary destination, we pick the successor with the
    628 /// fewest predecessors.  This should reduce the in-degree of the others.
    629 ///
    630 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
    631   TerminatorInst *BBTerm = BB->getTerminator();
    632   unsigned MinSucc = 0;
    633   BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
    634   // Compute the successor with the minimum number of predecessors.
    635   unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
    636   for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
    637     TestBB = BBTerm->getSuccessor(i);
    638     unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
    639     if (NumPreds < MinNumPreds) {
    640       MinSucc = i;
    641       MinNumPreds = NumPreds;
    642     }
    643   }
    644 
    645   return MinSucc;
    646 }
    647 
    648 static bool hasAddressTakenAndUsed(BasicBlock *BB) {
    649   if (!BB->hasAddressTaken()) return false;
    650 
    651   // If the block has its address taken, it may be a tree of dead constants
    652   // hanging off of it.  These shouldn't keep the block alive.
    653   BlockAddress *BA = BlockAddress::get(BB);
    654   BA->removeDeadConstantUsers();
    655   return !BA->use_empty();
    656 }
    657 
    658 /// ProcessBlock - If there are any predecessors whose control can be threaded
    659 /// through to a successor, transform them now.
    660 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
    661   // If the block is trivially dead, just return and let the caller nuke it.
    662   // This simplifies other transformations.
    663   if (pred_empty(BB) &&
    664       BB != &BB->getParent()->getEntryBlock())
    665     return false;
    666 
    667   // If this block has a single predecessor, and if that pred has a single
    668   // successor, merge the blocks.  This encourages recursive jump threading
    669   // because now the condition in this block can be threaded through
    670   // predecessors of our predecessor block.
    671   if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
    672     if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
    673         SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
    674       // If SinglePred was a loop header, BB becomes one.
    675       if (LoopHeaders.erase(SinglePred))
    676         LoopHeaders.insert(BB);
    677 
    678       LVI->eraseBlock(SinglePred);
    679       MergeBasicBlockIntoOnlyPred(BB);
    680 
    681       return true;
    682     }
    683   }
    684 
    685   // What kind of constant we're looking for.
    686   ConstantPreference Preference = WantInteger;
    687 
    688   // Look to see if the terminator is a conditional branch, switch or indirect
    689   // branch, if not we can't thread it.
    690   Value *Condition;
    691   Instruction *Terminator = BB->getTerminator();
    692   if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
    693     // Can't thread an unconditional jump.
    694     if (BI->isUnconditional()) return false;
    695     Condition = BI->getCondition();
    696   } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
    697     Condition = SI->getCondition();
    698   } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
    699     // Can't thread indirect branch with no successors.
    700     if (IB->getNumSuccessors() == 0) return false;
    701     Condition = IB->getAddress()->stripPointerCasts();
    702     Preference = WantBlockAddress;
    703   } else {
    704     return false; // Must be an invoke.
    705   }
    706 
    707   // Run constant folding to see if we can reduce the condition to a simple
    708   // constant.
    709   if (Instruction *I = dyn_cast<Instruction>(Condition)) {
    710     Value *SimpleVal =
    711         ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
    712     if (SimpleVal) {
    713       I->replaceAllUsesWith(SimpleVal);
    714       I->eraseFromParent();
    715       Condition = SimpleVal;
    716     }
    717   }
    718 
    719   // If the terminator is branching on an undef, we can pick any of the
    720   // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
    721   if (isa<UndefValue>(Condition)) {
    722     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
    723 
    724     // Fold the branch/switch.
    725     TerminatorInst *BBTerm = BB->getTerminator();
    726     for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
    727       if (i == BestSucc) continue;
    728       BBTerm->getSuccessor(i)->removePredecessor(BB, true);
    729     }
    730 
    731     DEBUG(dbgs() << "  In block '" << BB->getName()
    732           << "' folding undef terminator: " << *BBTerm << '\n');
    733     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
    734     BBTerm->eraseFromParent();
    735     return true;
    736   }
    737 
    738   // If the terminator of this block is branching on a constant, simplify the
    739   // terminator to an unconditional branch.  This can occur due to threading in
    740   // other blocks.
    741   if (getKnownConstant(Condition, Preference)) {
    742     DEBUG(dbgs() << "  In block '" << BB->getName()
    743           << "' folding terminator: " << *BB->getTerminator() << '\n');
    744     ++NumFolds;
    745     ConstantFoldTerminator(BB, true);
    746     return true;
    747   }
    748 
    749   Instruction *CondInst = dyn_cast<Instruction>(Condition);
    750 
    751   // All the rest of our checks depend on the condition being an instruction.
    752   if (!CondInst) {
    753     // FIXME: Unify this with code below.
    754     if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
    755       return true;
    756     return false;
    757   }
    758 
    759 
    760   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
    761     // For a comparison where the LHS is outside this block, it's possible
    762     // that we've branched on it before.  Used LVI to see if we can simplify
    763     // the branch based on that.
    764     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
    765     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
    766     pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
    767     if (CondBr && CondConst && CondBr->isConditional() && PI != PE &&
    768         (!isa<Instruction>(CondCmp->getOperand(0)) ||
    769          cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
    770       // For predecessor edge, determine if the comparison is true or false
    771       // on that edge.  If they're all true or all false, we can simplify the
    772       // branch.
    773       // FIXME: We could handle mixed true/false by duplicating code.
