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