    774       LazyValueInfo::Tristate Baseline =
    775         LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
    776                                 CondConst, *PI, BB, CondCmp);
    777       if (Baseline != LazyValueInfo::Unknown) {
    778         // Check that all remaining incoming values match the first one.
    779         while (++PI != PE) {
    780           LazyValueInfo::Tristate Ret =
    781             LVI->getPredicateOnEdge(CondCmp->getPredicate(),
    782                                     CondCmp->getOperand(0), CondConst, *PI, BB,
    783                                     CondCmp);
    784           if (Ret != Baseline) break;
    785         }
    786 
    787         // If we terminated early, then one of the values didn't match.
    788         if (PI == PE) {
    789           unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
    790           unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
    791           CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
    792           BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
    793           CondBr->eraseFromParent();
    794           return true;
    795         }
    796       }
    797 
    798     } else if (CondBr && CondConst && CondBr->isConditional()) {
    799       // There might be an invariant in the same block with the conditional
    800       // that can determine the predicate.
    801 
    802       LazyValueInfo::Tristate Ret =
    803         LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
    804                             CondConst, CondCmp);
    805       if (Ret != LazyValueInfo::Unknown) {
    806         unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
    807         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
    808         CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
    809         BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
    810         CondBr->eraseFromParent();
    811         return true;
    812       }
    813     }
    814 
    815     if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
    816       return true;
    817   }
    818 
    819   // Check for some cases that are worth simplifying.  Right now we want to look
    820   // for loads that are used by a switch or by the condition for the branch.  If
    821   // we see one, check to see if it's partially redundant.  If so, insert a PHI
    822   // which can then be used to thread the values.
    823   //
    824   Value *SimplifyValue = CondInst;
    825   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
    826     if (isa<Constant>(CondCmp->getOperand(1)))
    827       SimplifyValue = CondCmp->getOperand(0);
    828 
    829   // TODO: There are other places where load PRE would be profitable, such as
    830   // more complex comparisons.
    831   if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
    832     if (SimplifyPartiallyRedundantLoad(LI))
    833       return true;
    834 
    835 
    836   // Handle a variety of cases where we are branching on something derived from
    837   // a PHI node in the current block.  If we can prove that any predecessors
    838   // compute a predictable value based on a PHI node, thread those predecessors.
    839   //
    840   if (ProcessThreadableEdges(CondInst, BB, Preference, Terminator))
    841     return true;
    842 
    843   // If this is an otherwise-unfoldable branch on a phi node in the current
    844   // block, see if we can simplify.
    845   if (PHINode *PN = dyn_cast<PHINode>(CondInst))
    846     if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
    847       return ProcessBranchOnPHI(PN);
    848 
    849 
    850   // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
    851   if (CondInst->getOpcode() == Instruction::Xor &&
    852       CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
    853     return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
    854 
    855 
    856   // TODO: If we have: "br (X > 0)"  and we have a predecessor where we know
    857   // "(X == 4)", thread through this block.
    858 
    859   return false;
    860 }
    861 
    862 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
    863 /// load instruction, eliminate it by replacing it with a PHI node.  This is an
    864 /// important optimization that encourages jump threading, and needs to be run
    865 /// interlaced with other jump threading tasks.
    866 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
    867   // Don't hack volatile/atomic loads.
    868   if (!LI->isSimple()) return false;
    869 
    870   // If the load is defined in a block with exactly one predecessor, it can't be
    871   // partially redundant.
    872   BasicBlock *LoadBB = LI->getParent();
    873   if (LoadBB->getSinglePredecessor())
    874     return false;
    875 
    876   // If the load is defined in a landing pad, it can't be partially redundant,
    877   // because the edges between the invoke and the landing pad cannot have other
    878   // instructions between them.
    879   if (LoadBB->isLandingPad())
    880     return false;
    881 
    882   Value *LoadedPtr = LI->getOperand(0);
    883 
    884   // If the loaded operand is defined in the LoadBB, it can't be available.
    885   // TODO: Could do simple PHI translation, that would be fun :)
    886   if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
    887     if (PtrOp->getParent() == LoadBB)
    888       return false;
    889 
    890   // Scan a few instructions up from the load, to see if it is obviously live at
    891   // the entry to its block.
    892   BasicBlock::iterator BBIt = LI;
    893 
    894   if (Value *AvailableVal =
    895         FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
    896     // If the value if the load is locally available within the block, just use
    897     // it.  This frequently occurs for reg2mem'd allocas.
    898     //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
    899 
    900     // If the returned value is the load itself, replace with an undef. This can
    901     // only happen in dead loops.
    902     if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
    903     if (AvailableVal->getType() != LI->getType())
    904       AvailableVal =
    905           CastInst::CreateBitOrPointerCast(AvailableVal, LI->getType(), "", LI);
    906     LI->replaceAllUsesWith(AvailableVal);
    907     LI->eraseFromParent();
    908     return true;
    909   }
    910 
    911   // Otherwise, if we scanned the whole block and got to the top of the block,
    912   // we know the block is locally transparent to the load.  If not, something
    913   // might clobber its value.
    914   if (BBIt != LoadBB->begin())
    915     return false;
    916 
    917   // If all of the loads and stores that feed the value have the same AA tags,
    918   // then we can propagate them onto any newly inserted loads.
    919   AAMDNodes AATags;
    920   LI->getAAMetadata(AATags);
    921 
    922   SmallPtrSet<BasicBlock*, 8> PredsScanned;
    923   typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
    924   AvailablePredsTy AvailablePreds;
    925   BasicBlock *OneUnavailablePred = nullptr;
    926 
    927   // If we got here, the loaded value is transparent through to the start of the
    928   // block.  Check to see if it is available in any of the predecessor blocks.
    929   for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
    930        PI != PE; ++PI) {
    931     BasicBlock *PredBB = *PI;
    932 
    933     // If we already scanned this predecessor, skip it.
    934     if (!PredsScanned.insert(PredBB).second)
    935       continue;
    936 
    937     // Scan the predecessor to see if the value is available in the pred.
    938     BBIt = PredBB->end();
    939     AAMDNodes ThisAATags;
    940     Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
    941                                                     nullptr, &ThisAATags);
    942     if (!PredAvailable) {
    943       OneUnavailablePred = PredBB;
    944       continue;
    945     }
    946 
    947     // If AA tags disagree or are not present, forget about them.
    948     if (AATags != ThisAATags) AATags = AAMDNodes();
    949 
    950     // If so, this load is partially redundant.  Remember this info so that we
    951     // can create a PHI node.
    952     AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
    953   }
    954 
    955   // If the loaded value isn't available in any predecessor, it isn't partially
    956   // redundant.
    957   if (AvailablePreds.empty()) return false;
    958 
    959   // Okay, the loaded value is available in at least one (and maybe all!)
    960   // predecessors.  If the value is unavailable in more than one unique
    961   // predecessor, we want to insert a merge block for those common predecessors.
    962   // This ensures that we only have to insert one reload, thus not increasing
    963   // code size.
    964   BasicBlock *UnavailablePred = nullptr;
    965 
    966   // If there is exactly one predecessor where the value is unavailable, the
    967   // already computed 'OneUnavailablePred' block is it.  If it ends in an
    968   // unconditional branch, we know that it isn't a critical edge.
    969   if (PredsScanned.size() == AvailablePreds.size()+1 &&
    970       OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
    971     UnavailablePred = OneUnavailablePred;
    972   } else if (PredsScanned.size() != AvailablePreds.size()) {
    973     // Otherwise, we had multiple unavailable predecessors or we had a critical
    974     // edge from the one.
    975     SmallVector<BasicBlock*, 8> PredsToSplit;
    976     SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
    977 
    978     for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
    979       AvailablePredSet.insert(AvailablePreds[i].first);
    980 
    981     // Add all the unavailable predecessors to the PredsToSplit list.
    982     for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
    983          PI != PE; ++PI) {
    984       BasicBlock *P = *PI;
    985       // If the predecessor is an indirect goto, we can't split the edge.
    986       if (isa<IndirectBrInst>(P->getTerminator()))
    987         return false;
    988 
    989       if (!AvailablePredSet.count(P))
    990         PredsToSplit.push_back(P);
    991     }
    992 
    993     // Split them out to their own block.
    994     UnavailablePred =
    995       SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split");
    996   }
    997 
    998   // If the value isn't available in all predecessors, then there will be
    999   // exactly one where it isn't available.  Insert a load on that edge and add
   1000   // it to the AvailablePreds list.
   1001   if (UnavailablePred) {
   1002     assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
   1003            "Can't handle critical edge here!");
   1004     LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
   1005                                  LI->getAlignment(),
   1006                                  UnavailablePred->getTerminator());
   1007     NewVal->setDebugLoc(LI->getDebugLoc());
   1008     if (AATags)
   1009       NewVal->setAAMetadata(AATags);
   1010 
   1011     AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
   1012   }
   1013 
   1014   // Now we know that each predecessor of this block has a value in
   1015   // AvailablePreds, sort them for efficient access as we're walking the preds.
   1016   array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
   1017 
   1018   // Create a PHI node at the start of the block for the PRE'd load value.
   1019   pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
   1020   PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
   1021                                 LoadBB->begin());
   1022   PN->takeName(LI);
   1023   PN->setDebugLoc(LI->getDebugLoc());
   1024 
   1025   // Insert new entries into the PHI for each predecessor.  A single block may
   1026   // have multiple entries here.
   1027   for (pred_iterator PI = PB; PI != PE; ++PI) {
   1028     BasicBlock *P = *PI;
   1029     AvailablePredsTy::iterator I =
   1030       std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
   1031                        std::make_pair(P, (Value*)nullptr));
   1032 
   1033     assert(I != AvailablePreds.end() && I->first == P &&
   1034            "Didn't find entry for predecessor!");
   1035 
   1036     // If we have an available predecessor but it requires casting, insert the
   1037     // cast in the predecessor and use the cast. Note that we have to update the
   1038     // AvailablePreds vector as we go so that all of the PHI entries for this
   1039     // predecessor use the same bitcast.
   1040     Value *&PredV = I->second;
   1041     if (PredV->getType() != LI->getType())
   1042       PredV = CastInst::CreateBitOrPointerCast(PredV, LI->getType(), "",
   1043                                                P->getTerminator());
   1044 
   1045     PN->addIncoming(PredV, I->first);
   1046   }
   1047 
   1048   //cerr << "PRE: " << *LI << *PN << "\n";
   1049 
   1050   LI->replaceAllUsesWith(PN);
   1051   LI->eraseFromParent();
   1052 
   1053   return true;
   1054 }
   1055 
   1056 /// FindMostPopularDest - The specified list contains multiple possible
   1057 /// threadable destinations.  Pick the one that occurs the most frequently in
   1058 /// the list.
   1059 static BasicBlock *
   1060 FindMostPopularDest(BasicBlock *BB,
   1061                     const SmallVectorImpl<std::pair<BasicBlock*,
   1062                                   BasicBlock*> > &PredToDestList) {
   1063   assert(!PredToDestList.empty());
   1064 
   1065   // Determine popularity.  If there are multiple possible destinations, we
   1066   // explicitly choose to ignore 'undef' destinations.  We prefer to thread
   1067   // blocks with known and real destinations to threading undef.  We'll handle
   1068   // them later if interesting.
   1069   DenseMap<BasicBlock*, unsigned> DestPopularity;
   1070   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
   1071     if (PredToDestList[i].second)
   1072       DestPopularity[PredToDestList[i].second]++;
   1073 
   1074   // Find the most popular dest.
   1075   DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
   1076   BasicBlock *MostPopularDest = DPI->first;
   1077   unsigned Popularity = DPI->second;
   1078   SmallVector<BasicBlock*, 4> SamePopularity;
   1079 
   1080   for (++DPI; DPI != DestPopularity.end(); ++DPI) {
   1081     // If the popularity of this entry isn't higher than the popularity we've
   1082     // seen so far, ignore it.
   1083     if (DPI->second < Popularity)
   1084       ; // ignore.
   1085     else if (DPI->second == Popularity) {
   1086       // If it is the same as what we've seen so far, keep track of it.
   1087       SamePopularity.push_back(DPI->first);
   1088     } else {
   1089       // If it is more popular, remember it.
   1090       SamePopularity.clear();
   1091       MostPopularDest = DPI->first;
   1092       Popularity = DPI->second;
   1093     }
   1094   }
   1095 
   1096   // Okay, now we know the most popular destination.  If there is more than one
   1097   // destination, we need to determine one.  This is arbitrary, but we need
   1098   // to make a deterministic decision.  Pick the first one that appears in the
   1099   // successor list.
   1100   if (!SamePopularity.empty()) {
   1101     SamePopularity.push_back(MostPopularDest);
   1102     TerminatorInst *TI = BB->getTerminator();
   1103     for (unsigned i = 0; ; ++i) {
   1104       assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
   1105 
   1106       if (std::find(SamePopularity.begin(), SamePopularity.end(),
   1107                     TI->getSuccessor(i)) == SamePopularity.end())
   1108         continue;
   1109 
   1110       MostPopularDest = TI->getSuccessor(i);
   1111       break;
   1112     }
   1113   }
   1114 
   1115   // Okay, we have finally picked the most popular destination.
   1116   return MostPopularDest;
   1117 }
   1118 
   1119 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
   1120                                            ConstantPreference Preference,
   1121                                            Instruction *CxtI) {
   1122   // If threading this would thread across a loop header, don't even try to
   1123   // thread the edge.
   1124   if (LoopHeaders.count(BB))
   1125     return false;
   1126 
   1127   PredValueInfoTy PredValues;
   1128   if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
   1129     return false;
   1130 
   1131   assert(!PredValues.empty() &&
   1132          "ComputeValueKnownInPredecessors returned true with no values");
   1133 
   1134   DEBUG(dbgs() << "IN BB: " << *BB;
   1135         for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
   1136           dbgs() << "  BB '" << BB->getName() << "': FOUND condition = "
   1137             << *PredValues[i].first
   1138             << " for pred '" << PredValues[i].second->getName() << "'.\n";
   1139         });
   1140 
   1141   // Decide what we want to thread through.  Convert our list of known values to
   1142   // a list of known destinations for each pred.  This also discards duplicate
   1143   // predecessors and keeps track of the undefined inputs (which are represented
   1144   // as a null dest in the PredToDestList).
   1145   SmallPtrSet<BasicBlock*, 16> SeenPreds;
   1146   SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
   1147 
   1148   BasicBlock *OnlyDest = nullptr;
   1149   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
   1150 
   1151   for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
   1152     BasicBlock *Pred = PredValues[i].second;
   1153     if (!SeenPreds.insert(Pred).second)
   1154       continue;  // Duplicate predecessor entry.
   1155 
   1156     // If the predecessor ends with an indirect goto, we can't change its
   1157     // destination.
   1158     if (isa<IndirectBrInst>(Pred->getTerminator()))
   1159       continue;
   1160 
   1161     Constant *Val = PredValues[i].first;
   1162 
   1163     BasicBlock *DestBB;
   1164     if (isa<UndefValue>(Val))
   1165       DestBB = nullptr;
   1166     else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
   1167       DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
   1168     else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
   1169       DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
   1170     } else {
   1171       assert(isa<IndirectBrInst>(BB->getTerminator())
   1172               && "Unexpected terminator");
   1173       DestBB = cast<BlockAddress>(Val)->getBasicBlock();
   1174     }
   1175 
   1176     // If we have exactly one destination, remember it for efficiency below.
   1177     if (PredToDestList.empty())
   1178       OnlyDest = DestBB;
   1179     else if (OnlyDest != DestBB)
   1180       OnlyDest = MultipleDestSentinel;
   1181 
   1182     PredToDestList.push_back(std::make_pair(Pred, DestBB));
   1183   }
   1184 
   1185   // If all edges were unthreadable, we fail.
   1186   if (PredToDestList.empty())
   1187     return false;
   1188 
   1189   // Determine which is the most common successor.  If we have many inputs and
   1190   // this block is a switch, we want to start by threading the batch that goes
   1191   // to the most popular destination first.  If we only know about one
   1192   // threadable destination (the common case) we can avoid this.
   1193   BasicBlock *MostPopularDest = OnlyDest;
   1194 
   1195   if (MostPopularDest == MultipleDestSentinel)
   1196     MostPopularDest = FindMostPopularDest(BB, PredToDestList);
   1197 
   1198   // Now that we know what the most popular destination is, factor all
   1199   // predecessors that will jump to it into a single predecessor.
   1200   SmallVector<BasicBlock*, 16> PredsToFactor;
   1201   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
   1202     if (PredToDestList[i].second == MostPopularDest) {
   1203       BasicBlock *Pred = PredToDestList[i].first;
   1204 
   1205       // This predecessor may be a switch or something else that has multiple
   1206       // edges to the block.  Factor each of these edges by listing them
   1207       // according to # occurrences in PredsToFactor.
   1208       TerminatorInst *PredTI = Pred->getTerminator();
   1209       for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
   1210         if (PredTI->getSuccessor(i) == BB)
   1211           PredsToFactor.push_back(Pred);
   1212     }
   1213 
   1214   // If the threadable edges are branching on an undefined value, we get to pick
   1215   // the destination that these predecessors should get to.
   1216   if (!MostPopularDest)
   1217     MostPopularDest = BB->getTerminator()->
   1218                             getSuccessor(GetBestDestForJumpOnUndef(BB));
   1219 
   1220   // Ok, try to thread it!
   1221   return ThreadEdge(BB, PredsToFactor, MostPopularDest);
   1222 }
   1223 
   1224 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
   1225 /// a PHI node in the current block.  See if there are any simplifications we
   1226 /// can do based on inputs to the phi node.
   1227 ///
   1228 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
   1229   BasicBlock *BB = PN->getParent();
   1230 
   1231   // TODO: We could make use of this to do it once for blocks with common PHI
   1232   // values.
   1233   SmallVector<BasicBlock*, 1> PredBBs;
   1234   PredBBs.resize(1);
   1235 
   1236   // If any of the predecessor blocks end in an unconditional branch, we can
   1237   // *duplicate* the conditional branch into that block in order to further
   1238   // encourage jump threading and to eliminate cases where we have branch on a
   1239   // phi of an icmp (branch on icmp is much better).
   1240   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
   1241     BasicBlock *PredBB = PN->getIncomingBlock(i);
   1242     if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
   1243       if (PredBr->isUnconditional()) {
   1244         PredBBs[0] = PredBB;
   1245         // Try to duplicate BB into PredBB.
   1246         if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
   1247           return true;
   1248       }
   1249   }
   1250 
   1251   return false;
   1252 }
   1253 
   1254 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
   1255 /// a xor instruction in the current block.  See if there are any
   1256 /// simplifications we can do based on inputs to the xor.
   1257 ///
   1258 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
   1259   BasicBlock *BB = BO->getParent();
   1260 
   1261   // If either the LHS or RHS of the xor is a constant, don't do this
   1262   // optimization.
   1263   if (isa<ConstantInt>(BO->getOperand(0)) ||
   1264       isa<ConstantInt>(BO->getOperand(1)))
   1265     return false;
   1266 
   1267   // If the first instruction in BB isn't a phi, we won't be able to infer
   1268   // anything special about any particular predecessor.
   1269   if (!isa<PHINode>(BB->front()))
   1270     return false;
   1271 
   1272   // If we have a xor as the branch input to this block, and we know that the
   1273   // LHS or RHS of the xor in any predecessor is true/false, then we can clone
   1274   // the condition into the predecessor and fix that value to true, saving some
   1275   // logical ops on that path and encouraging other paths to simplify.
   1276   //
   1277   // This copies something like this:
   1278   //
   1279   //  BB:
   1280   //    %X = phi i1 [1],  [%X']
   1281   //    %Y = icmp eq i32 %A, %B
   1282   //    %Z = xor i1 %X, %Y
   1283   //    br i1 %Z, ...
   1284   //
   1285   // Into:
   1286   //  BB':
   1287   //    %Y = icmp ne i32 %A, %B
   1288   //    br i1 %Z, ...
   1289 
   1290   PredValueInfoTy XorOpValues;
   1291   bool isLHS = true;
   1292   if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
   1293                                        WantInteger, BO)) {
   1294     assert(XorOpValues.empty());
   1295     if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
   1296                                          WantInteger, BO))
   1297       return false;
   1298     isLHS = false;
   1299   }
   1300 
   1301   assert(!XorOpValues.empty() &&
   1302          "ComputeValueKnownInPredecessors returned true with no values");
   1303 
   1304   // Scan the information to see which is most popular: true or false.  The
   1305   // predecessors can be of the set true, false, or undef.
   1306   unsigned NumTrue = 0, NumFalse = 0;
   1307   for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
   1308     if (isa<UndefValue>(XorOpValues[i].first))
   1309       // Ignore undefs for the count.
   1310       continue;
   1311     if (cast<ConstantInt>(XorOpValues[i].first)->isZero())
   1312       ++NumFalse;
   1313     else
   1314       ++NumTrue;
   1315   }
   1316 
   1317   // Determine which value to split on, true, false, or undef if neither.
   1318   ConstantInt *SplitVal = nullptr;
   1319   if (NumTrue > NumFalse)
   1320     SplitVal = ConstantInt::getTrue(BB->getContext());
   1321   else if (NumTrue != 0 || NumFalse != 0)
   1322     SplitVal = ConstantInt::getFalse(BB->getContext());
   1323 
   1324   // Collect all of the blocks that this can be folded into so that we can
   1325   // factor this once and clone it once.
   1326   SmallVector<BasicBlock*, 8> BlocksToFoldInto;
   1327   for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
   1328     if (XorOpValues[i].first != SplitVal &&
   1329         !isa<UndefValue>(XorOpValues[i].first))
   1330       continue;
   1331 
   1332     BlocksToFoldInto.push_back(XorOpValues[i].second);
   1333   }
   1334 
   1335   // If we inferred a value for all of the predecessors, then duplication won't
   1336   // help us.  However, we can just replace the LHS or RHS with the constant.
   1337   if (BlocksToFoldInto.size() ==
   1338       cast<PHINode>(BB->front()).getNumIncomingValues()) {
   1339     if (!SplitVal) {
   1340       // If all preds provide undef, just nuke the xor, because it is undef too.
   1341       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
   1342       BO->eraseFromParent();
   1343     } else if (SplitVal->isZero()) {
   1344       // If all preds provide 0, replace the xor with the other input.
   1345       BO->replaceAllUsesWith(BO->getOperand(isLHS));
   1346       BO->eraseFromParent();
   1347     } else {
   1348       // If all preds provide 1, set the computed value to 1.
   1349       BO->setOperand(!isLHS, SplitVal);
   1350     }
   1351 
   1352     return true;
   1353   }
   1354 
   1355   // Try to duplicate BB into PredBB.
   1356   return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
   1357 }
   1358 
   1359 
   1360 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
   1361 /// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
   1362 /// NewPred using the entries from OldPred (suitably mapped).
   1363 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
   1364                                             BasicBlock *OldPred,
   1365                                             BasicBlock *NewPred,
   1366                                      DenseMap<Instruction*, Value*> &ValueMap) {
   1367   for (BasicBlock::iterator PNI = PHIBB->begin();
   1368        PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
   1369     // Ok, we have a PHI node.  Figure out what the incoming value was for the
   1370     // DestBlock.
   1371     Value *IV = PN->getIncomingValueForBlock(OldPred);
   1372 
   1373     // Remap the value if necessary.
   1374     if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
   1375       DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
   1376       if (I != ValueMap.end())
   1377         IV = I->second;
   1378     }
   1379 
   1380     PN->addIncoming(IV, NewPred);
   1381   }
   1382 }
   1383 
   1384 /// ThreadEdge - We have decided that it is safe and profitable to factor the
   1385 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
   1386 /// across BB.  Transform the IR to reflect this change.
   1387 bool JumpThreading::ThreadEdge(BasicBlock *BB,
   1388                                const SmallVectorImpl<BasicBlock*> &PredBBs,
   1389                                BasicBlock *SuccBB) {
   1390   // If threading to the same block as we come from, we would infinite loop.
   1391   if (SuccBB == BB) {
   1392     DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
   1393           << "' - would thread to self!\n");
   1394     return false;
   1395   }
   1396 
   1397   // If threading this would thread across a loop header, don't thread the edge.
   1398   // See the comments above FindLoopHeaders for justifications and caveats.
   1399   if (LoopHeaders.count(BB)) {
   1400     DEBUG(dbgs() << "  Not threading across loop header BB '" << BB->getName()
   1401           << "' to dest BB '" << SuccBB->getName()
   1402           << "' - it might create an irreducible loop!\n");
   1403     return false;
   1404   }
   1405 
   1406   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
   1407   if (JumpThreadCost > BBDupThreshold) {
   1408     DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
   1409           << "' - Cost is too high: " << JumpThreadCost << "\n");
   1410     return false;
   1411   }
   1412 
   1413   // And finally, do it!  Start by factoring the predecessors is needed.
   1414   BasicBlock *PredBB;
   1415   if (PredBBs.size() == 1)
   1416     PredBB = PredBBs[0];
   1417   else {
   1418     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
   1419           << " common predecessors.\n");
   1420     PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
   1421   }
   1422 
   1423   // And finally, do it!
   1424   DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName() << "' to '"
   1425         << SuccBB->getName() << "' with cost: " << JumpThreadCost
   1426         << ", across block:\n    "
   1427         << *BB << "\n");
   1428 
   1429   LVI->threadEdge(PredBB, BB, SuccBB);
   1430 
   1431   // We are going to have to map operands from the original BB block to the new
   1432   // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
   1433   // account for entry from PredBB.
   1434   DenseMap<Instruction*, Value*> ValueMapping;
   1435 
   1436   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
   1437                                          BB->getName()+".thread",
   1438                                          BB->getParent(), BB);
   1439   NewBB->moveAfter(PredBB);
   1440 
   1441   BasicBlock::iterator BI = BB->begin();
   1442   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
   1443     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
   1444 
   1445   // Clone the non-phi instructions of BB into NewBB, keeping track of the
   1446   // mapping and using it to remap operands in the cloned instructions.
   1447   for (; !isa<TerminatorInst>(BI); ++BI) {
   1448     Instruction *New = BI->clone();
   1449     New->setName(BI->getName());
   1450     NewBB->getInstList().push_back(New);
   1451     ValueMapping[BI] = New;
   1452 
   1453     // Remap operands to patch up intra-block references.
   1454     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
   1455       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
   1456         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
   1457         if (I != ValueMapping.end())
   1458           New->setOperand(i, I->second);
   1459       }
   1460   }
   1461 
   1462   // We didn't copy the terminator from BB over to NewBB, because there is now
   1463   // an unconditional jump to SuccBB.  Insert the unconditional jump.
   1464   BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
   1465   NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
   1466 
   1467   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
   1468   // PHI nodes for NewBB now.
   1469   AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
   1470 
   1471   // If there were values defined in BB that are used outside the block, then we
   1472   // now have to update all uses of the value to use either the original value,
   1473   // the cloned value, or some PHI derived value.  This can require arbitrary
   1474   // PHI insertion, of which we are prepared to do, clean these up now.
   1475   SSAUpdater SSAUpdate;
   1476   SmallVector<Use*, 16> UsesToRename;
   1477   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
   1478     // Scan all uses of this instruction to see if it is used outside of its
   1479     // block, and if so, record them in UsesToRename.
   1480     for (Use &U : I->uses()) {
   1481       Instruction *User = cast<Instruction>(U.getUser());
   1482       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
   1483         if (UserPN->getIncomingBlock(U) == BB)
   1484           continue;
   1485       } else if (User->getParent() == BB)
   1486         continue;
   1487 
   1488       UsesToRename.push_back(&U);
   1489     }
   1490 
   1491     // If there are no uses outside the block, we're done with this instruction.
   1492     if (UsesToRename.empty())
   1493       continue;
   1494 
   1495     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
   1496 
   1497     // We found a use of I outside of BB.  Rename all uses of I that are outside
   1498     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
   1499     // with the two values we know.
   1500     SSAUpdate.Initialize(I->getType(), I->getName());
   1501     SSAUpdate.AddAvailableValue(BB, I);
   1502     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
   1503 
   1504     while (!UsesToRename.empty())
   1505       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
   1506     DEBUG(dbgs() << "\n");
   1507   }
   1508 
   1509 
   1510   // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
   1511   // NewBB instead of BB.  This eliminates predecessors from BB, which requires
   1512   // us to simplify any PHI nodes in BB.
   1513   TerminatorInst *PredTerm = PredBB->getTerminator();
   1514   for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
   1515     if (PredTerm->getSuccessor(i) == BB) {
   1516       BB->removePredecessor(PredBB, true);
   1517       PredTerm->setSuccessor(i, NewBB);
   1518     }
   1519 
   1520   // At this point, the IR is fully up to date and consistent.  Do a quick scan
   1521   // over the new instructions and zap any that are constants or dead.  This
   1522   // frequently happens because of phi translation.
   1523   SimplifyInstructionsInBlock(NewBB, TLI);
   1524 
   1525   // Threaded an edge!
   1526   ++NumThreads;
   1527   return true;
   1528 }
   1529 
   1530 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
   1531 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
   1532 /// If we can duplicate the contents of BB up into PredBB do so now, this
   1533 /// improves the odds that the branch will be on an analyzable instruction like
   1534 /// a compare.
   1535 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
   1536                                  const SmallVectorImpl<BasicBlock *> &PredBBs) {
   1537   assert(!PredBBs.empty() && "Can't handle an empty set");
   1538 
   1539   // If BB is a loop header, then duplicating this block outside the loop would
   1540   // cause us to transform this into an irreducible loop, don't do this.
   1541   // See the comments above FindLoopHeaders for justifications and caveats.
   1542   if (LoopHeaders.count(BB)) {
   1543     DEBUG(dbgs() << "  Not duplicating loop header '" << BB->getName()
   1544           << "' into predecessor block '" << PredBBs[0]->getName()
   1545           << "' - it might create an irreducible loop!\n");
   1546     return false;
   1547   }
   1548 
   1549   unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
   1550   if (DuplicationCost > BBDupThreshold) {
   1551     DEBUG(dbgs() << "  Not duplicating BB '" << BB->getName()
   1552           << "' - Cost is too high: " << DuplicationCost << "\n");
   1553     return false;
   1554   }
   1555 
   1556   // And finally, do it!  Start by factoring the predecessors is needed.
   1557   BasicBlock *PredBB;
   1558   if (PredBBs.size() == 1)
   1559     PredBB = PredBBs[0];
   1560   else {
   1561     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
   1562           << " common predecessors.\n");
   1563     PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
   1564   }
   1565 
   1566   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
   1567   // of PredBB.
   1568   DEBUG(dbgs() << "  Duplicating block '" << BB->getName() << "' into end of '"
   1569         << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
   1570         << DuplicationCost << " block is:" << *BB << "\n");
   1571 
   1572   // Unless PredBB ends with an unconditional branch, split the edge so that we
   1573   // can just clone the bits from BB into the end of the new PredBB.
   1574   BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
   1575 
   1576   if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
   1577     PredBB = SplitEdge(PredBB, BB);
   1578     OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
   1579   }
   1580 
   1581   // We are going to have to map operands from the original BB block into the
   1582   // PredBB block.  Evaluate PHI nodes in BB.
   1583   DenseMap<Instruction*, Value*> ValueMapping;
   1584 
   1585   BasicBlock::iterator BI = BB->begin();
   1586   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
   1587     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
   1588   // Clone the non-phi instructions of BB into PredBB, keeping track of the
   1589   // mapping and using it to remap operands in the cloned instructions.
   1590   for (; BI != BB->end(); ++BI) {
   1591     Instruction *New = BI->clone();
   1592 
   1593     // Remap operands to patch up intra-block references.
   1594     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
   1595       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
   1596         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
   1597         if (I != ValueMapping.end())
   1598           New->setOperand(i, I->second);
   1599       }
   1600 
   1601     // If this instruction can be simplified after the operands are updated,
   1602     // just use the simplified value instead.  This frequently happens due to
   1603     // phi translation.
   1604     if (Value *IV =
   1605             SimplifyInstruction(New, BB->getModule()->getDataLayout())) {
   1606       delete New;
   1607       ValueMapping[BI] = IV;
   1608     } else {
   1609       // Otherwise, insert the new instruction into the block.
   1610       New->setName(BI->getName());
   1611       PredBB->getInstList().insert(OldPredBranch, New);
   1612       ValueMapping[BI] = New;
   1613     }
   1614   }
   1615 
   1616   // Check to see if the targets of the branch had PHI nodes. If so, we need to
   1617   // add entries to the PHI nodes for branch from PredBB now.
   1618   BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
   1619   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
   1620                                   ValueMapping);
   1621   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
   1622                                   ValueMapping);
   1623 
   1624   // If there were values defined in BB that are used outside the block, then we
   1625   // now have to update all uses of the value to use either the original value,
   1626   // the cloned value, or some PHI derived value.  This can require arbitrary
   1627   // PHI insertion, of which we are prepared to do, clean these up now.
   1628   SSAUpdater SSAUpdate;
   1629   SmallVector<Use*, 16> UsesToRename;
   1630   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
   1631     // Scan all uses of this instruction to see if it is used outside of its
   1632     // block, and if so, record them in UsesToRename.
   1633     for (Use &U : I->uses()) {
   1634       Instruction *User = cast<Instruction>(U.getUser());
   1635       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
   1636         if (UserPN->getIncomingBlock(U) == BB)
   1637           continue;
   1638       } else if (User->getParent() == BB)
   1639         continue;
   1640 
   1641       UsesToRename.push_back(&U);
   1642     }
   1643 
   1644     // If there are no uses outside the block, we're done with this instruction.
   1645     if (UsesToRename.empty())
   1646       continue;
   1647 
   1648     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
   1649 
   1650     // We found a use of I outside of BB.  Rename all uses of I that are outside
   1651     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
   1652     // with the two values we know.
   1653     SSAUpdate.Initialize(I->getType(), I->getName());
   1654     SSAUpdate.AddAvailableValue(BB, I);
   1655     SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
   1656 
   1657     while (!UsesToRename.empty())
   1658       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
   1659     DEBUG(dbgs() << "\n");
   1660   }
   1661 
   1662   // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
   1663   // that we nuked.
   1664   BB->removePredecessor(PredBB, true);
   1665 
   1666   // Remove the unconditional branch at the end of the PredBB block.
   1667   OldPredBranch->eraseFromParent();
   1668 
   1669   ++NumDupes;
   1670   return true;
   1671 }
   1672 
   1673 /// TryToUnfoldSelect - Look for blocks of the form
   1674 /// bb1:
   1675 ///   %a = select
   1676 ///   br bb
   1677 ///
   1678 /// bb2:
   1679 ///   %p = phi [%a, %bb] ...
   1680 ///   %c = icmp %p
   1681 ///   br i1 %c
   1682 ///
   1683 /// And expand the select into a branch structure if one of its arms allows %c
   1684 /// to be folded. This later enables threading from bb1 over bb2.
   1685 bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
   1686   BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
   1687   PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
   1688   Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
   1689 
   1690   if (!CondBr || !CondBr->isConditional() || !CondLHS ||
   1691       CondLHS->getParent() != BB)
   1692     return false;
   1693 
   1694   for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
   1695     BasicBlock *Pred = CondLHS->getIncomingBlock(I);
   1696     SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
   1697 
   1698     // Look if one of the incoming values is a select in the corresponding
   1699     // predecessor.
   1700     if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
   1701       continue;
   1702 
   1703     BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
   1704     if (!PredTerm || !PredTerm->isUnconditional())
   1705       continue;
   1706 
   1707     // Now check if one of the select values would allow us to constant fold the
   1708     // terminator in BB. We don't do the transform if both sides fold, those
   1709     // cases will be threaded in any case.
   1710     LazyValueInfo::Tristate LHSFolds =
   1711         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
   1712                                 CondRHS, Pred, BB, CondCmp);
   1713     LazyValueInfo::Tristate RHSFolds =
   1714         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
   1715                                 CondRHS, Pred, BB, CondCmp);
   1716     if ((LHSFolds != LazyValueInfo::Unknown ||
   1717          RHSFolds != LazyValueInfo::Unknown) &&
   1718         LHSFolds != RHSFolds) {
   1719       // Expand the select.
   1720       //
   1721       // Pred --
   1722       //  |    v
   1723       //  |  NewBB
   1724       //  |    |
   1725       //  |-----
   1726       //  v
   1727       // BB
   1728       BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
   1729                                              BB->getParent(), BB);
   1730       // Move the unconditional branch to NewBB.
   1731       PredTerm->removeFromParent();
   1732       NewBB->getInstList().insert(NewBB->end(), PredTerm);
   1733       // Create a conditional branch and update PHI nodes.
   1734       BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
   1735       CondLHS->setIncomingValue(I, SI->getFalseValue());
   1736       CondLHS->addIncoming(SI->getTrueValue(), NewBB);
   1737       // The select is now dead.
   1738       SI->eraseFromParent();
   1739 
   1740       // Update any other PHI nodes in BB.
   1741       for (BasicBlock::iterator BI = BB->begin();
   1742            PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
   1743         if (Phi != CondLHS)
   1744           Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
   1745       return true;
   1746     }
   1747   }
   1748   return false;
   1749 }
   1750